PSI uses the well-known Parallel Seismic method to establish the depth of existing foundations (specifically piles) where the superstructure precludes access to the pile heads.

The test requires the installation of a plastic access tube in parallel, and as close as possible, to the tested pile. The tube should be carried down to a depth exceeding the assumed pile length by a margin of 8-10 m and filled with water. In unsaturated soils the tube should be firmly grouted in the hole to achieve good coupling with the surrounding soil.