Study on mechanical properties and microstructure of porous organic polymer reinforced low-grade sand under wetting–drying cycles

Abstract

Polymer materials are finding increasing application in soil stabilization, but little is known about the changes in performance and microstructure of polymer membranes under wetting–drying (W-D) cycles. This paper introduced a self-developed porous organic polymer material (SPOP) to investigate the mechanical properties of treated sand under various SPOP contents (1 %, 2 %, 3 %, and 4 %) and W-D cycles (0, 4, 8, 12, 16, and 20) using uniaxial compression tests and triaxial tests. The changes in the microstructure of SPOP-sand mixtures were observed by scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results indicated that the addition of SPOP significantly increased the mechanical strength, elastic modulus, and cohesion of natural sand under W-D cycles. The mechanical properties exhibited an upward trend before decreasing as the increment of SPOP contents and peaked at 3 % SPOP content. Specimens with high SPOP content were more susceptible to moisture. On the other hand, the mechanical properties of SPOP-sand mixtures decreased significantly for all specimens under the first 4 cycles, followed by a decrease with increasing cycles exponentially. The NMR test showed the proportion of micropores within the specimens decreased by 27.81 %–48.09 % after wetting. The micropores were transformed into mesopores and macropores, and the porosity of specimens was increased. Based on the SEM images, there were three mechanisms of polymer strengthening sand, including wrapping, pore filling, and bridging effects. Alternate W-D cycles damaged the inter-particle bridging effect, thus weakening the bonding strength between the polymer membrane and sand particles. Soil aggregates inside the specimens were loosened, and polymer membrane was softened and broken under cyclic wetting and drying, causing the unstable structure of the specimen when applied external pressure.