Dynamic Pile Testing with the Pile Driving Analyser provides many quantities such as hammer performance, pile stresses, etc., immediately during the test. However with regard to static soil resistance the nature of soil, solid particles, air and water, means that the soil resistance measured on the pile shaft and at the pile base during a Dynamic Load Test is the total resistances consisting of a static soil resistance and a dynamic soil resistance. The CAPWAP program is required to separate the static soil resistance from the total soil resistance.
Pile Analysis engineers start the CAPWAP procedure by building a mathematical model of the test pile and the soil surrounding it. The pile model consists of a series of elements usually around 1m in length having the physical and mechanical characteristics of the pile. Each pile model element below ground has a soil resistance consisting of an elastic-plastic spring (static soil resistance) and a linear dashpot (dynamic soil resistance). In Dynamic Pile Testing the pile top force, velocity and pile response is measured and is therefore known, while the soil that produced the response is unknown.
The CAPWAP analysis begins with an Pile Analysis expert evaluating the dynamic load test data and considering any soils information available then inputting pile shaft resistance and pile base resistance in terms of a starting soil model for the pile shaft elements and the pile base. One of the measured quantities, velocity, is applied to the pile top and the CAPWAP program computes the resultant complementary quantity, force computed, which is compared graphically with the force measured. This process is repeated, progressively adjusting the model static soil resistance, until a best match is obtained between the computed pile top force and the measured pile top force (Fig.1). Once this best match has been reached the CAPWAP modelling procedure is complete and the resulting model is a mathematical representation of the pile and the soil surrounding the pile. This model can then be examined to give information about the expected performance of the pile under static loading conditions i.e. end bearing, shaft resistance and the distribution thereof, pile top and toe static load versus deflection et al.