SEISMIC DESIGN OF A RETAINING WALL WITH RELIEF SHELVES

Abstract

The purpose of retaining walls is to resist lateral soil pressure or hold backfill soil materials. The lateral pressure behind the retaining wall might be caused by liquid pressure, earth filling, sand, or other granular materials. Retaining walls are constructed in hilly areas, river fronts, basement structures, to support the plains, to support the slops etc. The present study focuses on analyzing the response of earth retaining walls with relief shelves and without relief shelves subjected to static and dynamic loading conditions. Special types of structures to retain the earth consists of retaining walls with pressure relief shelves. When the retaining wall is designed with a pressure relief shelf toward the backfill side, this reduces the total lateral earth pressure acting on the wall, which ultimately leads to a reduction of the wall section and an increase in overall structural stability. The provision of shelves helps to economically design the retaining structures in comparison to massive earth retaining structures without any shelves. The present study emphasizes on the advantages of a relief shelf retaining wall over a conventional retaining wall in terms of reduction of lateral earth pressure and increased factor of safety. A parametric study has been undertaken with a normal gravity retaining wall as well as gravity wall with relief shelves consisting of sand as the backfill material. The height of the wall has been varied (i.e., 6m, 7.5m and 9m) to assess the lateral earth pressure as well as computing the Factor of Safety (FOS) for overturning and sliding modes of failure. Further parametric studies consist of variation in number, length, thickness, location of the relief shelves. The results depicted that the provision of shelves significantly reduced the total lateral earth pressure in comparison to gravity wall without shelves. The study also shows that with the increase in number of shelves there is a significant drop in the total lateral earth pressure both for static as well as seismic loading conditions. The internal friction angle of the backfill soil has also been varied to understand the effect of the lateral earth pressure on retaining walls both with shelf and without shelf under static as well as seismic. It was observed that with the iv provision of one relief shelf to a wall height of 6m, the percentage decrease in lateral earth pressure under static condition was around 49% in comparison to wall with no relief shelf. The percentage reduction of lateral thrust for a wall height of 6m with one shelf was around 63% under static loading condition when the friction angle has been varied from 200 to 450. Further, it was also observed that the percentage decrease in total lateral earth pressure for a 6m wall with one relief shelf under both static and seismic loading conditions was 60% when the friction angle of the backfill material has been changed from 200 to 450. The study also comprises the assessment of lateral earth pressure on the retaining walls using a pseudo-static approach I.e., Mononobe-Okabe Method by varying the horizontal seismic coefficients (i.e., Kh = 1, 1.5 and 2). . It was found that with increase in horizontal pseudo-static seismic coefficient (Kh) the earth pressure increases and as a result the factor of safety of overturning and sliding decreased. The later part of the study focuses on the validation of parametric results with Finite Element models prepared in a commercial software, i.e., ABAQUS. The finite element analysis has been carried out in various steps and ultimately the lateral earth pressure was extracted for the active case of retaining wall, i.e., wall moving away from the backfill. Various finite element models of gravity retaining wall have been prepared and the lateral earth pressure on the wall have been evaluated. Seismic analysis has also been performed on different types of retaining wall structures using the concept of pseudo-static method. The finite element results have been compared with the parametric study and it shows the finite element results is in good agreement with parametric study. The importance of relief shelves in reducing the lateral earth pressure can be easily observed either from the results of parametric study or finite element models.