# Engineering Properties and Environmental Impact of Soil Mixing with Steel Slag Applied in Subgrade
## 4 Conclusions  
In this study, the properties and effects of improved soil mixed with SL were evaluated based on physical and mechanical aspects and the environmental impact. The following conclusions were obtained:  
(1) The conventional physical properties of intact soil were significantly affected by SL. Increased SL content led to increased maximum dry soil density, while optimum water content decreased. The liquid-plastic limit of SSL was lower than intact soil, and its plasticity index was also reduced.  
(2) The addition of SL improved the CBR value of intact soil after 96 h-immersion. The CBR value of the improved soils increased first and then decreased, reaching the maximum CBR value of 60% at 50% SL, meeting the requirement for Class I expressway.  
(3) Based on the compaction, Atterberg and CBR tests, the subgrade soil mixed with 50% SL had the highest CBR strength, good compaction characteristics and better stability. It also had the characteristics of a high utilization rate of SL.  
(4) Strength parameters such as cohesion and friction angle of SSL under the optimal proportion significantly improved. The addition of cement further improved the soil strength.  
(5) The dynamic modulus of the test soil improved by SL obviously increased, and the dynamic modulus of the test soil after adding cement was even more obvious, consistent with the static strength results.  
(6) The pollution trace elements of SSL and SSLC were lower than that of SL, and could meet the class I standard of surface water and class II standard of ground water, which proves that SSL has the potential to be a green subgrade filler.  
In conclusion, the physical and mechanical properties of SSL were better than intact soil, and the environmental pollution was low with no potential impact. The primary problem limiting the application of SL is its expansibility. In this paper, SL was used after aging for six months. Unlike other road structures, subgrade has stronger deformation compatibility, which is the focus of our subsequent studies.  
Conceptualization, Y.Z. and W.W.; Methodology, Y.Z. and T.J.; Validation, S.L.; Formal Analysis, T.J. and S.L.; Investigation, T.J. and S.L.; Writing-Original Draft Preparation, Y.Z.; Writing-Review and Editing, T.J., S.L. and W.W.; Project Administration, Y.Z. and W.W.; Funding Acquisition, Y.Z. and W.W. All authors have read and agreed to the published version of the manuscript.  
Figure 13: The permeability test results of the test soils.  
**Funding:** This work was funded by the Science and Technology Base and Talent Special Project of Guangxi Province (grant number AD19245014), the Scientific and Technological Project of Science and Technology Department of Jilin Province (grant number: 20210508028 RQ), National Natural Science Foundation of China (grant number: 52208438), and the Scientific Research Project of Department of Education of Jilin Province (grant number: JJKH20221019 KJ)).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.