Seismic velocities and geologic logs from borehole measurements at eight strong-motion stations that recorded the 1989 Loma Prieta, California, earthquake

Abstract

Drilling and Sampling ProceduresBoreholes were located as close as possible to the strong motion accelerographs usually within 10 meters.In the case of Oakland 2-Story office building site, the borehole was located in Snow Park about 150-200 meters from the instrument location.The Presidio borehole is located approximately 100 meters from the strong-motion instrument.At each site a hole approximately 6 to 10 inches in diameter was drilled using rotary wash drilling with bentonite mud.For purposes of ecomony, samples were not taken in any of the boreholes except for Oakland 2-Story (Snow Park).At this site seven "Undisturbed" samples were taken inside Shelby tubes (3-inch outside diameter) using a Pitcher barrel.These samples were allowed to drain of free water and sealed with wax plugs and endcaps.These samples were sent to Jonathan Bray at Purdue University for testing.The borings at Woodsite Fire Station and Gilroy #7 were cased 3-inch inside diameter, class 200, polyvinyl-chloride pipe capped at the bottom.APEEL #1 and APEEL #2 were cased with 4-inch pipe and the other sites were cased with 5-inch inside diameter pipe.The annular space around the casing was tremie grouted by pumping a water-cementbentonite mixture through a 1-inch steel pipe installed next to the casing.This provides good coupling between the casing and the wall of the borehole, and provides a sanitary seal preventing contamination of ground water.Grouting was done in stages of about 50-00 meters to prevent collapse of the casing.The California Division of Mines and Geology plans to install a strong-motion instrument package at the bottom of each 5-inch hole to supplement surface recordings. Geologic LogsGeologic logs are based on descriptions of drill cuttings, samples, reaction of the drill rig, and inspection of nearby outcrops.Sediment samples are described using the field techniques of the Soil Conservation Service (1951).Descriptions include sediment texture, color, and the amount and size of coarse fragments.Texture refers to the relative proportions of clay, silt, and sand particles less than 2 millimeters in diameter.This is determined visually and by feel without using laboratory tests.As such, this system is easier to use in the field than other classification systems.The dominant color of the sediment and prominent mottles are determined from the Munsell soil color charts.Descriptions of rock samples include rock name, weathering condition, color, grain size, hardness, and fracture spacing.Classifications of rock hardness and fracture spacing are those used by Ellen et al., (1972) in describing hillside materials in San Mateo County, California.Most information needed for describing relatively well-sorted soils and such properties of rock as lithology, color, and hardness are readily obtained from cuttings.Inspection of samples and nearby outcrops is necessary for determining the nature of poorly-sorted materials and fracture spacing.Reaction of the drill rig is useful in determining approximate sediment texture and in determining degree of fracturing because the rate of penetration in rock is highest for very closely fractured and crushed materials and drilling roughness generally is at a maximum in closely to moderately fractured rock.In-situ consistency of soil is determined largely from standard penetration measurements and rate of drill penetration. Site GeologyFour sites, APEEL #1, APEEL #2 (Portside Park), Oakland 2-Story (Snow Park) and Palo Alto 2-Story are located near the margin of San Francisco Bay on thick sections of Holocene and Pliestocene estuarine and alluvial sedimentary deposits.These deposits consist largely of fine-grained sediments with occasional thick layers of sand and/or gravelly sand.Two of the boreholes were drilled into the underlying bedrock.At APEEL #1, serpentinite was encountered at 188.5 meters (618 feet) and at APEEL #2, greywacke sandstone of the Franciscan assembladge was encountered at 84.7 meters (278 feet).The geology at Gilroy #7 consists of 17.4 meters (57 feet) of poorly sorted gravelly clay and sandy clay alluvium overlying hard Tertiary shale and sandstone.Two sites are underlain by bedrock.The Presidio site is underlain by serpentinite.Woodside Fire Station is underlain by firm to hard sandstone and mudstone which may be a part of the Butano sandstone.The strong motion instrument at Corralitos sits on 32 meters (105 feet) landslide deposits consisting of very closely fractured black shale.Major slide planes apparently were encountered at 15 meters (50 feet) and 32 meters (105 feet).The landslide deposits are underlain by sandstone which is probably part of the Purisima formation.Cracks in the road above the site suggest that the landslide may have moved slightly during the earthquake. Travel-time DataShear waves* were generated at the ground surface by an air-powered horizontal hammer (Liu, et al., 1988) striking anvils attached to the ends of a 2.3-meter-long aluminum channel.The hammer can be driven in both horizontal directions to generate positive and negative shear pulses.The switch that determines zero time is a piezo-electric sensor attached to the shear source.The source is offset from the borehole to prevent the direct arrival from traveling down the grout next to the casing.The source offset is 2 to 5 meters depending on the depth of the borehole.Shallow holes (30 meters or less) are generally offset 2 meters, while boreholes deeper than approximately 100 meters are offset 5 meters.Travel times are corrected (for slant offset) to vertical by the cosine of the angle of ray incidence.P-waves are made by striking a steel plate with a sledge hammer at the same intervals described above.The recorder is triggered by the sledge hammer making electrical contact with the steel plate.Measurements are made by lowering a single three-component geophone into the borehole and clamping it to the casing-wall with an electrically actuated lever arm.A second three-component geophone is placed at the surface approximately 30 centimeters from the shear source and is used as a check of the switch triggering the recorder for zero time.Depending on geologic information, measurements are repeated at 2.5 or 5.0 meter intervals.The 2.5 meter spacing is used when the layering of the sediments is thin (under 10 meters) and generally from the surface to 30 meters depth.The data are recorded on magnetic tape cassettes in digital form on a twelve channel recording system. DATA INTERPRETATION and PROCESSINGThe flow-chart, Figure 2, describes the processing and interpretation proceedures.The magnetic tape cassette contains 18 recorded traces from each depth.These include data * In this report shear-wave(s) and S-wave are used interchangeably as well as P-wave and compressional-wave.* see text for explanation of velocity limits AP18.C 9-24-93 8:40a