INSTRUMENTED FOOTING LOAD TESTS IN SOFT SENSITIVE BOTHKENNAR CLAY

The paper describes the loading to failure at a site on the south bank of the Forth Estuary in terms of load displacement history and bearing capacity. The site consists of a thin dessicated crust over 14 to 20 metres of lightly over consolidated estuarine clays and silts above a layer of gravels. Field behaviour is compared with theoretical predictions and those based on high quality laboratory testing. Details are given of the instrumentation consisting of one magnet extensometer, two earth pressure cells, three pneumatic piezometers and four settlement points cast into the footing at the centre of each edge. Results showed the Bothkennar clay to be highly non-linear, inelastic and hysteretic even at low strains. Vertical deformations were concentrated closer to the edge of the footing than predicted by elastic theory. Classical bearing failure mechanisms do not accurately model the observed progressive, punching type failure mechanism. Back analysis of the field behaviour predicted shear strengths up to 30 per cent less than peak undrained shear strengths from triaxial compression samples. Small variations in properties can initiate local yielding under load. Laboratory and field tests of shear strength and axial strain show similar results.