中文摘要：

冻融状态影响土壤的抗剪强度从而威胁季节性冻土地区的工程安全、边坡稳定以及土壤流失。通过直剪试验测定了不同冻融状态和初始含水率对青藏地区（S1）和北京地区（S2）土体抗剪强度的影响。结果显示,2种土在未冻和已融状态下的抗剪强度相似,且均随着土含水率的增加而减小,但S1土抗剪强度比S2土大7.5%-9.7%;在冻融状态下,S1土抗剪强度随着土含水率的增加而增大,而S2土则随之减小。S1冻融土抗剪强度在低含水率（≤13.5%）时小于未冻土和已融土,而在高含水率（≥24.5%）时则反之;S2冻融土抗剪强度小于未冻土和已融土。在冻融状态下2种测试土的内摩擦角显著小于未冻土和已融土,而黏聚力整体上则大于未冻土和已融土。与未冻土或已融土相比,2种土在冻融状态下的强度相对较低,宜作为季节性冻土地区工程设计以及土壤流失防治的基本状态。

英文摘要：

The freeze-thaw conditions affect soil shear strength and hence threaten engineering safety, soil slope stability, andsoil loss in seasonal frozen-soil region. Shear strength was tested for 2 kinds of soils （sandy loam in Qinghai-Tibet Plateau （S1）and silt loam in Beijing region （S2）） under 3 freeze-thaw statuses （unfrozen soil, frozen soil, and thawing soil after frozen） anddifferent initial water contents （8.0%-31.0% in mass fraction） by direct shear apparatus in the laboratory. The unfrozen soilspecimen did not suffer freeze-thaw processing in the laboratory, the thawing soil specimen was sheared after frozen at -18 ℃ for 24 h, and the thawed soil specimen was done after frozen at -18℃ for 24 h and then thawed at 27℃ for 12 h. The shearvelocity was set to 2.4 mm/min. Experimental results showed that the shear strength of the 2 types of soils increasedapproximately linearly with the increasing of normal stress, and the shear strength envelope of unsaturated thawing soil couldbe expressed by total stress Mohr-Coulomb failure criteria under tested normal stress, freeze-thaw status and soil water contentconditions. The shear strengths of unfrozen and thawed soils were similar to each other and their difference was averagely3.4%-3.7% after one freeze-thaw cycle, and they decreased with the increased initial soil water content for the 2 types of soils.The shear strength of soil sample S 1 was 7.5%-9.7% greater than that of S2 under the unfrozen and thawed statuses. Under thethawing status, the shear strength of S1 increased with the initial soil water content but that of S2 decreased with it. The shearstrength of the thawing S1 soil was smaller than that for the unfrozen and thawed soils at low soil water content （13.5%） but itwas on the contrary when the water content was higher; the shear strength of the thawing S2 soil was smaller than that for theunfrozen and thawed soils under all the soil water contents. Under the unfrozen, thawed, and thawing statuses, the internalfriction angle