LITERATURE REVIEW
2.4 BEARING CAPACITY OF FIBRE-REINFORCED SOIL .1 Based on California Bearing Ratio Tests
2.4.2 Based on Laboratory or Field Model Tests
Wasti and Butun (1996) performed laboratory model tests using strip footing plate (20 mm thick, 50 mm wide and 250 mm long) placed over sand beds reinforced with randomly distributed polypropylene fibre and mesh elements. Two sizes of mesh elements (30 × 50 mm and 50 x 100 mm) having the same opening size (10 × 10 mm) and one size of fibre element (50 mm length) cut from the meshes, were used in varying inclusion amounts (0.075, 0.10 and 0.15% by dry weight). In general, the reinforcement of sand by randomly distributed inclusions caused an increase in the ultimate bearing capacity values and the settlement at the ultimate load. A larger mesh size was found to be superior to the other inclusions, in terms of increase in ultimate bearing capacity. Though the mesh elements had an optimum percentage of inclusion, the fibres exhibited a linearly increasing trend of bearing capacity, for the range of fibre contents used.
Consoli et al. (2003a) carried out field plate load tests (300 mm diameter, 25 mm thickness) on a thick homogeneous stratum (1200 mm thickness) of compacted low plasticity silty sand-clayey sand, with and without polypropylene fibre reinforcement. In addition to the field test program, laboratory triaxial compression tests were performed to determine the static stress-strain response of the compacted soil reinforced with the randomly distributed polypropylene fibres. The plate load test on reinforced soil stratum was performed to relatively high pressures, and gave a noticeable stiffer response than that carried out on the non-reinforced stratum. The laboratory test results also showed that the strength increased continuously regardless of the confining pressure applied, and did not reach an asymptotic upper limit, even at axial strains as large as 25%.
Consoli et al. (2003b) reported the results of field plate load tests (300 mm diameter, 25 mm thickness) carried out directly on a homogeneous residual soil stratum (low plasticity sand silty red clay), as well as on a layered system formed with two different top layers of 300 mm thick sand-cement (7% content) and sand-cement-polypropylene fibres (24 mm length and 0.5% content) overlaying the residual soil stratum. The utilization of a cemented top layer increased the bearing capacity, reduced displacement at failure, and changed soil behaviour to a noticeable brittle behaviour. The addition of fibre to the cemented top layer maintained roughly the same bearing capacity but changed the post-failure behaviour to a ductile behaviour. A punching failure mechanism was observed in the field for the load test bearing on the sand-cement top layer, with tension cracks being formed from the bottom to the top of the layer. In the case of the sand-cement-fibre top layer, a completely distinct mechanism was observed with the failure occurring through the formation of a thick shear band around the border of the plate, which allowed the stresses to spread through a larger area over the residual soil stratum
Consoli et al. (2009a) also reported the results of plate load tests carried out on both unreinforced and reinforced Osorio sand with polypropylene fibres (24 mm long, 0.5% by dry weight), compacted at relative densities (Dr) of 30, 50 and 90%. The soil load-settlement behaviour was significantly influenced by the fibre inclusion, changing the kinematics of failure. For the densest (Dr = 90%) fibre-sand mixture, a significant change in the load- settlement behaviour was observed at very small (almost zero) displacement. However, for the loose to medium dense sand (Dr = 30% and 50%), significant settlements (50 mm and 30 mm respectively) were required for the differences in the load-settlement responses to appear.
The overall behaviour seemed to support the argument that inclusion of fibres increases strength of sandy soil by a mechanism that involves the partial suppression of dilation (and
hence produces an increase in effective confining pressure, and a consequent increase in shear strength).
Hataf and Rahimi (2006) carried out laboratory model tests to investigate the effects of content and aspect ratio of waste tyre shreds as reinforcement on the bearing capacity of sand. Tyre shreds of rectangular shape and widths of 2 and 3 cm with four aspect ratios (2, 3, 4 and 5) were mixed with the sand at five contents (10, 20, 30, 40 and 50%) by volume.
Addition of tyre shreds to sand increased BCR (bearing capacity ratio) between 1.17 to 3.9.
The BCR was found to increase with shred content only up to an optimum volume. For a given shred width, shred content and soil density it seemed that aspect ratio of 4 gave higher BCR. The maximum BCR of 3.9 was attained at shred content of 40% and dimensions of 3 × 12 cm.
Tafreshi and Norouzi (2012) carried out laboratory tests to obtain the bearing capacity of a square footing (100 × 100 mm in size and 20 mm thick) resting on a sand bed (600 mm height) in a test tank (700 x 700 mm in plan). The sand bed consisted of an unreinforced cap layer underlain by an intermediate layer reinforced with tyre shreds (2.5, 5 and 7.5% by volume) followed by another unreinforced layer. The efficiency of reinforcement was noted to vary with tyre shred content, thickness of the reinforced soil layer and thickness of the soil cap. At footing settlement level equal to 5% footing width, the maximum improvement in bearing capacity of reinforced bed was 2.68 times of the unreinforced bed. This optimum value of improvement was with tyre shred content of 5%, thickness of reinforced layer equal to 0.5 times of footing width and thickness of soil cap equal to 0.25 times of footing width.
Nasr (2014) conducted physical model tests to investigate the behaviour of a strip footing (100 mm width, 499 mm length and 25 mm thickness) resting on fibre-reinforced cemented sand in the active zone of a cantilever sheet pile wall, and compared the test results with the observations from two-dimensional non-linear finite element analyses. Fibre
inclusion (0.25 to 1% content) and cement kiln dust (3 to 12% content) into the soil caused an increase in ultimate bearing capacity of footing and significant reduction in the lateral deflection of the sheet pile wall. At high fibre content above 0.75%, the increase in bearing capacity and reduction in lateral deflection were 42% and 51%, respectively. The addition of fibres increased thevertical surface settlement obtained at the ultimate bearing capacity.
2.5 MODELS OF FIBRE-REINFORCED SOILS