Pitfalls and Fallacies in Foundation Design

Design of the load to be placed on footings and piled foundations conventionally includes calculated bearing capacity—a peak, or ultimate, resistance—usually defined as a plastic, or semi-plastic, response to increased load. Such bearing capacity can be found in loading tests on model footings, yet no full-scale tests on footings have show that an ultimate resistance mode has developed unless the load was placed off center—intentionally or not. For pile design, it is recognized that the pile toe is a footing with a long stem and none of the very large number of full-scale tests performed with the pile toe response measured separately from the shaft response, has shown a toe bearing capacity. Instead, the toe responses were similar to those found in tests on full-scale footings, i.e., a gradual, though less than linear, and increase of movement for increasing load. Pile capacity is usually established (interpreted) from the pile-head loadmovement of full-scale tests on single piles. However, the design analysis of the response of a pile to load is usually based on modeling a pile as a series of short elements, each with its ultimate resistance, or peak resistance. In most cases, the shaft resistance for each element beyond that peak is either strain-hardening or strainsoftening. Therefore, as demonstrated by examples, unless the pile is very short or next to infinitely stiff, the accumulated value of the ultimate shaft resistances of the element is not equal to the pile capacity established from the measured loadmovement curve of the pile. Moreover, for pile groups comprising more than about five by five rows of piles, i.e., wide piled foundations, the maximum shaft resistance of the piles in a group is limited to the weight of the soil in-between the piles, and the response of the piles differ between interior and perimeter piles. Design of wide piled foundations and piled rafts must be based on settlement analysis and no contribution from contact stress can be assumed.