ARCHIVED REPORTS_1989 - Laserfiche WebLink

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medium velocity (2880 to 3080 ft/sec) materials extended from 2 to 7 feet down to depths of 15 to 25 feet, where medium velocity (3660 ft/sec) materials were encountered. Again, the medium velocity materials extended to depths in excess of 96 feet. An anomalously low velocity (2100 ft/sec) zone was encountered at depths of a few feet to about 15 to 20 feet beneath the ground surface in the vicinity of the central 75 feet of line S-3. This zone corresponds to measurably lower blow counts from boring B-3, and may represent less cemented or weaker materials or perhaps a filled, buried channel. An anomalously high velocity (6250 ft/sec) zone was encountered just east of the middle of line S-4 at a depth of a few feet down to perhaps 10 or 15 feet beneath the ground surface. This zone may represent a more cemented or gravelly matrix in that location. Many of the lines have data with scatter, indicative of variations in the degree of cementation or content of rock fragments (gravels or cobbles). Rippability The low to medium velocity materials should be rippable with relative ease. The medium velocity materials may require heavy excavation equipment, but should be rippable without blasting to facilitate mass excavation. Figure 5 relates ripper performance to seismic wave Velocities. Subsurface materials at this site are alluvial fan deposits and include clays, silts, sands, and gravels as well as various -combinations of these constituents. Groundwater Because seismic velocities remained well below 5000 ft/sec, it is unlikely that saturated aquifers will be encountered to depths of at least about 96 feet beneath the project site. Thin, perched groundwater zones may be present, having escaped detection with the methods employed in this survey. Method and Equipment The seismic refraction survey procedure used for all lines consisted of placing 12 geophones in as straight a line as practical (in plan) spaced at 25 -foot intervals along as constant a slope as practical (in profile). A large sledge hammer was impacted 12.6 feet off both ends and at the center of each seismic line. The hammer impact generated seismic compression waves which were refracted through subsurface materials and received by the deployed geophones. Signals from the energy source initiation (time break) and from the geophones were monitored (amplified and filtered) simultaneously by a seismograph and displayed graphically in analog form by an oscillograph. Permanent records produced by the oscillograph were field checked, catalogued, and returned to the office for data reduction and interpretation. Seismic refraction lines were surveyed for location and