INTERPRETATION OF THE RESULTS OF VERTICAL LOAD TEST ON PILES-PITFALLS AND REMEDIES

Abstract

The vertical capacity of piles is estimated using static analysis, dynamic analysis and load tests. The first mentioned two methods are supposed to provide only an approximate estimate of pile capacity. Moreover, they do not predict the load settlement behaviour. Load tests are expensive and the results may be valid only for the particular pile and site conditions under test. Further, the ground conditions which prevail at the time of load test may not remain the same subsequent to load test due to causes such as change in moisture content in the top layers, thixotropic hardening etc. Under these circumstances the load settlement behaviour of the pile observed from the load test can not be used directly to estimate the pile capacity. Hence there is a need to develop a procedure of estimating load settlement behaviour of axially loaded Piles, which takes into account the changes that might occur in the ground conditions subsequent to the lo-ad test. In order to achieve the above objective a computer program is developed based on Coyle and Reese (1966) analytical Procedure. Provision is made in the program to accommodate different site conditions. The program can be run on a 'Personal Computer', which is easily accessible. The various parameters which govern the load settlement behaviour of pile, like adhesion factor 'a', remoulding effect, thixotropic strength gain etc. are discussed in detail. Enough data are presented to take care of the above parameters and the same are' used in the numerical examples solved in the present investigation. ( x ) Making use of the program results are obtained for a number of numerical problems to bring out the following 1. The generalised nature of the program 2. Comparision of the computed and the observed load settlement data. 3. Effect of thixotropic strength gain with time. 4. Effect of the desiccated soil on the vertical capacity of the pile. Based on the results obtained, it is found that a load test conducted immediately after pile driving in soft clay is likely to underestimate the actual capacity substantially and a load test on a short pile in a cohesive soil having a desiccated top crust may substantially overestimate the vetical load capacity. The results obtained for some of the numerical examples suggest that the changes in the ground conditions that could take place subsequent to the time of load test may have significant influence on the load settlement behaviour of the pile. Therefore, the results of the load test may not become valid under these changed conditions. Such situations are illustrated and a procedure of moderating the load settlement curve observed in a load test to take care of the changes that might occur in the pile and the soil conditions subsequent to,the time of load test is suggested. The procedure of moderating the pile load test data can be carried out efficiently using the program developed which includes a graphics package.