Seismic velocities and geologic logs at seven strong-motion stations that recorded the 1989 Loma Prieta, California, earthquake; Part IV

Abstract

Drilling and Sampling ProceduresBoreholes were located as close as possible to the accelerograph stations that recorded the strong-motion from the Loma Prieta Earthquake.In the case of Anderson Dam, Calaveras Reservoir, Sunol Fire Station, and Webb Ranch the boreholes were drilled approximately 30 meters from the strong-motion instrument.Callens Ranch and Hollister Airport borehole were located within 20 meters of the instrument and Williams Ranch borehole was within 10 meters of the strong-motion accelerograph.At each site a hole approximately 6 to 10 inches in diameter was drilled using rotarywash drilling with bentonite mud.For purposes of economy, samples were not taken in any of the boreholes.The borings at all stations in this report were cased with 3-inch inside-diameter, class 200, polyvinyl-chloride pipe capped at the bottom.The annular space around the casing was tremie grouted by pumping a water-cementbentonite mixture through a 1-inch steel pipe inserted 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 50-60 meters to prevent collapse of the casing. 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. S-and P-wave 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 zero time determined by the triggering of the * In this report S-wave and shear-wave are used interchangeably as well as P-wave and compressional-wave.recorder by the contact switch.After the measurement at the bottom of the borehole all measurements for this group of stations were made at 2.5 meter intervals.The data are recorded on magnetic tape cassettes in digital form on a twelve-channel recording system.