ESCI Seminar Series: TBM Tunnelling in the Himalayas, Tapovan-Vishnugad Hydroelectric Power Plant, India

Date: 5 March 2014 Time: 12.00 pm

Speaker: Bernard Millen, GEOCONSULT Consulting Engineers, Salzburg, Austria

The 520MW Tapovan-Vishnugad Hydroelectric Power Plant, a run of the river scheme, is being constructed by NTPC Ltd., India in the Himalayas. As part of this project a circa 12.1km long Head Race Tunnel (HRT) is being excavated, out of which circa 8.6km are being mined by a Herrenknecht double-shield Tunnel Boring Machine (TBM). The rest of the HRT is been constructed by drill & blast methods. GEOCONSULT is acting as a Consultant to NTPC Ltd. for the TBM part of the HRT. The TBM has an excavation diameter of 6.57m. The internal finished diameter of the HRT is 5.64m. The reinforced hexagonal segmental concrete lining – installed during TBM driving - has a thickness of 300mm. The annulus between the ground and lining is filled with pea gravel and subsequently grouted. During plant operation, the HRT will in fact be a pressure tunnel and in the area close to the surge shaft - above the power house - internal water pressures will reach circa 9-10bars. The project area lies within the Dhauliganga and Alaknanda Valleys in the state of Uttarakhand, India and consists of high strength medium to high grade metamorphic rocks belonging to the Central Himalayan Crystalline Series (Heim & Gansser 1939, for a recent review see Yin 2006). Overburden above the HRT reaches 1100m and at the start of the project the ground was considered to basically consist of jointed high strength quartzite, gneiss and schist. The tender documents mention faults and possible water-inflow, however, these subjects were not specifically dealt with due to limited outcrops and accessibility along the HRT alignment and according to contract the contractor was to make his own assessment about the geological conditions prevalent in the project area.

To date three TBM trapping events - all associated with subsurface water in-flow - have severely hampered the HRT excavation resulting in time delays and cost increases. The first trapping event occurred in December 2009 at chainage (Ch) 3016m at a depth of some 900m in a heterogeneous fault zone (Brandl et al. 2010; Millen & Brandl 2011). During trapping the front & outer telescopic shields were jammed in and dented by major wedge slides. Approximately 24hrs later, massive surges of high pressure subsurface water, containing faulted rock material, broke two crown segments of the segmental lining immediately behind the tailskin with the initial flow rates reaching circa 700L/s compounding the trapping problem. The second and third trapping events – which to date have not been reported on – happened in February & October 2012 at Ch 5840 & 5859m respectively in the same circa 20m wide fault zone at a depth of some 700m. This fault zone lies at a very acute angle to the tunnel axis meaning the TBM had (will have) to drive through this zone for at least 35m. At the time of writing the TBM was still trapped at Ch 5859m. When the second event occurred, the face and surrounding conditions were initially dry and due to over excavation and collapse a cavity of several cubic metres had developed around the cutter head and front shield of the machine in the soil-like stiff clay-rich fault gouge & breccia of which the fault zone consists of. As in the first trapping event, water inflow (1-2L/s) started some 20hrs later. The situation then greatly deteriorated as the water rapidly eroded the water sensitive fault gouge & breccia causing further cavity development, ground creep and ground in-flow through the cutter head and shield openings trapping the TBM.

Described are investigation methods and exploratory programmes instigated to assist the TBM recoveries. Geological & hydrogeological models of the trapping mechanisms are presented. The requirements of site investigations for tunnelling in mountainous areas are also considered. Due to adverse geology, tunnel or underground construction in active mountainous areas is notoriously difficult and presents special challenges. It is therefore important that geological surveying, to stop geological surprises and allow for preventive measures to be established, is sufficient during the feasibility and design & tendering phases of a project. In particularly for a TBM driven tunnels - which are not as flexible as a conventionally driven tunnels - forward destructive probe drilling from the tunnel face is certainly not an alternative to an adequate pre-investigation. However, considering the planned development of underground infrastructure in the Himalaya and other mountainous areas of the world in the near future, more rigorous efforts are required to overcome the shortcomings in tunnelling geological investigations in mountainous areas.

The lecture is intended for the wider audience - geologists, engineers & infrastructure planners - but in particularly for students of the same branches.