Stabilization of Soft Clays Exposed to Freeze–Thaw Cycles Using Chitosan

Nowadays, chitosan biopolymer has received much research attention in geoenvironmental practices like soil erosion reduction, hydraulic conductivity, and heavy metal absorption in contaminated soil. Nevertheless, the effect of chitosan incorporation on freeze–thaw resistance of soft clays has not been evaluated comprehensively. In this research, different concentrations (2%, 4%, 6%, and 8%) of biocompatible chitosan, which is synthesized from the waste of shrimp shells, are utilized to investigate its potential in resistance of clay specimens subjected to 1, 2, 3, 6, 12, and 24 freezing and thawing cycles. The results demonstrate that the stress–strain behavior and compressive strengths of the specimens significantly depend on the amount of added chitosan solution. The unconfined compressive strength grows as the chitosan content increases from 2% to 6%. The 8% chitosan destroys the balance in the overall electrical neutrality of the mixture and does not make a noticeable influence in comparison to 6%. Moreover, the interparticle interactions are strongly linked to the curing time and moisture content of specimens, and chitosan solution can provide an extra interaction among clay particles in the early days. Increasing the curing time for the treated specimens leads to an increase in compressive strength while its efficiency remains constant over time. The durability index used to quantify the resistance of the treated specimens to freeze–thaw damage reveals that by three freezing and thawing cycles, the index reduces to 0.64 and 0.66 for samples in optimum and saturated water contents, respectively. This reduction will be negligible for higher cycles (e.g., 0.53 and 0.50 for 24 cycles). In addition, according to the analysis by scanning electron microscopy, the number of freezing and thawing cycles has a substantial influence on enlarging the voids and clay disintegration, which damages the stabilized soft clays.

The freeze–thaw cycles change the geotechnical properties of soft clays and result in a further alteration in soil structure and loss of strength. These effects should be considered for a safe design in cold regions, particularly those underlined by permafrost. This research is performed to investigate and quantify the behavior of using an environmentally friendly material known as chitosan, which is one of the low-cost biopolymers extracted from discarded crustacean shells. In soft soils, the size of pores is approximately large (4–5 μm) and cycles of freezing and thawing cause an extra increase in the size of the voids. Mixing 6% chitosan at the optimum water content and curing for 28 days cause the chitosan to react with clay minerals and reduce the size of the voids to approximately 2–3 μm, resulting in a solid structure that is 90% more durable than the untreated soils.