EARTH PRESSURE BEHIND RETAINING WALL WITH REINFORCED BACKFILL

Abstract

Reinforced earth is comparatively a newer construction material used extensively in CIVILENGINEERING works. The concept of reinforced earth lies in mobilising the friction between the soil and reinforcement incorporated at designed placement levels. Soils, in general, possess 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 psuedo cohesion. In conventional type of reinforced earth construction, the reinforcement is attached to the wall facing. Present study aims at analysing the behaviour of a rigid retaining wall supporting backfill with a uniform surcharge load and reinforced with strips which are not attached to the wall. This way of reinforcing the backfill is intended to reduce the magnitude of generated lateral earth pressure on the retaining wall. Since the tension realised in unattached reinforcement would be comparatively small, cheaper reinforcing material like bamboo could also be used. The theoretical treatment of the problem 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, for two different placement locations of the reinforcement in the backfill. In the first 11 case, the reinforcement is laid right from the back face of the wall and is termed as the normal placement of reinforcement (NPR) whereas, in the second case, reinforcement is placed across the assumed trace of failure surface by extending half of the length on either side and is named as 'effective placement of reinforcement (EPR)'. Separate expressions for intensity of lateral earth pressure, resultant earth pressure and height of its point of application above the base have been derived in terms of strength parameters of soil, characteristics and distribution of the reinforcement in the backfill and the resulting equation expresses lateral pressure as sum of the component due to backfill and surcharge load. Effect of reinforcement in the analyses has been considered in terms of non-dimensional parameters 'Dp' and 'L/H' where Dp is expressed as a 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 reinforcing strips and L denotes the length of reinforcement. The analytical results are obtained for values of angle of internal friction ( <f>) equal to 20°, 25°, 30°, 35° & 40°, Dp equal to 0.0, 0.2, 0.5, 1.0, 1.5 and 2.0 and L/H is varied from 0.0 to 1.0 with an interval of 0.20. In case of normal placement of reinforcement the pressure intensity, resultant pressure and height of its point of application when expressed in terms of non-dimensional parameters indicate that the lateral earth pressures and their moment about toe oi retaining wall reduces with the increased amount of reinforcement, whereas its height of point of application increases marginally for strip lengths upto 0.6 times the height of wall. The values of nonIll dimensional earth pressure coefficients <PyAyh2 s Pq/qH) get reduced significantly for the strip lengths of 0.4 to 0.6 times the height of wall. Increasing the length of reinforcing strips beyond 0.6 times height of wall is not advantageous in further reducing the earth pressure on the wall. For angle of internal friction *> 30° of the backfilled soil, major reductions in the earth pressure coefficient take place even at shorter strip lengths ( < 0.6 x height of wall). The increase in the height of point of application of resultant lateral earth pressure due to fill is for strip lengths upto 0.6 times height of wall but the point of application of resultant lateral pressure due to uniformly distributed surcharge load rises above toe of wall for strip lengths equal to height of wall either in case of the values of angle of internal friction of retained ono ♦•« soil ranging between 20° to 3in0ou or for lesser amount of reinforcement <Dp v< 0.5) in the fill. Otherwise the height of point of application of resultant lateral pressure due to surcharge loading also reduces tremendously for strip lengths in excess of 0.6 times height of wall. The pattern of variation of the non-dimensional moment coefficient (product of non-dimensional coefficients for resultant lateral pressure and its height of point of application) is almost similar to that of resultant pressure coefficient. 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. Both lateral earth pressure and height of point of application reduce significantly with increased amount of reinforcement in IV case of effective placement of reinforcement. The reduction in the height of point of application of active earth pressure is comparatively much more in case of surcharge loading then due to earth fill. Non-dimensional design charts clearly indicates that for Dp values of 1.0 and above (1.5 & 2.0) considered in the analysis and for values of angle of internal friction of 30° and more maximum resulting design earth pressures are obtained at smaller strip lengths varying between 0.3 to 0.4 times the height of wall. Theoretical findings have been verified by medium scale model tests in the case of normal placement of reinforcement. Pullout tests have been conducted on both type of reinforcing materials viz. aluminium and bamboo strips, embedded in sand (SP) used in model test. Influence of the length of strip and the height of overburden of soil on the coefficient of soil-strip friction (f*) was investigated. Results have shown that coefficient of friction (f*) increased linearly with the increase in length of strip used used in the experiments but reduced, with increasing over burden pressure for the range of overburden pressures employed in the tests) in a non-linear pattern. Limited number of drained triaxial tests have been conducted on sand samples (SP) reinforced with aluminium discs under varying confining pressures. Improvement in the strength of reinforced soil samples and their failure pattern with the amount of reinforcement and the possibility of application of Broms (1977 b) equation in predicting the axial load for reinforced samples,were investigated. Strength of reinforced sand samples improve significantly, except when the reinforcement is placed at two L ends of sample. The samples have failed in slippage mode of failure though the reinforced samples have shown gain both in cohesion and in the angle of friction. It is expected that findings of this study would help in designing cost effective rigid retaining walls with reinforced backfill particularly as measures of landslide control in the hilly region. ***