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

PPT Presentation

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The presentation explains how enhanced geologic site characterization can improve pile-foundation design. Rather than relying primarily on existing records and boreholes, it recommends combining borings with geophysical data to develop a more reliable subsurface model and identify spatial variations.<br /><br />A case study at a facility development project illustrates the value of this approach. Borehole-based mapping of a sand bearing layer did not adequately predict pile behavior: test piles reached required capacity at shallower depths than expected, creating risks of overlong or insufficiently long piles. Standard penetration-test (N-value) results were not consistently representative of shear-wave velocity (Vs), particularly in saturated loose sand and silt.<br /><br />To supplement the borings, the project used active and passive Multichannel Analysis of Surface Waves (MASW) and seismic refraction. The surveys mapped the bearing layer, characterized Vs to depths of at least 60 feet, and sought deeper information for seismic site classification. Construction activity and limited access constrained survey locations and data collection.<br /><br />Results showed that the top of the bearing sand generally corresponded to a Vs boundary of about 650–700 feet per second, but soil stiffness varied across the site. Profiles identified velocity inversions, dense outwash or till at roughly 60–70 feet below ground, and a likely drainage channel containing younger, loose, fine-grained sediments.<br /><br />The two-dimensional Vs profiles enabled more accurate pile-length and driving-criteria adjustments, allowing production piles to be ordered with greater confidence. Monitored installation was successful, and the geophysical data also supported seismic site classification and foundation design for rotating equipment.

Keywords

geologic site characterization

pile-foundation design

subsurface model

boreholes

geophysical data

MASW

seismic refraction

shear-wave velocity

seismic site classification

pile length