Question

20. A 1-foot diameter plate load test is used to develop recommendations for a larger foundation (3-ft by 3-ft). What is a limitation of that test and applying the results to developing design for the foundation?

Answer 1

Plate load test is carried out to determine the ultimate bearing capacity of soil and the probable settlement under a given load. Moreover the advantages plate load test has certain limitations also. It has a limited depth of influence. There is scale effect as the size of test plate is smaller than actual foundation. Normally the foundations are larger than test plates, the settlement and shear resistance of soil against shear failure will depend on the properties of much thicker stratum. When one foot diameter plate is used instead of test in a 3foot2 area, it cannot provide the accurate results because in practical condition the influence zone of 3foot2 foundation is up to greater depth. The test could only give the bearing capacity of soils with depth up to two times the diameter of plate. Thus the results of test could be misleading if the character of the soil changes at shallow depths. It may not provide information on the potential for long term consolidation of foundation soils.

The test predicts the behavior of soil located only at a depth less than twice the depth of the width of the bearing plate. But in practical, the influence zone of a foundation is up to greater depth. The plate load test is performed for a short time period, so it cannot predict the settlement for a longer period. The plates of greater area are economically not feasible. The size of the bearing plate also affects the settlement of the footing. For clayey soils the bearing capacity (from shear consideration) for a larger foundation is almost the same as that for the smaller test plate. But in dense sandy soils the bearing capacity increases with size of foundation. Thus tests with smaller size plate tend to give conservative values in dense sandy soils. It may, therefore, be necessary to test with plates of at least three sizes and the bearing capacity results extrapolated for the size of the actual foundation.

Plate load test is carried out to determine the ultimate bearing capacity of soil and the probable settlement under a given load. Moreover the advantages plate load test has certain limitations also. It has a limited depth of influence. There is scale effect as the size of test plate is smaller than actual foundation. Normally the foundations are larger than test plates, the settlement and shear resistance of soil against shear failure will depend on the properties of much thicker stratum. When one foot diameter plate is used instead of test in a 3foot2 area, it cannot provide the accurate results because in practical condition the influence zone of 3foot2 foundation is up to greater depth. The test could only give the bearing capacity of soils with depth up to two times the diameter of plate. Thus the results of test could be misleading if the character of the soil changes at shallow depths. It may not provide information on the potential for long term consolidation of foundation soils.

The test predicts the behavior of soil located only at a depth less than twice the depth of the width of the bearing plate. But in practical, the influence zone of a foundation is up to greater depth. The plate load test is performed for a short time period, so it cannot predict the settlement for a longer period. The plates of greater area are economically not feasible. The size of the bearing plate also affects the settlement of the footing. For clayey soils the bearing capacity (from shear consideration) for a larger foundation is almost the same as that for the smaller test plate. But in dense sandy soils the bearing capacity increases with size of foundation. Thus tests with smaller size plate tend to give conservative values in dense sandy soils. It may, therefore be necessary to test with plates of at least three sizes and the bearing capacity results should be extrapolated for the size of the actual foundation.