The Subjective Aspect of Dynamic Load Testing

Dynamic load testing (or high strain dynamic testing as the test is called in ASTM D4945) is widely considered to be a reliable method for determining the vertical compression resistance of deep foundation elements. To illustrate the reliability of this test method, comparisons between the outcome of a dynamic load test and a static load test on the same pile are often used in the literature. The comparisons seem to confirm that the outcome of a dynamic load test can accurately predict the static vertical compression resistance of deep foundation elements. Unfortunately, in many instances, little consideration is given to important details of the dynamic test, such as the number of blows applied in the test and the blow number analyzed, the soil model, the soil response to the blows (strain hardening vs. strain softening), or even the shape of the upward traveling wave. In recent years, this lack of transparency and the subjective aspect of the test result interpretation (something that is the case irrespective of the software used to perform the analysis) are increasingly seen as drawbacks of this test method, and around the world different approaches have been suggested to deal with these issues. This paper provides an overview of these different approaches and makes recommendations on how this testing method can remain a valuable tool for assessing the vertical compression resistance of deep foundation elements. Suggestions for a truly independent examination and certification of persons wanting to analyze dynamic load test results will be provided as well.