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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 K0 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.

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Vertical drains are widely used to accelerate the consolidation of soft clay deposits when preloading is used as a ground-improvement technique. Barron's theory is generally used for the design of vertical drains. One of the important input parameters required in Barron's theory is the coefficient of horizontal consolidation, ch. A reliable estimate of ch is, therefore, necessary for proper design. The values of ch can be determined by the radial-consolidation test, using either a central sand drain or a porous plastic peripheral drain. This paper compares the results of the radial-consolidation tests conducted using a 100 mm dia. Rowe consolidation cell under radially inward or radially outward flow conditions. The influence of the diameter of the sand drain (namely, 10, 15 or 50 mm) was also studied. It was found that the radially outward-consolidation test gave lower values of ch compared to the radially inward test. Also, it was observed that the measured values of ch decreased as the perimeter of the drainage boundary increased.

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