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Seismic zones are delineated on the basis of known distribution of damaging earthquakes and the Modified Mercalli intensity of these earthquakes, evidence of strain release, and consideration of major geologic structures and provinces believed to associate with earthquake activity. Geotechnical (Foundations) The project site lies within a reclaimed gravel pit of approximately 30 to 40 foot depth, within the Corral Hollow Creek alluvial fan. Sediments of the Corral Hollow Creek alluvial fan and the other alluvial fans are thin (0 to 150 feet), and consist of continental, unconsolidated material derived from the Diablo Range. The lithology is a heterogeneous mix of discontinuous layers of argillaceous sand, gravel, silt, and clay deposits, with lenses of poorly sorted coarse sand and gravel, and locally, thin beds of argillaceous limestone, marl, and marly silt. The fine grained beds are loose to semi -consolidated; the gravel is locally cemented either by calcium carbonate or gypsum so as to form resistant strata of comglomerate. In July of 1993, eight test borings were drilled at the site by RGH Geotechnical & Environmental Consultants (RGH). These borings penetrated to depths of from 9 to 25 feet. These exploratory borings by RGH found interbedded sand, silt, and gravel, with discontinuous silt and clay layers to a depth of 25 feet. Ground water was not encountered during the test borings. Local water wells indicate that ground water is present at a depth of approximately 70 feet. Additional information on ground water characteristics is presented in Chapter 4.5 Water Resources. Outside of the pit, the administration area is blanketed by up to 2 feet of sandy clay soil with a low expansion potential. The upper 1 foot of this soil is weak, porous, and compressible, and the lower portions are stiff and stronger. Within the pit, surface soils consist of approximately 1 foot of loose silty sands and soft sandy silts. Surface soils are underlain by discontinuous layers of dense to very dense silty sands and gravels to the maximum depth explored by the eight test borings (RGH, 1993). On the pit wall slopes, the top layer of soil is experiencing "creep", a gradual downhill movement. Soils on the slopes are incised by erosional gullies. Slope Stability In most areas, the pit walls are inclined at from about 2:1 (horizontal to vertical) to 4:1, and expose discontinuous layers of sandy silts and clays, clayey sand, and silty gravels. A cut slope at the northwest corner of the quarry is inclined at about 1.5:1. The ground surface within the quarry is relatively level, with occasional low mounds. The stability of the quant' slopes is addressed in the project Geotechnical Investigation Report prepared by RGH Geotechnical and Environmental Consultants (RGH) and dated August 16, 1993. The analyses utilized the computer program STABL5, which calculates factors of safety against instability of slopes. ER -93-1 _59- (9-27-93) I u 0 r n C 0 [I I 0 91 TABLE 4.4.3 CORRELATION OF SEISMIC RISK ZONES WITH EARTHQUAKE EFFECTS _. SEISMLG RISK MAXIMUM _: MAXIMUM; INTENSITY ZONE ACCELERATION MAGNITUDE 0 0.04 g 4.25 no damage 1 0.08 g 4.75 V and VI 2 0.16 g 5.75 VII 3 0.33 g 7.00 VIII and up 4 0.50 g 6.5 VIII and up Seismic zones are delineated on the basis of known distribution of damaging earthquakes and the Modified Mercalli intensity of these earthquakes, evidence of strain release, and consideration of major geologic structures and provinces believed to associate with earthquake activity. Geotechnical (Foundations) The project site lies within a reclaimed gravel pit of approximately 30 to 40 foot depth, within the Corral Hollow Creek alluvial fan. Sediments of the Corral Hollow Creek alluvial fan and the other alluvial fans are thin (0 to 150 feet), and consist of continental, unconsolidated material derived from the Diablo Range. The lithology is a heterogeneous mix of discontinuous layers of argillaceous sand, gravel, silt, and clay deposits, with lenses of poorly sorted coarse sand and gravel, and locally, thin beds of argillaceous limestone, marl, and marly silt. The fine grained beds are loose to semi -consolidated; the gravel is locally cemented either by calcium carbonate or gypsum so as to form resistant strata of comglomerate. In July of 1993, eight test borings were drilled at the site by RGH Geotechnical & Environmental Consultants (RGH). These borings penetrated to depths of from 9 to 25 feet. These exploratory borings by RGH found interbedded sand, silt, and gravel, with discontinuous silt and clay layers to a depth of 25 feet. Ground water was not encountered during the test borings. Local water wells indicate that ground water is present at a depth of approximately 70 feet. Additional information on ground water characteristics is presented in Chapter 4.5 Water Resources. Outside of the pit, the administration area is blanketed by up to 2 feet of sandy clay soil with a low expansion potential. The upper 1 foot of this soil is weak, porous, and compressible, and the lower portions are stiff and stronger. Within the pit, surface soils consist of approximately 1 foot of loose silty sands and soft sandy silts. Surface soils are underlain by discontinuous layers of dense to very dense silty sands and gravels to the maximum depth explored by the eight test borings (RGH, 1993). On the pit wall slopes, the top layer of soil is experiencing "creep", a gradual downhill movement. Soils on the slopes are incised by erosional gullies. Slope Stability In most areas, the pit walls are inclined at from about 2:1 (horizontal to vertical) to 4:1, and expose discontinuous layers of sandy silts and clays, clayey sand, and silty gravels. A cut slope at the northwest corner of the quarry is inclined at about 1.5:1. The ground surface within the quarry is relatively level, with occasional low mounds. The stability of the quant' slopes is addressed in the project Geotechnical Investigation Report prepared by RGH Geotechnical and Environmental Consultants (RGH) and dated August 16, 1993. The analyses utilized the computer program STABL5, which calculates factors of safety against instability of slopes. ER -93-1 _59- (9-27-93) I u 0 r n C 0 [I I 0 91