Pit slope manual, supplement 5-1: plane shear analysis
Authors: Major G , Kim H -S , Brown-Ross D 
Published: April 2018

The purpose of this supplement is to provide methods of analyzing slope instability arising from sliding on discontinuities, that are feasible at the present time. A description of graphical methods is included because many cases can be adequately handled that
way and also because they provide an understanding of the essentials for making good use of the computer programs. The simplest case of plane shear instability is that of sliding along a single plane. The method of analysis is simple, the main difficulty being to obtain adequate input data. The
attitude of the plane throughout the slope, its roughness, its shear strength properties, and the ground water conditions affecting it, all require intensive site investigations. A variation of this problem is when sliding can occur along a series of planes of one set assisted by a set of cross
joints making up what is called a stepped-path. Again the major effort required is in obtaining data on the discontinuities such as length, spacing, areal distribution, and the strength of any connecting rock substance between discontinuities. In some cases, sliding can involve the interaction of
two blocks along one or two planes. Analyses are presented for cases where strike of the planes is substantially parallel to the face of the slope. Where the strike of the discontinuities that can be sliding planes is oblique to the slope face, single plane sliding or the above two-block case are
less probable. Analysis is required to determine if there are planes that can combine to form a 3-D wedge with an angle of intersection sufficiently steep to cause sliding. Once the geometry and strength properties are known, graphical deterministic analyses for such cases are straightforward. To
determine probabilities of instability, a computer solution is almost always required. Appendix A provides a computer program, PLAFAM, for analyzing the single plane or steppedpath mode of instability. The probability of in stability schedules are determined by using the Monte Carlo approach, je,
the simulated sampling of geometric and strength properties for repeatedly calculating instability, and statistically appraising the results. Appendix B provides documentation of the computer program TWOBAM for the analysis of twoblock sliding. Analysis of such a system is somewhat more complex than
for conventional plane shear stability having to be expressed as the ratio of weights of the active and passive blocks. Probability schedules are produced using the Monte Carlo approach. Appendix C provides documentation for WINTAM, a 3-D wedge analysis program. The analysis is essentially of a
rigid body sliding down the line of intersection formed by two planes or discontinuities. Probability schedules are obtained using the Monte Carlo approach. Appendix D contains program documentation for GENSAM, a general slip surface analysis. In some cases of potential instability sliding, if
sliding occurs, it will be along two planes, both striking parallel to the slope face but dipping into the pit at different angles. Also, as a result of various geologic inhomogeneities, the shape of the slip surface can be even more complex. GENSAM is a method of analysis, similar to the
traditional slices method for rotational sliding, which can be applied to bilinear and curved surfaces of any nature. It can, of course, also be used for circular surfaces and thus provides an alternate approach for the cases described in Supplement 5-2. Probability schedules are obtained using the
Monte Carlo approach. Appendix E provides program documentation for MONTEC, the Monte Carlo subroutines that are used in all the above stability programs. The package of subroutines perform random sampling and statistical calculations and plot the results.