This study was carried out to evaluate the effect of steel slag (SS) as a by-product as an additive on the geotechnical properties of expansive soil. A series of laboratory tests were conducted on natural and stabilized soils. Steel slag (SS) was added at a rate of 0, 5, 10, 15, 20, and 25% to the soil. The conducted tests are consistency limits, specific gravity, grain size analysis, modified Proctor compaction, free swell, unconfined compression strength, and California Bearing Ratio. The Atterberg limit test result shows that the liquid limit decreases from 90.8 to 65.2%, the plastic limit decreases from 60.3 to 42.5%, and the plasticity index decreases from 30.5 to 22.7% as the steel slag of 25% was added to expansive soil. With 25% steel slag content, specific gravity increases from 2.67 to 3.05. The free swell value decreased from 104.6 to 58.2%. From the Standard Proctor compaction test, maximum dry density increases from 1.504 to 1.69 g/cm3 and optimum moisture content decreases from 19.77 to 12.01 %. Unconfined compressive strength tests reveal that the addition of steel slag of 25% to expansive soil increases the unconfined compressive strength of the soil from 94.3 to 260.6 kPa. The California Bearing Ratio test also shows that the addition of steel slag by 25% increases the California Bearing ratio value from 3.64 to 6.82%. Hence, steel slag was found to be successfully improving the geotechnical properties of expansive soil.

Fissures are an important factor to induce slope instability of expansive soil channel, which destroys the integrity of soil mass and deteriorates soil mass. Currently, the research is limited to the fissures in the plane direction, and it is very important to reveal the development mechanism of fissures in expansive soils along the depth direction by studying the development law of fissures in expansive soils with different thicknesses. In this study, taking expansive soil on channel slope of the Middle Route Project of South-to-North Water Transfer as an example, crack expansion tests with thickness of 10 mm, 20 mm, 30 mm and 40 mm are carried out based on self-designed crack expansion test device. An innovative test method for volumetric fracturing rate is proposed and the following conclusions are drawn: (1) the later the cracking time of soil body is, the lower the water content of cracking and the higher the water content after stabilization when the soil body is thicker; (2) When the fissures develop in soils of different thicknesses, their plane fissure rate changes with time in accordance with the logistic law; (3) Volumetric fracturing increases significantly with thickness; (4) The development of fissures is the form of stress release of soil mass, and the release along depth direction is the main form for soil mass with large thickness. (5) It is of great significance to study the law of fracture development in depth direction for further exploring the mechanism of fracture propagation.