To investigate the influences of discrete fiber strips on the mechanical properties of reinforced sand, a series of direct shear tests were conducted. A method to strictly control the initial orientation of fiber strips in specimen preparation was developed. Under the same normal pressure, the peak strength of sand specimens was proportional to the fiber content and was inversely proportional to the fiber initial orientation angle. The influences of initial fiber orientation on peak strength may depend on the stress mobilization in fibers. When the fiber strips distributed at a certain orientation angle were subjected to tensile stress in shearing, they could play an effective role in the peak strength gain of sand and vice versa. Due to the restriction of fibers on the volume dilation of sand specimens, the residual strength of reinforced sand also increased. However, the initial stiffness of reinforced sand was smaller than that of pure sand, which may be related to the precompression of flexible fiber strips and the density inhomogeneity of specimens induced in the specimen preparation process. In addition, the ductility of sand specimens was improved by the introduction of fiber strips, intuitively reflected by the increase in displacement failure. This may also be attributed to the restriction of fiber strips on the deformation of sand specimens.
To investigate the instability behaviours of municipal solid waste under the reduction of effective means stress, consolidated undrained (CU), constant shear drained (CSD) and constant shear undrained (CSU) tests were conducted on artificial waste specimens. In CU tests, the effective stress paths were modified by considering the particle compressibility, and the waste specimens did not exhibit pre-failure softening behaviours. Instability of the waste was observed in CSD tests, but was not observed in CSU tests. When the waste specimens became unstable, the axial strains increased rapidly and the volume was severely dilated. Softening of the waste specimens, manifested in the inability to maintain constant deviatoric stresses, was also observed. For waste specimens with similar void ratios, the instability points were located around an instability line (IL), and the slope of the IL decreased with an increase in the void ratio. Some factors, including the stress path and deformation characteristic of waste, may lead to the instability of waste specimens in CSD tests by influencing the effects of reinforcements. In engineered landfills, high pore pressure is often generated in waste mass and then leads to the obvious reduction of the effective stress of waste; thus, waste instability needs to be considered in the safety evaluation of waste slopes.
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