# Engineering Properties and Environmental Impact of Soil Mixing with Steel Slag Applied in Subgrade
## 2 Materials and Experimental Methods
### Experimental Methods
#### 2.2.2 Mechanical Properties and Deformation Stability of Soils  
In order to evaluate the mechanical properties and stability of SSL, static and dynamic tests were carried out and the volume stability test was also conducted for the deformation stability of SSL. In addition, considering that a small amount of cement has been used to improve the strength and stability of soil in many studies [27; 28; 29], this paper also carried out the laboratory test of SSL incorporating 2% cement (i.e., SSLC).  
(1) Static triaxial test  
The static triaxial test is usually adopted to evaluate internal friction and cohesion. The shear strength values of soil, SSL, and SSLC could be measured by the static triaxial test. Based on the optimal water content and maximum dry density, the specimens with a diameter of 39.1 mm and a height of 80 mm were prepared by simulating the reality with the compactness of 96%. In this study, the TKA-TTS-1 full-automatic triaxial apparatus shown in Figure 5a was selected, and the confining pressure was set as 50, 100, and 150 kPa, with a longitudinal loading speed of 0.02 mm/min. When the failure or longitudinal deformation reached 5%, the static triaxial test would stop, and the stress-strain curve could be drawn. Based on the limit equilibrium method, the cohesion (c) and friction angle (\(\varphi\)) could be solved.  
(2) Dynamic modulus test  
In this study, the LFTD1821 dynamic triaxial testing machine was used for the dynamic modulus test according to the Chinese specification (GB/T 50123-2019), as shown in Figure 5b. The dynamic modulus test is commonly used to evaluate the dynamic characteristics of soil materials [30; 31]. The specimen size was the same as that of the static triaxial test, and the specimens were also prepared with a compactness of 96%. The consolidation was completed at the coefficient 1.1. Sinusoidal cyclic loading with a frequency of 1 Hz at the confining pressure of 100 kPa was applied. The selected parameters frequency and confining pressure are the most commonly used, and closest to the engineering situation. In order to obtain the deformation response of the specimen under different dynamic pressures, the longitudinal deviatoric stress was continuously increased from small to  
Figure 5: The experimental procedure of mechanical properties and deformation stability: (**a**) static triaxial test; (**b**) dynamic modulus test; (**c**) volume stability test.  
large (i.e., varying from 5 kPa to 350 kPa, at the amplitude of 5-50 kPa). Ten cycles were performed for each deviatoric stress state. The stress-strain curve data in the last cycle was extracted for calculation of the dynamic modulus.  
(3) Volume stability test  
For SL, the volume stability is the most critical factor to determine the engineering application. According to the requirements of the Chinese specification (GB/T24175-2009), the water immersion expansion rate test was carried out to measure the volume change of SL, SSL, and SSLC after immersion. The soil specimens were prepared according to the particle size requirements and formed by heavy compaction. Referring to the immersion conditions of the CBR test, the specimens were put into a thermostatic water bath and the data recorded. The water temperature was then kept at 90 \(\pm\) 3 \({}^{\circ}\)C for 6 h and afterwards heating stopped. Data after 18 h (the next day) was recorded before heating. The above process was repeated and the final data was recorded after 10 d, in which the volume change during the process could be obtained. The details of the volume stability test are shown in Figure 5c.