Mechanical properties of saline soil solidified with lime, fly ash and modified polyvinyl alcohol under freeze-thaw cycles

The repeated freeze-thaw cycles with seasonal alternations have an obvious effect on soil structure. To reduce the temperature sensitivity of saline soil and then use it in engineering, a combined treatment method is proposed, where lime, fly ash and modified polyvinyl alcohol (MPA) are used as solidified materials. Unconfined compressive strength (UCS) tests and microstructure characterization are firstly used to evaluate the solidified effect and obtain the parameters range of solidified materials. Then, the shear strength tests for determining cohesion and internal friction angle are conducted. Experiments are conducted by considering separate and combined treatments of materials mentioned above. The results indicate that the combined treatment with lime, fly ash and MPA can improve the strength of saline soil. After combined treatment, the UCS is 1130.25 kPa, which is 5.18 times than that of saline soil (218 kPa). The strength of combined solidified saline soil meets the requirements of engineering specification (JTG 3430-2020). The stable value of UCS of combined solidified saline soil is 700 kPa under freeze-thaw cycles. The fluctuation is about 5% after three freeze-thaw cycles. The cohesion and the internal friction angle of combined solidified saline under the optimal proportion can be 208.2 kPa and 38.56°, respectively after three freeze-thaw cycles. The sensitivity of the factors is in an order of decreasing importance: curing time, lime content, MPA content, dry density, salt content, and freeze-thaw cycles. With an increase of lime, fly ash and MPA content, the strength of combined solidified saline soil increases and then tends to become stable. The optimization of solidification parameters can effectively weaken the influence of freeze-thaw on coastal saline soil. Based on tests results of compressive strength and shear strength, it can be concluded that the optimal combination of solidified parameters is 14% of lime, 30% of fly ash, 1% of MPA, 28 days of curing time, and a dry density of 1.65 g/cm³.