PERMFILE41761 - Laserfiche WebLink

Home Browse Search

'F ~~ ~,Nv~! ~~L~fa~,s ~x~,.~..~~ III IIIIIIIIIIIIIIII .. ~ Analytical Methods for Predicting Subsidence Above Solution-Mined Cavities C: Y. Chang Keshavan Nair Woodward-Lundgren & Associates, Oak/and, Calilarnia ABSTRACT A genera! rational approach is described jor analyzing complex problems dealing with subsidence prediction. Qualitative comparison ojresu(ts obtained from the linear elastic analysis with observed subsidence phenomena in- dicated that the computed subsidence was generally much smaller than the observed subsidence and that the sub- sidence exlended over a (argei area than the actual sub- sidence in the field. This conclusion led to the necessity of developing techniques for computing the subsidence that may result from failure in the surrounding rock mass in the form of (!) rock failure in the immediate vicinity of the cavity or (1J failure along certain geo/ogic discontinuities such as faults, joints, or bedding planes. A computer program using finite element techniques has been developed to analyze the conditions ehal are created in a rock mass surrounding a solution mined cavity with the object ojpredicting the subsidence. The program, in addi- lon to the ability to conduct a linear elastic analysis, can account jor the inability of the rock mass to withstand tensile stresses and the occurrence ojslippage along bedding planes. The jarmulalion ojthe analysis and the capabilities of the program are discussed in the paper. The program does not consider time-dependent problems; this is treated in a comparton paper by Nair et al. of this Symposium. The analyticalmethods developed were employed to an- alyze two case histories ojso(ution mined cavities. A com- parison with observed subsidence indicates that the method can be utilized to predict subsidence above solution mined cavities with reasonable accuracy. INTRODUCTION The creation of an underground opening causes move- ments and induces stress changes in the surrounding rock mass. The movements are manifested as subsidence at the ground su rface above the opening. The subsidence and the associated horizontal strain are of concern to the mining industry because of the possible damage that can result to structures located on the ground surface. Therefore, in the planning and conducting of mining operations, it is desir- able to develop a reliable method of forecasting the proba- ble subsidence which may occur above mined areas. There are a number of factors that are known to influ- ence the magnitude and [he nature of the subsidence oc- curring above mined areas. The most significant factors are (1) the rock profile; (2) the rock properties; (3) the location, size, and shape of the opening; (4) [he presence of faults, shear zones, bedding planes, and other geological discontinuities; (5) the presence of other openings; (6) the initial stress state; and (7) any artificial support in the openings including packing or filling. Because of the com- plexity of the problem, a general approach was considered necessary for the development of a rational method of subsidence prediction. This approach, which is summa- rized in Figure I, consists of the following major steps: Establishment of objectives and performance criteria In any design process, it is necessary to establish the objectives of the design and to translate these objectives into performance criteria (measures of performance). For example, the performance criteria in subsidence problems may be in terms of limiting the total or differential settle- ment and the horizontal strain at the surface. In defining the response variables, it should be recognized that these should be in terms of the performance criteria in order that a comparison can be made. Definition of input and output (response) variables Based on the available geologic information, the initial stress state in the rock mass is defined and the stress distribution on the cavity Cace which determines the boundary loads that will be applied to the system to simu- late the creation of the opening is assumed. In addition to 101