SEISMIC VELOCITIES OF SAN FRANCISCO SEDIMENTS

Data gathered from seven borings in the southern San Francisco Bayshore area show that each of the three Quaternary geologic formations-Holocene Bay Mud, Young (late Quaternary) Alluvium, and Older (mid-Quaternary) Alluvium-has a characteristics set of soil types, geotechnical properties, and seismic wave velocities. The Bay Mud consists of soft, normally consolidated, estuarine, plastic silts (MH) and fat clays (CH) with an average shear wave velocity of 80- meters/second (260 ft/sec). The Young Alluvium consists of well-graded sands (SW) and gravels (GW) deposited in alluvial fans, interbedded with poorly graded fluvial channel sands (SP) and stiff, over-consolidated, nonplastic silts (ML) and lean clays (CL) deposited on floodplains between fans. The Older Alluvium is also composed of well-graded sands and gravels from alluvial fans, but has a higher degree of consolidation than the Younger Alluvium deposits & a higher V sub s, 550 meter/sec (1800 ft/sec) wave velocity. The seismic data further further suggest that the compressional wave velocities are controlled primarily by the position of the water table rather than the soil type or degree of consolidation. The average compressional velocity of dry soil is about 800 meters/second (2620 ft/sec versus 1800 meters/second (5900 ft/sec) for water-saturated soils. The shear wave velocities, on the other hand, are quite sensitive to change in soil type or consolidation but show little variation across the water table. For Quaternary soils, therefore, one should not rely on Poisson's ratio to estimate V sub s from measurements of V sub p alone, but show measure shear wave velocity directly. The major geotechnical factors controlling the shear wave velocity appear to be: a) the soil type, with coarse-grained soils having a cosistently higher V sub s than fine-grained types; and b) the degree of consolidation, with the over-consolidated silts and clays of the Young Alluvium having consistently higher V sub s than the normally-consolidated silts and clays of the Bay Mud, and c) the age, with a higher V sub s for the Older Alluvium than for the Young Alluvium. /Author/

Media Info

  • Features: Figures; References; Tables;
  • Pagination: p. 1007-23

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Filing Info

  • Accession Number: 00179794
  • Record Type: Publication
  • Report/Paper Numbers: Proceeding
  • Files: TRIS
  • Created Date: Oct 12 1978 12:00AM