Evolution of excess pore pressure and flow liquefaction responses of fibre reinforced sand under undrained cyclic loading

By conducting undrained cyclic triaxial tests on fibre-reinforced very loose and loose saturated sand, we investigated the build-up of excess pore pressure and the flow liquefaction responses. The test results show that unreinforced very loose and loose saturated sand has a high potential for liquefaction, with flow liquefaction occurring in all unreinforced samples under undrained cyclic loading. The presence of fibre reinforcement has a positive impact on the resistance to flow liquefaction of sand. Fibres provide both a densifying effect and a confining effect to the sand skeleton. However, the confining effect of fibres depends on the loading path imposed on the samples and the deformation mode of the samples. The presence of fibres alters the evolution law of the residual excess pore pressure in saturated sand. When fibres impose a strong confining effect on the sand skeleton, the evolution of residual excess pore pressure along with the normalized loading cycles follows a curve with an 'inverted L' shape, being significantly different from an 'S' shape curve which is followed by the unreinforced sand. Under the two-way symmetrical and one-way cyclic loading, the significant fibre stress contribution is mobilized, leading to the effective stress of the sand skeleton being much greater than 0 after the 100% build-up of excess pore pressure. As a result, the strength loss of the reinforced sample remains below 11% and thus the fibres prevent liquefaction from developing.