Stability evaluation for the siting of municipal landfills in karst

Abstract During the siting, construction, and maintenance of facilities in the karst terrain, the stability of the residual soils that overlie the cavitose limestone is often a concern. The development of arching in the residual soils and the associated distribution of stress is important to the stability. The economics of the landfill construction are usually improved by excavating a portion of the residual soil to increase the capacity of the landfill. However, it is generally accepted that for a given diameter of potential limestone cavity, the residual clay soil becomes less stable as the thickness of the soil cover decreases. Geotechnical engineers are often called upon to evaluate the stability of the residual soils as a part of the permitting process or prior to the construction of critical structures such as municipal landfills; yet, simple, rational methods are not available. Simplified methods are needed to estimate the thickness of the soil required to provide stability against soil collapse and the subsequent formation of sinkholes or distress to the surface structures. A simplified method is proposed, which consists of the evaluation of stability with respect to two potential modes of failure within the residual soil. Stability with respect to the first mode depends upon the development of arching in the residual soil, while the second failure mode corresponds to the yielding and plastic flow of the soils into the soil dome and/or rock void. The finite element method was used to conduct parametric studies of both stability modes for a typical residual clay soil over a range of cavity diameters. The results are presented in the form of a design chart. The application of the chart is demonstrated by an example. If a candidate site with a proposed cover soil depth appears to be stable for the anticipated range of cavity diameters, then a more comprehensive subsurface investigation and additional analysis can be conducted to evaluate the stability with respect to the proposed waste loading.