The usual way to carry out rock slope stability analyses is:
to make a rock mass discontinuity pole counting to identify the predominant directions of rock discontinuities;
with strength data for the discontinuities, to carry on plane, wedge and toppling failure analyses for the slopes involved. Although pole counting analyses are very useful and they always should be done, they do not insure that the actual failure plane or planes are identified, i.e. the plane in a predominant direction is not necessarily the weakest plane. To try to overcome this difficulty, the Author has developed a simple DOS based FORTRAN program: ALLWEDGE (50kB), which identifies all kinematically possible wedges for a rock slope and analyzes them with Mohr- Coulomb strength parameters (c´ and φ´) for the discontinuities and the Hoek and Bray (1977) complete method of wedge analysis without a tension crack. The program allows to process up to 400 discontinuities (theoretically 79,800 wedges for each slope) and 30 slopes, common rock unit weight, slope water conditions and earthquake horizontal and vertical accelerations. It also allows to include for each slope: external forces, maximum width (which limits wedge height) and to list only the wedges with a factor of safety less than a predetermined value. It also can calculate the stabilizing tension and its direction to reach a predetermined factor of safety. Output files could be very large. An example is presented for both the traditional method and the all-wedge method and conclusions are derived.
The usual procedure to carry out rock slope stability analyses is:
to make a rock mass discontinuity pole counting to identify the predominant directions of rock discontinuities.
with strength data for the discontinuities, to carry on plane, wedge and toppling failure analyses for the slopes involved. Although pole counting analyses are very useful and they always should be done, they do not insure that the actual failure plane or planes are identified, i.e. the plane in a predominant direction is not necesarily the weakest plane. The Author has analyzed actual wedge and plane rock slope failures in which the actual failure planes were not the ones with the predominant direction, due to the fact that either the discontinuity field survey was not as complete as required or that the critical discontinuities were not easily detected in these field surveys.
In view of these difficulties, the Author proposes the following procedure for rock slope wedge and plane failure stability analyses:
to identify from the discontinuity survey the discontinuities that should not intersect (which can be called "genetic"): i.e. bedding planes in sedimentary rocks.
with all the discontinuities, carry on wedge kinematic analyses to identify all possible wedges for a slope.
with all identified kinematically possible wedges, carry on stability analyses and obtain factors of safety.
