Analysis of wedge stability in the excavation of dam diversion tunnels: a case study

In tunnel design and execution, one of the critical phases is the stability analysis of tunnels and identifying mechanisms of over-excavation and collapse, or, in other words, recognising areas prone to wedge formation. This paper discussed and examined the collapsing wedges while excavating the Moshampa Dam diversion tunnels. Considering the excavation depth and the conditions of jointed rock masses in these tunnels, the most prominent failure mechanism is wedge collapse or the detachment of rock fragments from the walls. Therefore, the relationship between collapse-prone points or areas prone to wedge formation, identified based on pre-construction conditions and laboratory parameters, has been integrated and compared with actual site conditions. A kinematic analysis was performed using the UNWEDGE software to analyse the results and identify potential wedges. The results revealed a significant and interesting consistency between the kinematic analysis results and the actual post-construction conditions, confirming that these collapses occurred due to geological conditions during excavation. Since cohesion, friction angle, and the geometric characteristics of the joints (strike and dip) play a crucial role in kinematic analyses, the consistency between the results of over-excavation and the outcomes of wedge collapses highlights the importance of these parameters. This, in turn, emphasised the need for precision in field data collection, joint surveying, in-situ and laboratory testing, and generally, the completion of geotechnical and rock mechanics data.