TUNNELLING OF THE £177M Docklands Light Railway extension under the River Thames to Woolwich Arsenal, south east London, has hit delays, it emerged last month.
Contractor Amec started the first of two 2.5km long drives with a bespoke Lovat earth pressure balance TBM in May.
The tunnel was meant to arrive in Woolwich in September but it is understood the machine has suffered heavy wear to its picks while passing through a stratum of flintladen chalk.
An Amec spokesman said the picks had now been replaced and the machine restarted. 'It is churning out 20m of progress per 12 hours at the moment, which is the best rate of production we have achieved to date, ' he said. 'The drive is behind schedule but making up time.'
Completion of the fist bore is now expected next month.
Amec predicted that abrasion from the flints could pose a problem and designed three locations at which the TBM could be stopped for repairs.
It used permeation grouting to stop up water bearing fissures in two areas north and south of the river.
The third location was a ventilation shaft on the south bank of the Thames.
If the TBM suffered excessive wear, these three refuge points would have enabled Amec to access the cutting head and replace worn picks, without being inundated with water. Even so, the operation would have required the use of compressed air.
It was unclear where the TBM had stopped, or what the impact on schedule was likely to be.
The TBM was to have been stripped down and shipped back to the north bank of the Thames, ready for the second drive in December.
A spokeswoman for client Transport for London said it had been reassured lost time would be made up and the project would still be completed by early 2009.