Engineering Properties and Environmental Impact of Soil Mixing with Steel Slag Applied in Subgrade

The purpose of this study was to evaluate the feasibility of the large-scale application of steel slag (SL) in subgrade. Subgrade materials with three kinds of SL proportions were first prepared. Then, a compaction test, liquid-plastic limit combined-measurement test, and a California bearing ratio (CBR) test were applied to determine the best proportion between SL and intact soil (S), i.e., SL/S. Subsequently, static and dynamic tests and a volume stability test were carried out for soil mixed with SL at the optimum proportion (SSL). In addition, a composition analysis of infiltration fluid and a permeability test of SSL were performed. The test results showed that compared to S, the physical properties of SSL were significantly improved, especially the liquid-plastic limit, as well as the soil water stability. The optimum proportion of SL was determined as 50% of soil by mass. At the optimum proportion, SSL had the highest CBR value of 60%, which had both economic and engineering compaction performance, leading to a large-scale utilization rate of SL. The static and dynamic characteristics showed that the addition of SL would greatly improve the shear strength and dynamic modulus of soil, mainly expressed as the increase of internal friction angle. The volume stability of SSL could also meet the requirements of the Chinese specification. After adding 2% cement, the strength and stability of SSL was further improved. In addition, the environmental impact test proved that the infiltration liquid did not pollute surface water nor underground secondary water. Although the permeability coefficient of SSL with the optimum proportion of 50% was higher than that of pure soil, it still belonged to the normal value of clay and silty clay, and good impermeability would ensure the controllability of potential trace elements. Based on the test results of mechanical properties and environmental impact, SSL proved to have the potential for green road material engineering properties. This study proposes a reliable and practical method to promote the utilization of steel slag.