Effect of mixing and compaction methods on the physical properties, strength and microstructure of phase change material-modified soils under freeze-thaw process

The complex freeze-thaw process in seasonal frozen ground regions leads to the deterioration of soil properties. In this study, the microcapsule phase change material (mPCM) was applied to strengthen the properties of loess in Lanzhou, China. The variations of volume, surface hardness, wave velocity, strength, and microstructure were investigated under different mixing methods (i.e., drying and wetting mixing methods) and compaction methods (i.e., three layers unidirectional and single layer bidirectional compaction methods). The results showed that the increasing rate of volume for mPCM-modified soils was greater than that for soils without mPCM after the 3rd F-T cycle. The mPCM reduced wave velocity and increased the surface hardness, unconfined compressive strength (UCS) and splitting tensile strength (STS). The mixing methods had less effects on the stress-strain curves, UCS and STS than the compaction methods. For the mPCM-modified soils, the peak strengths of samples compacted by the three layers unidirectional compaction method were larger than that of samples compacted by the single layer bidirectional compaction method. The addition of mPCM filled the pores between soil particles and changed the micro morphological characteristics. The mPCM effectively reduced the percentage of large pores and increased the percentage of medium pores, mitigating the damage of F-T cycles to the pore structure. Compared to the single layer bidirectional compaction method, the three layers unidirectional compaction method results in a more uniform soil sample. Among these preparation methods, the soil samples with wet mixing method and three layers unidirectional compaction method have larger values of UCS and STS.