Bearing capacity of three closely-spaced footings on sand

Closely–spaced footings may be encountered on cohesionless soils in the form of grillage foundations and railway ties. Close spacing is known to permit higher loads than can be carried by similar isolated footings. This effect is due to interference between failure zones in the sand, and may increase the capacity by 150% when o = 35° and the centreline separation of the footings is approximately twice the footing width B. The method of stress characteristics has been used to calculate the theoretical bearing capacity of a series of parallel footings, and these have been compared with laboratory tests on three parallel surface footings at various spacings on two different sands. The testing programme also investigated the effects of footing roughness and load distribution. When interference occurs, pre-failure settlements become larger, and post-failure behaviour more ductile. The theoretical solutions show similar trends to the model tests, but suggest that interference stops at rather smaller spacings. D...