TUNNELING OF the £210M Docklands Light Railway extension across the Thames from King George V Dock to Woolwich Arsenal is now underway, contractor Amec said this week.
The 6m diameter Lovat earth pressure balance tunnel boring machine (TBM) was assembled in its launch pit last month (NCE 6 April).
Following commissioning, it broke through the glass-fibre reinforced end wall of the pit two weeks ago.
The TBM's 10m long cutter head is now fully buried, deep in river terrace deposits as it drives forward and down (see diagram).
Progress will increase to an estimated 3m/day towards the end of this month, once the TBM has advanced enough to allow erection of a belt conveyor behind it, said Amec tunnels director Peter South. This will speed spoil removal from the face.
On its drive south, the TBM will pass through alluvium, river terrace deposits, fissured 'flint and water-laden' chalk, and the Thanet sands.
The chalk will pose the greatest challenge, said South.It will confront the TBM with high water pressures and the potential for major wear on the cutting face.
Water pressure at the bottom of the launch pit is 1 bar, and Amec is expecting to encounter pressures of 3.5 bar at the tunnel's lowest point, 35m beneath the Thames.
To combat wear meted out on the TBM's cutters by the flint cobbles, manufacturer Lovat has built a one-off head for the job, as specifi ed by Amec, said South.
Long reach cutters are coupled to hydraulic wear indicators so that as the picks are ground down, hydraulic pressure within the cutters falls, alerting Amec to the need for replacement.
South said that unlike a conventional TBM head, this model is equipped with doors that can be closed to seal off the cutting face. This will enable access to the area behind the cutter head for inspection and repair, while maintaining earth pressure to support the face itself.
'For cutter head inspection we aim to stop the machine at three chainages on the drive where we hope to find dry ground. This will enable us to drain down the chamber so that we can get men in to carry out work, ' said South.
Amec hopes to avoid using compressed air to access the face especially in the ssured chalk section were there is too great a risk of pressure loss, he added.
'We have compressed air arrangements on standby, but the use of compressed air is far from ideal in this kind of ground, and there are serious health and safety considerations: At 3.5bar working time is limited, and decompression times are large.' The 1.8km long drive is due to complete in September. Amec will then ship the TBM back to the north bank of the Thames to carry out a parallel bore, starting in January.