Effects of protein-based biopolymer on geotechnical properties of salt-affected sandy soil

Salt-affected soils cannot meet the needs of engineering projects due to their deficiency in providing desirable geotechnical properties. Cement stabilization is widely used to improve the engineering properties of salt-affected soils, but cement has many backward effects, especially on the environment, limiting its application as a binder. This study evaluates the potential effects of salt on protein-based biopolymer treated sand. The influence of salt content, biopolymer content, and curing time on the strength and stiffness development of salt-affected sand was explored with unconfined compressive strength (UCS) testing. The UCS results showed that an increase in casein biopolymer content led to an increase in the unconfined compressive strength and stiffness; however, the addition of salt had a reverse effect on UCS results. By adding 2% casein solution, the compressive strength reached 1021.34 kPa, which is significantly greater than that of untreated soil with a value close to zero. When the salt content rose from 0.5 to 10% (for 2% casein content), a substantial strength loss (more than 48%) was observed in the UCS value from 978 to 501 kPa. This might be due to the salt existence in soil which adversely affected the biopolymer connections by blocking the bonds and bridges with soil particles. This adverse effect was gradually mitigated by the biopolymer increment until adding 3.5% sodium caseinate, then a higher percentage of the biopolymer was involved in further enhancement of compressive strength. Microscopic observation revealed that sodium caseinate acted as a binding agent between soil particles, while salt disrupted the sodium caseinate performance. To evaluate the physical properties of the sandy soil, permeability and wind tunnel tests were conducted. The inclusion of sodium caseinate as a protein-based biopolymer resulted in lowering the hydraulic conductivity and increasing the erosion resistance of salt-affected sand. Curing time had positive effects on strength development, increasing the erosion resistance, and reducing the permeability. Overall, sodium caseinate could adequately improve the engineering properties of salt-affected sand.