ABSTRACT
Shallow foundations are usually designed on the assumption that they act in isolation. When two footings (or a group of footings) are closely spaced, however, there is a beneficial interaction that can be quantified in terms of the ‘efficiency’, i.e. the ratio of the overall (group) bearing capacity to the sum of the individual (isolated) bearing capacities. The literature on this topic has been surveyed by Hazell (2004). For footings on sand, numerous theoretical and experimental studies have shown that the effect of interaction becomes highly significant for friction angles greater than about 30° and spacings less than about one footing width B. In contrast, the undrained bearing capacity of closely spaced footings on clay has received very little attention, perhaps because the early theoretical work of Mandel (1963) showed that the beneficial effect of interaction was insignificant, even for fully rough footings. This was confirmed experimentally by Hazell (2004), though only a few of his tests were conducted on clay.
