BEHAVIOUR OF RETAINING WALLS WITH REINFORCED <H> SOIL BACKFILL

Abstract

The concept of reinforced earth was introduced by Henry Vidal in 1966. Reinforced earth is a composite material which is formed by the association ofsoil and tension resistant reinforcing elements. The concept of reinforced earth lies in mobilising the friction between the soil and reinforcement incorporated at designed placement levels. Soils in general posses very low tensile strengths and these may tend to be insignificant in cases of granular soils like sands. Owing to internal friction, the forces which develop within the reinforced soil mass are absorbed by the reinforcement. Thus the role of reinforcement is to resist tension and to impart the material a kind of pseudo cohesion. The reinforcement thus suppresses the normal tensile strains in the soil mass through frictional interaction. Due to its economy, ease in construction and flexibility in nature, such a material has varied application in CIVILENGINEERING construction from retaining walls, water front structures to industrial and special structures. The present study has beenconducted to investigate the behaviour of a rigid retaining wall with inclined back face supporting reinforced cohesive-frictional backfill with a uniform surcharge load and also the line load. The analysis of the wall considers the static equilibrium of a horizontal element of soil under the action of various forces acting on it within a Coulomb wedge assumed to have developed under lateral thrust. Expressions for intensity of lateral earth pressure have been derived in terms of strength parameters of soil, reinforcement characteristics and its distribution in the backfill. The resulting equation expresses lateral pressure intensity as sum of the lateral pressure intensity components due to surcharge (line load and uniformly distributed load), cohesion and backfill. Effect of cohesion, developed due to c-<\> soil has also been considered in the analysis. Effect of reinforcement in the analysis has been considered in terms of L/H and a non-dimensional parameter 'Dp', where Dp is expressed as the ratio of product of width (w) of reinforcement, its coefficient of friction (f*) with respect to retained soil and the height of wall (H), to the product of designed horizontal (Sx) and vertical (Sz) spacings of reinforcement strips and L denotes the length of reinforcement. The analytical results are obtained for values of angle ofinternal friction (<j>) equal to 20°, 25°, 30° and 35°, Dp equal to 0.25, 0.50, 0.75 and 1.0, L/H varying from 0.0 to 1.0 with an interval of 0.20. Coefficient of cohesion (c) has been expressed in non-dimensional form as c/yH. Similarly, coefficient of adhesion with wall has been expressed as Cw /yH. Numerically c and c« are considered equal and their values are taken as 0, 0.05 and 0.10. An another term Dc has been defined as coefficient of adhesion with reinforcement and its values are taken as 0, 0.15 and 0.3. The values of non-dimensional earth pressure coefficient P/(l/2)yH2 and moment coefficient M/(l/6)yH3 have been evaluated for various values of uniformly distributed load coefficient (q/yH) and line load coefficient (Q/yH2). It has been found that values of non-dimensional earth pressure coefficients and moment coefficients reduce significantly for the reinforcing strip lengths of 0.4 to 0.8 times the height of wall for all the cases i.e. when the backfill soil is cohesive-frictional soil and also bearing the uniformly distributed surcharge (i.e, y + q + c), for the non-cohesive frictional backfill (i.e. y only), for frictional backfill with uniformly distributed load surcharge (i.e. y + q) or cohesive-frictional soil (i.e. y + c). Increasing the length of reinforcing strips beyond 0.8 times the height of wall is not advantageous in further reducing the earth pressure on wall. ill The height of point of application of resultant earth pressure reduces for the reinforcing strip length greater than 0.6 times the height of wall for (y+ q + c) and (y+ q) cases. The height of point of application reduces beyond the reinforcing strip length equal to 0.4 times the height of wall for (y - only) case and (y + c) case. The pattern of variation of the non-dimensional moment coefficients is almost similar to that of resultant earth pressure coefficients. This shows that even though the point of application goes slightly up in reinforced backfill as discussed above, yet its net effect in terms of moment on the wall is much less as compared to unreinforced backfill. The effect of line- load has also been discussed in the analysis. Non-dimensional charts have been prepared for getting the total earth pressure on the wall for different combinations of uniformly distributed load and line load in nondimensional forms. The theoretical findings have been verified by model tests conducted in two steel tanks. Bottom ash and c-<|) soils have been used as the backfill materials. For the tests conducted on cohesive-frictional soil, steel tank of size 1.55m X 0.87m X 1.10m was used whereas that for bottom ash was of size 1.50m X 1.50m X 1.10m (L X B X H). A steel plate (12 mm thick) was used as the model wall. For cohesive-frictional soil, the model wall size was lm high X 0.87m wide whereas that for bottom ash was 1.50m X 1.00m. The height of backfill for both the cases was 0.99m. Geogrid has been used as the reinforcing material in all the tests. These tests incorporate the effect of length of reinforcement and distribution of reinforcement, amount of uniformly distributed load and distance of line load from the wall face. The geogrid strips of 200 IV mm width with different lengths were used with different spacings. The model tests results are in good agreement with the theoretical results. Physical and engineering properties of bottom ash and c-<j> soil were evaluated in the laboratory before conducting the model tests. Based on the study it is concluded that the optimum length of reinforcing strips lie between 0.6 Hto 0.8 Hwhere His the height ofretaining wall. It is expected that findings of this study would help in designing cose effective rigid retaining walls with reinforced cohesive - frictional soil as backfill which is generally available in natural form at most ofthe sites. The study will also establish the use ofbottom ash (which is at present a waste material) as a backfill material behind a retaining wall at sites which are close to thermal power stations.