TUNNELLING OF the £177M Docklands Light Railway extension under the River Thames to Woolwich Arsenal, south east London, has hit delays, NCE has learned.
Contractor Amec started he first of two 2.5km long drives with a bespoke Lovat earth pressure balance tunnel boring machine (TBM) in May (NCE 11 May).
It was meant to arrive in Woolwich last month.
But it is understood that the machine suffered heavy wear to its picks while passing through a stratum of flint-laden chalk.
An Amec spokesman said that the picks have now been replaced and the machine been restarted.
'It's churning out 20m of progress per 12 hours at the moment, which is the best rate of production we've achieved to date.
'The drive is behind schedule but making up time, ' he said.
Completion of the first 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 as NCE went to press 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 to start the second drive in December.
A spokeswoman for client Transport for London said that it had been reassured that lost time would be made up, bringing the project in on its original completion date, early 2009.