PURPOSE OF HIGH-STRAIN DYNAMIC TEST (PDA)
The high-strain dynamic (PDA) test method is applied to evaluate the load capacity of single vertical piles, single inclined piles, regardless of size and construction methods (driving, static jacking, drilling…) through determination of pile’s force and velocity due to an axial force made by a heavy hammer to create a large displacement in the pile head.
METHOD OF HIGH-STRAIN DYNAMIC TEST
The high-strain dynamic (PDA) test method is based on the principle of transferring stress wave in elastic bar. Stress wave caused by driving hammer moves down from the top to the toe of the pile. Its strength and velocity depend on hammer energy and mechanical characteristics of pile materials. The transferring process is affected by soil resistance around the pile and characteristics of pile material. By measuring and analyzing this process, distribution of the soil intensity can be determined at different levels and defects of pile can be discovered.
When applying an impaction at the pile top, a stress wave will be generated and propagated along the pile shaft at a constant wave speed C, which is function of the material elastic modulus E and mass density r, (C2 = E/r). The necessary time for stress wave transmitting to pile toe and reflecting to pile top is proportional with the distance to the source of reflection t = 2L/C.
When the stress wave (Wi) meets the impedance change from Z1 = r1.A1.C to Z2 = r2.A2.C, one part of the wave will reflect upward (Wu) and the other part will propagate downward (Wd) such that both continuity and equilibrium are satisfied:
Wd = Wi [2.Z2/(Z2 + Z1)]
Wu = Wi [(Z2 – Z1)/(Z2 + Z1)]
At the free pile toe (Z2 = 0), the impact will be completely reflected, but with an opposite sign and for a uniform pile (Z1 = Z2) the impact wave will transmit with constant amplitude
By attaching the accelerators and strain sensors for measuring value of ware speed and force at pile top for different times, it can estimate the defect state and resistance distribution of soil axially pile shaft.
- TCVN 11321:2016 “Piles – High-strain dynamic testing”.
- ASTM D4945-00 “Standard Test Method for High-Strain Dynamic Testing of Piles”.