Yielding in soft clay at Bäckebol, Sweden

Using a simple elastic-plastic model of soil behaviour it is possible to predict the stress paths that will be followed and the pore pressures that will be observed at points on the centre-line below a circular load applied to the surface of a bed of soil. At Bäckebol, near Göteborg, Sweden a test was conducted by Sällfors (1975) in which a circular tank was slowly filled with water, and changes in pore water pressure were measured in the clay beneath the tank. Some of the assumptions of the simple model are not valid at Bäckebol, but the results can be back-analysed in order to deduce the shape of part of the yield surface for the soil. If the in situ stress conditions and past stress history are known for a site then the simple model can be used to predict the way in which yielding can be expected to spread in the soil below a circular load. It is found that very different sequences of elements reaching yield are obtained for different assumed site conditions: the spread of yield is shown for the known conditions at Bäckebol and also for three plausible artificial sites. Finally comparison is made between predicted and measured rates of pore pressure generation. It is concluded that treatment of soil as ari elastic-plastic material can provide a useful framework against which to interpret the results of field loading tests on soft clay.