The swelling properties of expansive soils can be reduced by the addition of modifiers. Nevertheless, the performance deterioration after modification occurs when weathering for a long term. Therefore, in this study, the effect of drying-wetting cycles on swelling behaviour and compressibility of modified expansive soils with the iron tailing sand and calcium carbide slag has been investigated. The swelling potential initially increases and subsequently decreases with the increasing number of cycles, reaches the peak at the seventh cycle, and tends to equilibrium after the tenth cycle. These results show that drying-wetting cycles will destroy the soil structure. The compressibility of modified expansive soils increases with the drying-wetting cycles, where an empirical formula between compressibility and the cycle number was established. Microstructural analysis is performed using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results of microstructural analysis show a tendency of degradation process.

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Aiming at researching shear strength parameters of expansive soil modified by industrial waste iron tailings sand, the enhancement of expansive soil is explored from macroscopic and microscopic aspects. After characterization and testing by various means, the results show that expansive soil modified by iron tailings sand will increase the maximum dry density of the improved soil and reduce its optimal moisture content, which is beneficial in tuning the moisture content at the construction site. In addition, iron tailings sand can improve the shear strength of expansive soils. The influence of iron tailings sand on cohesion increases first, then decreases, and reaches the peak value at 30%, while the effect on internal friction angle exhibits a continuously increasing trend. Furthermore, according to mercury intrusion tests and microangle analysis, the addition of iron tailings sand can reduce the tiny pores and enhance the occlusal force of the soil. Simultaneously, it increases the number of large pores, maximizing the macroscopic strengthening of iron tailings sand towards the expansive soil.