EXCEPTIONALLY HIGH water pressures and difficult ground conditions are causing major delays on Singapore's deep tunnel sewerage system project, NCE has learned.
Contract TO5, the 12.6km, 5m diameter bore known as the Kranji tunnel, is one of a number of sections now up to a year behind as engineers battle with complex heterogeneous hard rock and soft ground tunnel faces.
Problems on TO5 have prompted a range of tunnelling methods, including ground freezing, to be used by local contractor SembCorp to overcome water pressures of up to 5 bar and to control the inflow of material during tunnelling.
Conditions have seen the spoil mucking out conveyor systems of the contractor's two Herrenknecht earth pressure balance tunnel boring machines (TBM) inundated by near liquid spoil. One machine is attempting to bore an 8km northern drive while the other works a 4km drive south.
Abrasive ground means that the TBM cutting heads need to be replaced frequently, sometimes after just 40m. However, high pressures have prompted the use of ground freezing rather than more conventional compressed air techniques to control the ground. According to sources on the project this maintenance often sees TBMs out of action for two weeks.
This time consuming procedure is necessary because Singapore safety standards and legislation bans the use of compressed air over 3.5 bar.
'At these kinds of pressures the working time is very limited anyway, ' a source told NCE.
'You can perhaps do twenty minutes work, and the compression and decompression times are very long.'
It is also understood that delays have been caused by transition zones in the rock forcing TBMs to cope with both hard and soft rock in the same tunnel face. Such zones were frequent in the first 200m of tunnelling where hard and soft rock were often just 2m apart.
However, client for the project, the Singapore government's Ministry of the Environment, was unwilling to comment on delays this week. Supervising engineers Parsons Brinckerhoff and CH2M Hill also declined to comment.
The Kranji tunnel was to be completed by March 2004 as part of the giant project to construct a single sewer across Singapore to Changi in the south east of the island. In total this will see some 80km of up to 6.5m diameter tunnel built at depths up to 50m, plus another 170km of smaller tunnels. The first 125km should be complete by 2008 with the entire project open by 2015.
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