Geophysical Investigations Enhance Geologic Characterization in Support of Pile Foundation Program-Paper

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The paper describes how seismic geophysical surveys improved subsurface characterization for an industrial facility in eastern Rhode Island. The site contained variable-thickness organic silt and peat over sand deposits with uneven density, making pile lengths difficult to estimate from limited borehole data. During test-pile installation, some piles reached capacity sooner than expected, raising concerns about cutoffs, code-required reinforcement, and ordering production piles of suitable lengths.<br /><br />To refine the ground model, investigators used four multichannel analysis of surface waves (MASW) surveys and one seismic refraction survey. Construction activity restricted survey locations and created substantial noise, which was managed using an excavator as a seismic source for refraction and active and passive data collection for MASW. The surveys mapped compressional-wave and shear-wave velocity structures to depths of approximately 60 feet or more.<br /><br />Results broadly confirmed the borehole-based trend for the top of the bearing sand, generally associated with a shear-wave velocity boundary of about 575–600 feet per second. However, the profiles revealed significant spatial variation in soil stiffness, frequent velocity inversions in loose saturated sediments, and dense outwash or till at roughly 60–70 feet below ground. One survey line identified low-velocity deposits consistent with a buried drainage channel filled with younger, loose sediment.<br /><br />The findings supported a depositional model involving marine sediments and helped engineers make limited adjustments to pile lengths and driving criteria. With improved confidence in the design, production piles were ordered and installed successfully; monitoring confirmed that required reinforcement remained below cutoff and that cutoff quantities were as anticipated. The shear-wave data also supported seismic site classification and helped determine dynamic soil properties for rotating-equipment foundations.

Keywords

seismic geophysical surveys

subsurface characterization

MASW

seismic refraction

pile design

shear-wave velocity

bearing sand

soil stiffness

marine sediments

seismic site classification