LITERATURE REVIEW
2.2 SHEAR STRENGTH BEHAVIOUR OF FIBRE-REINFORCED SOILS .1 Direct Shear Tests on Sandy Soils
2.2.3 Triaxial Tests on Sandy Soils
0.3% and 1.1% by weight of dry soil. The reinforcing effect was more noticeable for soil with lower grain size and also with the lower shear rate of undrained condition.
reinforced with glass fibres and polypropylene pulp and mesh elements. Results indicated that shorter inclusions required a greater confining stress to prevent bond failure. Soil- inclusion friction interaction depended mainly on the extensibility of the inclusions. The mesh elements were superior to glass fibres in improving sand strength especially in the case of fine sand. The fine sand showed a more favourable response to fibre reinforcement than the medium sand. At constant fibre content, the increase in principal stress at failure and secant modulus were proportional to fibre length.
Maher and Ho (1993) investigated the effect of randomly distributed glass fibre reinforcement on cemented sand through triaxial static compression, cyclic compression and splitting tension tests. The inclusion of fibres significantly increased the peak compressive strength of the cemented sand. The increase was more pronounced at higher fibre contents and lengths, and at lower confining stresses. The post-peak strength loss of the cemented sand increased with increasing fibre content and length, and reduced with increasing confining stress. However, fibre addition increased peak internal friction angle, peak cohesion intercept and energy absorption capacity of the cemented sand.
Ranjan et al. (1994) carried out triaxial compression tests on a fine sand (SP) reinforced with plastic fibres and observed the influence of fibre properties (weight fraction and aspect ratio) and confining stress. Fibre inclusion increased the peak shear strength of the sand and reduced post-peak strength loss. It was found that the strength envelopes for the fibre-reinforced sand had a curvilinear or bilinear shape with a change of slope at a certain critical confining stress. The critical confining stress decreased with increasing fibre aspect ratio.
Consoli et al. (1998, 2002, 2007 and 2009b) investigated the effects of fibre inclusion (glass, polyethylene terephthalate and polypropylene fibres) on both uncemented and cemented sands (silty sand and fine grained Osorio sand) under drained conditions. Fibre
reinforcement decreased stiffness, increased both peak and residual shear strengths, and changed the cemented soil's brittle behaviour to a more ductile one. Addition of fibres increased the initial contraction at low strain level and decreased the subsequent dilation of specimen at higher strain level, and both the effects were more prominent with increasing confining pressure.
Consoli et al. (2004) found that the behaviour of fibre-reinforced uncemented and cemented fine sand specimens was influenced by the stiffness (elastic modulus) of the fibre type (polypropylene, polyester and glass fibres). Inclusion of relatively stiff fibres (polyester and glass) slightly reduced the stiffness and increased the peak friction angle of both the cemented and uncemented sand, and also slightly reduced the peak cohesive intercept and brittleness of the cemented composite. On the other hand, relatively flexible polypropylene fibre reinforcement dramatically reduced the stiffness and brittleness (changing the mode of failure of the cemented sand from brittle to ductile for longer fibres), while increasing the ultimate strength of the cemented composite.
Later, Consoli et al. (2009c) reported the effect of aspect ratio (120 to 2174) of polypropylene fibres by varying fibre length (12 to 50 mm) and diameter (from 0.023 to 0.1 mm) on stress-strain behaviour of the uncemented sand. It was found that fibres with high aspect ratios (above about 300) produced higher strengths and strain-hardening behaviour.
They suggested the usability of such mixtures as geomaterial for structures like embankments over soft soil which withstands large deformations without losing its strength. However, very thin fibres (such as 0.023 mm diameter), longer than 24 mm, tangled during mixing, and were found to significantly reduce the effects of the fibres on the soil behaviour.
Michalowski and Cermak (2002) conducted drained triaxial compression tests to investigate the role of the inclination of monofilament polyamide and galvanized steel fibres in the increase in strength of sandy soils. Two sands were used, one with grains significantly
smaller than the fibre diameter, and the second with grains larger than the fibre diameter. The contribution of the fibres to the soil strength was the largest when they were placed in the direction of largest extension (horizontal direction) of the fibre-reinforced soil. Vertical fibres under compression had an adverse effect on the initial stiffness of the reinforced soil specimen and did not contribute to the strength. Specimens with a random distribution of fibres exhibited a smaller increase in strength than those with horizontal fibres, because a portion of randomly distributed fibres was subjected to compression.
Later, Michalowski and Cermak (2003) reported the effects of fibre concentration, aspect ratio and the relative size of the grains with fibre length. The effect of reinforcement was more in fine sand than that of coarse sand at low fibre content (0.5%) and vice versa at higher fibre content (1.5%). The reinforcement was also more effective when the fibre length was large compared to the size of the grains, to be at least one order of magnitude larger than the size of the grains, to gain the reinforcement benefit. They introduced the concept of macroscopic internal friction angle to describe the failure criterion of fibre-reinforced sand.
Further, they presented a model for prediction of the failure stress in triaxial compression, in which the failure envelope has two segments: a linear part associated with fibre slip, and a nonlinear one related to yielding of the fibre material. Their analysis indicated that yielding of fibres would take place well beyond the stress range actually occurring in practice.
Ahmad et al. (2010) conducted both drained and undrained triaxial tests on specimens of silty sand reinforced with oil palm empty fruit bunch fibres. Both uncoated and coated (with acrylic butadiene styrene thermoplastic) fibres of different lengths (15, 30 and 45 mm) and varying content (0.25 and 0.5%) were used. Inclusion of the fibres significantly increased the peak shear stress and failure strain. Coated fibres increased the shear strength of the soil much more compared to uncoated fibres, as the coating increased interface friction between fibre and soil particles by increasing the surface area. Reinforced silty sand containing 0.5%
coated fibres of 30 mm length exhibited approximately 25% increase in friction angle and 35% in cohesion under undrained loading conditions compared to those of unreinforced soil.
Sivakumar Babu and Chouksey (2011) conducted consolidated undrained triaxial test and other tests on a fine sand and a cohesive red soil reinforced with plastic chips (12 mm long and 4 mm wide), obtained from waste plastic water bottles. It was found that the addition of plastic chips (0.5, 0.75 and 1.0% by dry weight of soil) significantly increased the shear strength of the soils in terms of increased friction angle, and their compressibility reduced significantly.
Hamidi and Hooresfand (2013) conducted conventional triaxial compression tests to investigate the effect of polypropylene fibres on cemented sand of 50% and 70% relative densities. The cement content was 3% (dry weight of the soil) and specimens were cured for seven days. Fibres of 12 mm in length and 0.023 mm thick were added at 0.5% and 1% (dry weight) of the sand-cement mixture. The initial stiffness of the cemented sand was noted to decrease with fibre content. The addition of fibres increased peak and residual shear strengths of cemented soil and changed its brittle behaviour to a more ductile one. With increasing fibre content, the compressive volumetric strain increased and residual dilation decreased.
The effectiveness of fibres in shear strength improvement was greater at 70% relative density.
Li and Zornberg (2013) conducted triaxial compression and fibre pullout tests to evaluate how the fibre tension was mobilized with varying shear strain levels for sand specimens of varying relative densities. It was found that comparatively high strain was required for full mobilization of fibre-induced tension. Based on these results, they proposed a refinement of the discrete framework of Zornberg (2002) for prediction of the equivalent shear strength of fibre-reinforced soil, by calculating the equivalent shear strength twice, at strain levels corresponding to both the peak and residual shear strength of the soil matrix. The
equivalent shear strength of the fibre-reinforced soil corresponds to the maximum of these two values.