National Grid’s Eade Road construction site in Manor House, north London, is one of two main tunnelling locations, each with its own TBM, for the operator’s current underground development totaling 32km length of tunnels.
The site, compact and inaccessible, takes advantage of brown-field land created from some land owned by National Grid prior to commencement of the tunnel works, combined with the a former textiles factory that had to be demolished.
The site is on a natural slope. Preparation of the site included some demolition and the creation of a suitable level surface by retained backfilling, and so creating enough storage space. The site is squeezed between a garage, textile and clothing industry factories and warehouses, a residential area and a canal in the Manor House-Haringey area. Not a square metre is wasted on the site itself, and a clever road traffic handling system ensures a quick turnaround of both materials deliveries and spoil removal to minimize disruption to neighbouring roads.
The second tunnelling start site is at Acton Lane, Willesden, where, as T&TI visited in mid-March, a Caterpillar TBM has been commencing its first of three 3mdiameter drives. The site will then move to Channel Gate Road.
Surface space
While space within the 200m by 15m Eade Road site is at a premium, it could be considered an advantage that everything is nearby. This enables major material handling to be carried out by two tower cranes with overlapping radii of operation. The larger, with a 50m radius and 7t rated capacity, unloads pre-cast lining segments and other materials from road delivery trucks using a variety of appropriate cradles. The segments are placed, as delivered, in double stacked rows of the same type at one end of the site. This crane is also used to sort the segments into sets of six (five and the crown) to form a ring, the set being carried in one cradle. Once the sets have been formed and space in the second storage area becomes available near the shaft, each is carried over to the second storage area.
The second tower crane has a smaller operating radius of 30m, but a higher lifting capacity of 13t for the heavy installation work that was required to set up the TBM. This crane lowers each set of segments down the shaft for loco-hauled transport to the TBM back up.
When in full production the TBM progress will require a high throughput of road trucks through the site both for materials deliveries and soil removal. With supplies, the target progress of 23 rings per shift at Eade Road, plus a two-ring contingency, will require 150 segments (25 rings) to be delivered on 20 articulated trucks. Grout mix and pulverised fuel ash (PFA) require another two or three road tankers a day to fill on-site silos.
Using the same haul road, but with waiting space for around eight trucks, spoil loading and removal requires 90 tipper trucks a day, even with sufficient spoil storage on site for 36h.
John Trounson, project manager for National Grid UK Construction, tells T&TI that although the Eade Road compound is definitely compact, the construction compounds at St John’s Wood and St Pancras are even more so.
National Grid has adopted a policy of utilising brown-field sites for new substations and other construction sites wherever possible. In addition to Eade Road and other shaft areas for the London Power Tunnels project in north and west London, National Grid has been cleaning up old town gas sites at Poplar and Bromley-by-Bow in east London to free the land for redevelopment. Soil from the tunnelling has been reused for these.
Purpose
The initial London Power Tunnels project has several strategic purposes for National Grid. Firstly it will renew the aging main electricity supply routes in the capital, replacing cables of old design that are now approaching the end of their life. Secondly it will provide for the continuously changing power needs of the conurbation. And, thirdly it is part of a new strategy by National Grid to link the many new and varied power sources with demand in regional grids rather than along limited main supply lines. The new strategy will facilitate the connection of existing and new power generation sources such as the cross- Channel cable link with France, wind-farms and other renewable sources as we rely less on coal-fired power stations. Thus London and other centres of demand can be fed with power from multiple directions.
Initially the new London Power Tunnels will carry two 400kV bi-directional circuits, although the larger 4m-diameter tunnel between Hackney sub-station and St John’s Wood substation is designed for more cables. The tunnel cables will supply various sub-stations along the route including via a spur to the UK Power Networks sub-station at Islington, and the Crossrail power supply via Kensal Green.
John Trounson explains that, considering the high level of tunnelling activity in the London area at present, National Grid decided to advance its shaft-sinking schedule, partially to avoid any shortage of skills that may be caused by the demands of projects such as Crossrail.
Cranes
Heavy lifting plays a key role in the set up and operation of the Eade Road and other drive shaft sites of the project. This started mainly with the delivery and assembly of the TBM requiring an 800t mobile crane to unload the cutterhead and front can and lower it down the shaft. Once this was moved into the previously excavated stub tunnel, a 140t mobile crane was called upon to install the steering can. Once the initial TBM excavation had been completed, the components for the 127mlong back-up system could be installed.
The semi-permanent Wolff tower cranes are positioned for their operating radii to overlap to facilitate segment ring delivery, sorting, storage and delivery down the shaft. The 30m crane is also required for changing locos, loco batteries and other heavy components up and down the shaft.
Safety and environment
Of course, all responsible construction operations have to have a safe method of working and site security controlling access, without clear and strict policies on safety and environmental matters, the potential for accident, hazards and general chaos on such a compact site with overlapping activities could be high. The system used at Eade Road is particularly sophisticated in that it includes a hand recognition system at entry points, plus a smart card system called Mosaic for additional identity checking, both interlocked to the barrier release. Thus instant checks can be made on whether the proposed entrant is recognised and has been appropriately trained.
The security system requires a permit to enter the site to ensure that each entrant has already received the necessary safety induction and is recognised.
The site operates for 24h a day, five days a week on 7-7 shifts except for Fridays. Surface operations are limited at night time, hence the need for a large spoil bunkering area and materials storage. One site, at Wandsworth, is only being worked daytime.
The tunnel itself is safe in that no noxious gases are expected from the strata, but as it is a dead end, numbers allowed in are limited and controlled by a tag system and banks men on entry to an Alimak hoist and at the shaft bottom. In the tunnel itself there are ‘hop-up’ platforms at 60m intervals as refuges from train operations.
Drives and shafts
All the drives in this development are either 4m or 3m in inside diameter and supported by Buchan precast trapezoidal (4-m) and wedge-block (3-m) segmental lining including Bekaert steel-fibre reinforcement. The ground is chiefly stiff London Clay although some of the alignment is close to the expected transition to the sand and gravel composing the Lambeth Group beds, and in one location is expected to enter it. Even though most of the drives are in London Clay, bolted segmental lining was chosen for the 4-m tunnel in preference to the expanded lining normally found in London Clay drives due to the proximity of mixed soft ground.
There are currently six planned drives, including a spur to the UK Power Networks sub-station in Islington. Two will be driven from the Eade Road site to St John’s Wood (9km) and Hackney (3km). Another, recently commenced, is being driven using the Caterpillar TBM from Willesden, through the Kensal Green sub-station shaft, to St John’s Wood to join with the Herrenknecht drive. The Caterpillar TBM will then be transferred southwards to the drive shaft at Wandsworth, just south of the River Thames. From here it will first drive south to Wimbledon and then be returned to Wandsworth to drive under the River Thames, past a headhouse shaft at Earls Court, and onwards back to Kensal Green to the North. A third TBM is under consideration for the Islington spur drive but a decision has not yet been made at the time of writing.
At the time of T&TI’s visit, eight shafts were complete and four more were under construction. All but two were due for completion by late March. Most shafts are constructed by conventional mechanical excavation with underpinned support lining rings. One was sunk by caisson and another required a diaphragm wall construction in less stable ground.
Stub tunnels or adits were excavated according to requirements about a year ago, especially for installing the TBMs. These were excavated by a Schaeff hydraulic mini-excavator and lined with sprayed concrete. The permanent linings may either be precast rings or cast in situ concrete, whichever is best suited.
The designs of both TBMs incorporate provision for the change of ground. The operational mode has not yet been decided for the second and third drives to be undertaken by the Caterpillar TBM as T&TI goes to press, pending extra site investigation in the Earls Court area to determine the nature of the Lambeth Beds.
In addition to the lengthy drives there are a total of 14 shafts being sunk, reduced in number from the originally planned 16. Only three of these (Eade Road, Channel Gate Road and Wandsworth) are designated as drive shafts, but others give opportunities for TBM maintenance and serve purposes for the final construction.
Alternate shafts will be equipped with extraction fans in headhouses. Artificial ventilation will be necessary not only to facilitate inspections and maintenance in the long tunnels, but also to remove heat emanating from the cables.
At some shaft sites new sub-stations will be built, and the shaft on the Seven Sisters Road links with the UK Power Networks distributor sub-station. From Kensal Green the sub-station will supply power for the Crossrail trains. These will be the first substations built by National Grid in the London area for 30 years.
Open and closed
In recognition of the risk of passing through permeable beds of sand and gravel, both TBMs operate in a choice of semi-closed modes, although they are not expected to have to be converted to full EPB mode. At the present time the Herrenknecht TBM driving the 9km from Eade Road to St John’s Wood and is operating in screw conveyor mode from the face invert so that apertures can be quickly closed off in case of the Lambeth Beds containing water being hit. It will then be returned to Eade Road for the 3km-drive to Hackney.
Conversely, the Caterpillar TBM starting its first drive is operating in open belt conveyor mode from the centre of the face since all the drive is expected to be well within the stiff London Clay.
So currently the choice is a matter of optimum spoil handling rather than face support on the TBMs. When in closed auger mode the rate of spoil extraction from the face can be better regulated by the screw conveyor, but it does not form a seal to hold face pressure as in full EPB.
Spoil
Whether carried from the TBM face by auger-screw conveyor or belt conveyor, soil is loaded into cars, hauled by a 20t Clayton battery locomotive, within the TBM backup train. At the shaft bottom each car is placed in a tippler for discharge into a hopper feeding a short belt conveyor that, in turn, feeds a vertical conveyor to carry the spoil up the shaft. At the shaft top the vertical conveyor discharges onto a highangle belt conveyor (HAC) that lifts the spoil to a sufficient height to create a muck pile in the storage area. From this area a wheel loader loads the spoil out to road-trucks waiting on the one-way site haul road.
Andy Frith, Costain construction manager, explained, "The target progress of up to 23 rings per shift will place high demands on the mucking out facilities, but we are set up for 25 rings. There is about 20m3 of solid ground to be excavated per ring, equivalent to 900m3 loose per shift. This will need 45 trucks a shift to remove from site, and therefore 90 trucks per day, but we do have 36h of storage to avoid night traffic."
Community relations
With all working sites in congested urban areas and most of the drives passing under residential areas, community/public relations were to play a major role in the project. Of course the project will benefit most Londoners but in the construction phase that might not always be obvious without an educational programme.
There are also major third-party matters to be considered. At the time of the visit the Eade Road TBM was passing under its first-major third party interaction – London Underground’s Piccadilly Line.
One common event is a TBM naming competition with community schools. In the case of Eade Road, nine-year-old Tottenham Cub Scout Libby O’Shea who won a competition organised by National Grid named the Herrenknecht TBM ‘Cleopatra’. The Caterpillar TBM was named ‘Evelyn’ by 9-year-old hastity Nyadu-Bekoe of St Joseph’s Primary School, Willesden.