M.Sc Student Abukhadra Fady Coupled PS Analysis - Effect of Critical Slip Surfaces on Pseudo Static Coefficients Used in Slope Stability Calculations Department of Civil and Environmental Engineering Professor Emeritus Rafael Baker (Deceased) Abstract

The most practical and simple approach recommended by most codes for analysing stability of slopes, under seismic conditions, is the pseudo-static (PS) method. In PS analysis the earthquake is replaced by equivalent static body force proportional to PS coefficient, constant in slope space. Most researchers and codes recommend for choosing PS coefficients are based, only, on seismological information (maximum earthquake acceleration, magnitude and epicentral distance).

In this study two other factors that affect the PS coefficient assessment were studied:

1. The critical slip surface area. This factor depends on the slope's geometry and material properties (when the slope angle decreases, the ratio increases or when the slope's height increases, the critical slip surface increases).

2. The earthquake wavelength at the bedrock.

Earth slopes do not behave as rigid bodies; therefore the distribution of acceleration, throughout the slope, induced by an earthquake, may be amplified or de amplified.

Therefore assessing the PS coefficient based only on seismological information (maximum earthquake acceleration, magnitude and epicentral distance) is not appropriate, since not all slopes with different slope geometries and materials characteristics will respond in the same way to a given earthquake. Hence, in the present study, we ascribed the PS coefficient to maximum average acceleration within the sliding mass which gives better representation than the maximum earthquake acceleration at the bedrock.

The main results and conclusions of this study are:

1. The area and depth of the critical slip surface influences the PS coefficient value, calculated from the maximum average accelerations produced within the critical slip surface. When the area and depth of the potential sliding mass increase, the PS coefficient decreases. It is shown that recommendations of most design manuals and codes for PS coefficient are conservative for conditions resulting with deep critical slip surface; for conditions resulting in shallow critical slip surface they may not be conservative (for high base earthquake frequencies). This results indicates that in PS analysis one should consider the coupling between seismological and geotechnical information's of the problem.

2. Earthquake wavelength, also, has a large influence on the average seismic coefficient calculated within the critical slip surface. For smaller earthquake wavelength at the bedrock (high frequency) more oppositely distributed accelerations will be produced in the same sliding body, resulting in a small average acceleration and therefore a small PS coefficient. For earthquakes with large frequencies, the PS coefficient will approach zero.