# Root Cause Analysis of a Collapse in a Hydropower Tunnel
### Collapse and Key Observations  
The collapse of the power or main tunnel occurred after the project had been in service for approximately 1 year. The scheme was shut down when the tunnel was completely blocked by the rockfall at the collapse zone. Debris was also noted for about 500 m downstream of the collapse, as shown in Figure 1. Approximately 12,000 m\({}^{3}\) of debris is estimated to have entered the power tunnel as a result of the collapse. The geometry or exact extent of the catastrophic collapse void could not be accessed or inspected directly because the throat was blocked, and it was unsafe to excavate the debris (see Figure 1).  
A second, much smaller collapse (estimated at about 40 m\({}^{3}\)) was also observed about 50 m downstream of the main collapse (Figure 1). There were also a number of areas on the tunnel crown and sidewall above the debris deposition zone downstream of the main collapse where significant shotcrete detachment and minor rock spalling were observed. Minor spalling and cracking of the shotcrete were also noted immediately upstream of the collapse and in several other areas located away from the collapse zone.  
Post-failure geotechnical holes were drilled to investigate the collapse zone and by-pass alignment. This geotechnical work concluded that the collapse void did not extend a significant distance laterally from the tunnel. The core was retrieved in holes drilled through the FDZ laterally and away from the tunnel side walls, but a void was intersected  
Figure 2: By-pass tunnel, collapse zone, and main power tunnel layout with geotechnical test holes and geological units encountered in test holes also shown in (**A**) plan view. (**B**) Photo showing white veins infilled with laumontite taken in the FDZ during construction of the by-pass tunnel.  
only above the crown (Figure 2). The by-pass tunnel was designed to be at least 30 m away (along strike) from the collapse zone in the main tunnel in order to avoid the collapse zone and the void in the crown. Given these large material volumes downstream of the collapse, the crown void must have extended a considerable distance above the crown of the tunnel. The exact shape of the collapse above the crown is, however, not known (shown schematically in Figure 1).  
With reference to Figure 2 at a high level, the prominent rock units immediately upstream and downstream of the collapse include the following:  
1. Hydrothermally altered red tuffs (HARTs);
2. Andesitic agglomerates (AAs);  
Within the collapse zone, the rock mass can be broken down further within a zone referred to in this paper as the fault-damaged zone (FDZ). Within the FDZ, the fault-damaged HART unit is observed to stratigraphically overlie a thin unit described as crystal lithtic tuffs (CLTs). The boundary between the HART and CLT is a discrete hard contact. The CLT unit has a gradational contact with the lower AA unit which is also fault damaged within the FDZ. The approximate distributions of these distinct lithologies (HART and the CLT grading into AA) relative to the collapse zone are shown in both Figures 2 and 3. Figure 2 also shows that the FDZ intersects the by-pass tunnel.  
Figure 3: Tunnel profile showing the fault-damaged zone (FDZ) and close-up of rock types in the collapse zone.