Shear strength is an important soil mechanical property that its improvement is one of the rehabilitation program for the reduction of soil erosion and degradation. The objective of this study was to investigate the effect of polyacrylamide (PAM) and the vetiver system as a cheap and long-term bioengineering method and their combination on unsaturated shear strength parameters (effective cohesion, c\\\\\\\' (kPa), angle of effective internal friction, φ\\\\\\\' (°) and angle of internal friction related to matric suction, φb (°)) of a loam soil. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water [0.2% (V0P2) and 0.4% (V0P4)], and simultaneous presence of vetiver and two concentrations of PAM (VP2 and VP4). Direct shear tests were performed at combinations of three normal stresses of 25, 50 and 100 kPa and four matric suctions of 0, 10, 30 and 50 kPa (12 tests per each treatment) to determine the shear strength parameters. It was found that vetiver and PAM decreased apparent angle of internal friction (φ) and φ\\\\\\\', and increased c\\\\\\\', total cohesion (c), φb, and as a result increased unsaturated shear strength. However, the positive effect of vetiver on shear strength was greater than that of PAM. It seems that PAM and vetiver enhanced the contact area, inter-particles bonds, aggregation, and inter-aggregate porosity, and as a consequence, increased the soil effective saturation, effective stress, cohesion and shear strength. Also, when matric suction increased the c and shear strength increased although the φ increased slightly. Simultaneous application of vetiver and 0.4% of PAM resulted in maximum shear strength, indicated although vetiver can increase the cohesive strength, it would be more pronounced when PAM was simultaneously applied. Simultaneous application of vetiver and PAM through increasing soil shear strength can be suggested as the rehabilitation program to reduce soil erosion and degradation.

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