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The Inside Track

A mobile grout plant and a double jointed drill mount are among several innovations speeding soil nailing work for the new M1 hard shoulder running scheme. Report and pictures by Adrian Greeman

Large-scale drilling and diaphragm wall firm Bachy Soletanche has not tackled soil nailing work before.

But for its first UK foray into the technique, for a major upgrade project on the M1 motorway, it has taken a good look at the way things are done and has devised some improved methods, which it hopes will also prove useful on future jobs.

New articulated drill mountings have been designed to fit nail drilling heads onto a medium size excavator base, and a grout mixing and delivery plant built to cut down on manual handling of cement bags and enclose the normal dusty mixing process.

The mobile batching plant with its bulk-loaded hopper has proved a hit with the workforce, particularly as it relieves them of most of the heavy humping of bags usually needed to make up grout.

Bachy is working in conjunction with contractor Carillion’s piling and foundations company, which does have some past soil nail experience, and this has helped to work out the improved methods and get the job going. Carillion contributes about 40% of the overall resource for the job.

Carillion site manager Tom Allen concedes that the piling firm, which works independently from its bigger parent, would not have had sufficient capacity to handle such a large job, some £18M worth, on its own.

But it is clear that its experience has fed into newcomer Bachy’s wish to essentially start from scratch in the way it tackles the job, helped by a good working relationship between the two joint venture partners.

The work sits within a major £300M restructuring of the M1 between junctions 10 and 13 by a main contract joint venture between Costain and Carillion.

This 25km long section of three-lane motorway passes close to Luton and is one of the most busily trafficked on the entire motorway network, handling more than 135,000 vehicles a day.

“Piles vary between 8m to 20m deep and between 750mm and 900mm in diameter, and they have gone fairly well,”

Unlike an equally busy section to the south that has been recently widened, here the Highways Agency has decided to use a “managed motorway” system to increase capacity, following what it says has been a successful trial on the M42 near Birmingham.

So rather than extend the motorway by another lane, as originally foreseen, it will use the hard shoulder in peak periods.

A dense network of gantry and roadside mounted signs, cameras and vehicle detectors, constantly overseen by the eastern division road operations centre at South Mimms, will control lane use and variable speed limits as required to adjust the road for varying traffic loads, or to switch lanes in the event of an accident or incident.

The system has increased capacities in the first section implemented on the M42 and evidence so far is that it also reduces accident levels while improving flows.

The construction advantage is that the road can be upgraded within the existing corridor with no new land take required.

Instead, the hard shoulders are being remade to pavement standards for running traffic.

The contract also includes major earthworks for two new junctions, but this is additional to the scope of the main carriageway work.

“For the road itself there is much reduced earthworks,” says Highways Agency project manager Lynne Stinson.

“Some remodelling of the embankment and cutting profiles is required because new emergency refuges are required every 500m and in some places additional space is needed for gantry mountings or cabling and control ducting.”

The refuges are short sections of new additional “hard shoulder” on the outside of the upgraded lane, where vehicles can pull over.

This is where Bachy and Carillion come in.

To make additional space, embankments are being steepened to 45 degrees.

Soil nailing is the main method for supporting this work.

The groundwork JV’s contract also includes several lengths of piled retaining wall up to 130m long, using contiguous CFA piles, for where a steeper-faced cutback is required. The retaining wall is usually up to 3m high.

The CFA piling rigs are also making the piles for around 50 of a total 90 gantries, which straddle the road every 800m.

Each of these is a large steel portal that spans the entire motorway and is hefty enough to need two large piles for the foundation on each side.

“Piles vary between 8m to 20m deep and between 750mm and 900mm in diameter, and they have gone fairly well,” says Bachy Soletanche contracts manager Nick Cordon.

The JV uses two rigs for the work - a Mait 180 and a Soilmec SR70 with 24m high masts.

So far work is on course and more than half the piles have been installed.

Ground changes along the alignment, with chalk at the south end, clay in the middle and finally Woburn sands.

None of it has been “particularly hard drilling”, says Cordon.

More than half the JV’s work involves the soil nailing, which is located at a variety of points.

“That means you can orientate the drill to just about any angle without having to worry about the excavator sitting on a slope or working at an angle,”

Mostly the nails are between 5m and 12m long and installed in rows at 1m spacings, sometimes as a single row or double rows for higher slopes, also with 1m spacings, but offset 500mm so that each is installed in the spaces of the row beneath.

Most of the nails are between 130mm and 150mm diameter and made with a 38mm diameter hollow tube reinforcement from Diwidag.

Grout is poured down the centre and flows back up the outside annulus.

“We also have some solid bar reinforcement,” says Cordon.

This slightly cheaper method can be used where the ground is good enough to drill self-supporting holes and the holes can be grouted easily around the bars.

Mostly that is in the harder chalk sections, while the clay and sands need the hollow tube.

For the drilling work the JV has used specialist drills from TEI Drilling in the US.

Four were ordered, three for different work locations and one held in reserve.

The JV mounted the 1t drill heads onto 28t excavators, which have been fitted with double swivel joints.

“That means you can orientate the drill to just about any angle without having to worry about the excavator sitting on a slope or working at an angle,” says Cordon.

It is faster and safer than otherwise would be the case.

The JV-devised set up includes a small work platform, also fixed to the excavator arm alongside the drill, and a safety cage over the drill.

The operator stands alongside and uses a radio control handset to work the drill.

Bachy has also developed its own mounted grout mixer and pump, which stores dry bulk cement , delivered by tanker, in a large tapered hopper and mixes it with water at the delivery hose connection.

It means that a consistent mix is achieved for grout and cuts down substantially on the dust and mess.

“Sometimes we still need a little extra grout and it is necessary to move cement bags around but it cuts out most of that,” says Cordon.

Nailing work has also gone well, the main problem being a relatively large amount of brick infill that was used in the construction of this original motorway section over 50 years ago.

The region is well known for brick making and more than 10,000 waste bricks were part of the fill used for the first section of M1.

“The bricks are slightly more difficult to drill through and take up more grout into the voids than the other ground,” says Cordon.

Once the nails are installed, the JV then has to fix a covering for the face comprising a Terram geofabric and galvanised steel mesh on top, both fixed down with attachments to the projecting ends of the nails.

Over that goes a 100mm deep pocketed geoweb covering.

The pockets in this are filled with topsoil to finish, which can be seeded and give a natural grassed embankment feel fairly quickly.

“That work is obviously a little more labour intensive,” admits Cordon.

He and Allen say the team has hit all the schedule targets so far.

The job will run until the late summer, around a year and a half since work started on site in February last year.

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