Can dynamic pile test (PDA test) be used to replace pile load test for testing driven pile capacity? The PDA system consists of two strain transducers and two accelerometers attached to opposite sides of the pile to measure the strain and acceleration in the pile. The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modulus. The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile. PDA test is commonly used for the following purposes: (i) Evaluation of driven pile capacity Soil resistance along the shaft and at the pile toe generates wave reflection that travel to the top of the driven piles. The time the reflections arrive at the pile top is a function of their locations along the pile length. The measured force and velocity at the pile top thus provide necessary information to estimate soil resistance and its distribution. (ii) Measurement of pile stress during driving Compressive stress at pile top is measured directly from strain transducers. (iii) Measurement of hammer energy delivered to the piles The hammer energy delivered to the pile is directly computed as the work done on the pile from the integral of force times displacement( ?Fdu ) and this can be calculated as force times velocity integrated over time (?Fvdt ). (iv) Determine if pile damage has occurred. Pile integrity can be checked by inspecting the measurements for early tension returns (caused by pile damage) before the reflection from the pile toe. The lack of such reflections assures a pile with no defects. There is a growing trend of using PDA test to check driven pile capacity instead of traditional static load test which is more expensive and time consuming. Some engineers may have reservation on the use of dynamic formula to evaluate driven pile capacity because of the fear of improper selection of quake values and damping factor. This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
Posted on: Fri, 12 Jul 2013 04:01:39 +0000