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Nailing the problem

Engineers in Nottinghamshire are working directly alongside thundering traffic to complete a motorway widening project. Paul Thompson reports.

Scheme: M1 Widening J25-J28
Client:Highways Agency
Value: £340 million
Main Contractor:MVM joint venture
Designer:Gifford – WSP
Supervisor:Scott Wilson/Arup
Soil Nailing sub-contract value: £1.8 million
Soil Nailing installation:Keller/Systems Geotechnique joint venture

The M1 between junctions 25 and 28 has always proven to be a bit of a bottleneck. Articulated lorries crawl up the uphill stretches while holidaying caravaners do their best to struggle past them, leaving only the outside lane to the rest of the travelling public. With four busy junctions and a service station in close succession it is easy to see why the 114,000 vehicles that travel this length of motorway each day sometimes create traffic snarl-ups of epic proportions.

For this reason, the Highways Agency has called in MVM – a joint venture partnership between construction heavyweights Morgan Est, Vinci and Sir Robert McAlpine – to help widen the motorway and get traffic moving smoothly.

But there is a difference between this and other motorway widening schemes on the network. Where on most projects traffic would be forced to weave in and out of never ending lines of cones until the work is close to completion, here contractors are working on the scheme without the aid of any permanent lane closures or contraflow.

While traffic whistles past at 50 mph on one side of the inside lane barrier, just a few metres away contractors are busily carrying out work that will see the motorway transformed from a three lane and hard shoulder in each direction to a four lane plus hard shoulder model.

Under the £340 million deal, MVM will deliver the newly widened motorway using only the corridor within which the existing M1 runs. This parcel of land is already owned by the Highways Agency, which enabled the entire project to pass through the planning process without the need for a protracted public enquiry.

And to bring the scheme in quickly, it has developed a system of working in two narrow lanes, one set on the northbound side of the motorway the other on the southbound, without compromising the traffic flow.

The plan is to hack into the bank of the existing cutting, using soil nails to pin it back where appropriate and use the extra space to provide the room for the fourth lane and hard shoulder. Where the motorway runs on top of an embankment, the extra lane will be built up alongside the existing hard shoulder.

That hard shoulder has become the project’s haul and access road with contractors and sub-contractors using it to service their crews working in specially allotted work spaces along the length of the contract.

These work spaces are allocated at weekly meetings with all the section heads of the various sub-contractors discussing what and where they will be working over the next few days. By ensuring each gang has its own dedicated work platform, the hope is that the haul road will not clog with delivery trucks, ensuring it remains open for each sub-contractor to use, as well as emergency vehicles should the need arise.

Any work that is likely to affect the flow of the haul road must be done outside of the sites normal working hours of 8am to 6pm on Monday to Friday and 8am to 1pm on Saturdays. Thankfully, of all the 40 or so bridges along the project route, there is only one – a farm bridge – that has had to be demolished and replaced. But a further 12 have had to be widened, limiting the effect on the haul road and helping keep the scheme on top of its autumn 2010 delivery date.

It is admits an MVM spokesman, vital for the smooth running of the project, to ensure that each sub-contractor and individual gang recognises the importance of keeping the haul road clear – without it there would be no chance of delivering the scheme while maintaining the full complement of lanes on the main carriageways.

Keller Group Joint Venture

The fact that Keller and Systems Geotechnique, both part of the Keller Group, are working together under the £1.8 million subcontract is testimony to its complexity and scale. It’s a question of needing the resources of the two companies to help complete the contract according to K/SG project manager Steve Worthington.

“There are in the region of 4,900 nails to be installed during the length of the contract and we will be working with around 60 staff on site. It’s a big contract and you need to be able to service that correctly and with the right level of expertise. Here that means working as a joint venture company,” he says.

MVM Joint Venture

The busy office hub for the huge scheme sits close to junction 27 of the M1 near Mansfield and accommodates staff from across the Morgan Est., Vinci and Sir Robert McAlpine joint venture partners. At times though, largely thanks to Vinci’s French workforce, the project office sounds more like a Parisian cafe than a Nottinghamshire building site. It is currently at its busiest with 150 to 200 staff including representatives from each of the joint venture partners.

As part of the deal, the project’s construction manager will always be a Vinci staff member, with the commercial manager from Morgan Est. and the project manager being from Sir Robert McAlpine.

Nailing down the M1

Steepening the fall of the slope as the motorway races through a series of cuttings can leave the new face dangerously unstable. Geotechnical contractor Keller is working alongside stable mate Systems Geotechnique that will see the joint venture install some 4,900 soil nails into the Nottinghamshire ground.

In many places across the site, MVM (another joint venture between Morgan Est, Vinci and Sir Robert McAlpine) found that when they trimmed back the toe of the existing batter the underlying Mercia Mudstone was not in as poor a state as had been anticipated. In several areas, particularly on the southbound side of the motorway around junction 27, the nails were deleted completely.

K/SG project manager Steve Worthington says: “At the moment the total number of nails we will have to place is down by about 30 per cent but those numbers could well come back again. The point is you never really know exactly what is there until it is has been uncovered. You have to retain some level of flexibility to be able to react to the conditions.

“The problem with Mercia Mudstone is that it can alter widely in its geological performance. It can be anything from a soft sandstone deposit all the way through to a very solid piece of rock.”

The slope is cut back to its required 45 degree angle and tested at 46 positions along the length of the scheme. These suitability tests are all about checking the capability of the ground to bear the bond between fully grouted soil nail and surrounding mudstone.

Boreholes range between 3 m and 10 m deep and are drilled using a 103 mm diameter bit at two different inclinations to horizontal - 20 degrees on the southbound batter, 15 degrees on the northbound. Nominally the first row of nails are installed 600 mm above finished road level and at 1000 mm vertical and 1200 mm horizontal centres.

The slightly oversize drill bit ensures the holes meet the minimum diameter requirement of 100 mm and can easily accommodate the 25mm diameter galvanised GEWI bar that site workers place inside the hole. This central bar is held at the correct cover from the sides of the hole with plastic spacers before the bar itself is slid into position, generally in one length.

If two pieces of bar are needed to reach the required length, a threaded coupler connects the two sections together. A tremmie pipe is then inserted to the base of the hole and a 40 N neat grout mix with a 0.45 water to cement ratio pumped to fill around the bar. Any section that cannot be filled is back filled and hand finished around the hole, flush with the angle of the slope.

K/SG section engineer Sean Nolan says: “We are using a 42.5 N CEM1 cement for the grout and have a required 28 day cube strength of 40 N. We are getting results in the high 30s after just seven days.”

Once grouted, the slope is covered in 80 mm square galvanised steel and plastic coated rockfall netting. Delivered in 25 m long and 2 m wide rolls, it is installed from top to bottom of the slope with a 500 mm projection at the crest and a similar 500mm projection at the toe of the batter. At the crest it is laid flat on the surface to be buried by the topsoil, but at the toe the projection is turned up before being folded into the toe drainage.

Contractors then thread a plastic membrane over the projecting nail to provide a further protective barrier between the netting and the 250 x 250mm nail plates that are installed over the bar. A tightened nut holds the entire system in place while the GEWI bars are cut at two threads above the nut and treated using a galvanising paint. The whole nail arrangement is then covered by a honeycomb topsoil retention system.

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