## 4 Design and Stability Analysis of Surrounding Rock-Supporting Structure of Shaft
### Analysis of Original Supporting Structure Calculation Results
#### 4.5.2 Damage Status and Cause Analysis of the Original Supporting Structure  
The damage condition of the supporting structure notably differed with burial depth and can be mainly divided into three damage types. (1) The first damage type was local disruption of the well wall, as shown in Figure 12a. At the site, only the concrete on the inside of the shaft wall had partially peeled off, there was no large-scale crushing or cracking, and the damaged part had no tendency to develop into the surroundings. This damage form was mainly distributed in the depth range of \(-200\) to \(-400\) m, and shotcrete was used for repair. (2) The second destruction type was well wall extensibility damage, as shown in Figure 12b. The field well wall concrete demonstrated shedding, extrusion, and destruction phenomena, and the inner edge produced cracks that showed an expansion tendency. This destruction form was distributed mainly in the \(-400\) to  
Figure 11: Characteristic curve of the surrounding rock support action. \(-600\) m depth interval, and the field treatment mainly involved the removal of broken parts and supplementation of the high-strength anchor. The repair treatment scheme involved concrete rejetting. After the treatment, the breakage conditions in this section were considerably improved and the support structure basically reached stability. (3) The third damage type was systematic destruction of the support structure, as shown in Figure 12c, and was mainly distributed in the depth interval from \(-600\) to \(-700\) m. The degree of damage to the support structure in this segment was more severe than that in the two shallower depth intervals. The damage to the support structure was no longer to a single well wall, but occurred as a global disruption to the overall support structure, in which the well wall appeared to have broken and fallen out over a large area, thus creating cracks that sharply expanded. Places where the breakage and fall-off were severe caused the well circles to squeeze and be exposed on the outside. The well circles reached the yield state for the third damage type, and the supporting structure lost its overall supporting capacity. The site removed the damaged part and re-sprayed the concrete and increased its thickness to \(700\) mm. After the repair was completed, the supporting structure continued to show similar damage and shaft excavation could not continue under normal construction conditions. The excavation section was, therefore, backfilled, the sand was backfilled to the damage location, and the construction was stopped. The key reasons for the failure and instability of the supporting structure based on the analysis of the damage condition, geological conditions, and current support scheme are described as follows:  
(1) The so-called \"three highs and one disturbance\" phenomenon of three highs (i.e., high ground stress, high temperature, and high karst hydraulic pressure) and one intense mining disturbance of a deep pluton outburst is directly responsible for the destruction of support structures, which are mainly represented by high-altitude stresses;  
(2) The strain-softening characteristics of deep rock mass are prominent, the strength is low, the excavation section is exceedingly deep in the shaft excavation process, the spacing between the supporting structure and working face is too small, and the stiffness of the supporting structure is too large. There is also insufficient space for stress relief in the surrounding rock, the in situ stress is too high, and the strength of the supporting structure cannot meet the support requirements and is destroyed;  
(3) Lack of timely rock capping after excavation of the vertical wells causes softening by water expansion after long exposure of plutons, which considerably weakens the anchor support effect. [PERSON] et al. [40] proposed that the main reasons for large deformation of surrounding rock are as follows.