Mechanics of upward propagation of cover-collapse sinkholes

Cover collapse sinkholes occur by failure of the walls of a soil void above cavernous bedrock. I examine conditions for failure in compression by crack propagation and coalescence, and for failure by sloughing in which pore water pressure produces a net tension perpendicular to the wall of the void. Compressive failure without relaxation of stress does not explain observed long-term stability of thick soils, nor failures in thin residual soils. If stress relaxation occurs with compressive failure, soil voids tend to stabilize and sinkholes are not predicted. Sloughing failure is not predicted for steady state pore pressure fields, even for a perched water table at the surface over a drained void. Transient pore pressure conditions that could produce sloughing may result from: (1) rapid drawdown causing soil consolidation and transient high pore pressure gradients; (2) approach of a wetting front in unsaturated soil, with high permeability behind the front; (3) dissolution of air from bubbles in the soil and exsolution near the void producing a low permeability zone near the void.