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Geotechnical Engineering and Geological Hazard Study Lincoln Elementary School Modernization Project Manteca, California Page 7 approximately 20,0:10 acct southwest 0m the site. Based on the closest proximity of these slopes to the site, we consider the potential risk from a landslide or loss of lateral support is remote. 5.3.8 Radon- Bearing Soil Radon -bearing soils are typically found near sites that overlie organic -rich marine black shale and certain igneous rocks. increased amounts of radon may be generated in the subsurface at these locations. T hough the site is not underlain by either of these rock types, we evaluated the potential exposure to radon - bearing soil during grading earthwork, and occupancy. The California Department of Health Services web page California Indoor Radon levels Sorted by Zip Code indicates that 5 radon tests have been conducted in the 95337 zip code area in San Joaquin County, Manteca, California. Of the 5 tests performed, I had results greater than or equal to 4 pCi.L, which is the advised action level. Based on our review, we consider the risk of encountering radon bearing soils to be low. 5.3.7 Asbestos Bearing Rock The nearest ultramafic rock outcrops, that may contain asbestos, are approximately 25 miles to the southwest in the Coast Range Mountains (Churchill, et al., 2000 and California Geologic Survey (CGS) Special Publication 124). Asbestos has been shown to remain in soils and can be transported. However, no imported rock was observed on site and natural transport would appear to be unlikely. The potential hazard from encountering asbestos bearing rock in surface and subsurface excavations at the site is considered low. The site location relative to mapped locations of ultramafic rocks is shown on Figure 6, Ultramafic Map, Appendix A. 5.3.10 Regional Subsidence and Hydrocollapse The Site is not within a region where regional subsidence or hydrocollapse are considered likely to occur. The closest actively subsiding area is approximately 50 miles to the south. Hydrocollapse is unlikely due to the age of local geologic fonnations. 6.0 SEISMIC CONSIDERATIONS 6.1 GROUND SHAKING Probabilistic values of ground motion corresponding to various levels of seismic hazards are available on- line from the CGS and the USGS. Both agencies use a probabilistic model to estimate ground motions corresponding to various levels of seismic hazard. Site soils are classified using the procedures specified in the 2013 California Building Code (CBC 2013) and Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers 7-10 (ASCE 7-10). The site alluvial soils within 50 feet of the surface have an average N -value of 17 with an extrapolated shear wave velocity (Vs) of 600 ft/s or 200 m/s which are classified according to the CBC 2013 and as Site Class D and exhibit a stiff soil profile based on the typical blow counts ranging from 15 to 50 blows per foot. However, the potential for liquefaction, Section 6.2 and ASCE 7-10, would cause the soils to be classified as Site Class F. except where the proposed structure is less than 40 feet in height and therefore has a fundamental period of less than one half second, T<0.5. The proposed structures are less than 40 feet in height, therefore Site Class D is appropriate for use at this site. Based on CBC 2013 and ASCE 7-10 procedures, the USGS model for a 10 percent probability of exceedance in 50 year hazard level (475 -year recurrence interval), the site Peak Ground Acceleration (PGA), spectral acceleration for a short (0.2 second) and 1.0 second period were calculated. The procedural methods use mapped spectral acceleration values (SS, St) for Site Class B conditions and site specific coefficients (Fa, Fv) to extrapolate from Site Class B to site specific conditions. The site is not 50