Sungei Pandan pipe jack completed ahead of schedule

The section of tunnel under the Sungei Pandan estuary was technically very challenging because of difficult ground conditions and a challenging combined horizontal and vertical curved alignment, which placed high demands on the tunnel boring machine (TBM) and on the project team. Rigorous risk assessments and detailed mitigation/contingency plans were undertaken at different stages of the project and the measures required to avoid and/or mitigate the risks were promptly implemented.

This forward planning is seen to be the main reason for the successful completion of the tunnel, ahead of schedule and without incident.

The risks related to the construction of shafts and tunnels were varied. Three shafts were constructed as part of the project to launch and receive the TBM with two of these shafts subsequently being incorporated into the permanent works.
The main risks related to the shaft construction were:

• Shaft wall instability from recent fill and soft soil materials;
• High ground water table;
• Shaft base heave;
• Limestone rock with fissures and cavities that have direct connection to the sea;
• Variable rock head level; and,
• Proximity to adjoining industrial premises and sea.

The design of the shaft took full account of the above risks. The most notable mitigation measure to prevent flooding of the shaft was to install a 2.7 m thick ‘jet grout’ temporary base slab prior to the excavation of the shaft. This work was done by a specialist sub-contractor to a very high standard using computer aided methods. The benefit of this slab became evident during excavation of the second shaft where a discrete leak generated inflows measured at 200 L/min. This leak was successfully repaired without causing any damage or settlement to the surrounding areas and if not for the jet grout the shaft would have flooded. Similar conditions were also experienced with the third shaft.

The tunnels meanwhile were constructed in two sections to suit the site constraints, using a 2 m ID EPB TBM up-skinned to the 2.4 m ID required for the project. The first section is a 57 m long road crossing. The second section is 605 m and crosses the Sungei Pandan estuary. The first section also served as a trial tunnel to study the performance of the TBM in the soft clay and sandy silt soils.

Construction of the second section was extremely demanding not only because it was under a navigational water way but also because of difficult ground conditions and stringent alignment control requirements. The alignment included two vertical curves of 700 m radius for the tunnel to dive under the river, together with a horizontal curve of 325 m radius and nearly 300 m in length, turning the tunnel through more than 50 degrees. The tunnel alignment was also required to deviate no more than 100 mm along the property boundary line that it followed onshore.

The main risks associated with this tunnel section were:

• Variable ground conditions (hard limestone rock and very soft soils);
• Tunnelling under a navigational channel and estuary;
• Limestone rock with fissures and cavities and direct sea connection;
• Vertical and horizontal curves along the pipe jack tunnel route; and,
• Sand, gravel and clay soils that can exert high friction on the jacking pipes.

Additional ground investigations were conducted at 10 m intervals through the marine section to better detail the rockhead profile. The results of this investigation showed that several pinnacles of limestone rock would be met along the tunnel route thus placing approximately 30 per cent of the tunnel route in part or full limestone rock. The presence of limestone rocks in Singapore was only recently identified and the geological maps prepared by PWD in 1976 (and used extensively by local engineers) do not show their existence.

As the Singapore limestone rocks are abrasive and extremely water bearing, it was envisaged that excavating through such a long length would require cutter tool changes under the river with the use of compressed air. Such work would need a stoppage of one to two weeks, depending on the severity of wear. Any prolonged stoppage could cause the pipes to become stuck in the ground and not be pushed any further.

This major risk was actively discussed between all parties and it was agreed to continue with the cautious approach to the management of ground and TBM risks. The solution agreed upon was to maintain the curved horizontal alignment and adjust the vertical alignment to minimise the rock interfaces, while at all times maintaining the required 5 m clearance between the tunnel and the river bed above.

Frequent survey checks were carried out to ensure any tendency for misalignment was detected early and corrective action was applied in a timely manner. The length of the jacking pipes was also reduced from the preferred 3 m to 2 m to negotiate the 325 m radius horizontal curve. Rigorous hydrostatic testing of the pipe joints was undertaken prior to fabrication to ensure water tightness.

The TBM supplier for the project was Herrenknecht AG, who also provided the operators. On their recommendation, additional wear plates were welded on to the TBM cutterhead (as a precaution against excessive wear and to protect the discs from impact loads in the mixed face conditions) immediately before the excavation of the under river section. As a result the drive was completed without the need for cutter changes.

The ground risks were mitigated by:

• Having a compressed air unit brought over from Germany in case access to the chamber of the TBM was needed;
• Installing six inter-jack stations to ensure no problems with ground friction, extended stoppage and steering through mixed face conditions;
• Using an automated lubrication system for the jacking pipes; and,
• The use of polymer injected to the TBM face that mixed with ground water and permeable ground (sand, gravel and limestone chippings) to form a plug in the TBM screw conveyor to keep the water out.

Project delivery
A major factor of the successful delivery of the project was the dedication and enthusiasm of the tunnelling project team.
The maintaining of a clear focus on the project objectives and the ongoing collaboration amongst client, consultants, contractor, designers and site supervision staff was a key factor that helped to realise the safe completion of the Sungei Pandan pipe jack tunnel.

The Sungei Pandan 400 kV cable tunnel is an excellent example as to how a difficult project can be successfully delivered to a high quality standard, whilst mitigating project risks and yet still complete ahead of schedule.

This article was published with kind permission of Trenchless Australasia magazine.