Abstract: The apparent cohesion caused by matric suction makes the mechanical properties of unsaturated sand significantly different from those of dry sand. In order to study the soil arching effect of the unsaturated sand tunnel, trapdoor tests with different water contents and buried depths were carried out. The time-varying characteristics of soil arching evolution under different working conditions were revealed by analyzing the failure mode of sand and the change law of earth pressure in the process of baffle falling, and the influence of water content and buried depth on soil arching effect was expounded. Meanwhile, the distribution mode of earth pressure above the baffle was analyzed based on the arc arch theory of large principal stress trace and considering the suction between particles. The discrete element numerical simulation was carried out based on PFC (particle flow code) adhesive rolling resistance linear model, and the soil arching effect under different working conditions was analyzed from the micro perspective. The results show that the failure mode of dry sand develops rapidly from triangle to trapezoid, and the failure mode of unsaturated sand is triangle and related to water content. The earth pressure changes in three stages and the earth pressure decreases to the extreme value and then rises when the sand is dry. The extreme value of soil pressure in the unsaturated conditions is greatly reduced compared with that of dry sand. The soil pressure is less affected by the burial depth when the water content is larger. Cracks appear at the edge of the loosened area, and natural arch is formed after the local collapse. The numerical simulation shows that with the baffle falling, the direction of principal stress deflects obviously. The contact force chain changes from the loose area to the stable area from weak to strong. The soil pressure distribution of model test and numerical simulation is consistent with the theoretical analysis. The results show that the porosity of dry sand is consistent with the soil pressure, and the porosity increases rapidly when the cracks appear in the water bearing condition, while the contact fabric changes obviously with the water content.

Key words: soil arching effect, unsaturated sand, failure mode, soil pressure, particle flow code