# Root Cause Analysis of a Collapse in a Hydropower Tunnel
###### Abstract  
This paper describes the investigation and findings from the root cause analysis (RCA) of a significant collapse that occurred in a hydropower tunnel at a confidential location. This collapse involved about 12,000 m\({}^{3}\) of material being deposited in the tunnel from a narrow 20 m width failure zone encountered in the haunch and crown area of the main power tunnel. This paper describes contributing factors which include the following: (1) degradation of a highly zeoliitized (laumontite-rich) zone of rock within a bedding concordant fault zone, termed the fault-damaged zone or FDZ; (2) relatively high in situ rock stresses concentrated in the haunch and crown area of the collapse zone in the tunnel; (3) large transient water pressure differences in the rock above the collapse zone and upstream and downstream of the collapse zone; (4) cyclical repetition of the above-described factors resulted in the propagation of crown and sidewall collapse in and around the FDZ. Lessons learnt on this project and other projects with similar durability problems in volcanic rock are distilled in this paper. It is hoped that advances made in the understanding of the failure mechanism at the unnamed tunnel can be included in future tunnel investigations and design in volcanic rocks.  
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main power tunnel is shown in Figure 2. The by-pass tunnel intersected the same sequence of rocks (or FDZ) that was present in the collapsed zone in the principal or main power tunnel. This exposure was a result of the NW-SE strike of the meta-volcanic sequence of rocks, i.e., the two tunnels cross the FDZ as a result of the orientation of the tunnels and the general strike of the volcanic units. The following data collection and testing were completed as part of the RCA:  
* Detailed geological mapping and observations of rock conditions within the by-pass tunnel at the location of the same geological sequence as the collapse zone of the main power tunnel;
* Geotechnical drilling and core logging from an array of holes drilled from the upstream side of the collapse in the main tunnel;
* Collection of samples from the bore holes around the main tunnel and by-pass tunnel for XRD testing, clay-sized speciation testing, petrographic examination, and swell pressure testing;
* Detailed mapping of the debris pile as the failure debris material was removed downstream of the collapse.  
Figure 1: Tunnel profile with main collapse, collapse debris, and estimated collapse zone void shown. “\(7\)” indicates inferred top of main collapse debris.  
Figure 2: _Cont._  
Following data analysis, it was clear that the initiating cause of the collapse was the result of interplay between the mineralogy and geological structure within the 20 m wide failure zone that overload the tunnel liner. The analysis also showed that a combination of other factors (other than the mineralogical factor) contributed to the collapse.  
It should be noted that the interaction of the factors described in this paper was undetected and unforeseen at the time of construction despite underground mapping on a round-by-round basis and prescribed ground support being installed prior to tunnel advance. The contributing factors are discussed below in their order of significance to the collapse.