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Technical paper: Stability coefficients for highway cutting slope design

by G E Barnes, senior lecturer, Geotechnical Engineering, School of Civil Engineering and Building, Bolton Institute.

This paper was first published in GE’s May 1992 issue

Abstract

A new method is presented which gives the minimum factor of safety for long-term stability of a homogeneous slope of height H using effective stress parameters in the form F = a + b tanφ. The stability coefficients a and b are presented in tabular and graphical form for slope inclinations of 1:1, 2:1, 3:1 and 4:1and have been found to be related to the cohesion soil parameter, c’/γH, and the water table level parameter, hw/H. Pore pressures are represented as a quasi steady state condition by a water table at toe level beyond the toe and then inclined at various angles within the slope given by the depth below crest level, hw. This is considered to be of more practical use than the arbitrary pore pressure coefficient, ru, which has been used previously in slope stability charts. When the critical circle lies below the water table the slope is considered ‘wet’. The situation when the critical circle lies above the water table and the slope can be considered ‘dry’ has been determined. This enables the assessment of whether the presence of a water table is important or not. The coefficient a was found to depend on slope inclination, c’/yH, and whether the slope is ‘wet’ or ‘dry’. The coefficient b was found to depend on slope inclination, c’/yH, and water table level, h„. The location of the critical slip circle in terms of depth factor, D, is also presented for the case when φ30’. An example calculation is given on the use of the coefficients. 

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