Properties of soil after surcharge or vacuum preloading

This paper compares the results of laboratory studies carried out to determine the engineering properties, response under cyclic loading and the microstructure of soft clay after subjecting it to preloading by surcharge and vacuum. The samples for these tests were prepared at different initial consolidation pressures of 25, 50 and 100 kPa under K-0 conditions to simulate the overburden pressure due to depth. These samples were further subjected to an additional pressure of 80 kPa under surcharge or vacuum loading conditions and then tested for engineering properties such as compressibility, permeability and shear strength. The response under cyclic loading was also studied. The changes in microstructure due to preloading were evaluated through scanning electron microscope studies. The consolidation properties of the vacuum preloaded samples were not very different from those of the surcharge preloaded samples. The vacuum preloaded samples showed less anisotropy of permeability compared with the surcharge preloaded samples. The undrained shear strength of the vacuum preloaded samples was slightly higher than the corresponding surcharge preloaded samples. The pore water pressure response under cyclic loading of the vacuum preloaded samples was significantly lower than the corresponding surcharge preloaded samples. The vacuum preloaded samples generally had a more flocculated microstructure, whereas the surcharge preloaded samples showed a more dispersed structure.