Geotechnical characterization and collapsibility of a natural dispersive loess

During preliminary investigation phase of the national Chabahar-Zahedan railway line, serious geotechnical problems including non-uniform settlements, tensile cracks, and local collapse were reported in parts of the path near the coastline. A follow-up field investigation revealed that the in-situ soil at construction site has a metastable structure being sensitive to saturation and loading. Therefore, a comprehensive series of physical, chemical, electro-chemical, and geotechnical tests were conducted to classify and characterize the soil properties and behavior in its natural state. The natural soil was classified as a clayey loess with moderately dispersive to dispersive characteristics. Given its significant collapse potential, the soil was assigned into the category with severe trouble for construction purposes. Importantly, our tests revealed that a threefold reduction in pore water salinity results in > 4% rise in collapse potential of similar specimens. Reduction in salt concentration increases the thickness of diffuse double layer and hence collapsibility. In addition, strain softening shear behavior was observed for elevated confining stresses under undrained triaxial stress state. In fact, this brittle type failure mechanism associated with continuous generation of pore water pressure is undesirable as catastrophic failures could happen with no advance warning.