BRITTLE CONTRACTIVE BEHAVIOR IN SOIL, TAILINGS, AND ROCK MASS

Abstract

The understanding of the mechanisms of loss of strength responsible for many significant slope failures has developed over recent years. Unforeseen slope failures, including tailings dam collapses, have been one of the main catalysts for the improvements to geotechnical understanding. Sudden loss of strength or brittleness can occur in the extreme case of these processes, such as liquefaction. The mechanism is typically understood to involve the rapid transition from drained to undrained behavior commonly associated with contractive behavior. Other slope failure mechanisms, including those that occur in rock masses, involve strength loss. Rock deformation has been previously characterized as dilative and therefore not likely to produce high water pressure associated with contractive deformation. However, contractive behavior can be shown to occur in rock mass kinematics such as toppling. In toppling, the layers or columnar blocks are initially in contact with each other and are in a configuration allowing down-slope rotation. The rate of movement is moderated by the frictional strength of the rock interfaces. Face-to-face contact between layers is lost as space opens up during rotation. This stage of failure can occur rapidly as the assemblage of layers loses its frictional strength and is effectively in a relatively free-falling condition. The layers may regain frictional strength once they have rotated into a condition of symmetry that restores the contact of rock interfaces. The new orientation of the toppled layers does not have the potential energy of the pre-toppling condition, and the slope may become locally self-stabilized. Between the initial and final conditions, there is a dilative and a contractive phase. The contraction of the rock mass has the potential to cause ejection of air and water, as is sometimes observed in rock slope failures. The contraction of the rock mass during deformation also has the potential to cause high water pressures, leading to a reduction in stability and a change to the failure mechanism.