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G eotechnical Engineering and Geological Hazard study Lincoln Elen:enta:v School Modernization Proicct ]Manteca, California Page 5 Drilling (license C574720904) with a truck -mounted drilling rig, using solid stem auger and mud rotary drilling techniques. A 6 -inch (OD) auger was used to advance the boreholes that extended to a depth of 25 feet. Mud rotary methods were used to drill B I and B8 to a depth of 50 feet. Soil samples were collected from each borehole at selected intervals using a 3 -inch OD California Modified WM) sampler fitted with 2.5 OD brass liners, and a 2 -inch OD Standard Penetration Test (SPT) sampler. The samples were collected using a 140 -pound auto -hammer falling 30 inches to drive the sampler. Field blow counts were recorded as the number of hammer blows required to drive the sampler the final 12 inches on an 18 -inch drive. Recorded blow counts shown on the boring logs for the California modified sampler have been approximately correlated to SPT blow counts by using a factor of 0.63. A Condor representative observed the drilling and sampling of each boring. A Condor representative visually classified soil samples and cuttings at the time of drilling using the Unified Soil Classification System. At the time of the April 2015 field exploration, groundwater was encountered at a depth of 19.8 feet. The boreholes where groundwater was encountered were backfilled with tremmied neat cement. The remaining boreholes were backfilled to the ground surface with soil cuttings for the entire depth. The boring locations and generalized cross section are presented in Figures 3 and 3A, Appendix A. Detailed soil boring logs are presented in Appendix B. Laboratory test results are presented on the soil boring logs and in Appendix C. 5.1 EARTH T ATERIALS The subsurface soils encountered at the Lincoln Elementary School site consist of unconsolidated alluvium to the maximum depth explored of 51.5 feet below the existing site grade. Generally, denser soils were encountered below a depth of 20 feet. The alluvial soil primarily consists of varying percentages of sand and silt with layers of clay to the total depth explored at 51.5 feet. In general, silty sand was present in all borings from the su mace to a depth of 10 feet bgs. The silty sand was underlain by poorly graded sand to a depth of 15 feet bgs. The poorly graded sand was underlain by coarse and fine grained soils classified as sand, silt or clay to a depth of 40 feet in the deeper borings. Poorly graded sand was present in the deeper borings from 40 to 50 feet bgs. Fine grained soils identified as clay were present at a depth of 50 feet in both of the deep borings. in general the silty sand identified in the top 10 feet of the soil profile was classified as loose to medium dense based on uncorrected N -values of less than 10, while soils below a depth of 10 feet bgs were classified as medium dense to dense course grained soils or stiff fine grained soils. 5.2 LOCAL GROUNDWATER CONDITIONS At the time of our exploration groundwater was encountered at a depth of 19.8 feet in soil boring B2. All other shallow borings did not encounter groundwater. The 50 -foot deep borings were drilled with drilling mud and were therefore unable to measure groundwater. Three groundwater wells in the Manteca area were located on the Department of Water Resources database that graphically demonstrated the overall drop in shallow groundwater over the last 20 to 40 years. This trend is generally anticipated to continue due to the expanding urbanization of Manteca (less surface irrigation of agricultural lands) and continuing regional drop in overall groundwater levels. The northeast, upgradient well (OlS07E33H001M) is currently (2011) at a depth of 22.8 feet, and the southwest, downgradient well (02S07EO4EOO I M) was last recorded in 1961 at a depth of 10.2 feet. The northwest well of 01S07E321100IM was last recorded in 1961 at a depth of 9.6 feet. Comparatively the northwest well 01 S07E33HOO I M was at a depth of 9.5 feet in 1969, and is now (2011) at 22.8 feet. The measured depth of approximately 20 feet at the project site fits well with this data. Groundwater in the past 60 years has been as shallow as 6.6 feet in June of 1956 in well O1S07E32HOO1M located approximately 0.75 miles to the northeast. See Appendix B for a historical listing of depths. While the depth to groundwater is expected to fluctuate in response to both seasonal rainfall and irrigation of surrounding farmland, there is a practical limit to the anticipated X i CONDOR