Engineers Australia

Stability of excavations in unsaturated fissured clay

Peter W. Mitchell

Abstract

The problems of the stability of vertical or near vertical excavations in unsaturated fissured clay are discussed. A consideration of the stability of vertical excavations in non-fissured clay, in clay with tension cracks, and by considering the effects of lateral stress relief on excavation in clays without pre-existing fissures, gives greater excavation depths than would be regarded as acceptable in clays. The overall stability of the soil mass containing fissures is governed by the residual shear strength along the joints, particularly with polished slickensides. A common failure mechanism in vertical and near vertical excavations occurs when a slickenside intersects a vertical shrinkage crack. Using the residual shear strength parameters and a planar failure mechanism, the stability of vertical and steeply sided excavations, either supported or unsupported, in fissured clay is discussed. Three case examples are outlined, and recommendations for the successful construction of temporary batters in unsaturated fissured clay are given.