传统的土拱效应理论是基于上覆土体为无黏性土或饱和黏性土建立的，但非饱和状态是实际土体的常见情形，其力学特性受自然环境变化的影响。针对这一问题，基于太沙基（Terzaghi）平面土拱效应分析模型和土体单元主应力轴旋转理论，建立了考虑基质吸力的土拱效应松动土压力分析模型。依次给出了基质吸力在上覆土体内呈均匀分布、梯形分布、正三角分布和倒三角分布时的松动土压力和侧压力系数表达式。为验证该分析模型的正确性，采用FLAC建立了Trapdoor数值模型，理论计算与FLAC模拟结果非常吻合。最后，着重分析了上覆土体的饱和度、厚度、Trapdoor宽度、地下水上升和降雨等因素对松动土压力的影响。研究发现，松动土压力随土体饱和度先减小后增大，当达到进气值所对应的饱和度时松动土压力最小。

The traditional theory of soil arch effect is developed on either cohesionless soil or saturated cohesive soil. In practice the unsaturated soils are commonly encountered, whose mechanical properties are influenced by natural environment transformation. Based on the Terzaghi’s assumption and the principle of principal stress axes rotation on soil arch effect, an analytical model is proposed to evaluate the loosening earth pressure of unsaturated soils considering matric suction. The analytical solutions of the loosening earth pressure and the lateral pressure coefficients are derived for uniform, trapezoidal, upright-triangular and reverse-triangular matric suction distributions respectively. A numerical model by FLAC is developed to verify the analytical model. The theoretical results are consistent well with the numerical results. The influences of such factors as saturation degree, thickness of overlying soil, Trapdoor width, groundwater level and rainfall, on loosening earth pressure are discussed. It is found that the loosening earth pressure decreases firstly and then increases with the saturation degree of soil increasing. The minimum loosening earth pressure is reached at the saturation degree corresponding to the air entry value of the soil.