# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 4 Successive Failures and the Additive Effect of Tunnel Pressure Changes  
While the initiation of the collapse is largely ascribed to the mineralogical factor, the propagation of the collapse and full choking or blockage leading to the shutdown of the scheme is likely as a result of the cyclical repetition of the mineralogical factor, the pervasive in situ- stress field, and as a result of changes in transient water pressures. Figure 15: Loads on the elastic and plastic liners compared to the strength envelop with shotcrete and bolts in the collapse zone for (**A**) elastic and (**B**) plastic (using approach presented in [48,49]). While an operational hydraulics analysis is beyond the scope of this paper, observations in the tunnel post collapse suggest that successive rockfalls in the FDZ may have been a contributing factor, resulting in high transient water pressure differences (with guidance provided by [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49]) in and around the collapse zone, and these pressures helped extend and expand the collapse zone (shown schematically in Figure 7). The following probably contributed to the propagation of the catastrophic collapse, leading to the final project shutdown:  
* As described in Section 3.2.3 and Figures 11 and 12, the swell pressures are highest in the HART rocks and close to or at the transition into the FDZ. It is also apparent when these results are compared with the mineralogical plots (Section 3.2.1 and Figures 9 and 10) that the HART rocks have the highest swelling clay percentages and the lowest laumontite percentages;
* The tunnel is below the hydraulic grade line [22], so the tunnel would have been under positive pressure during operations;
* When the initial and successive rockfalls occurred and partially blocked the tunnel, the velocities above a particular partial blockage would have increased significantly, and the higher velocity would be able to transport material downstream from the collapse zone. This material was deposited downstream of the blockage, where the velocity drops, as shown in Figures 1 and 6. This process continued as the collapse propagated upwards and more material collapsed in a series of rockfalls, with the higher flow velocities preventing the collapse zone from fully \"choking\" and stopping water flow. This is believed to be part of the reason why the volume of the collapse debris is so great;
* After each rockfall occurred and the tunnel partially filled with debris (as shown in Figure 7), the velocity increased at the collapse (partial choke point) as described above. This would have had the effect of lowering the pressure in the tunnel via the Venturi effect. The pore pressure in the rock (in particular, in the FDZ) would remain the same as before the collapse, so the differential pressure would destabilize the material in the fault-damaged zone, causing further collapse;
* At the final blockage, the tunnel downstream of the collapse is understood to have briefly completely drained downstream of the collapse; thus, the pressure in this section of the tunnel would have been lowered to atmospheric pressure or less. This unplanned depressurization would further result in a pressure differential between the rock around the tunnel and the interior of the tunnel, which would exacerbate the shotcrete and rock spalling. This may have caused the downstream so-called'second collapse zone'(Figure 1) in conjunction with the relatively high in situ stress field, where the fall-out material was observed to be sitting on top of the debris from the main collapse. While it is not known when individual rockfalls occurred, it is understood that the blockages only became significant enough to be detected in the degraded performance of the scheme a few days prior to the shutdown.