# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 3 Factors That Contributed to the Catastrophic Failure
### Mineralogical Factor
#### 3.2.3 Swelling Pressures in Support of Mineralogical Factors  
Swelling pressure assessments were made on the basis of laboratory testing on samples of rock from the by-pass tunnel construction and from core samples from the investigation holes around the collapse. Details of the laboratory methods are beyond the scope of this paper, but for completeness, the authors note that the following references were reviewed and used when undertaking testing on powdered samples [2; 7; 10; 21; 44; 45; 46; 47]. These data were used to help analyze the adequacy of the support for the RCA and the later design of the by-pass tunnel. These swelling pressure results of these tests are presented in Figures 11 and 12. Details of the design of the by-pass tunnel are also beyond the scope of this paper given that the focus is on the RCA of the collapse. It is sufficient to say that the authors of this paper were responsible for the by-pass tunnel design. Two types of tests were used in the course of the RCA investigations to estimate the potential swell pressures of the rock surrounding the subject tunnel. These include the following:  
* Constant volume swelling pressure tests carried out in the laboratory on powdered rock samples. These data are referred to as expansion swell pressures (ESPs); and
* Use of a project-specific correlation between swelling pressures and the ethylene glycol (EG) state and liquid limit (LL). These data are referred to as chart (EG/LL) design swell pressures (CDPs) in this paper. While the details are beyond the scope of this paper, the ESP method was found to provide a conservative estimate of actual rock mass swell behaviour and has been found to give generally higher result than the CDP test method. For those interested in the details of testing for swelling pressure in rock, please see [2; 7; 10; 21; 43; 44; 45; 46; 47]. The ESP swelling  
Figure 11: Swell pressure data vs. distance from the FDZ from the geotechnical test hole sample. Red numbers on x-axis indicate upstream distance from FDZ midpoint. pressures were de-rated by a site-specific factor of 0.6 to achieve better equivalence, with the latter signifying CDP-correlated values. Once brought into equivalence, the resulting values from either method are considered comparable and were used for ground support design in the by-pass tunnel. It is important to note that while this section of the paper is focused on swell pressures, for the by-pass tunnel design, the increase in loading on the liner as a result of laumontite-induced FRD rock degradation was also considered, not just swelling pressures. The results summarized in Figures 11 and 12 are important to the RCA because they indirectly corroborate the mineralogical factor considered critical to the initiation of the collapse in the main tunnel, i.e., swell pressures alone cannot be used to understand the collapse, and the mineralogical factors described above provide the key to understanding the initiation of the collapse. Figures 11 and 12 also provide useful insights that lend credence to the hypothesized failure mechanism in the collapse zone. Figures 11 and 12 show that while there is scatter in the data, the following conclusions can be drawn when referenced against mineralogical data for the same area (see Figures 9 and 10):  
* The swell pressures are highest in the HART rocks and close to or at the transition into the FDZ. The average swell pressure (average of both ESP and CDP results combined) in this zone is approximately 0.64 MPa in the by-pass tunnel and 0.4 MPa in HART rocks upstream of the collapse zone. It is apparent when these results are compared with the mineralogical plots that HART rocks have the highest swelling clay percentages and the lowest zeolite (laumontite) percentages;
* It is clear from these data that the measured swell pressures are directly proportional to the recorded swelling clay content, i.e., the higher the swelling clay content, the higher the swell pressure. Figure 12: Swell pressure data vs. by-pass tunnel station samples.