Study on direct shear strength properties of sand mixed with polyurethane prepolymer and sisal fiber

Sandy soil has engineering characteristics, such as loose structure, non-plasticity, poor stability, and compressibility, which can readily cause engineering problems. In practical engineering, physical and chemical methods are usually used to improve the overall performance of sand to ensure the safety of the engineering. However, some reinforcement methods are complex in technology, expensive in materials, and poor in environmental protection. Polyurethane reinforcing agent was introduced in the present study to improve soil reinforcement. Sand samples with different dry densities, sisal fiber content, and polyurethane pre-polymer content were prepared, and a series of direct shear tests were carried out to evaluate the shear strength of the sand. The microscopic mechanism of the sand mixture was studied by scanning electron microscopy. The experimental results showed that the dry density of the sand mixture had a positive effect on the shear strength within the test range. Increasing the concentration of polyurethane prepolymer can increase the shear strength of sandy soil due to the more stable network structure. The cohesion reaches up to about 148 kPa as polymer content is 3%. The shear strength of sand increases first of all and then decreases with an increase in sisal fiber content, 0.4% sisal fiber is an optimum content for sand reinforcement.