Lateral earth pressures in constrained landslides

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The problem of the limiting landslide pressure on an obstacle was first formulated in 1944 by Robert Haefeli of ETH Zurich, who recognised that the kinematics of the problem does not allow for classical active and passive earth pressure theories to be applied. He derived an approximate solution using a limit equilibrium approach with a number of rather arbitrary assumptions and simplifications. Since then, the Haefeli solution has been widely applied for the design and analysis of landslide retaining structures. The paper revisits this old landslide pressure problem by means of a rigorous upper- and lower-bound limit analysis and derives the exact landslide pressure solution for a planar landslide with a weak slip surface parallel to the slope. Being applicable to a wide range of natural and man-made obstacles and, unlike the classical theories, not affected by the wall friction and soil dilation, the upper-bound solution is rather robust. The landslide pressures from this solution increase with the strength of the sliding layer and are significantly higher than the active, but much lower than the passive, earth pressures. Of even higher practical importance, however, is that due to their oversimplifying assumptions, the widely used approximate solutions appear to get close to the exact solution only over a very narrow range of slope and friction angles. It appears that for mildly inclined weak slip surfaces and high strengths of the sliding layer, analysis and design of retaining structures based on well-known approximate solutions can become dramatically unsafe.