# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 3 Factors That Contributed to the Catastrophic Failure
### Structural, Geological, and Lithological Factors  
Based on regional geologic information, the main power tunnel in the vicinity of the collapse intersects an east to northeast dipping (30\({}^{\circ}\) to 50\({}^{\circ}\) regional dip) sequence of volcaniclastic rocks. The investigation holes were drilled from the upstream end in the main tunnel and were generally angled upwards and around the main tunnel to understand the extent of the collapse zone and to determine if any major faults had been missed during construction. These holes were drilled roughly perpendicular to the regional stratigraphy and downwards in the geological sequence (from younger to older rocks due to the presence of a synclinical regional structure [23]). The regional strike of these units is such that the main tunnel transects the same sequence of rocks present in the by-pass tunnel, although somewhat obliquely (Figures 6 and 7). As discussed above, three distinct lithologic packages can be projected along the strike to intersect both the collapse zone and the by-pass tunnel. Figures 3 and 7A show schematically the pre-collapse distribution:  
1. Hydrothermally altered red tuff (HART) upstream of the collapse zone;
2. HART within the FDZ and crystal lithic tuff (CLT within the FDZ and AA within the FDZ;
3. Andesitic agglomerates (AAs) downstream of the collapse. Figure 6: Oblique view showing the interpreted FDZ including FDZ-HART (green) and FDZ-CLT (blue) geological units. In addition to core logging, petrographic examination and XRD testing were conducted on cores from the investigation holes, and these data were used to identify and determine the spatial location of HART, CLT, and AA lithologies as well as the FDZ. Figure 7: Failure mechanism and failure progression for (**A**) pre-collapse, (**B**) initial liner failure and Rockfall, (**C**) additional collapse, rockfall progressing into the FDZ, (**D**) temporary tunnel blockage, and (**E**) final collapse. From a regional perspective, the HART stratigraphically overlies the CLT and AA. The contact between the CLT and AA is gradational and is interpreted as representing a gradual change in the deposition style within this epiclastic volcanic unit. The boundary between the HART and the CLT, on the other hand, is a discrete contact. The general coincidence between the FDZ, associated lithologies, and the collapse zone is shown schematically in Figures 3, 6 and 7a-c. The fault-damaged zone (FDZ) straddles the lithological boundary and includes fault-damaged components from the HART, CLT, and AA lithologies. In addition to identifying the lithological packages, as discussed above, the investigation holes in the main tunnel were also used to delineate the upstream boundary of the collapse in the tunnel crown and laterally from the tunnel. From this work and observations downstream, it was possible to ascertain that there was an approximate 20 m wide crown void above the collapse material. It was not possible to determine the exact shape or extent of the void above the crown, but as shown in Figure 7, the progressing collapse and the final crown void shape where controlled and occurred upward into the upstream dipping FDZ. The drilling investigation programme also determined that there was limited lateral collapse away from the sidewalls of the tunnel. Based on uniaxial compressive strength (UCS) testing in the investigation holes, the rock mass on either side of the FDZ has very different strengths. Therefore, there is an inherent competency contrast associated with the lithologic packages adjacent to the FDZ:  
* Intact HART in the drill cores that were examined and tested have UCS values of around 25 MPa;
* AA cores have UCS values of around 75 MPa. In general, the downstream AA rock is much more competent than the HART. The rock on both sides of the contact between the two lithologic packages was damaged through localized faulting or bedding-type slippage (shown schematically in Figure 8), which occurred between the two mechanically contrasting rock types.