It’s unusual for the disruption of a tunnel project to be welcomed by local residents, but in the Harlesden neighbourhood of northwest London, UK, this has been exactly the case. Landscaping works and the constant presence of site personnel have prevented the loitering and other activities that were causing problems for the neighbourhood’s residents.

And it’s a project by the client UK Power Networks (UKPN) that is to thank. The electricity network operator has set out to construct 1.4km of tunnels to house four 132kV cables between two substations in the Willesden and Brentfield Park neighbourhoods.

Location, location
The Willesden reception shaft is located in an existing National Grid substation, while the main drive shaft is in Brentfield Park, a green field site close to substations at Leicester Road, Taylor’s Lane and Gibbons Road, as well as E-ON’s Leicester Road Power Station. A third, safety/ventilation shaft is located on the external perimeter of E-ON’s site at Taylor’s Lane. This will also be used to bring the product cable in.

"The location of the drive shaft was pre-determined," says Pollard. "You need a fairly large site to set yourself up, and this kind of work couldn’t be done from a substation simply due to the amount of assets involved. The Taylor’s Lane shaft is novel like this, originally going to be in a substation, we found cables completely obliterating the route, so we had to come out the side and divert water mains involving special arrangements with Thames Water.

"The Brentfield Park [greenfield] site became available, a few other sites had been looked at, but this hit all the right buttons for UKPN, and the Brent local council was a willing party as a result of promised landscaping works."

Contracts
The main civils contract is worth GBP 10.2M of a GBP 15M (USD 23.85M) total project pot. Awarded to J. Murphy & Sons at the back end of 2011 as an NEC Option A lump sum contract. Pollard adds, "It’s quite novel in tunnelling, but that’s the way the client has gone, and our Newcross contract coming up with them will be the same.

"An Option ‘A’ Contract transfers considerable volume of risk to the contractor with regards to ground conditions whereas the majority of tunnelling projects undertaken by JMS have been shared risk NEC Option B Target Cost or IChem E or similar."

Murphy is working alone, without a JV partner and no subcontractors for main works, e.g. excavation.

Design is more of a mixed bag with the tunnels designed in-house at Murphy, while Donaldson Associates handled M&E design on behalf of Murphy, providing a functional specification, which is then delivered as the main contractor’s own design, based on knowledge of client requirements. Shafts and surrounding structures (as well as the end-use cable brackets) are handled by Parsons Brinckerhoff as the client’s designer.

Shafts
The two main shafts were sunk by 5t excavators – mini diggers with catalytic converters – and are lined with underpinned FP McCann rings down to the tunnel eye level where the line switches to SCL for the ease of taking on that work. This is mainly to avoid putting in large steel jams and picture frames, which is quite awkward and cumbersome. At the interface between rings and SCL the team placed Tam tubing, something could be injected at a later date if any cracking appears but probably won’t be necessary as the tunnel is very dry, with good quality London clay throughout.

The original shaft design by Parsons Brinckerhoff envisaged precast rings all the way to the bottom, but Murphy raised the issue with the steel jam and the time it was going to take, and so requested the spraying programme for the concrete eyes be extended for the remainder of the shaft depth.

The smaller shaft was SCL only, and not circular, but tapering towards the top like a chimney.

For all shaft dimensions, see works outline box.

Tunnels
The 1.25km-long main tunnel is built with 2.59m id expanded linings and excavated with a Lovat M116SE Series 11400 machine that was bought and refurbished by Murphy following a Thames Water Ring Main project. Two 17m radius bends were required after launch.

Pollard recalls, "It was quite a challenge – 17m on the Lovat. A lot of pushing on one side is involved." The 100m-long higher-level tunnel is lined with a Buchan 2.44m ID one-pass bolted grouted lining. This tunnel has two very tight 20m radius curves to be negotiated. The backhoe machine needed to "snake out very, very tight, then swing back in again," says Pollard.

"That’s the minimum radius, you definitely wouldn’t want to go any further. This is all done to keep under the public highway, which we follow all the way through.

"You have got to stay inside the curve. That is the key thing I say to my engineers: start early, and stay inside. If you don’t do this, it’s almost impossible without going into the front of it and practically put a plough in and try to dog the front end over. We put beading on to give a bit of overcut, but if you put too much on you sacrifice settlement control.

"We’ve had good settlement figures, at about 3-4mm once moving. Of course when you first launch it is a bit higher, and while we were negotiating the 20m radius, the settlement was more in the region of 6-7mm. This is because the machine is going slowly and there is time for the ground to relax behind the machine. The quicker you go, the less settlement from expanding London Clay."

Stub
A stub tunnel has been excavated to future-proof the project. It’s a reception chamber to receive a TBM should there be any later need to connect into the construction. It is not possible to bring a TBM into the tunnel containing live circuits, so a branch-out is needed.

Machine launch
The machine was not launched from a frame. The team came up with a novel solution. When the high-level tunnel machine was launched, a thrust ring was put in and an eye sprayed to the length of the machine to put it in. "The other one, we’ve never done it before," says Pollard. "But we sprayed an eye big enough to put the machine in, which saved a thrust frame or pit irons. We had the conveyor belt in short mode and it just fit inside the 7.5m launch shaft.

"We built the shove ring behind the machine, Donaldson Associates did the check on temporary works. A webbed frame with gussets, fixed with hilted fixings and grouted saved many days at the pit bottom before launch. You need good ground conditions to do a sprayed concrete eye. We did it with Ellie [the backhoe machine] at the top, and we thought as long as the numbers worked we would do it at the bottom. You can’t shove with great pressure, just having to ease it in until you are say half a dozen rings in, and then you grout up that bit behind the shove ring."

The idea was from a combination of the Murphy site team, and the team from the company’s Kentish Town yard. "Sean Martin who was in charge of TBM refurb deserves special credit," adds Pollard. "It simplified launching the machines and shortened the programme by saving breaking the machine up and putting it down in pieces.

"The method allowed for a quick and simple, controlled start to the tunnelling operations. Lowered onto shaft on a Thursday in mid-July 2012, connected and shoved into the forward eye on Friday, tested and commissioned mechanically and electrically on Saturday and Monday, with tunnelling commencing on Tuesday."

Mucking out
Mucking out was done by a crane raising Mullhauser skips, a secondary rope being attached to the skip to tip it. A train could be turned around in 10-12 minutes with the setup on site. Spoil was removed by road and there was a muck bin capable of storing a full day’s mucking if needed, but three or four lorries worked each day on a constant turnaround.

Pollard adds, "We had an 11m backshunt built. Originally we envisaged a bunker pit and a 30m backshunt, and it was a time comparison. We played a few tunes, looked at the production rate of raising these new skips…and realised we could split the train and by shuttling around we could still keep the TBM fuilly stocked by having two trains working. A subtle change from the original idea that worked well and safely.

"We needed new stock and we looked into this…originally there would be six skips per ring and that was the reason for the full train with 30m backshunt. We needed new skips and we ended up buying these. We were doing 3 rings per shift so approximately 12 skips per hour. You could get train emptied and segments put on by the time the other train came out. We were busy at first but easier as the distance from to the machine increased."

Fibre free
Parts of the tunnel have been constructed without fibres – polypropylene or steel – at all. Simon Stroud of Parsons Brinckerhoff explains, "The junction chambers and connection adit were designed by the Parsons Brinckerhoff team using a thrust line approach. As such the design only required compression capacities to be satisfied. The benefits steel fibres bring relating to tensile capacity were therefore not required and as a result steel fibres were not included in either primary or secondary sprayed concrete layers allowing greater flexibility for J. Murphy & Sons in excavation and application sequences."

Pollard adds, "Now we know for a fact that some consultants won’t put sprayed concrete in without fibres. But Parsons Brinckerhoff are quite happy if they look at K0 factors and everything else that if the whole thing is acting in compression then it’s not needed. It’s not something we’ve ever done before, and talking to other engineers, designers often go ‘belts and braces’ [i.e., extra careful] with SCL. You read opinions in the magazines where sometimes a 200mm thick lining is fine, but when you go SCL 400mm is needed."

Looking back
In terms of geology, the area was fairly well known from the start. Site investigation had been done initially so Murphy was "quietly confident" according to Pollard. "Contractors tunnelled here many years ago and I heard a story they claimed the clay was too hard. Normally you would hear that it’s too soft, but not here. In the end it was exactly as we expected: good blue London Clay.

"The biggest challenge we expected from the job was to satisfy UKPN of what we could deliver. Not only them, but the subcontractors too. As part of a Safety and Culture Development programme aiming for zero harm, the foreman chairs 10-minute meetings at the start of each day. We discuss what happened yesterday and what’s to come in that day. UKPN is actually adopting something similar in other parts of its business. Pollard adds, "I had some concerns over potential crime in the area, and we have a minibus to take workers to and from the station. We also brought in special hoardings. They are manufactured by Kwik-klik and are plastic interlocking barriers that are graffiti resistant. A solvent cleaner just washes it off. While pine hoardings would just be waste material by the end of work, this can be reused, making it more environmentally friendly.

"Everything’s gone pretty well on the project. The staff has gone right through"