Investigating microscale structural characteristics and resultant macroscale mechanical properties of loess exposed to alkaline and saline environments

Due to the monsoon climate, saline soils extensively spread over the arid or semi-arid area (e.g. NW China) and are featured with notable dissolution collapsibility and salt expandability. The features cause road hump and subgrade corrosion towards endangering the safety of passers. In this study, the microstructural evolution of the loess when subjected to alkaline and saline environments was investigated using a series of microscale tests, including scanning electron microscopic (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (SEM-EDS), and mercury intrusion porosimetry (MIP). The correspondence of the microstructural evolution with the macroscale mechanical properties using the reduced triaxial extension tests was studied. The test results showed that the alkaline and saline environments led to the microstructural evolution of the loess, involving the particle morphology, microstructural characteristics, pore distribution, and directionality of the pores. The increasing thickness of the diffuse double layer, induced by sodium sulphate, reduced the suction and deteriorated the microstructural characteristics, causing a degradation of the macroscale mechanical properties. Under sodium hydroxide conditions, a distinct transformation from granular structures to an agglomerated structure was attributed to the formation of sodium silicate, and the surface-surface connections were formed at the same time, which also indicated an enhancement of the microstructural characteristics. The enhanced microstructural characteristics contributed to the improvement in the macroscale mechanical properties. The findings of this study provide some design guideposts for preventing and mitigating the degradation of subgrade when exposed to saline environments.

Automated summary

This study investigated the microstructural evolution of loess in alkaline and saline environments through a series of microscale tests, finding that the enhanced microstructural characteristics contributed to improved macroscale mechanical properties. The results provide design guideposts for preventing and mitigating subgrade degradation in saline environments.