Loess is mostly distributed in an unsaturated state in nature, and the complexity of the engineering properties of unsaturated soil is mainly due to the existence of matric suction. Therefore, matric suction must be considered in investigating the mechanical properties of unsaturated loess. However, soils are most often subjected to three principal stresses with different magnitudes in practical engineering. For the sake of examining and discussing the mechanical behaviour of unsaturated natural loess under a complex stress path, a suction-controlled true triaxial apparatus with a rigid-flexible boundary is used to test unsaturated natural loess under a complicated stress path. Four trials of isotropic consolidation tests are conducted on natural loess under suction-controlled conditions via the true triaxial apparatus. The consolidation yield characteristics of the natural loess under different matric suctions are investigated. Forty-eight trials of consolidated drained true triaxial tests under suction-controlled conditions are conducted on unsaturated natural loess to examine and discuss the influence of the matric suction and intermediate principal stress parameter (b-value). The consolidated drained trials are performed under a constant net mean stress and a constant matric suction with different intermediate principal stress parameters (b-values). Stress-strain curves and failure envelopes of the natural loess are also presented. The results indicate that the stress-strain-strength response of unsaturated natural loess depends on the matric suction and intermediate principal stress parameter under true triaxial conditions.

黄土在自然界中大多以不饱和状态分布，非饱和土工程性质的复杂性主要归因于基质吸力的存在。因此，在研究非饱和黄土的力学性能时必须考虑基质吸力。但是，在实际工程中，土壤最常遭受三个大小不同的主应力。为了研究和讨论复杂应力路径下非饱和天然黄土的力学行为，采用具有刚柔边界的吸力控制真三轴仪对复杂应力路径下的非饱和天然黄土进行了测试。通过真正的三轴仪器在吸力受控的条件下对天然黄土进行了四向同性固结试验。研究了不同基质吸引下天然黄土的固结屈服特性。在不饱和天然黄土上进行了吸力受控条件下的固结排水真三轴试验四十八次试验，以检验和讨论基质吸力和中间主应力参数的影响（b值）。固结排水试验是在恒定的平均净应力和恒定的基质吸力以及不同的中间主应力参数（b值）下进行的。还给出了天然黄土的应力-应变曲线和破坏包络线。结果表明，非饱和天然黄土的应力-应变-强度响应取决于真实三轴条件下的基质吸力和中间主应力参数。