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Rock Slope Analysis based on Digital Terrain Models

Abstract:

The Limit Equilibrium Method has justified itself over many years of successful application as one of the leading analytical models for analyzing the stability of slopes and foundations. However, two factors complicate the analysis of slope stability problems: firstly determining volumes and magnitude of sliding areas of potential unstable blocks based on the real topography requires a considerable effort, and secondly finding the critical blocks which are formed by an intersection of various discontinuities (e.g. bedding planes, joints, etc.) in a complex topography is, to say the least, demanding.

To overcome these difficulties the CAD-based computer program AutoBlock was developed. It can import an arbitrarily complex terrain surface which has been digitized beforehand using a topographic map. This surface is then extruded to a 3D solid which is intersected by various sets of discontinuities. These sets are defined by their orientation, spacing and their strength characteristics. By combining all possible locations of all discontinuities potential unstable blocks are determined. Only blocks which are kinematically admissible are taken into account: this requires that each failure surface either outcrops at the terrain surface or is limited by other failure surfaces. For each of these blocks the resultant of all acting forces and the factor of safety against sliding are computed.

AutoBlock is an add-on to the popular program "AutoCAD" which allows its possibilities and its power to be exploited. E.g., the potential unstable blocks may be displayed and selected based on various criteria such as minimum and maximum volume, sliding area or outcrop length. Irrelevant blocks may be excluded from fur-ther investigations. This visual "discussion" is probably the most important contribution of AutoBlock to the analysis of rock slopes and foundations. It may prove to be more effective than even the determination of the factor of safety.

An example illustrates the use of the program. A steep rock slope near Baden, Switzerland, was in danger of failing. As a result, the construction of houses was not permitted, but quarrying at the toe of the slope was tol-erated, which led to a partial failure of the slope. In a first step AutoBlock was used to estimate the strength properties of the discontinuities involved based on a back-calculation. Then the excavation of the quarry was simulated, which led to a new degree of freedom and was responsible for the collapse. Finally, it was investigated which excavation procedure should have been adopted to avoid a failure.

Authors:

Fritz, Pit and Bergamin, Stefan

Index Terms:

slope; stability; CAD; dam; large dam; analysis; Stabilitat; AutoBlock; Informatics; rock; TunnelingGroup; Bergamin, Stefan; Fritz, Pit

Remarks:

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Further Information:

Date published: 2004