EXCEPTIONALLY HIGH water pressures and difficult ground conditions are causing major delays on Singapore's deep tunnel sewerage project.
Contract TO5, the 12.6km, 5m diameter bore known as the Kranji tunnel, is one of several sections running up to a year behind as engineers battle with complex hard rock and soft ground tunnel faces.
Local contractor SembCorp has used a range of methods, including ground freezing, on TO5 to overcome water pressures ofup to 5 bar and to control the inflow of material during tunnelling.
Conditions have seen the spoil mucking out conveyor systems of the two Herrenknecht earth pressure balance TBMs inundated by near liquid spoil.
One machine is boring an 8km northern drive while the other works on a 4km drive south.
Abrasive ground means the TBM cutting heads need to be replaced frequently, sometimes after only 40m.
According to sources on the project this maintenance often sees TBMs out of action for two weeks.
It is understood that delays have also 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 only 2m apart.
The client for the project, the Singapore government's Ministry of the Environment, was unwilling to comment on delays.
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.