Contents lists available at ScienceDirect Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions Effect of freeze-thaw cycles on shear strength of saline soil Yan Han a , Qing Wang a, ⁎ , Ning Wang a , Jiaqi Wang a , Xudong Zhang a , Shukai Cheng a , Yuanyuan Kong b a College of Construction Engineering, Jilin University, Changchun 130026, PR China b School of Highway, Chang'an University, Xi'an 710064, PR China ARTICLE INFO Keywords: Saline soil Freeze-thaw (F-T) cycle Triaxial compression test Shear strength ABSTRACT Freeze-thaw (F-T) cycle is an important external factor affecting the mechanical properties of saline soils in cold regions. In this study, the effects of F-T cycles (0, 1, 5, 10, 30, 60 and 120) and salt contents (0, 0.5, 1.0, 2.0 and 3.0%) on the shear properties of saline soil in western Jilin Province of northeastern China, including stress- strain behavior, peak undrained shear strength (τ u ), resilient modulus (E R ), cohesion (c u ) and internal friction angle (φ u ), were investigated by conducting unconsolidated-undrained (UU) triaxial compression tests. The results demonstrated that, when the salt content was the same, the τ u , E R and c u basically showed a decreasing- steady-decreasing trend with increasing F-T cycles, and the dynamic evolutions of soil microstructure were mainly responsible for such variations. The Gouy-Chapman diffuse double layer together with matrix suction theories were employed to analyze the mechanism of the effects of salt content on the c u and φ u at different freeze-thaw cycles; the thicker diffuse double layer induced by higher sodium ion contents and the decreasing matrix suction were deemed as the main reasons that led the c u to continuous reduction; the φ u was believed to be successively affected by the enhanced lubrication effect and the salt crystallization process, and there was a threshold salt content (2.0%) which was influenced by the number of F-T cycles. Finally, by categorizing the different experimental combinations into relative undamaged, single-factor damaged and two-factor damaged states concerning the τ u , an empirical mathematical equation of high reliability (R 2 > 0.985) was established to describe the combining effects of F-T and salinity on the changes in τ u . 1. Introduction The stability of engineering structures in a seasonally frozen region is inseparably related to the mechanical properties of soil. The prop- erties of soil vary dynamically since the soils in such cold regions are subjected to freeze-thaw (F-T) cycle at least once a year (Liu et al. 2010) or even once a day before reaching a continuous negative temperature. Swelling stress that occurs in the soil when the volume expands as water in pores freezes into ice directly influences the basic physical properties of soil. Qi et al. (2006) have summarized the changes in some physical properties subjected to F-T regimes, including void ratio, density, permeability and Atterberg limits, and these physical proper- ties had been extensively studied in the early stage (Chamberlain and Gow 1979; Eigenbrod 1996; Konrad 1989; Viklander 1998). The freezing of saline soil is a complex process (Zhang et al. 2017a), and changes in mechanical properties are the comprehensive effects of the abovementioned changes which are inevitably influenced by F-T cycles indirectly. Soil mechanical properties generally involve strength and deformation. In detail, the stress-strain relationship, resilient modulus and ultimate strength were often discussed in the literature (Eskişar et al. 2015). To be more specific, the peak on a triaxial stress-strain curve of over-consolidated soil can be decreased or vanished as an ef- fect of F-T cycles (Graham and Au 1985; Leroueil et al. 1991), and it has been experimentally confirmed that the shape of the stress-strain curve is not so closely related to F-T cycles (Wang et al. 2007). In addition, many studies have reached a consensus that F-T cycles cause the re- silient modulus to significantly decrease (Johnson et al. 1978; Lee et al. 1995; Orakoglu et al. 2017; Qi et al. 2008; Simonsen et al. 2002), and the extent of the decline mainly depends on soil type. A type of modulus calculated with measured maximum stress and strain also behaved in- versely proportional to increasing F-T cycles (Wei et al. 2015). How- ever, after several F-T cycles without re-humidification, the initial tangent modulus of clayey soil blocks increased although with the generation of visible cracks (Aubert and Gasc-Barbier 2012), and like- wise, Liu et al. (2018) stated an analogous law of granular material subjected to closed-system F-T cycles. These contradictory conclusions might be attributed to different initial states of soil and experimental https://doi.org/10.1016/j.coldregions.2018.06.002 Received 10 September 2017; Received in revised form 26 May 2018; Accepted 8 June 2018 ⁎ Corresponding author. E-mail address: wangqing@jlu.edu.cn (Q. Wang). Cold Regions Science and Technology 154 (2018) 42–53 Available online 20 June 2018 0165-232X/ © 2018 Elsevier B.V. All rights reserved. T