The strength characteristics of unsaturated soil after freezing significantly influence the safety of engineering construction in frozen soil regions. In this study, the soil freezing characteristic curve (SFCC) and soil water characteristic curve (SWCC) of unsaturated lean clay were tested. Based on the similarity between SFCC and SWCC, the unfrozen water content of SFCC was used to replace the water content of the SWCC to obtain the expression for soil suction under different negative temperatures. A shear strength model of unsaturated frozen soil was established. According to unsaturated triaxial tests under different conditions, the effectiveness of the established unsaturated soil shear strength model was verified. The test results show that in the rapid freezing stage of the SFCC, the volumetric unfrozen water content with an initial matrix suction of 57 kPa exhibited the largest rate of change. The stress-strain curve of soil at positive temperature is strain hardening type, and that at negative temperature presents different types under the influence of temperature, initial matrix suction and confining pressure. The effective cohesion and internal friction angle of soil varied significantly under the influence of temperature and initial matrix suction. When the initial matrix suction was constant, the unsaturated shear strength of the soil increased nonlinearly with a decrease in temperature. When the temperature was ≥−5 °C, the shear strength increased with an increase in initial matrix suction, but decreased when the temperature was ≤−5 °C. The model fitting results were consistent with the measured results of the unsaturated triaxial test, which verifies the validity of the model. This research can provide a reference for studying the mechanical characteristics of unsaturated frozen soil under negative temperature.

非饱和土冻结后的强度特性显着影响冻土地区工程建设的安全。本研究对非饱和贫粘土的土壤冻结特征曲线（SFCC）和土壤水分特征曲线（SWCC）进行了测试。基于SFCC和SWCC的相似性，用SFCC的未冻含水量代替SWCC的含水量，得到不同负温下土壤吸力的表达式。建立了非饱和冻土抗剪强度模型。通过不同条件下的非饱和三轴试验，验证了所建立的非饱和土抗剪强度模型的有效性。测试结果表明，在SFCC快速冻结阶段，初始基质吸力为57 kPa时体积未冻结水含量变化率最大。正温土体应力应变曲线为应变硬化型，负温土体应力应变曲线在温度、初始基质吸力和围压的影响下呈现不同类型。土体的有效粘聚力和内摩擦角在温度和初始基质吸力的影响下变化显着。当初始基质吸力一定时，土体的非饱和抗剪强度随着温度的降低呈非线性增加。当温度≥-5℃时，剪切强度随着初始基体吸力的增加而增加，但当温度≤-5℃时​​则降低。模型拟合结果与非饱和三轴试验实测结果一致，验证了模型的有效性。该研究可为研究负温下非饱和冻土力学特性提供参考。