# 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  
During construction, the presence of both laumontite and montmorillonite was known in the subject tunnel. There were many expert opinions on how to deal with the presence of these minerals during construction. The support levels installed generally considered the presence of these minerals and overburden stress levels, but as shown in Figures 6 and 7, this was not successful. A part of trying to understand the potential impact during construction included routine XRD and swell pressure testing during construction in order to determine the abundance of these mineral, in particular montmorillonite, and the degree of swelling that may be anticipated as a result of these analyses. The reason attention was paid to the minerals present in the rock during construction was because a few other projects around the world have had problems with swelling clays and swelling pressures, e.g., refs. [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. A well-known tunnel project in southern Africa (the transfer tunnel of the Lesotho highland water scheme) had degradation problems along at least half of its 45 km length, and it was determined to be as a result of the presence of laumontite-rich zones as opposed to swelling pressure problems, see [25; 26; 27; 28; 29]. Familiarity with the Lesotho experience and experience on other tunnels in South America provides evidence that there is still no clear consensus within the tunnelling industry worldwide regarding the approach that should be adopted when designing for suspected mineralogically induced durability issues in rocks with laumontite and/or montmorillonite [25; 26; 27; 28; 29]. The reasons for this are believed to be that the mechanism involved with durability issues related to laumontite in particular is complex and is still not fully understood, as described above. Ref. [21] provides important advances in understanding issues related to the dehydration-rehydration mechanism of laumontite but does not fully resolve the mechanism at the tunnel presented in this paper or in the Lesotho experience. Despite not being fully understood, the common link between the subject tunnel and the Lesotho experience is the presence of elevated laumontite where issues occurred, as described in [25; 29]. It is also clear that mere knowledge of the presence of these minerals does not mean that there is a rational design basis used globally that should have been followed at the subject tunnel. While it is recognized that the basaltic rocks in Lesotho are not the same as the rocks in and adjacent to the FDZ at the subject tunnel (volcaniclastic rocks as described above), the circumstantial evidence from both geological environments does indicate that the presence of laumontite above certain proportions can exacerbate degradation or reduce rock durability. As discussed above, these signs were recognized in the subject tunnel, and additional support was provided where both swelling clays and zeolites (laumontite belongs to the zeolite family of mineral) were identified or suspected, but as evidenced by the collapse, the support levels were inadequate for the unforeseen conditions (elevated laumontite in the FDZ). It is also evident from the Lesotho experiences [25; 27; 29] that the tectonic environments and rock types are very different (both are volcanic in origin), but the durability problems also manifested in a completely different way in the Lesotho transfer tunnel. Degradation occurred over a length of 20 km in the Lesotho tunnel [25; 27; 29], with disintegration in the tunnel walls in a largely unlined water conveyance tunnel, but without any major collapses. This occurred as a result of the tunnel intersecting a regional-scale sub-horizontal laumontite-rich zone, which is the result of a vertical zeolite zonation within the relatively undisturbed basaltic volcanic pile [25; 27; 29].