Model studies of soil deformations over a moving basement

Centrifuge and single gravity model tests have been performed to study the deformations that occur in a layer of soil as a result of vertical movements of an underlying stiff basement which produce discontinuities of slope but not of displacement. The modes of deformation that have been observed are relevant to the ground movements that occur in areas of deep subsidence caused by extraction of minerals or hydrocarbons.

Plane soil models were tested on the centrifuge at 100 gravities in a strong box having a perspex window through which deformations of the soil could be observed while in flight. Single gravity tests have also been performed in which the same boundary movements were imposed. Results are reported from tests on layers of dense sand of two different gradings, and of soft clay. The clay was prepared in such a way that it was as nearly as possible normally consolidated throughout its depth.

In all cases some faulting of the soil occurred when the underlying basement was lowered — even though the basement did not contain a displacement discontinuity. In soft clay, normal faults extended from the ground surface at a steep angle behind the break in slope. In sand models, internal ruptures formed but these did not break through to the ground surface. The different patterns can be linked with the different strength and dilatancy characteristics of the two types of soil.