In order to renew the existing aged extra high voltage (EHV) transmission cable network as well as expand and enhance the power supplies to the central and southern parts of Singapore, SP PowerAssets is embarking on a major tunnel infrastructure construction programme. The tunnels will enable progressive installation of EHV cables and facilitate future maintenance or replacement of the cables with relative ease. The tunnels are of 6m internal diameter and approximately 35km long. They are planned for a 100-year design life with provision for 10 EHV circuit cables will enter and exit at the termination points as well at various shaft locations along the route to maximize usage of the tunnels.

Tunnel design
Tunnel and shaft layout arrangements have been developed to accommodate the different types of cables and circuit configurations. The internal space is sized to accommodate both oil-filled and XLPE (cross linked polyethylene) cables and joints within the tunnel.

The cable tunnel itself is separated along its entire length into two fire rated compartments together with fire rated emergency escape doors at 100m intervals. The partitioning continues all the way up the shaft and into the shaft top buildings. This will prevent any potential fault in a circuit in one compartment affecting circuits in the other compartment, which could result in major supply interruption. The need for segregation of circuits and the provision of emergency escapes in the long tunnel is a key consideration in designing the cable layout. The tunnel diameter is determined such that it can accommodate the required number of cable circuits, provide space for a maintenance vehicle in one compartment and accommodate all mechanical and electrical (M&E) services. The maintenance vehicle is meant for four passengers and transportation of tools and minor spares. It is battery-powered and will run on top of the walls of the invert cable trough guided on either side by fixed guide plates.

EHV power transmission cables could be installed in flat or trefoil formation in the tunnel (see figure 1). Flat formation with the necessary spacing between cables occupies more space than trefoil formation. In the sizing of the tunnel, consideration has been given for the installation of both formations.

Equipment and ventilation buildings
Ventilation is supplied to the cable tunnel by fans housed in equipment buildings located in between two ventilation buildings approximately 6km apart. The tunnel ventilation system has been designed to remove heat and smoke from the tunnel under all operating conditions, including fire emergency. The equipment building also accommodates a 22kV substation and switchgear for the power supply of tunnel M&E equipment; standby generator and underground diesel tank; control room with SCADA screen; pumps, pipe work and water storage tanks for cable cooling as well as for the water mist system; cooling towers; building services; battery charging facilities for the maintenance vehicle and a guard house. The equipment buildings are typically two storeys high with two basements and serve as main access locations for inspection and maintenance.

The ventilation buildings on the other hand are smaller in scale and complexity to the equipment buildings and act mainly as exhaust or air supply points. However, at the intermediate locations these also provide entry or exit for a maximum of four EHV circuits.

Shafts
The shafts typically have a 14m internal diameter. Their layout is complicated by the need for fire rated partitioning of the ventilation system. The shafts require four separate ventilation compartments, plus another separate section for access to the tunnel by both staircase and lift as well as the necessary framework to support the power cables. All low voltage power cables and pipes to the tunnel are routed through the shafts. During the construction stage the shafts act as launching and receiving points for the TBMs.

Alignment
The tunnel routes are selected on the basis of achieving the most direct route possible between the EHV cable termination points while complying with the requirements of the agencies and authorities. The tunnel alignment is located within road reserves as far as possible taking into account any existing obstruction and allowing for future underground infrastructure along the same corridor. Where this was not possible and the route has to be located outside the road reserves, wayleaves from third parties are required although acquisition of private land is avoided.

The main difficulty in the route selection lies in locating suitable sites for the permanent shafts and buildings and in negotiating tight bends along the route. A minimum radius of 150m has been adopted in the design of the tunnel alignment though the cables themselves can negotiate approximately 5m radius bends. The shafts cannot be located in all cases along the line of the tunnels because of land constraints. In such situation off-line shafts are being proposed.

The vertical alignment of the tunnel is also controlled by constraints similar to the horizontal alignment. These place the tunnels at depth and mostly in rock.

Implementation
The site investigations for the tunnels were carried out in 2009 and advanced engineering consultancy contracts were awarded as two parcels in October 2010. The first parcel of 18.5km long north-south cable tunnel and shafts are being designed by Mott MacDonald Singapore while the 16.5km long east-west tunnels are under a consortium of Worley Parsons and AECOM. Planning approvals for the cable tunnels have been obtained.

The construction works are proposed to be let as design-and-construct contracts and will be divided into a number of contract packages to meet programme requirements and allow for wider participation by contractors. The M&E works will be combined with the civil works contract packages to ensure better coordination during detailed design and construction. The supply and installation of EHV transmission cables will be let out as separate contracts. All contractors were required to go through a prequalification exercise on 29 April.

North-south cable tunnel
The north-south cable tunnels will form an extension to the existing 1.8km long twin tunnels between Senoko Power Station and Gambas Avenue. The Senoko Power Station is one of the three major power stations in Singapore. In total five new 230kV circuits will enter the new tunnel from the existing tunnels to transmit electricity to the load centres in central Singapore. The 18.5km long tunnel will terminate at a T-junction with the east-west cable tunnel at the May shaft in the south.

Six permanent shafts along the tunnel route spaced at a distance of 2.2km to 4.5km apart have been proposed. In addition to the permanent shaft sites, two other sites are being explored to facilitate the location of temporary shafts to aid the tunnel construction.

The investigation has established that the proposed route for the NS cable tunnels passes through three distinct geological formations known as the Bukit Timah Granite (BTG), Jurong Formation and Old Alluvium, with the majority of the tunnel in the BTG.

The thickness of the weathered, residual soil layer in the BTG varies between 12m and more than 64m. The ground investigation verified that the rock head levels are much lower in some areas than previously anticipated. The rock mass, classified as weathering grades GIII to GI, was encountered at between 47.18mRL to 110.63mRL and was shown by rock coring, borehole televiewer and geophysical surveys to have extremely variable characteristics. The fracture spacing varies from very close to massive and the orientation and dip of the natural fractures in the rock also varies considerably. Laboratory testing established that the BTG ranges in strength from weak to very strong, but samples were generally strong.

Old Alluvium was encountered in the southern-most 1,200m of the proposed route. It exhibits great spatial variation, particularly with respect to particle size distribution, but appears to have a relatively uniform thickness of weathered material.

The risks related to the tunneling operations include, high face pressures in deep tunnels, highly variable geology, geological faults and open joints, high rock strength, very high abrasivity, weak ground in some local areas, mixed face conditions and corestones, high pressure groundwater, hot geothermal groundwater in a local area, unfilled boreholes and potential damage to adjacent properties and utilities from ground settlement.

In order to meet the project schedule and to control ground surface settlements, closed face TBM excavation is expected for this route. There are enough TBMs to keep drive lengths below 3km.

It is considered that a one pass lining system consisting of precast concrete gasketted segments will be the suitable in respect of both programme and waterproofing. Particular attention is paid in the selection of gaskets to prevent water ingress into the completed tunnel.

East-west cable tunnel
The 16.5km long east-west cable tunnel extends from Ayer Rajah Substation to Paya Lebar 400kV Substation. The initial section from Ayer Raja is critical to bring the power to a substation at Rangoon which is under construction at present. Seven permanent shafts along the tunnel route spaced at between 1.8km to 3.9km apart have been proposed. In addition to the seven permanent shaft sites, two other sites have been safeguarded for temporary shafts to aid the tunnel construction.

The tunnel route alignment is underlain by a variety of geological formations, ranging from the Jurong Formation in the west, through BTG in the central zone, a second area of Jurong Formation, the Kallang Formation and then Old Alluvium to the east. The Jurong Formation is a variable rock of interbedded sandstones, siltstones and mudstones and has been weathered to considerable depths.

The bedrock is expected to be reasonably good for tunnelling, although rapidly varying rock strengths are likely. The most important unit of the Kallang Formation is the Marine Clay, with a maximum thickness of 35m, although 10m to 15m is more common. The clay is soft to very soft. This material is difficult for tunnelling, but is within the capabilities of modern closed face TBMs of the EPB type. Careful monitoring and balance of the ground and groundwater pressures will be required in order to avoid any adverse ground movements.

The Old Alluvium is very extensive in the east of Singapore and has a thickness up to 185m. The formation is variable, but consists predominantly of dense silty sand. This is generally expected to be a good tunnelling material for an EPB machine, though it exhibits high abrasivity which could cause a very high degree of wear of the machine’s cutterhead, cutting tools and screw conveyor.

Three major considerations, geology, gradient and other structures, dictate the choice of vertical alignment. The tunnel has, where possible, been maintained in one type of geological formation for the length of each proposed TBM drive, in order to minimise the complexity of manufacture and operation of these machines. However, mixed ground TBM drives are expected to be unavoidable at certain sections of the route. Gradients between 0.25 per cent and 2.5 per cent have been assumed to be most suitable for permanent drainage and for ease of construction of the tunnel drives using rail mounted transport. Vertical clearances of 5m in soft ground and 2m in rock are expected to be adequate where the crossing of other tunnels or structures is substantially perpendicular. The result of the above considerations is a vertical alignment ranging from approximately 25m below ground level to as deep as 60m in some locations.

From the consideration of geology, lengths of drives, availability of worksites, construction techniques and construction periods, it has been determined that the civil works should be formed into three separate contract packages with tunnel lengths of between 4.5km to 6.5km for each contract package.

Each contract package will consist of two TBM drives carried out simultaneously. While there are some differences in contract length, it is possible to classify one of the contracts as rock tunnel only, one as soft ground tunnel only, while for the third contract mixed tunnelling conditions probably cannot be avoided.


Figure 1, transmission cable formations for a typical circuit Figure 2, typical cross section of the cable tunnel Singapore’s skyline along the waterfront Typical cross section of ventilation shaft and adit Singapore’s population had reached 5,076,700 in 2010