Abstract:
Measurements of pile strain and motion under hammer impacts are the basis of Dynamic Load Testing (DLT) of deep foundations. For driven piles, the hammer blows driving the pile provide the testing impacts. For auger-cast piles, drilled shafts, and helical piles, a drop hammer of an appropriate size is used to apply the few impacts for the test. The data is typically presented as pile-top force, velocity, wave-down, and wave-up time records. The hammer impact generates the incident stresswave travelling down the pile, soil resistance, pile end and non-uniformities generate the reflected stresswaves travelling up the pile; all shaping and affecting the characteristics of the test records. Data evaluation methods range from basic visual qualitative inspection of the test records to sophisticated quantitative computerized numerical analyses. The results include assessments of hammer, pile, and soil performances, including pile structural integrity, geotechnical resistance and static load bearing capacity. In general, dynamic test records have similar features that are common to most cases; however, under certain circumstances they exhibit unusual characteristics. Typically a Dynamic Load Test record can be divided into five phases: pre-impact period that is useful is assessing diesel hammers performance, impact event usually designated at peak as time “0”, time 2L/c (L is pile length and c is stresswave speed) designating one cycle of stresswave travel time containing information regarding skin friction resistance and structural integrity, toe reflection occurring at time 2L/c indicating end bearing resistance, and finally the end portion of the record that designates total resistance and rebound effects. This presentation uses wave equation analysis simulations and field measured data from numerous projects to study and delineate the various features and characteristics in the test records which will allow the attendees to confidently “read” the test records, understand the data, and rationally apply the test results.