The deformation and microstructure characteristics of expansive soil under freeze–thaw cycles with loads

The expansive soil in seasonally frozen areas is vulnerable to repeated freezing and thawing. When exposed to freeze–thaw cycles, this soil may pose a danger to civil engineering structures and cause serious economic losses. Using a closed-system freeze–thaw test, the deformation characteristics of expansive soil under freeze–thaw cycles with different loads were studied. Nuclear magnetic resonance (NMR) and SEM technology were used to study the microstructure characteristics of the soils after the freeze–thaw cycles. The test results show that the soils under different load conditions have different deformation characteristics. The pre-freezing stage of soils under loads is significantly longer than that for soil without a load. Loads play a key role in the deformation caused by freeze–thaw cycles. There exists also a freezing load (pF) and melting load (pT), under which the soil volume change is zero in the frozen and melted state after the soil reaches equilibrium. NMR and SEM tests showed that the pore structure characteristics of the soils under different loads are significantly different. When a low load is applied during the freeze–thaw cycles, the freeze–thaw cycles will damage the structure of the soil, forming new cracks and hence resulting in a loose pore structure. However, if there is a high load, the freeze–thaw cycles contribute to the formation of a dense pore structure. A backfilled soil thickness of 1.5–2.5 m is proposed without considering the gravity of the supporting structure.