LITERATURE REVIEW
2.3 UNCONFINED COMPRESSIVE STRENGTH BEHAVIOUR OF FIBRE- REINFORCED SOILS
Freitag (1986) investigated the effects of three different synthetic fibres (spun nylon string, polypropylene rope fibre and polypropylene olefin concrete reinforcement fibre)
fibres had different diameters (0.1 to 0.2 mm) but same length of 20 mm, and 1% volumetric content was used in all specimens. The plain and reinforced specimens were compacted over a sufficiently wide range of water content to define the compaction curve. The maximum strength occurred somewhat dry of optimum. However, greater contribution of the fibres to the strength was observed for the specimens compacted wet of optimum compared to those compacted dry of optimum.
Maher and Ho (1994) conducted UC test and other tests on a kaolinite clay reinforced with different types of fibres (glass, polypropylene and softwood pulp) of varying tensile strength and elastic modulus. The inclusion of fibres increased the peak compressive strength and ductility of the clay, with the increase being more pronounced at lower-composite water contents. Increase in fibre length reduced the contribution of fibres to peak compressive strength, while increasing the contribution to energy absorption or ductility. This is opposite to the effect of increased fibre length on the strength of fibre-reinforced granular soils (Maher and Gray 1990) as observed from triaxial compression tests.
Nataraj and McManis (1997) investigated the influence of moisture content, specimen size and amount of reinforcement on the UCS of 25 mm long polypropylene fibrillated fibre- reinforced clayey soil. The strength reached maximum values at the optimum moisture content, and then decreased with a further increase in moisture content in all cases. The strength increased with an increase in specimen diameter from 33 to 70 mm, with or without reinforcement and thereafter, slightly decreased with 100 mm diameter. The post-peak strength loss was less for specimens prepared wet of the optimum moisture content, when compared with that of specimens prepared dry of the optimum. The unreinforced specimens revealed a shear failure plane, and with the addition of fibres, the specimens bulged in compression.
Puppala and Musenda (2000) investigated the influence of polypropylene fibre reinforcement on expansive soil stabilization. Two expansive soils, two types of fibres and three fibre dosages (0.3, 0.6, and 0.9 percent by dry weight of soil) were used in the testing program. Results indicated that the fibre reinforcement enhanced the UCS of the soil and reduced both volumetric shrinkage strains and swell pressures of the expansive clays. The fibre treatment also increased the free swell potential of the soils. The axial strain at peak shear strength of the fibre-reinforced soils was 1 to 4 percent more than that of raw soil, and this axial strain increased with the fibre dosage.
Ang and Loehr (2003) performed unconfined compression tests on compacted fibrillated polypropylene fibre-reinforced silty clay to evaluate how the specimen size affects the measured strength and stress-strain properties. Four different diameters (38 to 152 mm) of cylindrical specimens were compacted at moisture contents of 12, 14, 16, 18, and 20 % (from 4 % dry to 4 % wet of standard Proctor optimum moisture content) and at fibre contents of 0.0, 0.2, and 0.4 % of the dry weight of soil. Specimen size effects were most significant for specimens compacted at water contents dry of the optimum moisture content, and were almost negligible for specimens compacted wet of the optimum. While no clear threshold size could be established, it appeared as though use of specimens with diameters of 70 mm or greater would produce strengths that are reasonably representative of the true “mass”
strengths for fibre-reinforced soils.
Akbulut et al. (2007) investigated the suitability of the inclusion of different fibres (scrap tyre rubber, polyethylene and polypropylene) on the geotechnical behaviour of three clayey soils (CH). Three different diameters (35, 50 and 80 mm) of cylindrical specimens were used. The UCS values increased with increasing tyre rubber fibres up to 2% content and then decreased, and with polyethylene and polypropylene fibres up to 0.2%. For maximum
improvement of the UCS values, the fibre length had to be increased with the sample dimension.
Tang et al. (2007) conducted UC tests on uncemented and cemented polypropylene fibre-reinforced clayey soil with different fibre contents (0.05%, 0.15% and 0.25% by weight of soil) of 12 mm length. The unconfined compressive strength for both cemented and uncemented specimens increased with fibre content, along with reduction in loss of post-peak strength. The addition of fibres reduced the width of tension cracks developed at failure by impeding the opening and development of cracks, and thereby changing the brittle behaviour to ductile behaviour.
Attom et al. (2009) investigated the effect of nylon (0.2 mm diameter) and palmyra (0.4 mm diameter) fibres on the mechanical properties of three clayey soils of varying plasticity index and clay content. Fibres of aspect ratios equal to 75 and varying volumetric content (1, 2, 3, 4 and 5%) were used as reinforcement. The stiffness, UCS and ductility of the clay-fibre mixture increased with fibre content. The post-peak softening decreased with increasing fibre content. For all fibre contents, Palmyra fibres showed a higher increase in the relative UCS than nylon fibres when mixed with the soils. Further, the strength improvement with fibre reinforcement was greater for the clayey soil of higher activity index.
Jiang et al. (2010) investigated the effect of polypropylene fibre content (0.1, 0.2, 0.3 and 0.4% by weight), fibre length (10, 15, 20 and 25 mm) and soil aggregate size on the UCS of a clayey soil. The UCS of the fibre-reinforced soil showed an initial increase followed by a rapid decrease with increasing fibre content and fibre length, and the optimal fibre content and length were found to be 0.3% and 15 mm, respectively. The strength of both unreinforced and fibre-reinforced soil decreased continuously with an increase in aggregate size.
Mirzababaei et al. (2013) conducted UC tests on carpet waste fibre-reinforced clayey soils at variable conditions of moisture content and dry unit weight. At a constant dry unit
weight, increasing the fibre content resulted in a significant increase in UCS value. At a constant fibre content and moisture content, an increase in dry unit weight of the reinforced specimens led to a significant increase in UCS. At the same fibre content and dry unit weight, an increase in moisture content of the reinforced specimens caused a reduction in UCS.
However, at aconstant fibre content, a combined increase in dry unit weight and moisture content resulted in an increase in the UCS.
Only a few studies related to UC tests on fibre-reinforced sands have been found in the literature. Santoni et al. (2001) conducted unconfined compression tests on six different sands ranging from a fine sand to a coarse sand, reinforced with four different types of polypropylene fibres. Five primary conclusions were obtained. First, the inclusion of fibres significantly improved the unconfined compressive strength of the sands. The performance of the various fibre types from best to worst was fabrillated, tape, monofilament and mesh.
Second, an optimum fibre length of 51 mm (2 in.) was identified for the reinforcement of sand specimens. Third, a maximum performance was achieved at a fibre dosage rate between 0.6 and 1.0% dry weight. Fourth, specimen performance was enhanced in both wet and dry of optimum conditions. Finally, the inclusion of up to 8% of silt did not affect the performance of the fibre reinforcement.
Park (2009) examined the effect of concentration and distribution of polyvinyl alcohol fibre reinforcement of the same overall specimen fibre content of 1% on the UCS of artificially cemented sand (4% cement). The randomly distributed fibres were placed only at predetermined layers (1, 3 and 5 layers) among five compacted layers of the specimens (70 mm diameter and 140 mm length). The results showed that the UCS of the reinforced soil increased gradually as the number of fibre inclusion layers increased. A fibre-reinforced specimen, where fibres were evenly distributed throughout the five layers, was twice as strong as an unreinforced cemented specimen. A specimen with five fibre inclusion layers
was 1.5 times stronger than a specimen with one fibre inclusion layer at the middle of the specimen.
2.4 BEARING CAPACITY OF FIBRE-REINFORCED SOIL