The second, and final, breakthrough in the 4.88m diameter inclined pressure tunnel excavations for Parbati II hydro scheme in Himal Pradesh, India has taken place.
Despite occasional water ingress to the bore – a greater problem more recently in the ongoing headrace drive – there were no serious difficulties for the Mitsubishi telescopic double-shield TBM excavating the pressure tube. The machine cut through 1,545m of what was called “practically a soft rock” – volcanites with bands of chlorite schist – according to MM Madan, a director for client National Hydroelectric Power Corporation (NHPC).
Excavation of the second shaft began in mid-July last year after the TBM had holed through the first tube, in mid-March, and was then removed for reassembly (T&TI, June 2006, p7). The second drive took 136 days with a best daily advance of 24m and furthest monthly progress, in August, was 388m.
While the top daily advance of the first drive, 22m, was not far off the second there was a gulf in the best monthly rate, only 232m having been achieved over almost 180 days. Progress on the first bore was not helped by clogging of the dewatering line, the mucking invert getting jammed and then floods disrupting site access.
Subcontractor Jaeger-Seli excavated both tunnels for main contractor Gammon India and work is underway to install steel lining in the first shaft to 3.5m i.d.. The first and second pressure shafts, bored at 48m centres, have total lengths of 2050m and 2120m, respectively, and the inclined stretches of both are 30° to the horizontal.
In the difficult headrace bore, being undertaken by Himachal JV (HJV), water ingress has interrupted work on face 4, according to brief details from the client. Almost three quarters of the 31.5km long headrace has been completed said NHPC but in November last year work was disrupted by large inflows.
HJV has 21.2km of the headrace to excavate by TBM and drill & blast in its Lot 2 contract. Some 9km of the headrace bore is being driven by a refurbished, 6.8m diameter open face hard rock Jarva TBM, with Robbins having been brought in early by NHPC to help HJV as subcontractor.
Progress has been slow on the tough job for reasons including variable rock, cutter wear and refurbishment needs, and overbreak calling for both extra steel support and concrete backfilling. Rock has varied from good gneiss requiring rockbolts and mesh to weak, jointed schistose gneiss with chlorite schist calling for heavy support and the need to make good the profile. The bore, varying from circular to horseshoe-shaped in section, will be concrete-lined to 6m wide.
Other tunnels included in the 800MW run-of-river hydro project are four feeder tubes, totalling 8.4km long. Parbati II is scheduled for completion in September 2009 with an estimated full cost of US$880M (Dec 2001 prices), said the client.
