REVIEW OF LITERATURE AND SCOPE OF PRESENT STUDY
2.2 STUDIES ON GEOCELL REINFORCED SOIL BEDS
2.2.1 Sand beds
Mitchell et al. (1979) conducted model tests on geogrid cell reinforced sand in a square box of 915mm size. In order to back calculate the equivalent elastic modulus (Er) for the reinforced layer, with help of the elastic theory solutions for homogeneous elastic layers overlying rigid base, the grid cell reinforced sand layer was placed directly on a rigid concrete floor. The influence of specific variables such as, ratio of radius of loaded area to cell height and ratio of loaded area to cell width were studied by varying the cell height and the diameter of the loaded area. An approximate formulation has been proposed to find out the equivalent elastic modulus (Er) of the reinforced layer.
Er = E f E
( ) ( )
E f a
B f h B f a
h f N f E
u g m
j s
1 2 3 4 5 6
(2.1)
where a/h, is the layer geometry ratio; h/B, the grid geometry ratio; a/B, the loaded area-grid geometry ratio; Em, the modulus of the cell fill material; Eg, the modulus of the grid material; Es, the modulus of the subgrade; Eu is the modulus of the unreinforced sand layer and Nj is the number of grid joints per unit area. The test results indicate that bearing capacity increases with size of loaded area and thickness
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of grid cell layer. Grid cell reinforcement can lead to many fold increase in the effective modulus of a sand layer.
Experimentally, Khay et al. (1986) studied the reinforcing efficiency of various geotextile structures in the performance improvement of sand subgrade. The geotextiles included, cell, fibers and prefabricated sheets of polyamide threads.
Geocells used had an a/b (cell width to depth) ratio of 0.5 with varying depth of 10, 15 and 20 cm. The geocell reinforcement showed considerable performance improvement. The settlement of geocell mattress was appreciably low which suggests that it behaves as a slab.
Guido et al. (1989) conducted a parametric study using laboratory plate load tests on geoweb reinforced sand beds. The parameters studied were the texturisation roughness of the geoweb wall, number of layers of the geoweb reinforcement (N), depth of placement of the first layer of the reinforcement below the loaded plate (u), size (extent) of the geoweb reinforcement (b) and relative density (ID) of the fill material. The plate load tests were conducted in a square wooden box. Test results indicated that for untextured geowebs, at N = 4 or more, the bearing capacity ratio (BCR) did not show any increase indicating that, N = 4 is an optimum value.
However, for medium texturised cells, at N = 4, the BCR was still increasing. As the depth of placement decreased the BCR increased and settlement decreased. For the untextured geoweb at u/B ratio (i.e. B is the width of the footing) of 1.0, while for texturised geocell at u/B ratio of 1.25; the reinforced structure behaved like an unreinforced case. The optimum b/B ratio beyond which the increase in BCR is marginal was found to be 2 and 3 for untextured and medium textured case respectively. Studies carried out on the effect of relative density indicated that for loose soil the percentage improvement in load carrying capacity is relatively higher.
Koerner (1990) proposed a failure mechanism for the geocell reinforced foundation bed, based on plastic limit equilibrium theory as used for estimating the bearing capacity of shallow foundation. Since slip lines are interrupted by the vertical walls of the mattress, for such a failure to occur, the sand in particular cell must punch out of it, thereby loading the sand beneath the level of the geocell. This in turn fails in bearing capacity, but now with the positive effects of a surcharge loading and higher density conditions. Thus the increase in bearing capacity due to the provision of geocell can be taken as 2τ where, τ is the shear strength between geocell wall and soil contained within it and is defined as
τ = σhtanδ (2.2)
Where, σh is the average horizontal force within the geocell = qKa, q is the applied vertical pressure, Ka is the coefficient of active earth pressure, δ the angle of shearing resistance between soil and cell wall material.
Dash et.al. (2001a) reported the results of laboratory-model tests on a strip footing supported by a sand bed reinforced with a geocell mattress. The parameters varied in the testing program include pattern of geocell formation, pocket size, height and width of geocell mattress, the depth to the top of geocell mattress, tensile stiffness of the geogrids used to fabricate geocell mattress and the relative density of the sand. With the provision of geocell reinforcement, failure was not observed even at a settlement equal to 50% of the footing width and a load as high as 8 times the ultimate bearing capacity of the unreinforced sand. In addition to the tensile strength of reinforcement, the aperture size and orientation of ribs of the geogrid used to fabricate the geocell mattress substantially influence the improvement in the performance due to the reinforcement.
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The results from laboratory model tests conducted by Dash et.al (2001b) on strip footings supported by geocell reinforced sand beds with additional planar reinforcement show that a layer of planar geogrid placed at the base of the geocell mattress further enhances the performance of the footing in terms of the load-carrying capacity and the stability against rotation. The beneficial effect of this planar reinforcement becomes negligible at higher heights of geocell mattress.
Dash et. al. (2003a) through laboratory model tests studied the bearing capacity of circular footings supported by geocell-reinforced homogenous sand beds. Footing load–settlement response, deformations at the fill surface, strains in the geocell wall and pressure distributions below the geocell mattress were measured. The test results demonstrate that by providing geocell reinforcement in the sand bed, significant performance improvement in terms of increased bearing capacity and reduced surface deformation can be obtained. Measurements indicate that the footing pressure is redistributed more uniformly over a wider area on the subgrade soil, indicating that the geocell-reinforced sand bed behaves as a composite mass.
Dash et. al. (2004) have studied the relative performance of different forms of reinforcement (i.e. geocell, planar and randomly distributed mesh elements) in sand beds under strip loading. The results demonstrate that geocell reinforcement is the most advantageous form of soil reinforcement technique amongst the three. With the provision of geocell reinforcement, failure was not observed even at a settlement equal to about 45% of the footing width and a load as high as eight times the ultimate capacity of the unreinforced soil, whereas, with planar reinforcement, failure took place at a settlement of about 15% of the footing width and a load of about four times the ultimate capacity of the unreinforced soil. For the case with randomly distributed mesh reinforcement, failure was recorded at a load of about 1.8 times the ultimate
capacity of the unreinforced soil and at a settlement of about 10% of the footing width.
Dash et. al. (2008) have observed that with the provision of geocell reinforcement, the subgrade modulus of sand bed can be increased as high as eight times that of unreinforced case. A multiple-variable data regression is performed on the experimental data to establish the relation between the effect of reinforcing in terms of subgrade modulus improvement factor and the parameters that control the scheme of reinforcement. This equation will be of use in predicting the subgrade modulus of geocell-reinforced sand beds, and effective utilisation of geocell reinforcement in increasing the performance of sand foundations.
Wesseloo et. al. (2009) reported the results of uniaxial compression tests performed on geocell packs of different sizes comprising of single and multiple geocells. The packs were fully instrumented to record the deformations within. They observed that the strength of the geocell composite structure is indirectly proportional to the size of the individual cells and that the strength of the packs reduces with an increase in the number of cells in the structure.