ARCHIVED REPORTS_1988 - 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 cobb I e3 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 6000 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 practicat. , ( in plan ) spaced at 25-foot intervals along as constant a slope 4practical -* ( in profile ) . A large sledge hammer was impacted-A1 . 5 feet off both ends and at the center of each seismic kXna . 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