Experimental study on improving the engineering properties of coarse grain sulphate saline soils with inorganic materials

Coarse grain soils are a good kind of embankment fill material. However, when they contain a high-level soluble salt, the engineering consequences can include salt expansion and collapsibility. The main objective of this study was to improve the salt expansion, collapsibility, and strength characteristics of coarse grain sulphate saline soils using inorganic binders and to provide an optimal proportion for the improvements. Coarse grain sulphate saline soils with 2% soluble salt contents were improved using different ratios of slaked lime, slaked lime-fly ash, and slaked lime-volcanic ash. Meanwhile, salt expansion tests, collapsibility tests, saturated unconfined compressive strength (SUCS) tests, and SEM tests were performed for the improved soils using different proportions. The results demonstrated that adding slaked lime, slaked lime-volcanic ash, or slaked lime-fly ash could not only effectively reduce the salt expansion amount, but also reduce the sensitive temperature range of the salt expansion. The deformation rate of salt expansion and collapsibility were <1% after adding >11% slaked lime or 15% slaked lime-volcanic ash (or fly ash). The 7 curing day SUCS of the improved soil was not <0.35 MPa. Compared with the strength of the slaked lime improved soil and the slaked lime-fly ash improved soil, the strength of the slaked lime-volcanic ash modified coarse grain sulphate saline soil was stronger. Also, the addition of volcanic ash could increase the strength gain rate in these improved saline soils. Furthermore, the feasibility and rationality of using inorganic binder to improve coarse grain sulphate saline soil were clarified by discussing the microstructure and improvement mechanism of coarse grain sulphate saline soil before and after improvement.