# 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
It is contended that the desiccation of laumontite leads to the weakening and disintegration of the rock in the FDZ when exposed to the atmosphere (with confirmation provided by [21], who describes the mechanism of dehydration and disintegration of laumontite in some detail). This disintegration is followed by some minor swelling as desiccated laumontite reverts to its hydrated state [21]. This mechanism is further described below in Section 3.2.2 below [25; 26; 27; 28; 29], with reference to the Lesotho transfer tunnel experience. This cycle repeated itself during initial tunnel filling and subsequently with successive collapses. This is described in more detail below and in Section 5. The mineralogy of the rocks in the FDZ is not the only factor responsible for the collapse. However, it is considered to be the most important factor with respect to the initial shotcrete spalling, and it limited the initial width of collapse, as described above and below. Within and outside the FDZ, the nature of the zeolites and other minerals can be summarized as follows:  
* XRD testing and petrographic examination in and around the collapse also showed that laumontite is preferentially deposited in veins in the FDZ, and this may be related to differing temperatures and pressure conditions within the FDZ rocks during faulting based on review and knowledge gained from [30; 31; 32; 33; 34; 35; 36; 37; 38]. While it is beyond the scope of this paper, readers interested in detail on the effect of temperature, pressure condition, and stability conditions for zeolites and other low grade metamorphic mineral are recommended to review [30; 31; 32; 33; 34; 35; 36; 37; 38];
* This is in contrast to the HART rocks, in which several species of zeolite, namely wairakite and analcime, and a very low percentage of laumontite are present in veins;
* Wairakite, analcime, and other zeolites are present but are much less susceptible to desiccation [21], which is likely why the HART rocks have higher swell potential than the FDZ rocks (due to montmorillonite absorbing water, see [1; 2]) but do not disintegrate like the FDZ rocks. It is also apparent that a lot of the veins present in the HART rocks contain phyllosilicates that are not expansive clays, e.g., chlorite and muscovite;* The high montmorillonite percentages found in the HART rocks occur primarily in the very fine-grained matrix material, and the matrix material is bound together with hematite and other metallic oxides that may slow down the rate of swelling;
* The significance of this is that while minor laumontite is present in the HART rock, it is likely less available to air or water exposure compared to the FDZ rocks. While the HART rocks are prone to swelling because of the high montmorillonite content, this is likely a relatively slow process due to the lower permeability of the HART rocks [10];
* Laumontite is present in similar percentages in both the FDZ rocks and the AA rocks. Analysis showed that laumontite is locked into the welded rock fragments and altered plagioclase crystals in the AA rocks, and therefore, only a small proportion of the laumontite is available to react with air. While mostly below detection levels, montmorillonite is also likely present in the AA matrix but is again not readily available to react with water because the matrix material between the rock particles is typically not connected (based on thin section observations), thereby resulting in a fairly inert rock that is not prone to swelling. Another important point of clarity from the XRD clay speciation work and petrographic work is that the FDZ rocks, while pervasively zeolite veined, do contain sporadic montmorillonite in the fine-grained fragments present between veins, particularly the FDZ material of HART origin, albeit in significantly lower proportions compared to the HART rocks upstream of the FDZ. It is also significant, as indicated above, that the fine-grained matrix in the FDZ contains laumontite, which is also prone to desiccation.