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

Abstract

igure 1. Regional map showing the locations of boreholes (solid circles) included in this report. FIELD MEASUREMENTS Drilling and Sampling Procedures Boreholes 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 Logs Geologic 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 po...

Comment on "Comparison of Time Series and Random-Vibration Theory Site-Response Methods" by Albert R. Kottke and Ellen M. Rathje

“…The TI array was installed in 1992 by the CGS with support of the National Science Foundation (NSF) to study the response of soft-over-rock geologic structure to earthquake motion (Darragh et al, 1993;de Alba et al, 1994). Original downhole S-wave velocity measurements were performed in the 104 m deep hole by the U.S. Geological Survey (Gibbs et al, 1992). I performed new S-wave velocity averaging based on the P-S suspension logging conducted in the more recently drilled 122 m deep downhole .…”

Nonlinear numerical method for earthquake site response analysis II — case studies

“…The physical characteristics used in our simulations for the soil profile (see Figs. 4, 5 and Table 1) were determined from the geophysical and geotechnical studies performed on the site (Gibbs et al 1992;Hwang and Stokoe 1993;Pass 1994;Graizer et al 2000). They are detailed in Table 2.…”

Identification of Site Frequencies from Building Records

“…Recently compiled borehole and site characterization information is also available (Gibbs et al 1994a). Figure 1 shows plan view, instrumentation scheme of the building, and the location of the free-field stations associated with this building instrumentation.…”

“…acterization data related to variation of shear-wave velocities with depth (Gibbs et al 1994a). The depth to bedrock has been adopted from a map by Hensolt (1993) as 150 m (ϳ500 ft).…”