# Engineering Properties and Environmental Impact of Soil Mixing with Steel Slag Applied in Subgrade
###### Abstract  
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. steel slag; subgrade material; physical property; mechanical property and stability; environmental impact +
Footnote †: journal: Applied Sciences  
Due to its compressive strength and wear resistance, it has been recognized that SL can be used as an aggregate. In 1974, Canada constructed a test road with SL for the 401 expressway, and its excellent anti-slip properties were noticeable after four years. In the 1980s, Japan began combining asphalt pavement with SL to improve skid and wear resistance. Subsequently, SL aggregates have been widely applied in the US, Germany, Singapore and other countries [7]. In 2009, SL was used instead of basalt to pave the surface pavement layer in Beijing, and the first all SL municipal road was completed in Taiyuan, Shanxi in 2015 [8]. Asphalt mixed with SL can serve as a partial replacement for aggregates and significantly improve physical and mechanical properties and fatigue resistance [9; 10; 11; 12; 13]. More recently, waste/recycled materials have gradually been used for stabilizing subgrades. [PERSON] et al. [14] studied the utilization of environmentally clean recycled aggregates, such as recycled glass, for improving subgrades with problematic soils. [PERSON] et al. [15] carried out a comprehensive investigation on the utilization of carpet waste fibers in the reinforcement of clay soils. These relevant studies provided reference for the utilization of SL. Scholars have extensively attempted to use SL in road subgrade and subbase [16; 17; 18; 19]. [PERSON] et al. [16] performed dry shrinkage and temperature shrinkage tests on cement-stabilized steel slag by using SL instead of crushed stone, and found that a reasonable adjustment of SL gradation could effectively reduce the dry shrinkage and temperature shrinkage. [PERSON] et al. [17] examined the effect of different SL dosing on the mechanical properties of cement-stabilized crushed stone subgrade.