非饱和砂土隧道土拱效应模型试验及颗粒流数值模拟研究

Model test and particle flow numerical simulation of soil arch effect for unsaturated sandy soil tunnel

非饱和砂土基质吸力引起的表观黏聚力使其力学性质与干砂差异较大,为研究非饱和砂土隧道土拱效应,进行了不同含水率及埋深条件的Trapdoor试验,通过分析挡板下落过程中砂土破坏模式及土压力变化规律,揭示了不同工况土拱演化的时变特征,阐述了含水率及埋深对土拱效应的影响,同时基于大主应力迹线圆弧拱理论对挡板上方土压力分布模式进行了理论分析,并考虑粒间吸力作用基于PFC线性滚动阻力黏结模型,进行了离散元数值模拟,从细观角度对不同工况土拱效应进行了分析。结果表明:干砂工况破坏模式由三角形迅速发展为梯形,非饱和工况破坏模式为三角形且夹角与含水率相关;土压力呈三阶段变化,干砂时土压力降到极值后出现回升;非饱和工况土压力极值较干砂大幅减少,含水率较大时土压力受埋深影响较小,且松动区边缘出现裂缝,局部坍塌后形成自然拱;数值模拟表明,随挡板下落,主应力方向发生明显偏转,接触力链为由松动区向稳定区呈由弱到强结构,对模型试验及数值模拟土压力进行归一化处理,土压力分布与理论分析一致。干砂工况孔隙率与土压力规律一致,在含水工况出现裂缝处孔隙率快速增大,同时接触组构随含水率变化明显。

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.