## 4 Conclusions  
In this study, the resilient moduli of unmodified SC and modified SC after F-T cycles were measured. The effects of stress state and F-T cycles on the resilient modulus were investigated. Some conclusions were obtained as follows:  
1. The resilient moduli of both two materials are significantly influenced by their stress states. The resilient modulus always increases with the increase of confining pressure, and is inverse to the deviator stress, which is in accord with other research. F-T cycles are also an important factor affecting the resilient modulus of test soils. With increasing F-T cycles, the resilient modulus of unmodified SC decreases continually, while the resilient modulus of the modified SC decreases sharply after the first F-T cycle and then tends to be stable.
2. Damage ratio can be effectively adopted to describe the resilient modulus reduction after F-T cycles. For unmodified SC and the modified SC, their damage ratio variation trends after F-T cycles can be effectively fitted in the form of exponential decay equations.
3. The significance analysis demonstrates that the significance of confining pressure, deviator stress, and F-T cycles on resilient moduli for the two materials are great. For unmodified SC, the variance significance of confining pressure is the greatest, followed by F-T cycles and deviator stress. For the modified SC, the significance of confining pressure is also the greatest, then followed by deviator stress and F-T cycles.
4. From the SEM testing, the microstructure of unmodified SC is a whole platy structure with layered structures, while the microstructure of modified SC consists of a lot of fine particles and agglomerates, which possess greater stability after F-T cycles than unmodified SC. With increasing F-T cycles, the initial structure of soils is destroyed and the porosity also increases. The correlation analysis shows that the porosity and mean diameter of pores are strongly negatively correlated with the resilient modulus. The SEM testing also reflects the variation characteristics of pores for compacted soils after F-T cycles: the small pores gradually develop into medium and large pores.  
In summary, the resilient modulus of SC modified by OSA and FA is superior to that of unmodified SC after F-T cycles. The modified soil can be used for road construction in seasonally frozen areas. Additionally, more research will be conducted to investigate the strength and deformation characteristics of the modified silty clay.  
[PERSON] and [PERSON] conducted the resilient modulus test and prepared the original draft. [PERSON] performed the SEM testing and revised the manuscript. [PERSON] and [PERSON] consulted the research program and proofread the manuscript. This work was supported by the National Natural Science Foundation of China [grant number 51578263]; Transportation Science & Technology Program of Jilin Province [grant number 2015-1-11]; Scientific and Technological Planning Project of Ji Lin Province [grant number 20180201026 SF]; and Open Fund Project of Loess Earthquake Engineering Key Laboratory of China Earthquake Administration, Gansu Province, China [grant number KLLEE-17-002]. The authors declare no conflict of interest.