Ayres Knowledge Center Scour Evaluation: Bridges with Unknown Foundations | Page 30

PARALLEL SEISMIC FIELD PROCEDURES AND RESULTS
4.0
DISCUSSION
The PS test equipment used in this investigation included a 1.25 inch diameter, 42 kHz
hydrophone receiver suspended at 1 ft depth intervals inside the 2 inch ID PVC water-filled
casings for all tests to record the arrival of the impact energy to the known hydrophone depth
(from a nylon tape measure).
The results of 3 impacts at each depth and corresponding
hydrophone receiver responses were averaged to improve signal to noise ratios in the data. Steel
pile compressional wave velocities are constant with a velocity of 16,600 ft/s. Concrete and
timber piles typically have velocities of 12,000 to 15,000 ft/s, but timber piles can range from
10,000 to 18,000 ft/s depending on wood species. Faster velocities are indicative of higher
quality, moduli and strength for concrete and wood piles. A hammer was used to hit a 6 inch lip
on the south face of Pier 17 with a hydrophone laying near the underwater impact location
which was used to trigger the recording system (see Figure 2 below).
Figure 2 –Hammer Impact to underwater ledge using a hydrophone nearby as a
trigger for the system on Bridge 794004 Pier 17 SW Corner Pile
Olson Job No. 4174A-3
Supplementary Report
FDOT Bridge 794004
Page 4
Page SR-27