在地质背景复杂的西北地区进行路堑开挖的工程中遇到了大量以上部为黄土层、下部为砂岩层的典型二元结构路堑高边坡。为确定黄土-砂岩二元结构路堑高边坡的失稳变形机制,依托兰永线高速公路K35+092段工程,利用有限元分析软件GEO-studio建立了黄土-砂岩二元结构路堑高边坡开挖全过程的数值模型。通过对边坡在分步开挖卸荷过程中的坡顶水平位移、竖向位移、深层水平位移、边坡稳定性安全系数变化,以及边坡在开挖状态下应变模拟结果的分析,对黄土-砂岩二元结构路堑高边坡的破坏特征以及失稳破坏机制进行了较为系统的研究。研究结果表明:黄土-砂岩二元结构路堑高边坡的结构特点、地层岩性决定了其失稳变形机制,在开挖卸荷过程中,坡顶水平位移不断增大,土体内塑性应变累积,边坡稳定性逐渐降低;黄土-砂岩二元结构路堑高边坡中的软弱夹层充当“滑动垫层”,起到“润滑”以及“弱化边界”的作用,是边坡潜在的不稳定因素之一;随着路堑边坡的刷方卸荷,上覆黄土坡体发生滑移破坏并对下伏砂岩坡体产生巨大的冲击力,而下伏坡体顺层剪出破坏产生的“联动作用”再次引发上覆坡体的滑塌下错,致使边坡整体失稳。为避免类似黄土-砂岩二元结构路堑高边坡失稳滑塌的发生,可采用框架预应力锚杆支护结构对边坡进行加固。

A large number of typical dual-structured slopes with loess layer on the upper part and sandstone layer on the lower part are encountered in cutting excavation projects in northwest China with complex geological background. To determine the instability deformation mechanism of such high cutting slope, we built a finite element model for the entire excavation process of the dual structure using GEO-studio with segment K35+092 along the expressway from Lanzhou to Yongjing as research background. The damage characteristics and failure mechanism of loess-sandstone dual structure slope were investigated systematically in terms of the horizontal and vertical displacement and the change of safety factor during staged excavation unloading, as well as the stress and strain of slope in excavation state. The results indicate that the instability and deformation mechanism of loess-sandstone dual-structured high cutting slope is determined by structural characteristics formation lithology. Under the action of excavation disturbance, the displacement of slope crest increased, the plastic strain in soil accumulated, and the steady state of the slope gradually weakened. The weak interlayer in the high cutting slope acts as sliding cushion which plays the role of lubrication and weakening the boundary, thus is a potential destabilizing factor of the slope. With the unloading of the slope, the upper part of the slope, namely, loess, slips and damages the lower slope body; in turn the joint action caused by the shear failure of the lower part caused the slumping of the upper part, resulting in an overall instability of the slope. In order to avoid instability and collapse of similar high cutting slope with dual structures, we put forward some corresponding reinforcement recommendations.