Investigation of the dynamic soil properties to be used in interpretation of dynamic load test results

Abstract

Testing of piles using high strain dynamic load testing using Pile Driving Analyzer (PDA) has become very popular among Sri Lankan geotechnical engineers. In the high strain dynamic pile load testing, a pile instrumented at the top with accelerometers and strain gauges, is impacted with a heavy hammer to generate a large strain compression wave, which propagates along the pile shaft. The measured strain is converted to force using cross sectional area and elastic modulus of the pile, and acceleration is integrated into velocity. There are mainly two methods used in the analysis of the velocity and the force measurements at the pile top: (i) Case method - using the wave propagation theory and assuming linear elastic pile material, and dynamic soil properties; and (ii) CAPW AP method - using a combined wave equation soil model and a continuous pile model to iteratively determine the unknown soil parameters by matching the field recorded velocity and force measurements at the pile top. The PDA operator in the field will use the Case method, with assumed Case damping factor (Jc) and wave velocity (C) to check the mobilized capacity under a given hammer blow and the integrity of the pile. When the mobilized capacity estimated from the Case method exceeds the required capacity, the PDA operator will terminate the test. Thereafter, measured velocity and force records are down loaded to a computer and a more rigorous matching process is used to obtain more accurate strength and deformation characteristics of the pile tested. In this research, the soil material parameters used in the field with the Case method, namely Case damping factor and the wave velocity are investigated.