Considerations for measuring residual stresses in driven piles with vibrating wire strain gauges

Vibrating wire strain gauges are often the preferred technology for measuring strain in driven piles. However, measuring the residual strain after pile driving is challenging to accomplish using vibrating wire gauges. The driving process can cause a shift in the no-load reading from a relaxation of locked-in manufacturing strains in the pile or relaxation of the gauge wire tension. Also, there are temperature effects from installing piles below ground. A test pile program was developed using driven steel H-piles instrumented with vibrating wire strain gauges. The piles were subjected to dynamic forces by striking against a steel plate in an attempt to relax the locked-in manufacturing strain prior to installation. The strain gauges and thermistors were connected to a data logger during pile driving to record strain and temperature changes following installation. It was observed that applying a dynamic impact to the piles prior to installation resulted in a shift of 0–5 με (microstrain). Temperature effects from installing the piles in cooler ground resulted in a shift of strain in excess of 60 με in some strain gauges. It is concluded that temperature-induced shifts to strain must be measured following pile driving to interpret residual stresses.