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Title: Finite element analysis of slopes in jointed rock
Authors: University of California, Berkeley. College of Engineering. Office of Research Services.
Duncan, J. M. (James Michael)
Goodman, Richard E.
Keywords: Joints and jointing
Rock slopes
Rock mechanics
Finite element method
Publisher: U.S. Army Engineer Waterways Experiment Station.
Engineer Research and Development Center (U.S.)
Description: Contract Report
Abstract: The studies described in this report were performed to evaluate the effectiveness of finite element methods for rock slope analyses. Following a review of the state of engineering for rock slopes, three fundamentally different methods of analysis are described and illustrated. In the first method the slope is treated as a homogeneous, isotropic, linearly elastic body. Stresses and displacements are calculated by simulating excavation of a vertical slope in a rock mass with an initially horizontal surface. These analys es show that the severity of the stress conditions after construction are influenced to a large degree by the initial stress conditions prevailing before excavation. Using stress values calculated in this manner, analyses have been made to determine the severity of stress conditions on planes with 16 different orientations. These analyses are called "ubiquitous joint " analyses, because it is imagined that a joint with the orientation considered will pass through each point in the slope where the stress conditions have been evaluated. Examination of the stress conditions on planes with that same orientation and comparison with shear failure criteria for the joints indicate the likelihood of joint opening and slip in the region adjacent to the slope. Charts showing where joints would to open or slip have been prepared for 16 joint orientations, 4 different initial stress conditions, and 3 different values of joint cohesion. These charts may serve as a convenient means of determining the severity of stress conditions in various portions of a slope, for joints of various orientations. The stresses used in these analyses were calculated neglecting the influence of the joints, and it seems likely that joints would tend to reduce the severity of the stresses. Therefore these ubiquitous joint charts would be expected to give conservative indications of possible joint distress. One advantage of the ubiquitous joint approach over all of the other analytical methods considered is that the analysis can be made for joints with any arbitrary orientation in space, whereas other methods of analysis are capable of considering only joints whose normals are perpendicular to the slope axis. The second method of analysis consists of representing a jointed rock mass by an equivalent orthotropic continuum. Equations are derived which express the properties of the equivalent orthotropic material in terms of the properties of the joints and intact rock. A finite element computer program was developed for solution of orthotropic plane stress and plane strain problems, and several example problems have been solved. Analyses of jointed rock slopes with the rock represented by an equivalent orthotropic elastic continuum resulted in stresses not much different from those calculated for the case in which the rock was assumed to be isotropic. Thus these example analyses indicate that while joints may alter the stress system locally, an extensive joint system appears to have only a rather small influence on the stress conditions around slopes. The third method of analysis described in this report is one in which a single prominent joint is represented by a series of finite elements having properties different from the surrounding elements which represent intact rock. Analyses were conducted using both two-dimensional finite elements and a new one-dimensional finite element to represent joints. A variety of joint orientations was considered, in each case with the joint passing through the toe of the slope. A parameter study was conducted to determine the effects of joint flexibility and initial stress conditions on the joint shear and normal stresses, slope displacements, and the tension and shear stresses around slopes. An analysis was also conducted to illustrate that essentially the same results may be achieved using one-dimensional or two-dimensional elements to represent joints. The computer program used in the studies described is included in the Appendix, together with instructions for its use , and a bibliography of pertinent references is included. NOTE: This file is large. Allow your browser several minutes to download the file.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Contract Report

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