Freeze-thawing characteristics and microscopic mechanism of expansive soil treated with lignin fibers

Expansive soil is commonly used to construct long-distance water diversion and transfer projects in the northern areas of China. This kind of problematic soil is also characterized by the undesirable properties of multiple cracks and strong expansibility. The complex environmental field can be attributed to the deterioration of expansive soil below the canals in the high-altitude and cold regions. It is very necessary to treat the expansive soil for the safe operation of water diversion engineering. This study aims to explore the fiber treatment of expansive soil subjected to freeze-thaw cycles. Taking a canal in northern Xinjiang as an example, a series of laboratory tests were performed on the expansive soil treated by lignin fibers, including freeze-thaw deformation, direct shear, unconfined compressive strength, tensile, and scanning electron microscope (SEM) tests. A systematic investigation was implemented on the volumetric deformation, compressive strength, tensile strength, and fiber-treatment mechanism of expansive soil below the canals subjected to freeze-thaw cycles. The results showed that the samples were characterized by the expansion under freezing and the contraction under thaw. The volume of samples after contraction was still higher than the initial volume. The volumetric rate of treated samples subjected to freeze-thaw cycles decreased by 24% to 37%, compared with the untreated. The attenuation rate of shear strength exceeded 42.2% in the untreated samples after 15 freeze-thaw cycles, while that of the treated samples was outstandingly lower than 29.1%. The softening properties were observed in the compressive stress-strain relationship of the samples. There was no significant variation in the properties of compressive stress-strain relationships after the repeated freeze-thaw. Meantime, the elasticity modulus and compressive strength of the treated samples decreased firstly and then tended to be stabilized with the freeze-thaw cycles, whereas, those of the untreated samples were consistently decreasing. However, the tensile stress-strain relationship of samples was developed under the freeze-thaw cycles from the "linear elastic growth-strengthening-softening" to the "linear elastic growth-softening". The tensile strength, tensile modulus, and fracture energy of the treated samples were significantly higher than those of the untreated ones during cyclic freeze-thaw cycles. The compressive strength of the samples with the different fiber content and fiber length showed an approximately linear relationship with the tensile strength, and their ratios of tensile-to-compressive ranged from 0.112 to 0.182. The addition of lignin fibers effectively improved the volumetric deformation, shear strength, compressive strength, and tensile strength properties of expansive soil subjected to freeze-thaw cycles. Additionally, better freeze-thaw performance was achieved in the treated expansive soil, when the content and length of lignin fibers were 2%, and 1 mm, respectively. At the same time, there was a significantly lower damage degree of the samples treated by lignin fibers subjected to freeze-thaw cycles, compared with the untreated ones. The reason was that the presence of lignin fibers further limited the dislocation and redistribution of soil particles during cyclic freeze-thaw processing. The integrity of the internal structure in the expansive soil can be expected to explain the freeze-thaw performance of expansive soil treated by lignin fibers. The findings can provide a strong reference for the expansive soil engineering in seasonally frozen areas.