# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 3 Factors That Contributed to the Catastrophic Failure
### Mineralogical Factor  
#### 3.2.1 XRD Testing and Petrographic Examination  
The systematic sampling of the core from the investigation holes and from the by-pass tunnel was used for XRD testing (Rietveld method and clay speciation [24]) and petrographic examination to determine the mineralogical make-up of these rocks and to quantify, as far as possible, the percentages of the various mineral assemblages present. Figures 6 and 7 provide, amongst other things, a general description of the rock types that are present in the area of the collapse and in the collapse zone (FDZ), and Figures 9 and 10 show the distribution and results of XRD quantification of key minerals within and adjacent to the FDZ in both the investigation holes and the by-pass tunnel. White minerals are visible in veins in all the rocks in the area around the collapse. The XRD testing and thin section examinations established that the white mineral is typically from the zeolite family, and the dominant zeolites present typically include laumontite, analcime, and wairakite (see [21] for more details on zeolites). The key factor here, however, is that the rocks present in the FDZ and the coincident collapse zone have an elevated amount of laumontite compared to the HART rocks upstream. Remember that the FDZ comprises fault-damaged HART rocks; weak, fault-damaged crystal lithic tufts (CLTs); and fault-damaged anddesitic agglomerates (AAs), all of which have zeolite-rich veins and filled fractures (Figures 9 and 10). The mineralogical differences upstream, downstream, and within the collapse zone can be summarized as follows:  
* As shown in Figures 9 and 10, the upstream HART rock has very little to no laumontite (and are therefore not reactive to dehydration and disintegration--see [21]) but has very high percentages of montmorillonite--a swelling clay [2; 3; 4; 5; 6; 7; 8; 10]. As discussed above, despite the presence of montmorillonite in the HART rocks upstream of the collapse, these tuffs were not affected during the collapse (i.e., montmorillonite was only elevated outside the collapse zone);  
Figure 10: XRD weight percent (%) vs. by-pass tunnel station samples. Figure 9: XRD weight percent (%) vs. distance from the FDZ from geotechnical test hole samples. Red numbers on x-axis indicate upstream distance from FDZ midpoint. * The AA rocks downstream of the FDZ also have an elevated presence of laumontite (Figures 9 and 10), and it is known from petrographic studies of the RCA that the laumontite in AA rocks is present primarily as an alteration product of plagioclase in euhedral mineral grains and (welded) grain-supported rock fragments. The significance of this is that laumontite in the crystals is largely protected to exposure to air or water and therefore is not as prone to desiccation or swelling;
* In the FDZ rocks, laumontite is abundant in interconnected ubiquitously developed veins and in the fine-grained clay-sized matrix between the veins. Unlike the AA rocks, these laumontite-rich veins and fractures are not protected from exposure to air or water. Critical behavioural aspects relating to the abundance of interconnected ubiquitously developed veins of laumontite in the FDZ and the collapse zone include the following:
* When exposed to air (during construction and during successive collapses), the laumontite in the veins loses water and desiccates, causing shrinkage cracks followed by the disaggregation of the FDZ rock as a whole. Ref. [21] confirms that dehydration causes the disintegration of laumontite;
* Laumontite can lose up to one-eighth of its water of crystallization and changes to a desiccated laumontite or secondary leonhardite with a corresponding shrinkage of 1.5% in volume [21; 25; 26; 27; 28; 29];
* Conversely, leonhardite can expand (although not to the same degree as montmorillonite) as it acquires crystal water through wetting and can revert back to laumontite [21].