# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 3 Factors That Contributed to the Catastrophic Failure
### Mineralogical Factor
#### 3.2.2 Laumontite and Swelling Clay: Experience on Another Project
In the subject tunnel, as described above, a major collapse occurred in a narrow 20 m wide zone with very high laumontite percentages, certainly higher than those observed in the Lesotho transfer tunnel of around 10%, where damage was observed [25; 27; 29]. The FDZ (collapse zone) is, as described above, a localized very discrete zone that is the result of inter-bed slippage within a highly active faulted and folded mountain building tectonic environment, and this is very different to the geological environment in Lesotho (extrusive lava pile of around 1800 m thick and zeolite zonation due to paleo-geothermalgradients in the basaltic pile found in Lesotho) and elsewhere in other basaltic terrains globally [27; 39; 40; 41; 42]. At the confidential subject tunnel, the rock types are a result of explosive volcanic activity and subsequent mountain building processes and are not directly comparable to Lesotho but nevertheless are subjected to hydrothermal and low-grade alterations, with the development of laumontite as one of the end products of this metamorphism [30; 31; 32; 33; 34; 35; 36; 37; 38]. The zeolite zonation model and mineralogical criteria developed to address durability issues in Lesotho [25; 27; 29] are likely not universally applicable in all volcanic environments or rock types. The criteria developed in Lesotho are summarized below in Table 1. Table 1 is only partly applicable in the subject tunnel. This RCA has shown that rocks upstream of the collapse (HART rocks) have the highest swelling potential, yet these rocks are still standing, while rocks with much lower swelling potential and excessive laumontite (the FDZ) collapsed. The RCA also shows that the AA rocks downstream of the collapse have elevated levels of laumontite, similar to that encountered in the FDZ, and these rocks did not collapse. Reasons for this are proposed in Section 3.2.1. While the FDZ rock in the collapse zone has very similar and elevated levels of laumontite when compared to the AA rocks downstream of the collapse, the latter are competent and still standing. These subtleties were not present in Lesotho, and it has to be concluded that the tunnelling industry, in general, has not developed a consistent approach to dealing with these laumontite-related durability issues yet, principally because the failure mechanism is only partially understood (as described above). The durability issue related to laumontite manifests in different projects at different scales and different intensities. As a consequence, direct knowledge from Lesotho or the subject tunnel can only be used as an indicator and does not provide explicit design methods that fit all circumstances. Nevertheless, parts of Table 1 remain applicable as evidenced by the fact that while laumontite was present throughout much of the subject tunnel, these zones were below the empirical 10% laumontite by weight, above which durability issues may manifest. The lesson learnt from the subject tunnel RCA is that the FDZ had laumontite percentages significantly greater than 10% and had extreme durability issues, but the AA rock downstream of the collapse has similar elevated levels of laumontite, indicating that the collapse could not have been predicted purely on the basis of the presence or percentages of laumontite. Section 3.2.1 describes the petrographic difference between the occurrence of laumontite in the FDZ and the AA rocks and why the AA rocks are less susceptible to disintegration. It is also known, from the Lesotho experience, that laumontite-rich zones, associated with poor durability, can have a vertical thickness of more than 200 m, and the same problematic zone can extend for tens of kilometres in a lateral or horizontal sense [25; 27; 29]. It is known that the zonation of zeolites in basalt terrains is a function of the paleo-geothermal gradient present after extrusion, and this, in turn, is a function of the extent and thickness of the original basalt pile [25; 27; 29].