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HomeMy WebLinkAbout11.Geotechnical Engineering Report Geotechnical Engineering Report TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Prepared For: Tuscan Ridge Associates, LLC c/o NexGen Engineering & Consulting, LLC 1043 Nichols Court, Suite 200 Rocklin, California 95765 Geotechnical Engineering Report TUSCAN RIDGE SUBDIVISION Paradise, California WKA No. 12206.07 TABLE OF CONTENTS INTRODUCTION .................................................................................................................... 1 Scope of Services ....................................................................................................... 1 Supplemental Information ............................................................................................ 2 Figures and Attachments ............................................................................................. 2 Proposed Development ............................................................................................... 2 FINDINGS ............................................................................................................................... 3 Site Description ........................................................................................................... 3 Historical Aerial Photograph Review ............................................................................ 4 Geology ....................................................................................................................... 4 Subsurface Soil Conditions .......................................................................................... 6 Groundwater................................................................................................................ 7 CONCLUSIONS ...................................................................................................................... 7 Seismic Design Criteria ............................................................................................... 7 Soil Expansion Potential .............................................................................................. 8 Foundation Support ..................................................................................................... 8 Excavation Conditions ................................................................................................. 9 Soil Suitability for Engineered Fill Construction ............................................................ 9 Groundwater and Seasonal Moisture .......................................................................... 10 Pavement Subgrade Quality ........................................................................................ 11 Undocumented Fill....................................................................................................... 11 Seismic and Geologic Hazards .................................................................................... 11 RECOMMENDATIONS ........................................................................................................... 13 General ....................................................................................................................... 13 Site Clearing ................................................................................................................ 13 Subgrade Preparation ................................................................................................. 15 Engineered Fill ............................................................................................................ 16 Cut-Fill Transitions....................................................................................................... 18 Cut and Fill Slopes ...................................................................................................... 18 Utility Trench Backfill ................................................................................................... 19 Foundation Design ...................................................................................................... 20 Interior Floor Slabs ...................................................................................................... 22 Moisture Penetration Resistance ................................................................................. 22 Geotechnical Engineering Report TUSCAN RIDGE SUBDIVISION Paradise, California WKA No. 12206.07 TABLE OF CONTENTS (continued) Exterior Flatwork ......................................................................................................... 23 Pavement Design ........................................................................................................ 24 Retaining Walls ........................................................................................................... 26 Site Drainage ............................................................................................................... 27 Drought Considerations ............................................................................................... 27 Geotechnical Engineering Construction Observation Services .................................... 28 LIMITATIONS ......................................................................................................................... 28 FIGURES Vicinity Map .......................................................................................................... Figure 1 Site Plan .............................................................................................................. Figure 2 Geologic Map ....................................................................................................... Figure 3 Test Pit Soil Profile Graph .................................................................................... Figure 4 Logs of Current Test Pits .................................................................. Figures 5 through 15 Unified Soil Classification System ....................................................................... Figure 16 APPENDIX A – General Information, Field Exploration and Laboratory Testing Grain-Size Distribution Test Results .................................................................. Figure A1 Atterberg Limits Test Result .............................................................................. Figure A2 Expansion Index Test Result ............................................................................. Figure A3 Resistance Value Test Results .......................................................................... Figure A4 Resistance Value Test Results .......................................................................... Figure A5 Corrosion Test Result ........................................................................................ Figure A6 Sulfate Test Result ............................................................................................ Figure A7 Corrosion Test Result ........................................................................................ Figure A8 Sulfate Test Result ............................................................................................ Figure A9 Geotechnical Engineering Report TUSCAN RIDGE SUBDIVISION 3100 Skyway Road Paradise, California WKA No. 12206.07 May 6, 2021 INTRODUCTION We have completed a geotechnical engineering study for the proposed Tuscan Ridge residential subdivision to be constructed south of Skyway Road, between Chico and Paradise, California. The purpose of our study has been to explore the existing site, soil, bedrock and groundwater conditions, and to provide geotechnical engineering conclusions and recommendations for the design and construction of the planned residential development. This report presents the results of our study. Scope of Services Our scope of services for this project included the following tasks: 1. A site reconnaissance; 2. Review of United States Geological Survey (USGS) topographic maps, geologic maps and reports that included the project area, historical aerial photographs, and available groundwater information; 3. Review of previous environmental assessments completed by Wallace-Kuhl and Associates (WKA) at the project area. These assessment included the excavation of 40 test pits to a maximum depth of approximately 6½ feet below existing site grade (bsg). Practical refusal was encountered at each of the test pits in resistant volcanic mudflow deposits (lahars) of the Tuscan Formation; 4. Subsurface exploration, including the excavation of 11 supplemental test pits to a maximum depth of approximately three feet bsg. Like the previous test pits, practical refusal was encountered at each of the current test pits in resistant lahars of the Tuscan Formation; 5. Laboratory testing of selected soil samples to determine engineering properties of the soil; 6. Engineering analyses; and, 7. Preparation of this report. Geotechnical Engineering Report Page 2 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Supplemental Information Supplemental information used in the preparation of this report included review of the following environmental and hydro-geologic studies prepared for the subject property: • Phase I Environmental Site Assessment (WKA No. 12206.01 and 12206.04, dated February 26, 2019 and April 21,2020); • Hydrogeologic Investigation (WKA No. 12206.03, ongoing); and, • Site Inspection Report II (WKA No. 12206.04, dated April 15, 2020). Figures and Attachments This report contains a Vicinity Map as Figure 1; a Site Plan showing the previous and current test pit locations as Figure 2; a geologic map for the project area as Figure 3; a graph summarizing the soil profiles encountered at the test pits as Figure 4; and the current test pit logs as Figures 5 through 15. An explanation of the symbols and classification system used on the boring logs is contained on Figure 16. Appendix A contains general information regarding the exploratory methods used during our field investigation and the laboratory test results that are not included on the logs. Proposed Development We understand the 175-acre property (Site) will be subdivided into individual lots for low to medium density residential homes. The Site is identified by two Butte County Assessor Parcel Numbers (APNs): 040-520-100 and -103. We anticipate the proposed homes will be one and two-story, wood frame structures supported on shallow spread foundations with interior concrete slab-on-grade floor systems. Structural loading is anticipated to be relatively light, typical for the anticipated type of structures. Appurtenant construction is anticipated to include buried utilities, paved streets, retaining walls and various concrete flatwork. Topographically, the site is gently to moderately sloping with general drainage to the southwest with about 280 feet of relief based on the 2012 United States Geological Survey (USGS) 7.5-Minute Series Topographic map of the Hamlin Canyon, California quadrangle. The proposed project currently is in the conceptual stage, and grading plans were not available to us at the time this report was prepared. We anticipate the proposed lots will be graded to generally conform to the existing topography with maximum cuts and fills in the range of about five feet or less. Excavations for buried utilities are not anticipated to extend more than 10 feet below final site grade. Geotechnical Engineering Report Page 3 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 FINDINGS Site Description Prior to our current site investigation, we understand that the irregular-shaped property was once occupied by the Tuscan Ridge Golf Club golf course that was abandoned on or about 2018. In the summer and fall of 2018, the site was used as a Pacific Gas &Electric (PG&E) vegetation management Camp and then used as a basecamp for emergency response operations during the Camp Fire until April 2019. PG&E then used the site as a basecamp for debris removal until March 2020. At the time of our current field explorations, PG&E had removed all previous structures, equipment, stockpiled materials, and vehicles. Except for a strip of land adjacent to Skyway Road in the northwest portion and the northern portion of the site, it appeared that most of the area was disturbed to some degree during construction and operation of the basecamp. Large areas of the site were covered by crushed rock and aggregate base material that had been used to construct pads for various roads, parking areas and temporary structures. A vacant Quonset hut-type structure with a concrete floor slab was located in the eastern one-third of the property. Other structures on the property included the former golf clubhouse in the southwest portion of the site, a maintenance building, and former golf cart storage canopy. Santa Rosa Road extends south and southeast from Skyway Road to the former clubhouse location. Two ponds were noted near the southeastern property line and appeared to be approximately 4½ acres. The ponds were lined and surrounded by chain-link fence. Green vent pipes were located west of the former clubhouse. We understand the vent pipes are apparently associated with a leach field bioactive system installed for the basecamp operations. Several square concrete pads were noted in the central portion of the property. The pads each had numerous conduits protruding from them. The remainder of the site had a hummocky appearance and was covered by a moderate to heavy growth of weeds, grasses and scattered cobbles and boulders. Mature oak and other trees and brush were dispersed throughout. A gate with large piles of decorative boulders on both sides was located at the entrance to the site at the intersection of Santa Rosa Road and Skyway Road. Long rows of boulders extended east and west from the entrance along the north boundary of the site. Geotechnical Engineering Report Page 4 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 The site is bounded to the north by Skyway Road, beyond which is vacant land, Butte Creek Canyon and Butte Creek. The Paradise Rod and Gun Club and vacant land are located to the east of the site. Undeveloped vacant land is located to the south and west of the site. Historical Aerial Photograph Review We reviewed historical aerial photographs of the site available from our files, Google Earth Pro software (Google, 2018), and the website HistoricalAerials.com. The reviewed photographs were taken intermittently from 1941 to 2018. Review of the aerial photograph from 1941 shows the site to be essentially open grassland with scattered trees. An unpaved road generally traverses the south perimeter of the property. In a 1951 photograph, the site appears essentially the same, however, Skyway Road has now been constructed along the northern boundary of the property. In several photographs from 1951 to about 2002, the site appears to be essentially unchanged. In a 2003 aerial photograph, the Tuscan Ridge Golf Club and golf course have been constructed in the southern portion of the property, with the golf course in the central portion of the site still under construction. One of the two ponds currently located in the southwestern portion of the site is visible. The second pond appears to have been excavated but not filled with water. The northern portion of the site appears undisturbed. Santa Rosa Road, running north-south from Skyway Road, is visible. The golf course appears to be complete in a 2005 photograph with a club house and parking lot, at the south end of Santa Rosa Road, and several structures with a gravel covered driveway, parking, and storage in the east-central portion of the site. The site appears to be essentially unchanged in several photographs between 2005 and 2018. In an early 2018 photograph, the golf course appears to be abandoned with brown fairways and greens. In a December 2018 photograph, the central and southwestern portions of the site is occupied by what appears to be the beginning stages of the emergency basecamp erected after the Camp Fire by PG&E and other contractors. Geology The project site is located along the northeastern edge of the Great Valley geomorphic province of California. Situated between the granitic and metamorphic basement rock which forms the Sierra Nevada range and the sedimentary and volcanic rock units of the Coast Ranges, the province is a vast asymmetrical, synclinal trough formed by uplifting of the Sierran block to form the Sierra Nevada mountains with the western side dropping to form the valley. Erosion of the Geotechnical Engineering Report Page 5 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 adjacent Sierra Nevada and Coast Ranges has in-filled the valley with a thick sequence of unconsolidated to semi-consolidated Quaternary (Pleistocene and Holocene) age alluvial, basin, and delta plain sediments deposited by the Sacramento and San Joaquin rivers and their tributaries. The project site is located within the Sierra Foothills, east of the Chico Monocline, a broad upwarping caused by uplift on the east side of the Chico Monocline fault, located a few miles east of Chico. The primary geologic formation with the project area is the Tuscan Formation extending from Redding south to near Oroville, where surface exposures are seen on the east side of the Great Valley. Overall, the Tuscan Formation is composed of a series of volcanic lahars (mudflows) that include volcanic conglomerate, sandstone, siltstone, and pumiceous tuff layers that were deposited over a period of about 1 million years (Helley and Harwood 1985)1. The source areas of the lahars were the eroded ancestral volcanoes, Mount Yana and Mount Maidu, which were historically located northwest and south of Lassen Peak in the Cascade Range (Lydon 1968)2. As the lahars flowed westward off the ancestral volcanoes and onto the valley floor, they fanned out, causing deposition that varies in thickness and topographic elevation. Over time, ancient streams and rivers flowed downslope over the lahars, forming channels which were then infilled with reworked volcanic sand and gravel sediments. East of the of the Chico Monocline, the Tuscan Formation has been uplifted to form the south to southwest sloping Sierra Foothills east of Chico. Subsequent streams and other drainages have cut their way into the Tuscan to form deep, steep-sided, narrow canyons separated by equally long and narrow, fingerlike ridges or mesas. The total effect is a subparallel arrangement of canyons and southwestward sloping ridge-crests. The site is situated on one of the fingerlike ridges (Coon Ridge) between Butte Creek Canyon to the north and Nance Canyon to the south. Rock exposed at the surface of the site is mapped by Helley and Harwood (1985) as Unit C (denoted as Ttc) of the Tuscan Formation. Unit C is described as lahars with some interbedded volcanic conglomerate and sandstone locally, separated from overlying units by partially stripped soil horizon. Within the general project area, the lahars are described as 3 to 12 meters thick layers separated from each other by thin layers of volcanic sediments containing abundant casts of wood fragments and prominent cooling fractures. Per Harwood et al (1981)3, Unit C is described as predominantly lahars composed of angular to subrounded volcanic fragments (cobbles and boulders) in a matrix of gray-tan 1 Helley E.J. and Harwood D.S. (1985), Geologic Map of the Late Cenozoic Deposits of the Sacramento Valley and Northern Sierran Foothills, California, 1:62,500: United states Geological Survey Map MF-1790 2 Lydon P.A (1968), Geology of Lahars of the Tuscan Formation, Northern California, The Geological Society of America Volume 116 3 Harwood D.S. (1981), Geologic Map of the Chico Monocline and Northeastern Park of the Sacramento Valley, California, 1:62,5500: United States Geological Survey Map I-1238 Geotechnical Engineering Report Page 6 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 volcanic mudstone in excess of 150 feet in total thickness. A geologic map of the project area is presented as Figure 3. Surface exposures of the lahar are common all over the site. Areas where hard lahar is exposed at the surface or beneath a thin mantle of soil are referred to by the local contractors as “lava cap.” The individual lahar units dip at approximately one to five degrees to the southwest, which also generally conforms to the topography of the site. Many of the current and former tree lines visible on aerial photographs generally follow the boundaries between lahar units. Subsurface Soil Conditions The subsurface soil conditions at the project site were initially explored on March 15, 2019, by excavating 40 test pits using a track mounted excavator to depths ranging from a few inches to about 6½ feet bgs. On March 17, 2021, 11 additional test pits were excavated using a small excavator to depths ranging from about six inches to about three feet bgs. The approximate test pit locations are presented as Figure 2. In general, the site is mantled with relatively thin soil deposits, ranging from less than ½ foot to about 3½ feet (average of about 14 inches), underlain by lahar of the Tuscan Formation, Unit C. The soils generally are composed of clayey sand to sandy lean clay with variable concentrations of gravel, cobble and occasional boulder to clayey gravels. Based on laboratory testing, these soils are low plasticity clays with a very low to low expansion potential. At many of the test pits, the native soil and lahar were overlain by crushed gravel, aggregate base and disturbed fill soils placed during construction of the PG&E basecamp. The underlying Tuscan formation consists of variably weathered and strong lahar. The lahar is a fine-grained matrix of mud, volcanic ash, sand and gravel with inclusions of cobble and boulder. At each test pit explored, the lahar allows none to a few inches of penetration with the excavators before practical refusal to further excavation was encountered. The subsurface conditions described above are a generalized interpretation of the soil and bedrock conditions encountered. For specific information regarding the soil conditions encountered at each of the most recent exploration locations, refer to the exploration logs presented as Figures 5 through 15. Detailed test pit logs were not maintained during the explorations in 2019. A graph showing a summary of the previous test pit findings, along with our current findings, is presented as Figure 4. Geotechnical Engineering Report Page 7 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Groundwater Groundwater was not encountered at the time of our field explorations. Furthermore, no surface evidence of springs or seepage was observed. A well log completed for a well on the property suggests that groundwater in the project area is greater than 500 feet below the existing ground surface (bgs). This geotechnical evaluation assumes that high groundwater at the project site will not exceed this elevation. CONCLUSIONS It is our opinion that development of the site with a residential subdivision is feasible from a geotechnical standpoint, provided that the conclusions and recommendations presented in this report are incorporated into the project design and specifications. The principal geotechnical considerations are discussed in the following sections. Seismic Design Criteria The 2019 edition of the California Building Code (CBC) references the American Society of Civil Engineers (ASCE) Standard 7-16 for seismic design. Using the latitude and longitude for the approximate center of the project site, Table 1 provides 2019 seismic design parameters developed using a web interface developed by the Structural Engineers Association of California (SEAOC) and the California Office of Statewide Health Planning and Development (OSHPD) (https://seismicmaps.org). Since S1 is greater than 0.2g, the 2019 CBC coefficient values Fv, SM1, and SD1 presented are valid for seismic design, provided the requirements in Exception Note No. 2 in Section 11.4.8 of ASCE 7-16 apply, specifically if T ≤ 1.5TS. If not, a site-specific ground motion hazard analysis is required. Geotechnical Engineering Report Page 8 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Table 1 Latitude: 39.7129° N Longitude: 121.7086° W ASCE 7-16 Table/Figure 2019 CBC Table/Figure Factor/ Coefficient Value Short-Period MCE at 0.2 seconds Figure 22-1 Figure 1613.2.1(1) SS 0.714 1.0 second Period MCE Figure 22-2 Figure 1613.2.1(2) S1 0.297 Soil Class Table 20.3-1 Section 1613.2.2 Site Class C Site Coefficient Table 11.4-1 Table 1613.2.3(1) Fa 1.214 Site Coefficient Table 11.4-2 Table 1613.2.3(2) Fv 1.5 Adjusted MCE Spectral Response Parameters Equation 11.4-1 Equation 16-36 SMS 0.867 Equation 11.4-2 Equation 16-37 SM1 0.446 Design Spectral Acceleration Parameters Equation 11.4-3 Equation 16-38 SDS 0.578 Equation 11.4-4 Equation 16-39 SD1 0.297 Seismic Design Category Table 11.6-1 Table 1613.2.5(1) Risk Category I to IV D Table 11.6-2 Table 1613.2.5(2) Risk Category I to IV D Notes: MCE = Maximum Considered Earthquake; g = gravity Soil Expansion Potential The near-surface sandy clays and clayey gravels encountered during our explorations are low- plasticity materials with low expansion (shrink/swell) characteristics (EI = 14). Furthermore, the underlying lahar bedrock is non-expansive. Accordingly, measures to resist or control potential soil expansion pressures are not considered necessary on this project Foundation Support Based on the native subsurface conditions encountered, shallow spread foundations should provide adequate support for the anticipated one- to two-story single-family homes provided the recommendations presented in this report are incorporated into the project design and specifications. In areas of fill, these soils and/or an approved import soil should also provide adequate support for foundations provided they are placed and compacted in accordance recommendations provided in this report. Geotechnical Engineering Report Page 9 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Excavation Conditions The relatively thin layer of surface and near-surface soil and surficial cobbles and boulder at the site should be readily excavated using conventional earthmoving and trenching equipment. The underlying lahar or “lava cap”, however, can be particularly resistant, requiring heavy equipment such as a Caterpillar D10L fitted with a single tooth ripper for general earthwork and hydraulic shovels with case hardened steel ripper or rock trenching equipment, i.e., a “rock wheel”, to excavate utility and foundation trenches. Localized blasting or the use of a jack hammer may be required to remove large andesitic boulders in confined trenches. Shallow excavations (less than five feet deep) in the soil mantel covering the lahar should stand vertically for a period long enough for typical foundation and utility construction, unless they become wet or are disturbed. Sand, however, may cave and/or slough soon after it is exposed in the excavation. Where encountered, the contractor should be prepared to brace or shore the excavations, as necessary. Excavations into the lahar (lava cap) and any conglomerates, if encountered, should stand near vertical, although fractures in the rock may result in local instability. Temporarily excavations less than 20 feet in depth should be constructed in accordance with federal, local and OSHA standards (29 CFR Part 1926) under the guidance of the Contractors qualified “competent person.” For preliminary evaluation, the soils encountered would classify as Cal-OSHA Type C soil, while the lahar would classify as Type A soil. In no case should the information provided be interpreted to mean that Wallace-Kuhl & Associates is assuming responsibility for site safety or the Contractor’s activities. Excavated materials should not be stockpiled directly adjacent to an open excavation to prevent surcharge loading of the excavation sidewalls. Heavy or frequent truck and equipment traffic should also be avoided near excavations. If material is stored or heavy equipment is stationed and/or operated near an excavation, a shoring system must be designed to resist the additional pressure due to the superimposed loads. Soil Suitability for Engineered Fill Construction The soils encountered are considered suitable for use in engineered fill construction provided these materials do not contain rubble, rubbish, significant organic concentrations and are at a moisture content appropriate for compaction. Screening may be required to remove over-sized cobbles and boulder. Imported materials, if necessary, should be granular and approved by our office prior to importing the materials to the site. Geotechnical Engineering Report Page 10 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 The lahar (lava cap) includes andesite cobbles and boulders in a fine grained matrix of hardened mud, volcanic ash, sand and gravel. During excavation, it is expected that large fragments will be generated that will need to be broken-down or crushed to six inches or less in size in order to use the material for engineered fill. Resistant fragments will need to be placed in the lower portion of deep fills or screened and disposed outside of lots and pavement areas. Even with processing, fills composed of fractured lahar may need to be mixed with soil to avoid concentrations or nesting of rock fragments. Crushing of the fine-grained matrix of the lahar may produce materials suitable for uses such as aggregate base or pervious sand or gravel drainage material. While potentially feasible, it not known whether crushing will produce a material with the appropriate aggregate sizes or if the material will meet Caltrans standards for durability (Durability Index, R-value, Sand Equivalent) specified for aggregate base. If considered, we suggest that a trial be performed prior to bidding to evaluate equipment capabilities and procedures so that bidders can develop responsive bids. The trial should also include laboratory tests on the processed material to determine its physical properties. Groundwater and Seasonal Moisture Based on our observations and previous referenced data, no spring activity was observed and groundwater levels should not encroach near-surface or impede grading operations at the site. It’s not uncommon, however, to encountered seepage accumulating and/or flowing from between individual lahar units, within fractures of the lahar and/or as moisture perching atop and seeping over the lahar. Furthermore, if site grading is performed during or following extended periods of rainfall (winter and spring months), the moisture content of the near-surface soils may be significantly above optimum and unstable. Controlling and diverting seepage and stormwater runoff away from the proposed improvements will be a critical element in developing the Site. Since the project is in the conceptual stage and grading plans are not currently available, the layout for the proposed subdivision is unknown. In most instances, gravel filled utility trenches and pavement subgrades can be utilized to intercept and collect seepage, runoff and landscape irrigation. To prevent water accumulation in the trenches or pavement baserock, it will be necessary to install a passive drainage system that collects the accumulated water and diverts it to the storm drain system for the development. If an extensive storm drain system is not planned, it will be necessary to install drainage ditches or gravel filled trenches (French drains) that intercept the subsurface water and safely divert it away from the development. Geotechnical Engineering Report Page 11 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Typical remedial measures for unstable soil conditions include discing and aerating the soils during dry weather, mixing the soils with dryer materials, removing and replacing the soils with an approved fill material, stabilization with a geotextile fabric or grid, or mixing the soils with an approved hydrating agent such as a lime or cement product. Our firm should be consulted prior to implementing any remedial measure to observe the unstable subgrade condition and provide site-specific recommendations. Pavement Subgrade Quality The results of our laboratory tests indicate the near-surface soil should provide fair support characteristics for pavements as represented by Resistance ("R") values (California Test 301) ranging from 24 to 40. The R-value test results are shown on Plates A4 and A5. Given the anticipated grading and mixing of soils during earthwork construction, and R-value of 25 was used to evaluate pavements supported by native soil or engineered soil fill. The underlying lahar or crushed lahar fills should provide good support for pavements with an R-value of at least 50. Therefore, an R-value of 50 was used to develop pavement sections supported on this material. Undocumented Fill During construction of the emergency basecamp during and following the Camp Fire in 2018, PG&E performed extensive grading and placed large areas of aggregate base throughout the central and southern portions of the site. It’s unknown if the fill and aggregate was compacted as engineered fill, however, we speculate that that no quality control or testing was performed during grading. Based on this assumption, it is our opinion that the soils disturbed and the fill and aggregate placed will not be suitable in their current condition for support of the proposed improvements due to potential settlement issues. Seismic and Geologic Hazards Butte County has a history of relatively low seismicity in comparison with more active seismic regions, such as the Bay area or Southern California. The site is not located within an Earthquake Fault Study Zone (Hart and Bryant, 2007) or an Earthquake Hazards Zone designed by the California Geologic Survey (CGS). The evaluation of potential seismic hazards was not within the scope of this study. Based on our findings and previous hazards studies in the general project area, however, it is our professional opinion that the potential for geologic hazards, such as liquefaction, fault rupture or slope instability, is unlikely. Geotechnical Engineering Report Page 12 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Soil Corrosion Potential Two samples of near-surface soil were submitted to Sunland Analytical Lab of Rancho Cordova, California, for testing to determine pH, chloride and sulfate concentrations, and minimum resistivity to help evaluate the potential for corrosive attack upon buried concrete. The results of the corrosivity testing are summarized in Table2. Copies of the test reports are presented on Figures A6 through A9. Table 2 Analyte Test Method Sample Identification TP43 (0'-0.5') TP51 (0'-3') pH CA DOT 643 Modified* 7.14 6.18 Minimum Resistivity CA DOT 643 Modified* 1210 -cm 4560 -cm Chloride CA DOT 422 23.5 ppm 4.0 ppm Sulfate CA DOT 417 118.3 ppm 4.9 ppm Sulfate – SO4 ASTM D-516 108.8 mg/kg 5.0 mg/kg Notes: * = Small cell method, -cm = Ohm-centimeters, ppm = Parts per million, mg/kg= milligrams per kilogram The California Department of Transportation Corrosion and Structural Concrete Field Investigation Branch, 2015 Corrosion Guidelines (Version 2.1), considers a site to be corrosive to foundation elements if one or more of the following conditions exists for the representative soil samples taken: the soil has a chloride concentration greater than or equal to 500 ppm, sulfate concentration greater than or equal to 2000 ppm, or the pH is 5.5 or less. Based on this criterion, the on-site soils tested are not considered unusually corrosive to steel reinforcement properly embedded within Portland cement concrete (PCC). The California Amendments to Section 10.7.5 of the American Association of State Highway and Transportation Officials (AASHTO) bridge design specifications, 6th Edition (AASHTO 2012) considers soils to be corrosive to buried metals if the minimum resistivity is 1,000 ohm-cm or less. Based on this criterion, the on-site soils tested are also not considered significantly corrosive to buried metal. Table 19.3.1.1 – Exposure Categories and Classes, of American Concrete Institute (ACI) 318-14, Section 19.3 – Concrete Design and Durability Requirements, as referenced in Section 1904.1 of the 2016 CBC, indicates the severity of sulfate exposure for the sample tested is Exposure Class S0 (water-soluble sulfate concentration in contact with concrete is low and Geotechnical Engineering Report Page 13 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 injurious sulfate attack is not a concern). The project structural engineer should evaluate the requirements of ACI 318-14 and determine their applicability to the site. Wallace-Kuhl & Associates are not corrosion engineers. Therefore, if it is desired to further define the soil corrosion potential at the site, a corrosion engineer should be consulted. RECOMMENDATIONS General The recommendations presented below are appropriate for construction in the late spring through fall months. The on-site soils will become very moist and wet following rainfall in the winter and early spring months, and likely will not be suitable for earthwork without drying by aeration, chemical treatment, or geogrid stabilization. Should the construction schedule require work to start or continue during the wet months, additional recommendations can be provided, as conditions dictate. Site preparation should be accomplished in accordance with the provisions of this report. A representative of the Geotechnical Engineer should be present during all earthwork and ground improvement construction operations to evaluate compliance with our recommendations and the guide specifications included in this report. The Geotechnical Engineer of Record referenced herein should be considered the Geotechnical Engineer that is retained to provide geotechnical engineering observation and testing services during construction and shall include either the Geotechnical Engineer or his or her representative. Site Clearing Construction areas should be cleared of any existing surface and subsurface structures to expose firm and stable soils as determined by the Geotechnical Engineer’s representative. The area to be cleared should extend at least five feet beyond the edge of all exterior foundations and at least five feet beyond any exterior flatwork or pavements, where practical. Demolition debris should be removed from the site, or used as engineered fill, provided it is processed per the recommendations included in this report. Any existing underground utilities designated to be removed or relocated should include all trench backfill and bedding materials. The resulting excavations should be restored with engineered fill placed and compacted in accordance with the recommendations included in this Geotechnical Engineering Report Page 14 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 report. On-site wells, septic systems, or below-grade tanks should be properly abandoned in accordance with State and local requirements. Any existing pavements designated for removal may be broken up, pulverized, and reused as engineered fill where appropriate, or removed from the site. If pavement rubble is to be reused as engineered fill, the material should be pulverized to fragments less than three inches in largest dimension, mixed with soil to form a compactable mixture, and must be approved by the owner. Existing surface vegetation/organics and organically laden soil within construction areas should be stripped from the site. Debris from the stripping should not be used as general fill within structure, concrete slab or pavement areas. With prior approval from the Geotechnical Engineer, strippings may be used in proposed park and landscape areas, provided they are kept at least five feet from building footprints, pavements, concrete slabs and other surface improvements. Discing of the organics into the surface soils may be a suitable alternate to stripping, depending on the condition and quantity of the organics at the time of grading. The decision to utilize discing in lieu of stripping should be made by the Geotechnical Engineer, or his representative, at the time of earthwork construction. Discing operations, if approved, should be observed by the Geotechnical Engineer’s representative, and be continuous until the organics are adequately mixed into the surface soils to provide a compactable mixture of soil containing minor amounts of organic matter. Pockets or concentrations of organics will not be allowed. Any trees, bushes and other vegetation designated for removal should include the entire root- ball and roots larger than ½-inch in diameter. Adequate removal of debris and roots may require laborers and handpicking to clear the subgrade soils to the satisfaction of the Geotechnical Engineer’s representative. Any on-site ditches, swales or detention ponds should be fully drained of water and cleaned of organics. Saturated and unstable soils exposed should be removed to expose firm, native soil or rock, as verified by the Geotechnical Engineer. These soils will likely be saturated and will require aeration and a period of drying to allow proper compaction. Organically contaminated soils will not be suitable for use in engineered fill construction. Depressions resulting from site clearing operations, as well as any loose, soft, disturbed, wet, or organically contaminated soils, as identified by the Geotechnical Engineer’s representative, should be cleaned out to firm, undisturbed soils or lahar and backfilled with engineered fill placed and compacted in accordance with the recommendations in this report. It is important Geotechnical Engineering Report Page 15 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 that the Geotechnical Engineer’s representative be present during site clearing operations to verify adequate removal of the surface and subsurface items, as well as the proper backfilling of resulting excavations. Subgrade Preparation Based on our findings, a large portion of the Site is underlain by anywhere from a few inches to over three feet of undocumented, variably-dense, soil fill, gravel and scattered debris. In our opinion, these materials, in their current condition, will not be suitable for support of the proposed structures and pavements due to potential settlement issues. The most direct method to improve the subgrade conditions would be to overexcavate the undocumented and deleterious materials to expose firm soil or rock, remove any deleterious materials encountered, and restore the area with compacted engineered fill. The zone of overexcavation and compaction should extend at least five feet beyond any structural foundations or concrete slabs. In proposed exterior flatwork and pavement areas, the lateral zone of overexcavation and compaction can be reduced to two feet beyond the proposed improvements. Where building pads will be located over former ponds and depressions from the golf course development, the depressions should be widened as necessary to reduce the overall fill differential to less than two feet. The affected lots should be clearly shown on the project grading plans. The native, undisturbed soils and highly weathered lahar are relatively loose and we anticipate that clearing operations will likely cause additional disturbance to the upper soils. Therefore, in all areas that will support concrete slabs, engineered fill or pavement, the surface soils should be thoroughly scarified to a depth of at least 12-inches, brought to a uniform moisture content above the optimum moisture content, and compacted to not less than 90 percent of the maximum dry density per ASTM D1557 specifications. In pavement areas, the relative compaction of the upper 6-inches of final soil subgrade should be increased to 95 percent of the maximum dry density. Where moderately to unweathered lahar rock is exposed, no scarification should be necessary; however, these surfaces should be proof-rolled to a firm and unyielding condition. Any localized zones of soft or pumping materials observed should be scarified and compacted or be overexcavated and replaced with engineered fill. The performance of pavement is critically dependent upon uniform and adequate compaction of the soil subgrade, as well as all engineered fill and utility trench backfill within the limits of the pavements. Final pavement subgrade preparation (i.e. scarification, moisture conditioning and Geotechnical Engineering Report Page 16 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 compaction) should be performed after underground utility construction is completed and just prior to aggregate base placement. Pavement subgrades should be stable and unyielding under heavy wheel loads of construction equipment. To help identify unstable subgrades within the pavement limits, a proof-roll should be performed with a fully-loaded, water truck on the exposed subgrades prior to placement of aggregate base. The proof-roll should be observed by the Geotechnical Engineer’s representative. The prepared subgrade soils should be protected from disturbance until covered by capillary break material or aggregate base. Disturbed subgrade soils may require additional processing and recompaction just prior to construction of these improvements, depending on the level of disturbance. All subgrade preparation must be performed in the presence of the Geotechnical Engineer’s representative who will evaluate the performance of the subgrade under compaction loads and identify any loose or unstable soil conditions that could require remediation. We suggest that a rippability evaluation be performed prior to bidding to evaluate equipment capabilities and procedures for excavation and processing of the lahar and that the information be provided to the bidders. In addition, we suggest that construction bid documents should contain unit prices (price per cubic foot) for additional excavation due to unsuitable materials and replacement with engineered fill and for blasting. Engineered Fill From a geotechnical engineering standpoint, the on-site native soils blanketing the site and existing undocumented gravel and soil are considered suitable for use as engineered fill provided that they do not contain significant quantities of organics, rubble and deleterious debris, and are at a proper moisture content to achieve the desired degree of compaction. The lahar bedrock, boulder, or approved inert debris, i.e., concrete or asphalt-concrete pavement, that breaks into fragments less than six inches in maximum dimension can be used as engineered fills within the upper three feet of final soil subgrade beneath proposed floor slabs and pavement, and within the upper five feet of final soil subgrade beneath building foundations. The lahar and debris fragments should be thoroughly mixed with soil to avoid concentrating or nesting the material. Lahar or concrete or asphalt fragments ranging from six to 18 inches in maximum dimension may be placed below these depths provided they are also thoroughly mixed with soil. If the rock Geotechnical Engineering Report Page 17 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 and/or debris does not break down to a gradation compatible with in-place density testing, then compactive effort should be applied using track equipment weighing at least 20 tons (Caterpillar 815 or larger) until there is no perceptible increase in fragmentation of the particles or observable consolidation of the fill during repeated passes of the compaction equipment. In pavement areas, lahar or debris fragments greater than 18 inches in maximum size may be included in engineered fills below a depth of five feet, but only at the foundation level for the fill. The boulders or fragments should be staggered and spaced so that soil or crushed lahar fill can be machine placed and compacted between them to form an interstitial fill. As an alternative, flooding and jetting can be used to sluice cohesionless soil, i.e., sand, into voids between the boulders and fragments. Following sluicing, this fill course should be proof-rolled with heavy track equipment until there is no observable consolidation of the fill beneath the equipment. Fragments greater than 24 inches in maximum size should not be included in any fill. Engineered fill consisting of on-site soil, highly weathered lahar, existing undocumented fill material, or import materials should be placed in lifts not exceeding six inches in compacted thickness, with each lift being thoroughly moisture conditioned to at least the optimum moisture content and uniformly compacted to at least 90 percent relative compaction. The upper six inches of engineered fill placed in pavement areas should be uniformly compacted to at least 95 percent relative compaction at a moisture content of at least the optimum moisture content. Imported fill materials should be compactable, well-graded, granular soils with a Plasticity Index not exceeding 15 when tested in accordance with ASTM D4318; an Expansion Index of 20 or less when tested in accordance with ASTM D4829; and, should not contain particles greater than three inches in maximum dimension. Imported fill material to be used within pavement areas should possess a Resistance value of 40 or higher, when tested in accordance with California Test 301. In addition, with the exception of imported aggregate base and bedding/initial fill materials for underground utility construction, the contractor should provide appropriate documentation for all imported fill materials that designates the import materials do not contain known contaminants per Department of Toxic Substances Control’s guidelines for clean imported fill material (DTSC, 2001), and have corrosion characteristics within acceptable limits. Imported soils should be approved by the Geotechnical Engineer prior to being transported to the site. The Geotechnical Engineer’s representative be present on a regular basis during all earthwork operations to observe and test the engineered fill and to verify compliance with the recommendations of this report and the project plans and specifications. Geotechnical Engineering Report Page 18 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Cut-Fill Transitions We anticipate that some structures could be supported by building pads that transition between compressible fill or native soil and essentially uncompressible lahar. Because of the different physical properties and thus support characteristics of these two materials, there is a possibility that unpredictable and sometimes adverse differential settlement and concrete cracking could occur within this transition. In these situations, the lahar should be undercut, if feasible, and replaced with engineered fill to maintain a maximum differential fill thickness of two feet. As an alternative, foundations should be deepened to bear on the lahar and floor slabs should be reinforced to resist differential movement and cracking. The overexcavation should extend laterally at least five feet beyond the perimeter of the structure. Cut and Fill Slopes Although grading plans were not available at the time this report was prepared, we anticipate that slopes ranging from about five to 10 feet in vertical height may be planned. In our professional opinion, permanent cut and fill slopes should be inclined no steeper than two horizontal to one vertical (2(h):1(v)). This slope recommendation is based on our experience with similar conditions since no detailed slope stability analysis was performed to justify steeper slopes. Cut slopes in the lahar can likely be inclined at gradients of 1(h):1(v) or steeper if no adverse fractures are present. If slopes with gradients steeper than 2(h):1(v) are considered or slopes will be greater than 10 feet in vertical height, the Geotechnical Engineer should review the project grading plan and provide additional guidance regarding stable slope configuration and drainage design. Additional geotechnical exploration, testing and evaluation may be required. Given this 2(h):1(v) inclination, there is a modest risk that displacement and/or movement could occur in the event of strong seismic ground shaking. For the native soils, highly weathered lahar and compacted fill conditions anticipated, we expect this movement to be relatively shallow, requiring limited cleanup and dressing to restore the slopes to their original condition. If this risk is unacceptable, the slopes should be flattened to 3(h):1(v). Where fills will be constructed on ground that slopes at an inclination of 6(h):1(v) or steeper, a two foot deep toe key should be excavated into firm, competent soil/weathered rock. The keyway should be at least four feet wide at the bottom or a width equal to ½ the vertical slope height, whichever is greater, with the bottom inclined down and back into the slope at two percent. As filling progresses, benches should also be cut into firm, competent soil/lahar. Each bench should consist of a level terrace at least four feet wide with the rise to the next bench held to three feet or less. Geotechnical Engineering Report Page 19 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 It is difficult to construct fill on the specified slopes without leaving a loose, poorly-compacted soil zone on the slope face. To reduce sloughing and erosion, the fill slopes should be slightly over-built, then cut back to firm, well-compacted soils prior to applying vegetative cover. If slopes cannot be over-built and cut back, the finished soil slopes should be compacted to reduce, as much as practical, the thickness of the loose surficial veneer. The compaction may be done by making several coverages from top to bottom of the slopes with a track-mounted bulldozer, front-end loader, or sheeps foot compactor. Paved interceptor drains should be provided along the tops of slopes where the tributary area flowing toward the slope has a drainage path greater than 40 feet, measured horizontally. The interceptor drains should be sloped to a suitable drainage device and disposed off-site well below the toe of the slope. Drop inlets and drainage pipes should not be installed near the crests of slopes because leakage can result in maintenance problems or possible slope failure. The slopes should be inspected periodically for erosion, and if detected, repaired immediately. Interceptor drains should be cleaned before the start of each rainy season, and if necessary, after each rainstorm. To reduce erosion and gulling, all disturbed areas should be planted with erosion-resistant vegetation suited to the area. As an alternative, jute netting or geotextile erosion control mats can be installed per the manufacturer’s recommendations. Utility Trench Backfill Utility trench backfill should be mechanically compacted as engineered fill in accordance with the following recommendations. Bedding of utilities and initial backfill around and over the pipe should conform to the manufacturer’s recommendations for the pipe materials selected and applicable sections of the governing agency standards. If open-graded, crushed rock is used as bedding or initial backfill, an approved geotextile filter fabric should be used to separate the crushed rock from finer-grained soils. The intent of geotextile filter fabric is to prevent soil from migrating into the crushed rock (piping), which could result in trench settlement. As discussed in the Conclusions, controlling and diverting seepage and stormwater runoff away from the proposed improvements will be a critical element to development of the site. During or following wet weather, infiltrating storm runoff will likely create a temporary perched water condition and seepage above the hard lahar. If uncontrolled, the seepage could migrate beneath or into structures and beneath or through pavement aggregates, leading to moisture issues and instability. Gravel filled utility trenches and pavement subgrades can often be utilized to intercept and collect seepage, runoff and landscape irrigation, however, trenches should include a passive dewatering system that diverts the collected water into a sump or to storm drain manholes or drop inlets. An example of a passive system would include a perforated drainpipe enclosed in Caltrans Class 2 permeable rock and/or clean gravel and Geotechnical Engineering Report Page 20 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 geotextile filter fabric is often placed at a depth of five to eight feet in the storm drain trenches. If a storm drain system is not planned throughout the development, it will be necessary to install drainage ditches or gravel filled trenches (French drains) that intercept the subsurface water and safely divert it away from the proposed improvements. Once a grading and utility plan has been developed, the Geotechnical Engineer should review the plans and provide additional guidance as to the location and details for the drainage system. In building pad areas, utility trenches, i.e., sewer laterals, yard drains, water services, etc. should slope down and away from structures. Furthermore, low-permeable materials, i.e., silt, clay or an approved controlled low strength material (CLSM), should be used as backfill for utility trenches located within the building footprints and extending at least five feet horizontally beyond perimeter foundations to reduce water transmission beneath the buildings. Utility trench backfill should be placed in maximum 12 inch-thick lifts (loosely placed thickness), thoroughly moisture conditioned to at least the optimum moisture content, and mechanically compacted to at least 90 percent relative compaction. Within the upper six inches of pavement subgrade soils, compaction should be increased to at least 95 percent relative compaction at no less than the optimum moisture content. The lift thickness will be dependent of the type of compaction equipment used. Underground utility trenches that are aligned nearly parallel with shallow foundations should be at least three feet from the outer edge of foundations, wherever possible. As a general rule, trenches should not encroach into the zone extending outward at 1(h):1(v) inclination below the bottom of shallow foundations. Additionally, trenches parallel to shallow foundations should not remain open longer than 72 hours. The intent of these recommendations is to prevent loss of both lateral and vertical support of shallow foundations, resulting in possible settlement. Foundation Design The proposed one- and two-story residential structures may be supported upon continuous and/or isolated spread foundations extending at least 12 inches below lowest adjacent soil grade. Lowest adjacent soil grade is defined as the grade upon which the capillary break material is placed or exterior soil grade, whichever is lower. Continuous foundations supporting one- and two-story structures should maintain a minimum width of 12 inches; while isolated spread foundations should be at least 24-inches in plan dimension. Foundations should be continuous around the perimeter of the building to reduce moisture variations beneath the structures. Geotechnical Engineering Report Page 21 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Foundations bearing on undisturbed or compacted native soils, engineered fill, or a combination of those materials may be sized for maximum allowable “net” soil bearing pressure of 2,500 pounds per square foot (psf) for dead plus live load. Foundations bearing on sound lahar rock, as verified by the Geotechnical Engineer, may be sized for a maximum allowable “net” soil bearing pressure of 6,000 psf for dead plus live load. A one-third increase in the allowable bearing pressures may be applied when considering short-term loading due to wind or seismic forces. The weight of the foundation concrete extending below lowest adjacent soil grade may be disregarded in sizing computations. Total settlement of an individual foundation will vary depending on the plan dimensions of the foundation and the actual load supported. Based on the foundation criteria discussed above and the assumed foundation loads, foundations are anticipated to experience a maximum total static settlement on the order of about ½-inch or less, and differential settlement on the order of about ½-inch for 50 lineal feet or the shortest distance of the structure, whichever is less. All foundations should be adequately reinforced to provide structural continuity, mitigate cracking and permit spanning of local soil irregularities. The structural engineer should determine final foundation reinforcing requirements. Resistance to lateral foundation displacement may be computed using an allowable friction factor of 0.40, which may be multiplied by the effective vertical load on each foundation. Additional lateral resistance may be computed using an allowable passive earth pressure equivalent to a fluid pressure of 300 psf per foot of depth, acting against the vertical projection of the foundation. These two modes of resistance should not be added together unless the frictional component is reduced by 50 percent since full mobilization of the passive resistance requires some horizontal movement, effectively reducing the frictional resistance. Where excavations into the lahar are not reasonably feasible and the foundations cannot be embedded, foundation resistance to lateral and uplift forces may be achieved by rock tiedown anchors (such as grouting steel dowels) into the lahar. There are several approaches and anchor products available that would be suitable for this project. If dowels are used, a common approach would be to drill two to four inch diameter holes using air percussion to a depth of at least three feet; blowing out the hole to remove as much rock dust as possible; filling the hole with a non-shrink grout (such as Embeco 636) or an approved high strength epoxy; and then installing the dowel (such as a No. 8, grade 60 reinforcing bar). The uplift capacity of the anchor is typically assumed to be equivalent to the effective weight of bedrock within a cone or wedge defined by a 1(h):1(v) projection up from the outside edge and mid-depth of the grouted dowel. A bedrock effective unit weight of 125 pounds per cubic foot Geotechnical Engineering Report Page 22 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 and a minimum factor of safety of 2 may be used for estimating uplift. For anchors with overlapping cones, the effective weight of bedrock within the overall area of the overlapping cones should be used for determining uplift. The overlapping of the zones of influence between adjacent anchors results in anchor uplift capacity less than that for a single anchor. The actual anchor design and approach should be determined by the Contractor in coordination with the Structural Engineer. Additional rock cores or geophysical testing may be required to determine the final depth of the anchors and the design criteria. An uplift load test should be performed on some (typically 5 to 10 percent) of the completed anchors to verify the design capacity. The Geotechnical Engineer should review the final anchor design and a representative should observe the load test and anchor installation. All foundation excavations should be observed by the Geotechnical Engineer’s representative prior to placement of reinforcement and concrete to verify firm bearing materials are exposed and the proximity of anchors to natural rock discontinuities, such as fractures. Interior Floor Slabs Interior concrete slab-on-grade floors should be supported by the soil subgrade prepared in accordance with the recommendations contained in the Subgrade Preparation and Engineered Fill sections. The interior concrete slabs should be at least four inches thick, however, the project structural or civil engineer should determine final floor slab thickness, reinforcement and joint spacing. Temporary loads exerted during construction from vehicle traffic, cranes, forklifts, other construction equipment, storage of palletized construction materials, etc. should be considered in the design of the thickness and reinforcement of the interior concrete slabs-on-grade. Moisture Penetration Resistance It is likely that floor slab subgrade soils will become very moist or wet at some time during the life of the structures. This is a certainty when slabs are constructed during the wet season or when constantly wet ground or poor drainage conditions exist adjacent to structures. For this reason, it should be assumed that interior slabs with moisture-sensitive floor coverings or coatings will require protection against moisture or moisture vapor penetration through the slabs. Interior floor slabs for the planned buildings should, as a minimum, be underlain by a layer of free-draining crushed rock/gravel, serving as a deterrent to migration of capillary moisture. The Geotechnical Engineering Report Page 23 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 crushed rock/gravel layer should be between four- and six-inches-thick and graded such that 100 percent passes a one-inch sieve and less than five percent passes a No. 4 sieve. Additional moisture protection may be provided by placing a vapor retarder membrane (at least 10-mils thick) directly over the crushed rock/gravel. The water vapor retarder membrane should meet or exceed the minimum specifications as outlined in ASTM E1745 and be installed in strict conformance with the manufacturer’s recommendations. For portions of the interior floor slabs that are designated to support vehicular traffic, we recommend placing the vapor retarder membrane directly over compacted aggregate base. Floor slab construction practice over the past 30 years or more has included placement of a thin layer of dry sand or pea gravel over the vapor retarder membrane. The intent of the sand/pea gravel is to aid in the proper curing of the slab concrete. However, during the wet seasons moisture can become trapped in the sand or pea gravel, which can lead to excessive moisture vapor emissions from floor slabs. As a consequence, we consider use of the sand/pea gravel layer as optional. The concrete curing benefits should be weighed against efforts to reduce slab moisture vapor transmission. It is emphasized that the crushed rock/grave and the vapor retarder membrane suggested above provides only a limited, first line of defense against soil-related moisture issues and will not "moisture proof" the slab. Nor do these measures provide an assurance that slab moisture transmission levels will tolerable levels to prevent damage to floor coverings or other building components. If increased protection against moisture vapor penetration is desired, a concrete moisture protection specialist should be consulted. The design team should consider all available measures for slab moisture protection. It is commonly accepted that maintaining the lowest practical water-cement ratio in the slab concrete is one of the most effective ways to reduce future moisture vapor penetration of the completed slabs. Exterior Flatwork The final subgrade for exterior concrete flatwork (i.e., sidewalks, patios, etc.) should be prepared and constructed in accordance with recommendation provided in the Subgrade Preparation and Engineered Fill sections. Exterior flatwork should be underlain by at least four inches of aggregate base compacted to at least 95 percent relative compaction to provide stability during slab construction and to protect the soils from disturbance during construction. Exterior flatwork concrete should be at least four inches thick. Consideration should be given to thickening the edges of the slabs at least twice the slab thickness where wheel traffic is expected over the slabs. Expansion joints should be provided to allow for minor vertical movement of the flatwork. Exterior flatwork should be constructed independent of other Geotechnical Engineering Report Page 24 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 structural elements by the placement of a layer of felt material between the flatwork and the structural element. The slab designer should determine the final thickness, strength and joint spacing of exterior slab-on-grade concrete. The slab designer should also determine if slab reinforcement for crack control is required and determine final slab reinforcing requirements. Practices recommended by the Portland Cement Association (PCA) for proper placement, curing, joint depth and spacing, construction, and placement of concrete should be followed during exterior concrete flatwork construction. Pavement Design The subgrade soils and weathered bedrock in pavement areas should be prepared in accordance with the recommendations contained in the Subgrade Preparation and Engineered Fill sections. Based on laboratory testing, an R-value of 25 was used for design of pavements supported on the near-surface soil and/or engineered fill. An R-value of 50 was used for pavements supported on the hard lahar rock. The pavement sections presented in Table 3 have been calculated using traffic indices assumed to be appropriate for the project. The procedures used for pavement design are in general conformance with Chapters 600 to 670 of the California Highway Design Manual (Caltrans, 2019). The project civil engineer should determine the appropriate traffic index and pavement section based on anticipated traffic conditions. If needed, we can provide alternative pavement sections for different traffic indices. Geotechnical Engineering Report Page 25 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Table 3 Traffic Index (TI) Typical Street Classifications Number of Residential Units Served (20-yr. Design) Type A Asphalt Concrete (inches) R-value = 25 R-value = 50 Class 2 Aggregate Base (inches) Class 2 Aggregate Base (inches) 4.5 Average Residential < 24 2½ 6 4 2½* 6 4 5.0 Residential Collectors 25 – 40 2½ 8 4 3* 7 4 5.5 41- 90 2½ 9 5 3½* 7 4 6.0 Collectors and Minor Arterials 91 – 180 3 11 6 3½* 9 4 6.5 181 – 300 3 12 6 4* 10 4 7 301 – 500 3 13 7 4* 11 5 7.5 Local Industrial and Arterials 501 – 700 3½ 14 7 4½* 12 5 8.0 701 - 900 4 15 8 5* 12 5 Notes: * = Asphalt concrete thickness contains the Caltrans safety factor. All pavement materials and construction methods of structural pavement sections should conform to the applicable provisions of the Caltrans Standard Specifications, latest edition. All aggregate base should be compacted to at least 95 relative compaction. Efficient drainage of all surface water to avoid infiltration and saturation of the supporting aggregate base and subgrade soils is important to pavement performance. Weep holes could be provided at drainage inlets, located at the subgrade-aggregate base interface, to allow accumulated water to drain from beneath the pavements. Consideration should be given to using full-depth curbs between landscaped areas and pavements to serve as a cut-off for water that could migrate into the pavement base materials or subgrade soils. Geotechnical Engineering Report Page 26 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 Retaining Walls All retaining walls or below grade walls for the buildings should be designed to resist the lateral soil pressures of the retained soils. Retaining walls that are fixed/restrained at the top should be capable of resisting an “at-rest” lateral soil pressure equal to an equivalent fluid pressure of 60 psf per foot of the wall height (fully drained conditions). Retaining walls that will be allowed to slightly rotate about their base (unrestrained at the top or sides) should be capable of resisting an "active" lateral soil pressure equal to an equivalent fluid pressure of 40 psf per foot of wall height (fully drained conditions). For retaining walls with backfill sloped at a gradient of up to 2(h):1(v), add 20 and 15 psf per foot of the wall height to the at-rest or active equivalent fluid pressures provided above, respectively. Based on recent research (Lew, et al. 2010), the seismic increment of earth pressure may be neglected if the maximum peak ground acceleration at the site is 0.4 g or less. Our analysis indicates the maximum peak ground acceleration at the site will be about 0.38g; therefore, the seismic increment of lateral earth pressure may be neglected, and retaining walls may be designed using the lateral earth pressures presented above. If structural elements, i.e., foundations, roadways, etc., encroach the 1(h):1(v) projection from the bottom of retaining walls, the retaining walls should account for surcharge loads resulting from those structural elements. Additionally, any below-grade retaining walls should also account for surcharge loads resulting from construction equipment, vehicles, palletized materials, etc. that encroach the 1(h):1(v) projection from the bottom of the below-grade retaining walls. Surcharge loading under the circumstances described above should be evaluated by the retaining wall designer on a case-by-case basis and be included in their design of the walls. The retaining wall designer should evaluate the surcharge load distribution, magnitude of the surcharge resultant force to be applied on the walls, and the location of where the resultant force should be applied on the walls. Surcharge loading on the retaining walls will depend on the specific surcharge load type (e.g. point load, distributed load, etc.) and distance away from the retaining walls. Retaining wall or below grade walls should be fully drained to prevent the build-up of hydrostatic pressures behind the wall. Retaining walls should be provided with a drainage blanket of Class 2 permeable material, Caltrans Standard Specification, Section 68-2.02F(3), at least one-foot wide extending from the base of wall to within one foot of the top of the wall. The top foot above the drainage layer should consist of compacted on-site or imported engineered fill materials, unless covered by a concrete slab or pavement. Weep holes or perforated rigid pipe, as appropriate, should be provided at the base of the wall to collect accumulated water. Drainpipes, if used, should slope to discharge at no less than a one percent fall to suitable Geotechnical Engineering Report Page 27 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 drainage facilities. Open-graded ½- to ¾-inch crushed rock may be used in lieu of the Class 2 permeable material provided the rock and drain pipe are completely enveloped in an approved non-woven, geotextile filter fabric. Alternatively, approved geotextile drainage composites, such as MiraDRAIN®, may be used in lieu of the drain rock layer. If used, geocomposite drain panels should be installed in accordance with the manufacturer’s recommendations. If efflorescence (discoloration of the wall face) or moisture/water penetration of the retaining walls is not acceptable, moisture/water-proofing measures should be applied to the back face of the walls. A moisture/water-proofing specialist should be consulted to determine specific protection measures against moisture/water penetration through the walls. Structural backfill materials for retaining walls within a 1(h):1(v) projection from the bottom of the walls (other than the drainage layer) should consist of on-site or imported, compactable granular material that does not contain significant quantities of rubbish, rubble, organics and rock over four inches in size. Clay, pea gravel and/or crushed rock should not be used for structural wall backfill. Structural wall backfill should be placed in lifts not exceeding 12 inches in compacted thickness, moisture conditioned to at least the optimum moisture content, and should be mechanically compacted to at least 90 percent relative compaction. Foundations for support of retaining or below grade walls should be designed using the appropriate foundation design parameters provided in the Spread Foundations section included in this report. Site Drainage Final site grading should be accomplished to provide positive drainage of surface water away from the buildings and prevent ponding of water adjacent to foundations, slabs or pavements. The subgrade adjacent to the buildings should be sloped away from the building at a minimum two percent gradient for at least five feet, where possible. All roof drains should be connected to non-perforated rigid pipes, which in-turn are connected to available drainage features that convey water away from the buildings or discharging the drainage onto paved or hard surfaces that slope away from the buildings. Landscape berms, if planned, should not be constructed in such a manner as to promote drainage toward the buildings. Drought Considerations The State of California can experience extended periods of severe drought conditions. The ability for landowners to use irrigation as a means for maintaining landscape vegetation and soil moisture can be inhibited for unpredictable periods of time. For this reason, landscape and Geotechnical Engineering Report Page 28 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 hardscape systems for this development should be carefully planned to prevent the desiccation of soils under and near foundations and slabs. Trees with invasive shallow root systems should be avoided. No trees or large shrubs that could remove soil moisture during dry periods should be planted within five feet of any foundation or slab. Fallow ground adjacent to foundations must be avoided. Geotechnical Engineering Construction Observation Services Wallace-Kuhl & Associates be retained to review the final plans and specifications to verify that the intent of our recommendations has been implemented in those documents. Site preparation should be accomplished in accordance with the recommendations of this report. Geotechnical testing and observation during construction is considered a continuation of our geotechnical engineering investigation. Wallace-Kuhl & Associates should be retained to provide testing and observation services during site clearing, preparation, earthwork, and foundation construction at the project site to verify compliance with this geotechnical report and the project plans and specifications, and to provide consultation as required during construction. These services are beyond the scope of work authorized for this study; however, we can submit a proposal to provide these services upon request. In the event that Wallace-Kuhl & Associates is not retained to provide geotechnical engineering observation and testing services during construction, the Geotechnical Engineer retained to provide these services should indicate in writing that they agree with the recommendations of this report, or prepare supplemental recommendations as necessary. A final report by the Geotechnical Engineer providing construction testing services should be prepared upon completion of the project. LIMITATIONS Our recommendations are based upon the information provided regarding the proposed project, combined with our analysis of site conditions revealed by the previous field explorations and associated laboratory testing programs. We have used engineering judgment based upon the information provided and the data generated from our study. This report has been prepared in substantial compliance with generally accepted geotechnical engineering practices that exist in the area of the project at the time the report was prepared. No warranty, either express or implied, is provided. Geotechnical Engineering Report Page 29 TUSCAN RIDGE SUBDIVISION WKA No. 12206.07 May 6, 2021 If the proposed construction is modified or re-sited; or, if it is found during construction that subsurface conditions differ from those we encountered at the previous exploration locations, we should be afforded the opportunity to review the new information or changed conditions to determine if our conclusions and recommendations must be modified. We emphasize that this report is applicable only to the proposed construction and the investigated site, and should not be utilized for construction on any other site. The conclusions and recommendations of this report are considered valid for a period of two years. If design is not completed and construction has not started within two years of the date of this report, the report must be reviewed and updated, if necessary. Wallace - Kuhl & Associates Gary H. Gulseth, GE Senior Engineer GR Reddish brown, moist, medium plastic fines, clayey GRAVEL with sand and scattered gravelup to 18 inches in diameter (GC) Practical refusal at 3 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 3.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP41 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PMTP41 @ 2'-3' 14 Reddish brown, moist, low plasticity fines, sandy GRAVEL (GP) Practical refusal at 2 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP42 @ 0'-2' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 2.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP42 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM GR FILL: reddish brown to gray, moist, clayey SAND with gravel up to 2 inches in diameter (SC) Practical refusal at 0.5 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP 43 @ 0'-0.5' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 0.5 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP43 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM FILL: Yellowish brown, moist, silty fine to medium grained SAND (SM) Reddish brown, moist, low plasticity, silty GRAVEL with sand; fine grained gravel up to 0.75inch (GM) Practical refusal at 1 foot below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP44 @ 0'-0.5' TP44 @ 0.5'-1' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 1.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP44 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM 11 RVPIGR FILL: gray, moist, low plasticity, sandy lean CLAY with scattered gravel (CL) Reddish brown, moist, low plasticity, sandy GRAVEL; fine to coarse grained gravel up to 2 inches (GM) Practical refusal at 3 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP45 @ 0'-1' TP45 @ 1'-2' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 3.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP45 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM RV Reddish brown, moist, clayey GRAVEL; fine to coarse grained gravel up to 2 inches (GC) Practical refusal at 2.5 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP46 @ 1'-2' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 2.5 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP46 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM Reddish brown, wet, clayey GRAVEL with sand; fine to coarse grained gravel up to 2 inches(GC) Practical refusal at 0.5 feet below existing ground surface in Lahar of Tuscan Formation. Groundwater was not encountered. Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 0.5 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP47 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PMTP47 @ 0'-0.5' 18 Reddish brown, moist, low plasticity, sandy GRAVEL; fine gravel (GP) Practical refusal at 0.5 feet below existing ground surface in Lahar of Tuscan Formation. Groundwater was not encountered. Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 0.5 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP48 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PMTP48 @ 0'-0.5' 21 GR Reddish brown, moist, low plasticity, clayey SAND with gravel (SC) Practical refusal at 1 foot below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 1.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP49 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PMTP49 @ 0'-1' 16 RVEI Fill: Reddish brown, moist, clayey GRAVEL with cobbles (GC); 4 foot piece of Larhar Practical refusal at 3 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP50 @ 2'-3' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 3.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP50 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM GR Reddish brown, moist, poorly graded GRAVEL with clay and scattered cobbles ranging from4 to 12 inches in diameter (GP-GC) Practical refusal at 3 feet below existing ground surface in Lahar of Tuscan Formation.Groundwater was not encountered. TP51 @ 0'-3' Sheet 1 of 1Project Location: Paradise, California Project: Tuscan Ridge Subdivision WKA Number: 12206.07 FIGURE 3.0 feet SamplingMethod(s) Approx. SurfaceElevation, ft MSL Excavator GRAPHIC LOGADDITIONALTESTS3/17/21 DRY UNITWEIGHT, pcfDriving Methodand Drop LOG OF TEST PIT TP51 DEPTH, feetRemarks Soil CuttingsNot Encountered NUMBEROF BLOWSSAMPLENUMBERDrillingMethod Drill RigType Total Depthof Drill Hole Drill HoleBackfill KRL Kubota KX040-4 NexGen SAMPLE DATA ENGINEERING CLASSIFICATION AND DESCRIPTION Date(s)Drilled 24 ELEVATION, feetSAMPLECheckedByLoggedBy GHG DrillingContractor Groundwater Depth[Elevation], feet Diameter(s)of Hole, inches MOISTURECONTENT, %TEST DATA N/A BORING LOG 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 5/3/21 4:00 PM TUSCAN RIDGE SUBDIVISION Paradise, California RWO KRL GHG 04/2021 UNIFIED SOIL CLASSIFICATION SYSTEM 12206.07 APPENDIX A General Information, Field Exploration and Laboratory Testing APPENDIX A A. GENERAL INFORMATION The geotechnical engineering study for the Tuscan Ridge Subdivision, located between Chico and Paradise, California, was authorized by Mr. Scott Bates on March 15, 2021. Authorization was for a study as described in our proposal dated February 19, 2021, sent to our client the Reeder Sutherland, Inc. in Roseville, California; telephone (530) 401-3670. B. FIELD EXPLORATION The subsurface soil conditions at the project site were initially explored on March 15, 2019, as part of an environmental study by excavating 40 test pits using a track mounted excavator to depths ranging from a few inches to about 6½ feet below the existing ground surface (bgs). Eleven additional test pits were excavated as part of this current study on March 17, 2021, to a maximum depth of about 3 feet bgs. The test pit locations are shown in Figure 2. The test pits were excavated using a Kubota KX040-4 equipped with a 24 inch bucket provided by the client. Practical refusal was encountered at the each test pit in hard lahar (mudstone). Disturbed bulk samples were collected during the current field explorations and taken to our laboratory for additional soil classification and selection of samples for testing. The Logs of Test Pits containing descriptions of the soils encountered in each of the test pits excavated for this study are presented in Figures 5 through 15. A Legend explaining the Unified Soil Classification System (ASTM D2487) and the symbols used on the logs is contained in Figure 16. A graph showing a summary of the findings for all the test pits is presented as Figure 4. C. LABORATORY TESTING Selected undisturbed samples of the soils were tested to determine the natural moisture content (ASTM D2216) of the soils. The results of these tests are included in the test pit logs at the depth each sample was obtained. Five soil samples were tested to determine the Particle Size Distribution (ASTM C136 and D7928) of the soil. The results of the test is presented in Figure A1. One soil sample collected from test pit TP45 was tested to determine the liquid limit, plastic limit and plasticity index of the soil using the Atterberg Limits test (ASTM D4318). The result of the test is presented in Figure A2. WKA No. 12206.07 Page A2 One bulk sample of the near-surface fine-grained (plastic) soil collected at test pit TP50 was tested to estimate the expansion potential of the soil using the Expansion Index test (ASTM D4829) with result presented in Figure A3. Three bulk samples of anticipated pavement subgrade soil were collected at test pits TP45, TP46 and TP50 and subjected to Resistance-value ("R-value") testing in accordance with California Test 301. The results of the R-value tests, which were used in the pavement design, are presented in Figures A4 and A5. Two selected soil samples of near-surface soil was submitted to Sunland Analytical of Rancho Cordova, California, to determine the soil pH and minimum resistivity (California Test 643), Chloride concentration (California Test 422m), and Sulfate concentration (California Test 417, ASTM D516m). The results of these tests are presented in Figures A6 through A9. 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.1110 PARTICLE SIZE DISTRIBUTION GRAVEL COARSE MEDIUM FINE PARTICLE SIZE IN MILLIMETERSPERCENT FINER BY WEIGHTFINE #4 #60 #100 SILT CLAY TP41 TP43 TP45 TP49 TP51 COARSE 3" 1.5" 3/4" #10 SAND #20 #40 U.S. STANDARD SIEVE NUMBERS 3/8" U.S. STANDARD SIEVEOPENING #200 2'-3' 0'-0.5' 1'-2' 1'-2' 0'-3' PILLDepth(feet)USCS ClassificationSymbol FIGURE A1 Project: Tuscan Ridge Subdivision WKA No. 12206.07GRAIN SIZE 12206.07 - TUSCAN RIDGE SUBDIVISION.GPJ WKA.GDT 4/26/21 10:15 AM 2'-3' 0'-0.5' 1'-2' 0'-1' 0'-3' Test PitNumber Sample Depth GC SC CL SC GP-GC 35 12 Clayey GRAVEL with sand and cobbles FILL: clayey SAND with gravel Sandy lean CLAY with scattered gravel Clayey SAND with gravel Poorly graded GRAVEL with clay ATTERBERG LIMITS ASTM D4318 CL - ML ML and OL 40 CL CH MH and OH"A" Line Liquid LimitPlasticity Index10 0 20 30 40 50 60 70 80 0 102030 5060708090100110120 TP45 1.0’-2.0’---35 ---CL12 KEY SYMBOL LOCATION SAMPLE DEPTH NATURAL WATER CONTENT (%) LIQUID LIMIT (%) PLASTICITY INDEX (%) PASSING No. 200 SIEVE (%) UNIFIED SOIL CLASSIFI- CATION SYMBOL ATTERBERG LIMITS 12206.07 A2 RWO KRL GHG 04/2021 ATTERBERG LIMITS TUSCAN RIDGE SUBDIVISION Paradise, California DATE PROJECT MGR CHECKED BY DRAWN BY FIGURE WKA NO. 12206.07 A3 RWO KRL GHG 04/2021 EXPANSION INDEX TUSCAN RIDGE SUBDIVISION Paradise, California DATE PROJECT MGR CHECKED BY DRAWN BY FIGURE WKA NO. Sample Depth Pre-Test Moisture (%) Post-Test Moisture (%) Dry Density (pcf) Expansion Index EXPANSION INDEX Medium Low Very Low POTENTIAL EXPANSION EXPANSION INDEX TEST RESULTS MATERIAL DESCRIPTION: LOCATION: CLASSIFICATION OF EXPANSIVE SOIL * * From ASTM D4829, Table 1 ASTM D4829 TP50 2’ - 3’ 13.2 28.5 97 14 Reddish brown, clayey GRAVEL with cobbles 0 - 20 21 - 50 51 - 90 91 - 130 High Above 130 Very High 12206.07 TUSCAN RIDGE SUBDIVISION Paradise, California A4 RWO KRL GHG 04/2021 RESISTANCE VALUE TEST RESULTS RESISTANCE VALUE TEST RESULTS (California Test 301) MATERIAL DESCRIPTION: LOCATION: Dry Unit Weight (pcf) Specimen No. Moisture @ Compaction (%) Exudation (psi) Pressure Expansion (dial, inches x 1000)Value R (psf) 2 3 1 1 3 2 (psf) R Value ExpansionPressure (psi) Exudation (%) @ Compaction Moisture No. Specimen (pcf) Weight Dry Unit LOCATION: MATERIAL DESCRIPTION: TP45 (1’ - 2’) 112 116 118 15.5 14.5 13.6 208 286 431 2 4 12 9 17 52 12 22 36 R-Value at 300 psi exudation pressure = 24 TP46 (1’ - 2’) 117 115 118 14.9 15.7 14.4 344 205 505 9 0 43 29 17 37 (dial, inches x 1000) 2 0 10 R-Value at 300 psi exudation pressure = 26 Fill:Gray, sandy lean CLAY with gravel Reddish brown, clayey GRAVEL 12206.07 TUSCAN RIDGE SUBDIVISION Paradise, California A5 RWO KRL GHG 04/2021 RESISTANCE VALUE TEST RESULTS RESISTANCE VALUE TEST RESULTS (California Test 301) MATERIAL DESCRIPTION: LOCATION: Dry Unit Weight (pcf) Specimen No. Moisture @ Compaction (%) Exudation (psi) Pressure Expansion (dial, inches x 1000)Value R (psf) 2 3 1 Reddish brown, sandy cobbles TP50 (2’ - 3’) 118 116 114 12.5 13.3 14.2 494 330 234 14 5 1 61 22 4 74 47 21 R-Value at 300 psi exudation pressure = 40 TUSCAN RIDGE SUBDIVISION Paradise, California A6 RWO KRL GHG 04/2021 CORROSION TEST RESULTS 12206.07 TUSCAN RIDGE SUBDIVISION Paradise, California A7 RWO KRL GHG 04/2021 CORROSION TEST RESULTS 12206.07 TUSCAN RIDGE SUBDIVISION Paradise, California A8 RWO KRL GHG 04/2021 CORROSION TEST RESULTS 12206.07 TUSCAN RIDGE SUBDIVISION Paradise, California A9 RWO KRL GHG 04/2021 CORROSION TEST RESULTS 12206.07