1. School of Infrastructure Engineering, Nanchang University, Nanchang 330031, China 2. School of Civil Engineering, Central South University, Changsha 410075, China
The shallow expansive soils, closely related to the engineering disaster of expansive soils, are usually subjected to low confining pressures, and their mechanical properties are different from those under high confining pressures. Triaxial shear tests were conducted to compare the shear behavior and nonlinear characteristics of the shear strength of expansive soil under high and low cell pressures. The shear properties and reinforcement mechanism of shallow expansive soil improved by polypropylene fibers were explored. Results showed that the shear strength of shallow expansive soil exhibited significant nonlinearity, which could be represented by a power function. Moreover, it was observed that the nonlinearity of the shear strength of the expansive soil became more prominent with the increase of mass fraction of water and the decrease of cell pressure. Adopting shear strength parameters obtained from triaxial shear tests under high cell pressure led to an overestimation of the cohesion of saturated shallow expansive soil by 225.7% and an underestimation of the internal friction angle by 42.5%. The shear strength of expansive soil was significantly enhanced by adding fibers, and the degree of enhancement was related to the formation of three-dimensional fiber network within the soil. Longer fibers were more effective in reducing the attenuation rate of shear strength, which weakened the strain-softening effect of expansive soil. However, excessively long fibers may bend and twist within the soil, leading to the degradation of the fiber-expansive soil interface during soil deformation.
Tab.1Main technical parameters of polypropylene fiber
Fig.1Physical images in test process
Fig.2Stress-strain curve of expansive soils with water mass fraction of 17.5%
Fig.3Stress-strain curve of expansive soils with water mass fraction of 20.7%
Fig.4Stress-strain curve of expansive soils with water mass fraction of 24.4%
Fig.5Variation in shear strength attenuation of expansive soils with cell pressure
Fig.6Strength line Kf of expansive soil under high and low cell pressure sections
Fig.7Difference between strength envelope under Coulomb failure criterion and true strength envelope
Fig.8Fitting nonlinear characteristics of shear strength of expansive soil by power function
Fig.9Stress-strain curve of expansive soil improved by polypropylene fiber with a mass fraction of 0.3%
Fig.10Tensile phenomenon of fiber on shear plane of failure sample
Fig.11Peak strength of fiber improved expansive soil and its attenuation characteristics
Fig.12Strengthening mechanism of fiber improved expansive soil
Fig.13Effects of fiber length and mass fraction on peak strength of expansive soil
Fig.14Fiber network structure at fiber length of 3 mm and mass fraction of 0.3%
Fig.15Damage and destruction of polypropylene fibers
Fig.16Variation in shear strength parameters of fiber improved expansive soil with fiber mass fraction
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