The main objective of pile instrumentation is to establish the load distribution characteristics of soil along the pie shaft.

The test pile is subjected to loading and under the maintained load test. Special vibrating wire type of strain gauges are attached along the main steel reinforcement bar in the test pile.  These gauges work on the principle that the natural frequency of a wire changes as its length changes.  Thus, by measuring the change in frequency, the deformation can be obtained, and hence the strain. The readout unit gives the strain directly. From the measured strains, the forces in the pile can be calculated, and thus the load distribution along the pile shaft is known when the strain gauges are installed at intervals along the pile shaft length. i.e. the average unit friction between any two sections of the pile is calculated by the change in load divided by the circumferential area between the two sections.