A STUDY OF REINFORCED EARTH WALL AND RETAINING WALL WITH REINFORCED BACKFILL

Abstract

Economical improvement of mechanical properties of soil is one of the aims of a geotechnical engineer. Reinforced soil technique is a method by which overall stability of the soil can be improved. Rein forced soil is a composite material which is formed by the association of soil and tension resistant reinforcing elements. The reinforcement 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, slabs and water front structures to industrial and special structures. The present study has been conducted to investigate (i) the beha viour of reinforced earth wall, (ii) the behaviour of retaining walls having reinforced backfill and (iii) the frictional characteristics of reinforcing material with respect to soil. The most common use of reinforced soil is in the construction of reinforced earth wall. The design of a reinforced earth wall requires its analysis against external stability and internal stability. The exter nal stability comprises of checking the safety of wall against sliding, overturning and bearing failure considering the wall as a rigid block. In internal stability the competency of the reinforcements provided at different heights is examined against tension and pullout failure. In this investigations pseudo static analysis has been carried out for the complete analysis and design of a reinforced earth wall having uniform surcharge on the wall and backfill. Both horizontal and vertical seismic forces have been taken into account. The non-dimensional charts prepared for designing a reinforced earthwall located in a seismic area considering all the aspects mentioned above, are helpful for getting the design in short time. The design includes the determination of adequate length of reinforcement, its cross-section and vertical and horizontal spacing. One design example has been included to demonstrate the use of the design charts. The analysis of a reinforced wall requires identification of rupture surface during pullout failure, as the effective length of the reinforcement contributing in developing frictional resistance is the portion of the reinforcement lying outside the wedge. In this investigation an attempt has been made to test a laboratory model wall (1.0 m high). The wall was instrumented to measure the stress distribution in reinforcing strips for the surcharge free loading and surcharge loading conditions. Tests were performed for different lengths of the reinforcement varying the vertical spacing and surcharge intensities. From the test results, locus of maximum tie tension have been plotted which gave the rupture surface. Further for the verification of the location of rupture surface prototype tests on retaining wall of 4.0 m high having instrumented reinforcing strips have been performed. Finally a recommendation has been made about the rupture surface. Values of maximum tie tensions were also interpreted to give the pressure intensity at the location of the reinfor cement which in turn gave the observed earth press jre distribution. The results indicated that Rankine's theory gives earth pressure more closer to the observed ones and therefore it may be adopted for design. Another important parameter in the design of the reinforced earth w all is coefficient of friction between the reinforcing material and soil. (ii) In this investigation study has been made to get the friction characteris tics of different reinforcing materials and soil using pullout and sliding shear tests. Bamboo strips, Aluminium strips, Nylon niwars, Woven geotextiles and Netlon geogrids were chosen as the reinforcing materials and dry sand as soil. Effect of the length and width of the reinfor cement, strain rate and overburden pressure on the friction has been studied. In general it was found that the friction coefficient decreases with (i) increase in over burden pressure; (ii) increase in length of the reinforcement; (iii) decrease in strain rate; (iv) increase in width of reinforcement. Secondly the friction coefficient obtained by sliding shear test and pullout test were found different. In some of the cases correlation could be obtained between the friction coefficient obtained by the two tests incorporating the effect of length of reinfor cement and overburden pressure. An alternative form of soil reinforcement for retaining wall has also been investigated where the lateral pressures on the conventional retaining wall are sought to be reduced by reinforcing the back fill by unattached horizontal strips or sheets. Realising that tension, in loose strips, would be small, cheap reinforcing materials like bamboo strips or fabrics could be -employed as reinforcements. Theoretical anal ysis .has been developed for a wall with inclined back face and horizontal reinforced backfill carrying uniformly distributed surcharge. The analysis considers the force and moment equilibrium of a horizontal element of soil within a Coulomb wedge assumed to have formed under lateral thrust. The expression for lateral pressure intensity has been derived in terms of wall parameters, soil properties, surcharge intensity and character istics and distribution of reinforcement. The resultant earth pressure (iii) and its moment about the base of wall were obtained using numerical integration technique considering only positive lateral pressure intensities. Results have been presented in terms of non-dimensional charts. Theoretical analysis has also been developed for inclined retaining wall having horizontal unreinforced backfill with line load and uniformly distributed surcharge together by considering the equilibrium of the Coulomb wedge. The results of the analysis indicated that the effect of magnitude of uniformly distributed load affects the magnitude of the increment of the earth pressure due to a line load. Non-dimensional charts have been prepared for getting the total earth pressure on the wall for different combination of uniformly distributed load and line load in non-dimensional forms. For evaluating the height of the point of application of the total earth pressure, increment of earth pressure due to line load is taken to act at a height as per Spangler's approach. This is combined with the height of the point of the application obtained by the earlier analysis considering the equilibrium of the horizontal element of Coulomb wedge for wall having uniformly distributed load on its backfill. This gave the height of point of application of the total earth pressure taking uniformly distributed load and line load together and it is also presented in the form of non-dimensional charts. The analysis was extended empirically for reinforced backfill having line load by considering the line load as the equivalent uniformly distri buted load giving the same pressure increment in unreinforced case. For experimental verification of theoretical analysis of wall with reinforced backfill, model tests were performed. These tests incorporate the effect of distance of the line load from the wall face, length of the reinforcement and distribution of reinforcement. The model test data (iv) have indicated a pattern of variation of the observed moments about the base of wall similar to that of predicted moments. A critical review of the literature available on the pertaining items studied in this investigation have been included in the thesis. Suggestions for further research have also been made. (