Seminar - Deepankar Choudhury - Earthquake resistant design of retaining structures and foundations
Wednesday, June 2, 2005, 1.10 - 1.50 pm
Civil Engineering Lecture Room 3
Designs of foundations and retaining structures like retaining walls, abutments, sheet piles, diaphragm wall etc. are very important problems related to the geotechnical engineering. The devastating effects of earthquake make the problem more important in the earthquake prone regions. Determination of passive earth pressure both in magnitude and the point of application is the basic step to compute the design forces required to consider for the design of these geotechnical structures. In earthquake resistant design of foundations and retaining structures most of the design codes still follow a very old methodology proposed by Mononone-Okabe (1926, 1929), which has been shown to be improper and unsafe by several recent researchers. Moreover, this method considers the planar rupture surface even for passive condition, which was shown to be highly on unsafe side by Terzaghi (1943). Also, the method gives only the total value of the earth pressure and not the point of application required to compute the size of the section and reinforcement for the wall. Hence most of these design codes hardly specify anything about the geotechnical aspects of foundation and retaining structure design.
In this presentation the development of theoretical solutions for the seismic passive earth pressure coefficients, seismic bearing capacity factors of shallow foundations in general c-phi soil are discussed. Pseudo-static analysis is carried out for the seismic forces using curved rupture surfaces by adopting the method of limit equilibrium. Design charts for seismic passive earth pressure coefficients considering the effects of a wide range of parameters like wall batter angle (alpha), ground slope (beta), soil friction angle (phi), delta/phi ratio, adhesion to cohesion ratio (ca/c) and horizontal and vertical seismic accelerations (khg and kvg) are reported. Method of horizontal slices to determine the point of application of seismic passive resistance is proposed. Seismic bearing capacity factors for shallow foundations are obtained by considering one-sided failure mechanism with different curved failure surfaces, the mobilization of passive resistance on the rear side of failure is assumed partial. Search for the critical focus of the logarithmic spiral of the failure surface is made. Comparison of present results along with numerical example shows the merit of the proposed methods over the existing methods. A complete solution and design values for geotechnical structures like retaining walls and shallow foundations under seismic conditions will be presented in this seminar.
Keywords: Seismic passive earth pressures, retaining wall, point of application, bearing capacity, partial mobilization, design charts, design code.