Parallel Seismic Testing
The Parallel Seismic test is employed to determine the buried length of unknown foundation depths for bridges and buildings. Compressional and shear waves are generated by a hammer impact at the top of the pile cap, traveling through the pile and refracting into the surrounding soil. The arrival time of the refracted wave is measured using a triaxial accelerometer in a cased borehole, while the triggering time is derived from another accelerometer placed at the top of the pile cap near the hammer impact location. Foundation depth is determined by plotting the wave travel time history over the measurement layer, with abrupt changes indicating the pile tip’s depth. Once soil properties from boreholes, pile depths from parallel seismic tests, and pile cross-sectional geometry are known, the load carrying capacity of the foundation is determined accordingly.
The apparatus for the Parallel Seismic test is depicted in the accompanying figure. Borehole locations should not exceed 1 m from the pile to ensure sufficient clearness of the refracted wave from the pile through soil layers to the receiver inside the borehole. Generally, the borehole extends to a greater depth than the suspected foundation depth, serving dual purposes for both soil property investigation and parallel seismic testing.