London Electricity has undertaken a number of major system replacement and refurbishment programs over the last six years to upgrade electricity supplies in the capital city. One of those programs involved the installation of 132-kV cables in the central areas of London using tunneling methods. The tunnel design provided a cost-effective and environmentally friendly solution to cable installation works in a major urban area.
Stage 1: Leicester Square Substation In 1989, as part of the project to construct the underground 180-MVA substation under Leicester Square gardens, London Electricity needed to install three 132-kV cable circuits from an existing substation in Duke Street 3 km (1.9 mi) from the substation and adjacent to the main Oxford Street shopping area. Streets in this area are relatively narrow and are heavily congested with traffic 24 hours a day. The requirement to construct a 1.2 m (3.9 ft) wide by 1.5 m (4.9 ft) deep trench over a distance of 600 m (1969 ft) per cable section length in this important commercial area was not received with enthusiasm by Westminster City Council, the authority responsible for this area of London.
London Electricity invited Howard Humphreys & Partners, a firm of consulting engineers (now part of the Brown & Root organization) to perform a feasibility study to see if a tunnel could be constructed that followed the highway and avoided the mid-level sewers and the four underground train tunnels in the area. The study concluded that a 2 m (6.6 ft) dia tunnel could be constructed in the London clay at a depth of 20 m (65.6 ft). It also concluded that the cost of a tunnel, hand-driven within a hydraulically operated shield and lined with wedgeblock concrete rings would cost approximately 25% more than constructing a conventional trench, taking into account the considerable amount of other utility apparatus already occupying the highway.
The adverse impact on the business community and the difficulty of forecasting the timescale of constructing a conventional surface trench in this area lead London Electricity to proceed with the deep tunnel option.
Four intermediate access shafts were constructed along the 3-km (1.9-mile) route to assist with the cable installation and to allow easy access to the cable jointing positions. In order to accommodate the 132-kV joints, the tunnel diameter was increased to 3 m (9.8 ft) at the jointing positions. The section lengths were approximately 500 m (1640 ft).
London Electricity's Public Electricity Supply (PES) license allows it to excavate or tunnel under the public highway without seeking further planning permission. The contractors, J. Murphy and Sons, were able to accommodate this license condition successfully, but the tight corners on the route necessitated the use of bolted rings rather than the conventional wedgeblock arrangement (Fig. 1). The cables were buried in a 14 to 1 cement bound sand mixture to eliminate fire hazards and to assist with the heat dissipation through the tunnel lining.
Although original construction plans called for two 10-hr shifts per day, single shifts were used because residents near the site objected to the background construction noise level during the night hours. However, the civil construction of the tunnel still took only 12 months to complete at a cost of o3.9 million (US$5.8 million).
Stage 2: Duke Street/Carnaby Street Cable Tunnel After construction work was completed on the Leicester Square project, London Electricity had to replace a 66-kV paper-insulated cable route between Duke Street and Carnaby Street substations, which was approaching the end of its useful life. It was decided to spur off the existing tunnel and construct an additional length of tunnel to Carnaby Street to house this 66-kV cable. Figure 2 shows the four 66-kV cable circuits supported on steelwork fixed to the tunnel lining and one set of the 132-kV joints at a joint bay area.
London Electricity's policy is to apply two coats of a fire protection paint to all exposed 132- and 66-kV cables in tunnels and substation basements because the medium density polyethylene (MDPE) sheath material used on these cables is a combustible material. Figure 2 shows the 66-kV cables already painted with the fire protection paint. The 132-kV cables and joints were painted shortly after this photograph was taken.
Stage 3: Barbican Substation Uprating A second refurbishment project authorized in 1993 involved uprating an existing 33/11-kV substation in the Barbican development in London to 132/11-kV operation. Initially equipped with two 60-MVA, 132/11-kV transformers, the substation provided for a third unit to be installed within the next 10 years. The utility decided to adopt a similar design of cable tunnel to accommodate the two 132-kV cable circuits as it would allow the third circuit to be installed at very low cost. This tunnel was only 1.2 km (0.75 miles) long and had one intermediate shaft of 3.5 m (11.5 ft) dia.
Like the route on the Leicester Square project, the tunnel was to follow existing roads, but in this case this meant going beneath a section of road that was bridged by a car park. The tunnel had to steer a narrow course between the piles supporting the car park.
London Electricity had to back up its contractual guarantees on safety with a stringent procedure to test for any vertical or horizontal movement in the road structures above, which comprised 30-story-tall blocks of luxury flats.
Because the tunnel had only a 600 mm (23.6 inch) horizontal clearance from the piles and because it lay 4 m (13 ft) beneath a load bearing slab, the Corporation of London and many of the Barbican residents were understandably nervous. To satisfy the Corporation staff, who were responsible for the buildings, an initial structural survey of the buildings was carried out, and then mechanical strain gages were installed to record any movement in the buildings. The gages were checked on a daily basis during the most critical stages of construction.
No discernible movement was detected in the buildings and the settlement of the slab was half the 4 mm (1.6 inches) predicted. The success of this work is attributable to the use of a unique grout developed for the Channel Tunnel project, which provides very high load transfer properties and sets within 20 minutes of application. The grout reduced the time when there is no support beneath the surface to an absolute minimum.
The cable tunnels were fully serviced with lighting and power. At each shaft position sump pumps were installed to drain away any groundwater entering the tunnel. A leaky feeder was installed in each tunnel to provide telecommunication facilities. Hand-held mobile telephone units were able to dial into London Electricity's internal telephone system from any point in the tunnel. Contact with system control or other ground-based crews will be possible at all times during maintenance inspections.
Gas monitoring equipment was also installed at each shaft position. The intermediate shafts, which were capped on completion of construction with a single manhole cover (900 by 600 mm or 35 by 23.6 inches), provide access for personnel. A combination of covers provide plant access measuring 1.8 by 1.2 m (5.9 by 3.9 ft). These covers are locked with special keys. An automatic alarm system is being installed to detect any open pit cover.
Stage 4: Southwest London In 1994, London Electricity commenced work on a major refurbishment of the primary distribution system (132 and 66 kV) to upgrade electricity supplies to South West London at a cost of œ52 million (US$76.7 million). The main part of this project involved the construction of a 2.5 m (8.2 ft) dia. tunnel, 10 km (6.2 miles) long. The tunnel connects three main substations at Wimbledon and Wands-worth on the south side of the River Thames to Moreton Street, located in Pimlico on the north side (Fig. 3).
With the increased length of tunnel and the need to drive under the River Thames, London Electricity's consultant engineers, Howard Humphreys & Partners, opted for a larger diameter tunnel of 2.54 m (8.3 ft) allowing the use of a tunnel boring machine (TBM). The increased diameter meant the joints could be installed within the tunnel bore and no enlargement was needed at the joint bay position.
The TBM allows a faster construction speed and, since the machine chosen was a Lovat full faced machine, reduces the risk of delays because of the possible presence of scour holes in the clay. Scour holes are fissures of loose sand and gravels, often containing water, which can seriously affect ground stability when encountered. The use of a full faced machine allows the machine doors to be closed in seconds, thus protecting the machine operators and maintaining the ground stability until a decision can be made on how to overcome the unstable ground. Invariably, this will mean fixing a tailskin to the rear of the machine where the ring erection takes place to support the ground until the segmental bolted rings are in place and grouting is completed (Fig. 4).
The tunnel construction work was carried out from two main working shafts located on industrial sites away from any residences, with good access for material delivery and spoil removal. Four trains with six spoil wagons serviced the tunneling crew, and each train carried enough segments to form one ring at the face to carry away the spoil. Ring segments were one m (3.3 ft) wide on this tunnel.
The TBM had a specially designed arm at the back that lifted the segments into place, driving home the key segment with the aid of hydraulic rams. The machine then pushed itself forward via another set of hydraulic rams, engaging the last set of concrete rings installed. On a good day the team installed 50 rings during a 12-hr shift.
As the route of this tunnel again followed a major road system in South London, the intermediate shafts were constructed in side roads where disruption to traffic flow was minimized. The shafts were then connected to the main drive via short, hand-dug, side-entry tunnels.
Experience gained from the first two tunnel projects demonstrated that the installation of the 132-kV XLPE insulated cables in formed concrete troughs was both expensive and time-consuming. Therefore, London Electricity decided to affix the four 132-kV circuits required in the section of tunnel between Wandsworth and Pimlico directly to the tunnel lining, as shown in Figure 5. The cables were then covered with the 14 to 1 cement bound sand mixture, providing a level floor across the tunnel width.
Longer cable section lengths could be installed if the cable drums (weighing approximately 20 t or 22 tons) could be lowered to the base of the working shaft (7.5 m in dia or 24.6 ft). Motorized rollers spaced along the tunnel could then be used to install the cables rather than a conventional bond pull. Figure 6 shows one of the motorized rollers in situ pulling the first cable length along the tunnel.
Section lengths of 1000 m (3280 ft) were ordered from the cable supplier, Pirelli Cables Ltd., while the civil contractor, J. Murphy and Sons Ltd., constructed the tunnel and installed the cables. Motorized rollers spaced at approximately 50-m (164-ft) intervals along the tunnel over a distance of 3000 m (9843 ft) were used to install up to three section lengths in one pull. The cables were then placed into trefoil cleats spaced at 5-m (16.4-ft) intervals with short circuit restraining straps fixed at 1.25 m (4.1 ft) distances.
The cost of providing lighting in the tunnel was estimated at œ1 million (US$1.47 million) and, since the tunnel would only be accessed for inspection every six months, was considered to be uneconomic. Instead, a two-person electric vehicle equipped with inspection lights was developed. The vehicle was fitted with gas detectors that shut the machine off immediately when gas is detected. The vehicle carries a drum containing 500 m (1640 ft) of flexible lighting cable, which can be connected to a shaft power source and can provide floodlighting and power to any point along the tunnel should work be required. In order to support the vehicle, the top layer of cement bound sand has had a layer of stronger mix concrete skimmed over it.
Figure 7 shows the vehicle and cable drum prior to lowering it into the tunnel. Outrider arms can be lowered when the vehicle is in the tunnel to ensure it drives along the middle of the tunnel and does not damage the tunnel walls or attached equipment.
Cable Circuit Monitoring Ventilation of the tunnels and dissipation of the heat produced from the cables has required considerable attention as the amount of information available is limited and tends to be theoretical. Calculations for the first tunnel route to Leicester Square indicated a possible maximum temperature of 35øC (95øF) _ design value of 30øC (86øF) _ if two out of the three circuits were operating at the maximum cyclic rating of 450 A.
It was therefore decided to rely initially on natural ventilation from the two end shafts but to make provision to install a forced ventilation system if conditions dictated. Experience to date has shown that a very good flow of air is maintained throughout the tunnel and operating temperatures are well below the design criteria.
For the longer Wimbledon, Wands-worth to Pimlico tunnel, each intermediate shaft has a roadside cabinet equipped with fans capable of providing the shaft area with two exchanges of air per hr when access is required in the tunnel. The shaft at Wandsworth will have a housing placed over it on completion of construction and the facility exists to install suitably sized ventilation fans to provide a forced ventilation system, if required.
On this route a fiber optic cable has been placed in the middle of the trefoil grouping of each circuit, which will be used to continuously monitor conductor temperatures of the four circuits. A separate fiber that will be used to monitor tunnel ambient temperatures and operating experience over the next few years should provide valuable information on the parameters affecting cable heating and cooling cycles when installed in tunnel conditions.
The construction cost of the 10-km (6.2-mile) tunnel was œ14.5 million (US$21.4 million), and the cable supply and installation cost amounted to œ8.5 million (US$12.5 million). The first circuit was commissioned at the beginning of May 1996. All work was to be completed by the end of May. London Electricity currently has two more major tunnel projects under construction _ a 6-km (3.7-mile) route between Willesden and Fulham Palace Road that will initially house three 132-kV cable circuits and another 4.5-km (2.8-mile) route between St. John's Wood and Holborn substations. Different types of TBM are being used on the two projects because of differing ground conditions. In particular, ground conditions approaching Hol-born, where the London clay disappears and loose sands and gravels are known to exist, dictate that a high-level pipe jack will minimize the risk of ground subsidence and the need for expensive tunneling techniques.
In all these works London Electricity has emphasized informing the local business and residential communities about construction and has provided a 24-hr telephone contact to handle complaints speedily. Brochures describing construction plans are given to all involved around the shaft sites, and meetings with local residents have helped maintain good relationships with the local community and minimized their inconvenience.
The construction of tunnels to accommodate London Electricity's primary distribution cables has successfully reduced the impact of construction on a congested urban area and will allow future maintenance work on its cables to be carried out economically. In constructing an asset with a useful life of at least 100 years, London Electricity will also be able to refurbish its primary system cables in about 40 years with minimal installation costs. TDW
John Mathews is London Electricity's network projects manager responsible for the design and construction of all major project work associated with its electricity distribution network. During the 1970s he spent five years working in the Far East. He is a chartered engineer and a member of the Institution of Electrical Engineers.