Ayres Knowledge Center Scour Evaluation: Bridges with Unknown Foundations | Page 29

3.0
PARALLEL SEISMIC METHOD
The PS method involves impacting the exposed portion of the foundation or substructure
attached to the foundation or a location which, when impacted, couples sufficient energy to the
pile to generate a sound or stress wave which travels down the foundation. The wave energy is
tracked by a hydrophone receiver suspended in a water-filled, cased borehole. The PS tests
typically involve lowering the hydrophone(s) to the bottom of the borehole, impacting the
exposed portion of the foundation structure and recording the hydrophone(s) responses. The
hydrophone receiver(s) is then raised 1 foot to the next test elevation. This test sequence is
repeated until the top of the casing (or the top of the water level in the casing) is reached. The
pile depth is determined by plotting the hydrophone(s) response from all depths on a single
display or page. For soils of constant velocity surrounding the pile (such as saturated soils below
the water table and well above the pile tips as was the case at this site), a break in the slope of the
line occurs below the bottom of the pile indicating the pile depth. For soils with varying
velocities, a break sometimes cannot be clearly identified from the slope of the lines, but the
bottom of the pile can be identified by observing the traces of the hydrophone plot to identify
changes in the response, such as a reduction in signal amplitude, change in signal frequency, or
diffraction/reflection of tube wave energy from the tips of piles (diffraction is frequently the
indicator for the pile tip depth of steel or concrete piles).
about 3-5 ft
15 ft
Figure 1 - Parallel Seismic (PS) Method for Unknown Foundation Depth Determination
Olson Job No. 4174A-3
Supplementary Report
FDOT Bridge 794004
Page 3
Page SR-26