REVIEW OF LITERATURE AND SCOPE OF THE PRESENT STUDY
2.3 STUDIES WITH GEOCELL REINFORCEMENT
2.3.3 Triaxial Compression tests on Geocells
Bathurst and Karpurapu (1993) carried out triaxial compression tests on cohesionless soil encased in single geocell. The sample size was of 200 mm in diameter with an aspect ratio (height to diameter) of unity. The confinement provided by the reinforcement contributed to the increase in shear strength of the encapsulated soil by about 42% to 62%. The frictional angle however, was found to be the same as that of the unreinforced case. The authors used an elastic membrane model suggested by Henkel and Gilbert (1952) to estimate the magnitude of the additional confining pressure. This additional confining pressure creates an apparent cohesion which is responsible for the increase in the strength and stiffness of the soil. The model proposed by the authors gave a reasonably accurate estimate of the apparent cohesion of the soil-geocell composite sample.
Rajagopal et al. (1999) studied the influence of confinement provided by the multicell geocell system on the strength and stiffness behaviour of granular materials through triaxial compression tests. The geocells were fabricated from geotextiles of different strength and stiffness. They observed failure in most cases to be by bursting of the seams as the seam strength was much less than the geosynthetic materials. Their results showed the trend of failure to be as similar as reported by Bathurst and Karpurapu (1993) where the confinement induced apparent cohesive strength in the geocell encapsulated soils
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whereas the frictional strength remained equal to that of an unreinforced soil. They found the increase in the stiffness as well as the strength of the soil to be a function of cell diameter, strength and stiffness of geocell material. The triaxial test results also show that the three interconnected cells in the triaxial test can adequately simulate the multiple geocell system in the field.
Mengelt et al. (2006) evaluated the effect of geocell confinement on the resilient modulus and the plastic deformation of the infill soil. Fine grained as well as coarse grained soil, confined in single geocell, were tested through triaxial compression tests. The triaxial samples were prepared with a relatively smaller aspect ratio of 0.8 and the conventional aspect ratio of 2. The results obtained from samples prepared at these two different aspect ratios were comparable. They found the improvement in resilient modulus to depend on the type of infill material with the coarser material giving an improvement of 1.4 - 3.2%
as against the 16.5 - 17.9% improvement in case of the finer infill. Large deformation was observed in case of finer grained material suggesting that this would have likely mobilised greater reinforcing effect of the geocells.
Latha and Murthy (2007) investigated the effects of different reinforcement forms viz.
discrete, planar and cellular, on the behaviour of granular soil. Triaxial tests were conducted on samples of 38 mm diameter and 76 mm height. The geocells in the study were made from geotextiles and polyester film. They observed that the failure in case of planar and discrete reinforcement was by bulging but for geocell encased samples, the failure was by bursting of the seam. Even with seam having much lower tensile strength which dictates the response of the geocells, the performance improvement is comparable to that of planar reinforcement having high tensile strength. This suggests that with better welded seam, the geocells would give a much improved performance. They found
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polyester geocells to be more efficient than the geotextile geocells. They observed that by using the model proposed by Bathurst and Karpurapu (1993), experimental results are in agreement with the predicted values.
Wesseloo et al. (2009) performed large scale uniaxial compression tests on single and multiple cell geocell-soil composite system to develop a better understanding of the behaviour of geocell structure. The multiple cell structure considered in the study was a square grid of 2×2 cell, 3×3 cell and 7×7 cell. They observed that the deformation behaviour of the geocell-soil composite structures is dependent on the boundary conditions imposed on the structures. For the direct comparison of axial stress in multi cell structure, the axial stress is normalized with respect to the original cell diameter.
They found from normalized axial stress-strain behaviour of the structure, both the stiffness and strength decrease with the increase in the number of cell in the structure.
They observed peak axial stress of the structure to decrease with the increase in the number of the cells. This decrease in peak strength was quantified by ‘efficiency factor’
which is defined as the ratio of the axial stress in a single cell structure at a specified diameter and the axial strain rate to the axial stress in a multi-cell structure consisting of the cells with the same cell diameter tested at the same axial strain rate. For a 7×7 cell structure peak axial stress was found to be 55% lower than that of a single cell of same diameter at same strain rate. An equation which relates the efficiency factor with the periphery factor was formulated in order to compare results that were obtained from the different geometries. The periphery factor is based on the fact that the strength of composite is dominated by the hoop stress in the outer membrane. This relation shows that there is a logarithmic decrease in the efficiency with the increase in the number of cell in the structure. They concluded that the relation can be an important tool to
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investigate the results obtained from the laboratory which has less number of geocell as compared to the number of geocell used in the field.