Research on Deep Deformation Characteristics of Roof Arch and Loading Variation of Unboned Pre-stressed Cable at Large Underground Cavern with High Geostress

In order to explore the characteristics of those deformation induced deep in surrounding rock of the roof arch, a Displacement Analysis on the distribution features was carried out, as well as the development. Based on the elastic-plastic theory, the mechanism of deep shrinkage deformation of top arch was qualitatively analyzed. Then the loading variation of Unboned Pre-stressed tension stress, where the shrinkage occurrs, was discussed by analyzing the correlation between Pre-stressed tension stress and rocks strain. The research shows that rocks strain is in strange asymmetric distribution during the early excavation, the shrinkage strain appears in the upstream side, and the distribution range along the powerhouse axis is directly proportional to the drilling depth. Shrinkage strain occurrs in the range of 10.0 m far away from the wall, and is weaken in depth. The stress redistribution caused by excavation causes the arch effect area in the deep layer of the crown arch. The deep surrounding rock of the crown arch on the upstream side is located in the "arch seat", and its stress is loaded radially along the crown arch, resulting in shrinkage deformation. The downstream side crown is the unloading relaxation area. The shrinkage deformation zone is related to the size of the cavern. At the initial stage of excavation, the shrinkage deformation zone evolves to the deep, and the shrinkage strain of the surrounding rock in the relatively shallow part decreases, or even turns to tensile strain. The variation of cable stress go through three stages, including “0” load bearing, rising in step, and convergence. The value of the cable stress was approximate to pre-stressed tension one and the steel strand only bears the pretension load when the depth of Shrinkage zone beyond the anchor depth. The pre-stress loss rate was mainly affected by tensile strain of reinforced rock mass.