The rapid load testing of piles in fine grained soils
The behaviour has been examined of piles installed in clay subject to a rapid load testing method known as the Statnamic test. The Statnamic method is easier and quicker to mobilise than a static test and is less complex to analyse than dynamic pile load tests. This investigation consisted of a laboratory study of the effect of the rate of loading on pile behaviour in clay and a field test of a pile in glacial clay to calibrate the findings of the laboratory study. The effects of penetration rate and Statnamic loading on model pile behaviour have been studied using an instrumented clay calibration chamber. The effect of rate of loading on the pile's capacity was quantified using constant rate of penetration tests (CRP) at different pile penetration rates. This allowed viscous soil damping characteristics to be determined and a new Statnamic analysis method incorporating rate dependant soil behaviour to be developed. This rate dependant behaviour can be represented by modification of a non-linear rate law proposed by Randolph & Deeks (1992). A field pile testing facility was developed in glacial till. To test the success of the new Statnamic analysis, a class A prediction of static pile behaviour from prototype pile load testing was undertaken. Encouraging results were obtained for the prediction of ultimate static pile behaviour, but the analysis method under predicted soil-pile stiffness. A soil inertial component was added to the analysis, based upon instrumentation readings, which improved the predicted static soil-pile stiffness. Results from prototype pile testing show that the stiffness during Statnamic and static load tests was very similar up to 50% of the ultimate static pile capacity. Thus, rapid load testing may be used for verification of pile settlements at working loads in clays. At the present level of understanding of testing in clays, rapid load pile tests should not be carried out in isolation. Ideally, tests should be used in conjunction with a static test that will allow back figured parameters to be derived for analysis.