Work is underway in north India to repair the recovered TBM in the headrace tunnel of the Parbati II hydro scheme and plan consolidation of the weak ground ahead, which engulfed the machine last year. However, the work is also awaiting cost negotiations to be completed, T&TI understands.
Following the slurry inrush in November 2006, and months of recovery effort, the deadline for the construction of the entire power project in Himachal Pradesh province has been re-confirmed as September 2009 (T&TI, January, p9) – a reversion to the original programme. Less than half of the 9.05km drive has been excavated.
The 6.8m diameter open face hard rock Jarva TBM was excavating from Adit 2 to drive Face 4 of the headrace tunnel when inrushes of water and silt brought that work to a halt last year. The inflow of water continues to be significant, the rate reduced to 5000 litres/minute, and major drainage efforts continues, the client National Hydroelectric Power Corp (NHPC) briefed T&TI.
In total, just over 21.3km of the 6m diameter headrace is to be excavated by contractor Himachal JV using the TBM as well as drill and blast in its Lot 2 civil and hydro-mechanical works contract. In early 2005, Robbins was enlisted by the client to help HJV as subcontractor with the TBM drive. The client had previously supplied the TBM specification and geological data.
Before the TBM was inundated it had completed just over 4km of the drive. Cover along the section – between adits 1 and 2, which is mostly the TBM drive – ranges from 800m-1350m. There have been rock bursts but the worst geological problem though has been the weak, saturated ground that led to silt-laden inflows of up to 7000 litres/minute last year, burying all vital parts of the TBM.
The client said that before work can resume, plans are underway to consolidate the weak ground ahead of the recovered shield. NHPC declined to provide information on the overall cost issues under negotiation. Robbins confirmed that the TBM had been recovered and repairs were underway, but neither it nor HJV were prepared to comment further.
NHPC said that testing and commissioning work on the TBM was due to be completed this month. However, it will not be until the ground consolidation method, subcontractor and trials are in place that NHPC can assess how much longer the drive may take to complete. The TBM drive has another 5km to bore.
In total, the headrace will be 31.5km long and drill and blast is also being used. A number of faces have been completed but work still continues on some, such as on Face 3, which is only slowly closing in on the stalled TBM from the opposite direction due to difficult ground. It is not due to take on additional works. The Face 3 drive has less than 800m to go, and the Face 8 drive a bit less.
Work is underway to enable the TBM drive to resume at Parbati II, India Buried TBM at Parbati II
