Effect of Uncertainties in the Field Load Testing on the Observed Load–Settlement Response

Abstract

Field plate load test (PLT)/pile load test is being considered the most suitable technique to obtain load–settlement response of foundations resting on soil/rock. The load–settlement data obtained from PLT is routinely used in the evaluation of several design parameters of soil, such as bearing capacity, settlement of foundations, modulus of subgrade reaction, Young’s modulus, etc. This information is very much essential and would assist the engineers in the design decision process. However, due to various sources of uncertainties in testing procedures, viz. lack of suitable equipment and experimental know-how, inadequate codal provisions, etc., these tests often produce unrealistic results, and create chaos in the design decisions. In this paper, a failed field PLT is discussed and various irregularities associated with the field testing are identified, viz., non-maintained load during loading stages, man-made disturbance of the influence zone during the testing, improper placement of supports to datum bar, etc. Some of the above can be easily avoided by using little common sense. However, it has been noted from several field load tests that the load on the plate seldom made constant during the loading cycles. The effect of non-maintained loading on the load–settlement behavior is systematically analysed, through a series of laboratory PLTs, simulating the field conditions. By comparing the results obtained from the laboratory and field load tests, it is observed that the bearing resistance of the soil is highly overestimated due to non-maintained load. Equilibrium pressures are attained only for lower pressure range of 100–200 kPa after 15–20 min of load application, and for higher pressure range, the pressure on the plate is continuously decreased. Based on the load–resistance factored design approach, a resistance factor of 0.5 is obtained for bearing pressure, which suggests that there is significant variability in the bearing pressure. In conclusion, proper care should be taken during the load testing, and the codes of practice should be revised from time to time.