Photo Credit: Nathan Shoemaker
Shown here are the strain gauge transducer and accelerometer used for high-strain dynamic pile testing, there are two more gauges on the other side of the pile. The idea behind dynamic pile testing is that when the pile is struck with the hammer a wave propagates down the pile to the tip and back up. The wave is dissipated by dampening along the skin and toe. The strain data combined with the pile properties (modulus of elasticity and area) give axial force and the acceleration data can be integrated to find particle velocity, and since the wave speed of steel and concrete is reasonably known, the wave distance can be determined. Depending on the shape of the force and velocity graphs it can be determined if the pile is more of a friction or end-bearing pile and what the capacity of the pile is. The legitimacy of this testing can be proven by the reductions in factors of safety for deep foundations that AASHTO allows for when a high-strain dynamic pile testing program is implemented on a project.