INTERACTION OF FOOTINGS RESTING ON REINFORCED EARTH SLAB

Abstract

Technique of reinforcing the soil is one of the latest and fast growing techniques in the field of geotechnical engineering. The main advantages of implementation of this technique are the reduced cost and easy construction. It has, however, only in the sixties that a systematic study of soil strengthened by reinforcement was undertaken by, a French Engineer, Vidal (1966), who termed it as 'Reinforced Earth1. In comparison with other applications of geosynthetic reinforced soil, for example, geosynthetic-reinforced soil embankments or retaining walls, relatively less emphasis has been placed on the use of geosynthetics for reinforcing soil foundations. Most of the work reported in the present literature relates to experimental studies on isolated footings resting on reinforced earth. As the footings in the field generally interfere with each other to some extent and are rarely isolated. the problem of interaction between adjacent footings is of practical significance. There is a lack of reliable method of analysis and design of footings on reinforced soil. To bring the study on isolated and interfering footings on reinforced earth slab to a stage at which it can be adopted for designs, appropriate analysis supported by suitable experiments are necessary. The present study has, however, been taken as an attempt towards this goal. The study has been carried out in the following steps: (i) (1) The method of analysis of isolated strip footings resting on reinforced earth slab after Binquet & Lee (1975) has been modified based on more realistic assumptions. The analysis has been extended to (i) isolated square and rectangular footings (ii) interfering strip footings and (iii) interfering square & rectangular footings. The overall analysis requires the estimation of normal force on the plan area of reinforcement within and outside the assumed plane separating the downward and outward flow. Solutions for the same have been obtained and presented in the form of nondimensional charts for direct use of practicing engineers. The analysis needs the pressure settlement curve of unreinforced soil as prerequisite, which has been obtained using constitutive laws of soils. (2) Experimental programme essentially consists of model tests on square and strip footing resting on unreinforced/reinforced soil for isolated and adjacent footings case (S/B - 0.0, 0.5, 1.0 and 2.0). The effect of spacing between the footings, number of layers of reinforcement, continuous and discontinuous reinforcement below the interfering footings and extension of reinforcement beyond the edge of footing on pressure, settlement and tilt characteristics were studied. Total of seventy four tests were carried out, thirty seven on strip footings and thirty seven on square footings. The reinforcement used for tests was Tensar SS20 geogrid. The pressure settlement and pressure tilt (for interfering footings) curves were obtained and plotted in graphical form. For the determination of tie-soil friction angle, which is required for the analysis, pull-out tests have been conducted in a specially designed tank. The effect of length of reinforcing strip and normal pressure has been (ii) (3) studied. The pull-out load and displacement were obtained and plotted in a graphical form. Normal stress versus pull-out shear stress were plotted to find tie-soil friction angle. For the determination of rupture strength of reinforcement used, extension tests were carried out on samples of geogrid in longitudinal and transverse direction. Extension load versus deformation curves have been plotted. For the evaluation of constitutive laws of soil, drained triaxial shear tests were performed. The data from extensive laboratory investigation has been used for the validation of the proposed method of analysis and it has been found that predicted values were in good agreement with experimental data. Using the developed theory, the effects of depth of first layer of reinforcement, vertical spacing of reinforcement layers, number of reinforcement layers, size of reinforcement sheet, size of footing, tensile strength of reinforcement and depth of reinforcement were investigated for isolated and interfering footings in terms of 'pressure ratio' which is defined as ratio of average pressure of footing on reinforced soil to that on unreinforced soil for the same settlement. These results were compared with the available experimental data from previous investigators and was found in good agreement with the same. As the proposed theory doesnot give the ultimate bearing capacity of footings resting on reinforced earth slab, an empirical approach has been suggested to find out the same. (iii » The tilt of interfering footings has been studied based on model test results and conclusions have been drawn for the design of interfering footings. (4) Based upon the theoretical and experimental studies undertaken, the following conclusions have been drawn: The optimum depth of first layer of reinforcement has been found to be 0.25B and optimum vertical spacing between the adjacent layers of reinforcement has been found to be 0.15B for isolated and interfering footings. The length of reinforcement beyond the edge of footing to get the maximum benefit has been found to be 2.5B for isolated strip footings, 2B for square footings, and IB for interfering footings. With the increase in number of layers of reinforcement, the pressure ratio increases, if the reinforcement is provided within a depth of 1.5B. The size of footing does not affect the pressure ratio, if all the parameters are taken same in terms of size of footing and the rupture of ties does not take place. With the increase in tensile strength of reinforcement, the pressure ratio increases in case of field size foundations, where the tie break failure is more predominant. The provision of footing size reinforcement layers beneath the footing increases the bearing capacity and the effect is similar to the deep (ivj foundation placed at the depth of lowermost layer of reinforcement for both isolated and interfering footings, subject to a maximum vertical spacing of 0.25B. In case of interfering footings resting on reinforced soil, the magnitude of settlement and tilt decreases for a given load intensity as compared to the same footings on unreinforced soil for the same load intensity. With the provision of continuous reinforcing layers below the interfering footings, the direction of tilt of footings reverses as compared to the same on unreinforced soil and magnitude of tilt decreases, both in case of strip and square footings. At a factor of safety of greater than two, the tilt of interfering footings almost vanishes for NR a 2. In case of discontinuous reinforcing layers beneath the interfering footings any significant improvement in tilt has not been observed. The findings of present study are useful in understanding the behaviour of isolated and interfering strip, square and rectangular footings resting on reinforced earth slab and facilitate in providing complete parameters required for the design of the foundations. It is expected that the findings of this study will lead to more economical and safe design of geotechnical structures. Cvr