With the majority of the UK’s underground construction industry focused on the soon to start, multi-contract, Channel Tunnel Rail Link tunnels under east London, it’s been easy to overlook the fact that a single contractor, using three Lovat TBMs, has been steadily boring a 20km tunnel under the capital’s northern suburbs for over a year.
The project in question is The London Connection Tunnel, a US$60.2M, 3.05m i.d. cable tunnel being constructed by UK contractor J. Murphy & Sons for the Client, the National Grid Company plc. Regulated by OFGEM, National Grid is the sole license holder for the high voltage transmission of electricity and is building the complete US$280M London Connection Project, including the tunnel, as part of it’s five year capital expenditure programme. The tunnel length stretches an impressive 20km and will link two major electricity supply substations, one to the north at Elstree in Hertfordshire and one in St John’s Wood near the world famous Lords Cricket Ground. The London Connection has been designed to cope with the burgeoning demand for electricity in this densely populated area of London by installing a 400kV cable circuit. The tunnel has been designed to allow the option to add an identical cable circuit in the tunnel in 10-15 years time as demand increases. Without the scheme, National Grid would not be able to comply with the conditions of its license by the winter of 2006.
Project history
Jim Street, National Grid’s project manager explained the history, “the feasibility studies for the project started in the middle of 1998, an overhead power line is out of the question in any populated area and laying a cable in the road would have caused far too much disruption. This left a tunnel as the only really viable option”
The feasibility study started in 1998 and was carried out by UK consultant Mott MacDonald. By mid 1999 National Grid had committed to the tunnel, making the estimated funds available and started a full planning and public consultation process. During this period, National Grid decided to take on a consultant and placed the detailed design contract out to competitive tender. The then named Brown & Root (Halliburton KBR) won the award for tunnel design, assisting with the assessment of tunnel construction tenders and site supervision. “National Grid has not had to construct tunnels for many years and therefore we do not have the expertise in house. We had to buy that expertise in. Mott MacDonald proved we could build it, Brown & Root turned this into a detailed design” explained Street.
From the outset the client and consultant stipulated that the tunnel was to be bored using three TBMs to achieve the tight programme of tunnel handover in May 2003 and a project commissioning date of December 2004. Upon completion of the design, the tunnel construction contract went out to tender in May 2000 with four contractors, J. Murphy & Sons; Miller Tunnelling (now Morgan Est); Amey; and Amec returning bids for the Target Cost I-Chem Green Book contract with a 50/50 pain gain risk share between client and contractor. “National Grid traditionally uses a fixed price contract strategy and this is our first venture into a target cost contract. At the end of the day it was a question of the tenderers pricing the risk and agreeing the target with us and us going with the lowest sensible bid,” said Street.
Of the four companies bidding for the contract, J. Murphy & Sons won the award with the price of US$60.2M in January 2001 with work commencing on-site in April that year.
The machines and tunnel
At the design stage the 20km long tunnel was split into three similar length drives; one x 8km long, and two x 6km long. Also included was the construction of six shafts, Elstree being sunk as a caisson shaft and the rest by underpinning. The shafts at Kilburn Grange Park and Collindale are offline with each connecting to the main drive via a 70m access tunnel built through clay using a specially modified Brock excavator adapted for plate erection.
J. Murphy & Sons chose to use three self-owned Lovat EPBMs in open mode to bore the main tunnel, one for each drive. At the time of tendering for the contract, J. Murphy & Sons only owned two of the machines, ‘Helen’, which had been bought from Thames Water after cutting her teeth on a substantial section of the London Water Ring Main Tunnel, and Caroline, which had worked on several UK tunnelling projects. “At the time we didn’t have a third machine,” remembers Peter Jaques, divisional manager for the contractor, “we thought about using a new backhoe, but in the way these things happen, a third machine, ‘Doris’ that had worked on Amey’s Perry Hall contract, became available from Severn Trent (Water Authority). It’s the ideal machine.”
The three machines required slight modification for the drives and due to heavy time constraints, J. Murphy & Sons sub-contracted the work on Caroline and Helen to UK TBM specialist Dosco whilst carrying out the refurbishment on Doris itself.
Tunnelling began in September 2001 when the first machine,’ Doris’, went to face on the 6km long southern drive from the 8m deep x 10.5m diameter Elstree substation shaft towards the 28m deep x 10.5m diameter Canons Corner shaft. A month later in October 2001, the two remaining TBMs, Helen and Caroline began their drives in opposite directions from the projects main working site at the 35m deep x 10.5m diameter shaft at Cricklewood. ‘Caroline’ was charged with the 8km drive north to Canons Corner, whilst ‘Helen’ headed south on the 6km stretch to the 24m deep x 10.5m diameter reception shaft at St John’s Wood. For the first 1.2km at the northern end, the tunnel went through the Reading Formation consisting of mottled clays and sands. For the remainder the tunnel runs through London Clay predominantly consisting of stiff fissured blue-grey clay, apart from a section at Collindale where the tunnel runs close to the London Clay/Harwich Formation interface (mottled clays). The 6km long Elstree to Canons Corner drive is being supported by a six-plate 1.2m wide trapezoidal ring whilst the remaining 14km between Canons Corner and St John’s Wood utilises both standard and bolted 1.2m wide expanded wedgeblock linings. The segments are fully gasketed for waterproofing and additionally secured by spear bolts that are being left in. The contractor was responsible for the detailed design of the ring and placed this with the ring supplier, UK segment manufacturer Charcon. George Jackson, J. Murphy & Sons contracts manager explained, “for the section between Elstree and Canons Corner we thought the initial 0 -1500m could be water bearing, also further along the alignment the tunnel runs under Brockley Hill at a depth of 85m, so trapezoidal segmental lining was the sensible option for the whole 6km drive.”
Considering the scale of the project, the contractor at a very early stage considered building a site specific casting yard for segment production. This was abandoned due to limited space and the ever -present time constraint on construction.
Production cycle
As with most machine driven segmental tunnels, Murphy planned the production cycle around one train movement completing one ring of tunnel advance with the capacity and number of skips in the train, sized to suit the bulked volume of the ring to be excavated. The tunnel ring is carried separately on four ring bogies to suit the segment off loading arrangement in the TBM. The majority of tunnels constructed in London in recent years have used 1000mm wide rings, but Murphy decided to use tunnel rings 1200mm wide on this project as it was considered that this could result in an increased daily advance for the same number of production cycles. This has had an impressive knock on effect on the production cycle. The extended ring has had the advantage of increasing production by up to 20% and has been ideal for a tunnel of such a substantial length with long travel times between shaft and face. The average production cycle time for the tunnel has worked out at around 20 minutes.
The 3.05m i.d ring at 1200mm wide has a net muck volume of 10.9m3. In London Clay this would bulk to approximately 22m3 following TBM excavation. This muck is transported from the face to the pit bottom in five x 4.5m3 Mahlhauser side tipping cars pulled by a 7 tonne Schoma locomotive complete with a 10 man, man-rider car. The loco takes the muck to the drive shaft where it is finally side-tipped into a muck bin and hoisted to the surface using a 100 tonne crawler crane at Elstree. At Cricklewood the muck is tipped onto a conveyor and transported to a surface stock pile via a 100m long inclined side adit and transported by lorry to landfill sites at Gerrards Cross and Hemel Hempstead.
Current progress
At the time of T&TI’s visit to the site in May, some 13 months after work started, enviable rates of excavation had been achieved with almost 75% of the tunnel completed. Since Christmas 2001, boring has progressed some 11km with a week’s best advance for the machines of 853m. Doris had already broken through on her first 3km drive from Elstree reaching Centennial Business Park shaft in February 2002. The final 3km long drive to Canons Corner broke through as T&TI went to press. Helen was nearing the end of her 6km drive from Cricklewood to St John’s Wood with breakthrough scheduled for mid July. Caroline was on coarse to complete the 8km drive to Canons Corner at the same time. The machine had recently restarted work after having been stopped for three days for a slight routine modification to the cutter head. This was a result of the machine starting to bore through the expected horizontal boundary of the London Clay and the Harwich Sands near Edgware Brook. “There’s a surprising high quantity of claystone in the clay,” said Jackson, “and we’ve found they’ve been occasionally jamming in the TBM doors. Some have had to be manually broken up. The tunnel drive has been running along the London Clay/Harwich Formation interface, with roughly half the face in London Clay and the lower half in Harwich Formation. We’ve altered the position of some of the tool holders and looking at the cut track now, it seems to have had the desired affect.”
With the end of tunnelling insight J. Murphy & Sons can prepare for an intense programme to finish the remaining underground works within its contract. This includes the shaft fit out, completion of shaft headhouse buildings plus all M&E disciplines.
The programme takes the contractor well into next year with the final tunnel on coarse for the scheduled handover in late summer 2003.
Related Files
Map of the tunnel route
Longitudinal section
