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|An examination of slope stability computation procedures for sudden drawdown
|University of Texas at Austin. Geotechnical Engineering Center.
Virginia Polytechnic Institute and State University. Department of Civil Engineering.
Computer Applications in Geotechnical Engineering Project (U.S.)
Wright, Stephen G. (Stephen Gailord), 1943-
Duncan, J. M. (James Moyer)
|Effective stress analysis
Pore pressure coefficients
Shear strenqth envelopes
|Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
|Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; GL-87-25.
Abstract: "Sudden" or "rapid" drawdown is considered to occur when reservoir or other adjacent water levels are lowered at such a rate that little or no drainage occurs in an earth slope at the time the water level is being lowered. Slope stability analyses are routinely performed to calculate the factor of safety for earth slopes subjected to this condition. Several different procedures exist and are currently used by designers to compute slope stability for the case of sudden drawdown. These various procedures are based on fundamentally different approaches and are known to produce different results in at least some instances. The procedures, the assumptions employed, and the numerical results obtained using these procedures are the subject of this paper. The various procedures differ principally in the manner in which the shear strength is defined and can be grouped into two general categories. The first category includes those procedures which are based on the use of effective stresses. The effective stress procedures require that pore water pressures be estimated at the end of sudden drawdown and the differences among the various effective stress procedures are related directly to differences in the procedures used to estimate pore water pressures. The second category of procedures for sudden drawdown analysis is comprised of two-stage procedures : one set (stage) of calculations is performed for conditions existing immediately prior to drawdown and is used to estimate effective stresses and corresponding undrained strengths; the second set (stage) of computations is performed for conditions immediately after drawdown and employs undrained shear strengths estimated from the first set of computations. This study provides a basis for a number of important conclusions regarding the accuracy and the degree of conservatism of the procedures for analyzing rapid drawdown slope stability. From these resuits , the best procedure should have the following characteristics: (A.) The procedure should employ a measure of shear strength and pore pressure that reflects the actual properties of the soils in the slopes. (B.) The procedures should employ a soundly based technique for relating shear strength to effective consolidation pressures. (C.) The procedure should not include strength components due to negative pore water pressures to avoid overestimating factors of safety for conditions where partial drainage occurs dur ing drawdown. A method with these characteristics can be conceived as a combination of methods currently in use. It could be described as being a modification of the method currently used by the US Army Corps of Engineers, the modification being an adoption of Lowe and Karafiath's procedure for representing the undrained strength of the soil rather than using the R-envelope as is now done.
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