Even though plate load tests are very commonly used, they do not represent the behavior of prototype foundations totally for the following reasons:
The zone of influence for prototype foundation is larger and deeper than for small size plate. Thus, in stratified soil deposits the soils at greater depths will have a significant influence on the bearing capacity and settlement of prototype foundation. Whereas, for the plate its behavior is controlled chiefly by the upper strata. The figure shown below shows the effect of size of foundation and plate, on the zone of influence.
In truly uniform cohesive deposit the ultimate bearing capacity of the plate will be nearly equal to the ultimate bearing capacity of the foundation under undrained conditions of loading. But in such cases the settlement of the plate will not represent the long-term consolidation settlement of the foundation.
In granular and frictional soils the bearing capacity increases with the width of the foundation. Plate load test must be carried out on different sizes of plates to infer the effect of width of foundation on bearing capacity.
If the plate load test is carried out within the capillary zone the plate can bear higher ultimate load since the depth of capillary zone is more than or of the same order of magnitude as the size of the plate. The effect of capillary zone is, however, small in actual foundations, due to their large size. To avoid this error, the plate load test must be carried out at the water-table level.