III. GEOLOGIC PROBLEMS

The project site is located on an alluvial terrace along the west side of Los Gatos Creek at an elevation range of 288 to 295 feet above sea level. The site has been previously graded so that it is essentially flat, with a gentle slope downward to a runoff catchment, about four to five feet deep, on the east side.

1. The main potential hazard to structures and people on site would be from seismic activity. Because the site would be fenced and locked, direct public access would be prevented. No workers would be present on the site, except during brief periods of occasional maintenance and inspection activities. Therefore, the potential for injuries to people on the site during earthquakes would be only a remote possibility. The earthquake hazards are potentially significant only for the facilities themselves. To the extent that these would be rendered inoperable by an earthquake, resulting in a loss of power in the service area, the project could affect population in the service area. However, a major earthquake that could affect the site also is likely to affect a wide area in the South Bay. By providing better linkage of power transmission in the area, the project likely would result in a net improvement to system reliability during and following a major earthquake. In the event of the loss of operation of one of the other substations in an earthquake, the potential for providing electric power service could be compensated by operation of the other substations, including the proposed Vasona station.
   
   The San Andreas Fault lies about 5 miles north of the site, and there is evidence that the potentially active Shannon Fault crosses the southwestern corner of the project site (PG&E, 1997a). The Seismic Safety Element of the Town General Plan has mapped the Shannon Fault zone approximately 100 feet south of the project site (Los Gatos, 1994), however, this portion of the fault is poorly constrained and has been mapped in several locations. One study, Herd (1980), places the fault through the southern portion of the site. There may be the potential for fault rupture (small displacement - less than 0.5 feet) related to coseismic ground deformations (secondary effects such as ground lurching and settlement) from a large earthquake occurring on the San Andreas Fault, or possibly from a primary rupture on the Shannon Fault across the western or southwestern corner of the site (PG&E, 1997a). Compliance with the Uniform Building Code is required for design and construction of the facilities; this would reduce fault rupture effects to levels of acceptable risk and result in a less than significant impact from possible fault rupture.

2. The project site is located in the Santa Cruz Range foothills in the Coast Range Geomorphic Province, which is an area of high seismic activity. Several major northwest-trending fault zones are anticipated to generate major earthquakes that could induce significant ground shaking at the site, including the San Andreas Fault Zone (the dominant fault zone in California), and a number of smaller fault zones are located within 40 miles of the project site. In addition to the San Andreas and Shannon Faults, other potentially active faults are listed in Table III-1. According to the Seismic Safety Element of the Town General Plan (1994), the site is located in a "Major Geotechnical Hazard area with Landslide Potential". A major earthquake on any of the faults listed in Table III-1 could produce severe groundshaking at the site, affecting the proposed facilities (see discussion under (a), above. Compliance with the Uniform Building Code in facilities' design and construction, as required by state and local policy, would reduce groundshaking effects on those facilities to levels of acceptable risk, as is the specific purpose of the Code, and result in a less than significant impact from seismic hazard.

3. Earthquakes or aftershocks may cause secondary ground failures. Ground failures are caused by soil losing its structural integrity. Examples of seismically-induced ground failures are liquefaction, lateral spreading, ground lurching, and subsidence. Liquefaction (the rapid transformation of soil to a fluid-like state) affects loose saturated sands. Lateral spreading is the horizontal movement of loose, unconfined sedimentary and fill deposits during seismic activity. Ground-lurching is the horizontal movement of soil, sediments, or fill located on relatively steep embankments or scarps as a result of seismic activity, forming irregular ground surface cracks. The potential for lateral spreading or lurching is highest in areas underlain by soft, saturated materials, especially where bordered by steep banks or adjacent hard ground. Subsidence is vertical downward movement of the ground surface.
   
   Because of the site's upland location at the edge of substantial slope which would drain groundwater toward the valley bottom to the east, saturated soils are unlikely to be present at the site and. therefore, liquefaction and lateral spreading hazards are deemed low. The site is mapped as having "no to low liquefaction susceptibility" on a map "Liquefaction Susceptibility, San Jose West 7.5 Minute Quadrangle", prepared by William Lettis & Associates, Inc. and PG&E Geoscience Department, 1992 and rated as "low likelihood" for liquefaction by the Association of Bay Area Governments (1980). Secondary hazards from seismic activity which could affect the site include ground lurching and ground settlements (subsidence). Lurching would potentially occur in an easterly movement toward the hill slope adjacent to the site. Lurching would likely result in ground cracking and settlement, estimated on the order of inches. These existing hazards would not be affected in any way by the proposed project. The project would not add any new geotechnical hazard to the site because of grading or alteration of subsurface soil and ground water conditions. Required compliance with the Uniform Building Code in the facilities' design and construction would reduce impacts from ground failure to levels of acceptable risk. These Code compliance measures would not eliminate the earthquake hazard, but they would avoid creation of unacceptable hazards, and, therefore result in a less than significant impact from earthquakes.

4. Earthquakes can cause tsunami ("tidal waves"), seiches (oscillating waves in enclosed water bodies), and landslide splash waves in enclosed water bodies such as lakes and reservoirs. Earthquakes can also result in dam failures at reservoirs. The project site is not located near an enclosed body of water (PG&E, 1997a), and is not in the inundation zone from a dam failure at Vasona Reservoir (Los Gatos, 1994). Therefore, there would be no impact from tsunami, seiche, or seismically induced dam failure related to the proposed project.

5. According to the Seismic Safety Element of the Town General Plan, the site is located in a "Major Geotechnical Hazard area with Landslide Potential" (Los Gatos, 1994). However, there are no mapped major landslides in the immediate vicinity of the project site (Wright, 1974). The project site is essentially flat, and is located on an alluvial terrace above Los Gatos Creek. Just beyond the eastern border of the site, the land slopes downward with over a 30 percent grade to the Charter Oaks condominium complex. The southern and western edges of the site are bordered by Lark Ave. and Winchester Blvd., respectively.
   
   There is a slight potential for landslides on the slope just beyond the eastern border of the project site. The proposed construction at the site would not change the nature of this landslide potential. Failure of that slope could cause part of the noise wall to topple, but this would pose a small hazard to the condominiums, which are located fifty or nore feet beyond the wall at the foot of the slope. As the proposed substation facility will not be staffed and will be fenced off from public access, hazards to people related to landslide or mudflow potential will be minimal, and the impact is less than significant.

6. In general, unstable soil conditions include settlement and failure from low strength (i.e., the soils deform when loads are placed on them). Soils at the proposed site are not of the types characterized by low strength. Settlement can occur either uniformly or differentially. Uniform settlement of a structure can cause poor drainage and potential failure of underground utility connections. Differential settlement can damage a foundation and cause mechanical and structural problems within a structure. The magnitude of settlement of a fill will depend on the quality of the fill material and the manner in which it is placed, the thickness of the fill, the type of subsurface soil and the load placed on the material. Settlement is anticipated to be minor at the site. As engineering, design and construction practices are proposed by PG&E that conform to common engineering practice, such as conformance with Building Codes and ASTM (American Society for Testing and Materials) standards, including compaction of engineered fills, impacts resulting from settlement are anticipated to be minor and the hazard would be less than significant.
   
   The project site would require minimal additional grading of the flat site to construct the proposed substation and will not result in any changes in topography. Construction of the substation and containment basin would disturb site soils; site soils may be subject to erosion by rain splash and overland flow of stormwater for the duration of the construction activities. Site preparation would entail minor regrading, resurfacing and paving of portions of the site. Other construction activities which could result in temporary erosion would include construction of a cement block wall along the eastern perimeter of the site. Because the site is flat, and therefore the erosivity of the soil is low, soil erosion from construction activities would not likely result in significant hazards of gully formation. The site currently has no erosion gullies, indicating a low erosion hazard. Runoff from the site could entrain loose soil and discharge it into storm drains (see Water). While the hazard is deemed low, and less than significant, the impacts from erosion hazard could be reduced to an absolute minimum by implementation of commonly accepted erosion control measures such as those required by the Los Gatos Public Works Department (PG&E, 1997a).

7. The project would not require the removal of groundwater or any change in groundwater use, therefore, there would be no impact related to ground subsidence hazard (which is most commonly related to overdraft of an aquifer).

8. Expansivity, or shrink-swell, is the cyclic change in volume that occurs in fine-grained sediments because of expansion and contraction of clay caused by wetting and drying. Soils which are expansive (have shrink-swell potential) can damage foundations and structures. This problem can be overcome with proper foundation engineering (Helley, 1979). Surface soils on the site were observed to be compact gravel-sand-silt-clay mixtures with a low-to-moderate shrink-swell potential (PG&E, 1997a). PG&E’s proposed foundation engineering practices of filling and compaction to create engineered soils would reduce any impacts resulting from expansive soils to a less-than-significant level.

9. The site is essentially flat and has no unique geological features located on the site, therefore there would be no impacts related to unique geologic or physical features (ESA 1997).

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