Reassessment of Mononobe-Okabe Method and Propose a Complementary Solution for Seismic Design of Retaining Structures

Retaining structures, as a part of road transportation systems have sometimes a great affect on the total cost of construction in our highly seismic risk country. Accordingly, studying the lateral earth pressure during earthquakes is very important to provide and economical and technical design for retaining walls. Mononobe-Okabe method is one of the first theories proposed to determine the seismic lateral earth pressure and is still in use by geotechnical engineers and designers. M-O method is a seismic version of coulomb theory and based on pseudo-static earthquake loading for granular soils. This method applies earthquake force components by two coefficient called seismic horizontal and vertical coefficients. The M-O method is essentially a closed form solution based on some simple assumptions which supply relationships to calculate both active and passive pressures. The assumptions can make the problem too simple to be used by engineers, but in many cases is not applicable to the real cases which put it mostly far from having a safe or economical design. Simple geometry of backfill material, disability to consider the cohesion of soils and water table are some of its disadvantageous. The aim of this study is to remove the limits and to cover other problems that M-O has no answer for them. In the proposed method, in addition to soil cohesion, the interface cohesion between soil and wall material is included. The crack depth in ground surface for the case of cohesive materials of backfill can be also considered. The study is headed back to the fundamental theory of Coulomb and benefits from an iterative calculation of forces diagram of failure wedges to overcome the limitations encountered in M-O method.