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Chiltern's Fast-Track Soil Nailing

Embankment stabilisation is a key part of work to upgrade the Chilterns line between London and Birmingham. GE reports.

Marylebone Station in central London is the terminus to one of the capital’s more traditional lines.

Its atmospheric, if faded, grandeur feels a bit like stepping into a Jules Verne novel.

But Victorian standards can prove frustrating for the modern passenger on the Chiltern line.

Much of the route has line speeds which cannot match the high-speed West and East coast main lines and other fast routes - journeys to Birmingham from London take two hours.

Now £250M is being invested in the latest stage of an ambitious project to expand the railway network.

It will improve journey times for passengers travelling to London from Oxford and beyond.

Under Chiltern Railways’ Evergreen3 banner, an additional 80km of track will be upgraded to allow line speeds of 160km/h and slash the travel time between London and Birmingham by half an hour.

Along the way engineers are creating new cuttings, extending existing embankments and improving the bearing capacity for lines that will soon be carrying more services at greater speeds.

Geotechnical engineer Bam Ritchies has been given the task of supporting steepened cuttings and embankments near Gerrards Cross in Buckinghamshire and Northolt on the outskirts of London.

“Further nailing will strengthen and widen an existing embankment at Northolt and finally we are installing steel columns for a new king-post retaining wall at Northolt.”

Ritchies contracts manager Andrew O’Donovan is overseeing the installation of more than 4,000 soil nails across the two sites as well as driving boreholes for and installing steel kingpost wall columns at more than 160 locations at Northolt.

“There are three elements of the work we are carrying out on two different sites,” he explains. “Soil nailing is needed for the realigned toe of an existing cutting near Gerrards Cross station. The extra space will allow for an additional sidings track for one of the busier commuter stations which is the starting point for a number of services and requires stabling of trains in the off peak.

“Further nailing will strengthen and widen an existing embankment at Northolt and finally we are installing steel columns for a new king-post retaining wall at Northolt.”

The site team began work to stabilise the steepened cuttings at Gerrards Cross in early October.

There, 4m has been trimmed from the bottom of the batters and the slope has been re-profiled to 65 deg to help provide more working space for track realignment planned for the area.

Over three weekend line possessions the project team has installed a total of 269, 14m long, 32mm diameter hollow stem Dywidag soil nails, as well as 14, 39mm diameter nails of a similar length.

Some 19, 15m long, 38mm diameter nails have also been placed across the site.

“In all we have installed almost 4.5km of soil nails at Gerrards Cross,” says O’Donovan. “In one weekend we managed to install 1.7km - more than a mile.”

Soil_nailing_at_Gerrards_Cross

Road/rail excavator-mounted drilling equipment was used to install the nails which were simultaneously grouted while MacMatR soil netting was placed to help stabilise the reprofiled slopes.

“We were fortunate in that the road/rail plant access was only a few hundred metres away from our work site so we could get on with it quickly,” says O’Donovan.

“The nailing and mesh solution was the only practical one really. Sprayed concrete would have been too rigid and once the nails are covered with concrete you can’t go back to them,” he adds.

Ritchies is using a similar method further along the track near Northolt. Here, O’Donovan’s team is part way through the first phase of a complex installation that involves installing more than 3,500 nails by the time the second phase is completed in the spring.

The bulk of these will be the self-drilling 32mm diameter Dywidag nails while a further 904 of 38mm diameter will also be placed.

These are being used to stabilise the slopes of an extended existing embankment and also boost its load-bearing capacity.

The site at Northolt is split into four quadrants - north-east, north-west, south-east, and south-west - and throughout the northern sector, work is predominantly to strengthen the existing embankment.

In the southern sector, however, the design includes a gabion retaining wall section some 3m high.

Unusually this has slight forward slope, of around three degrees, which requires that they be supported.

“The nailing and mesh solution was the only practical one really. Sprayed concrete would have been too rigid and once the nails are covered with concrete you can’t go back to them,”

Gabion installer Maccaferri left holes in the gabion structures to allow nails to be installed through them and then into additional fill being placed behind them, to form a single structure.

The nails are drilled further through the imported fill and into existing material.

Ground conditions are well suited to self-drilling nails, according to O’Donovan.

His team is installing the 14m long, sacrificial bit nails using as many as four rigs at any one time.

“It’s all London Clay, other than the fill material, which is pretty good for our installation. It drills well and we get a good return of material when grout flushing,” he says.
Ritchies is also in the process of drilling more than 160 boreholes along the length of a proposed kingpost wall as part of its £1.8M contract with main contractor and sister company Bam Nuttall.

Using a drilling rig equipped with 450mm diameter, 1m long augers, the team will bore holes 6m to 8m deep and install and grout 254mm by 254mm by 107mm steel universal columns to help form the kingpost retaining wall at the site. These steel columns are up to 10.5m long.

The planks of the finished retaining wall, which are being installed by the main contractor, will span across three of the steel piles.

With Chiltern Railways set to begin accelerated services in May 2011 the team needs to make sure it nails down its deadlines just as efficiently.

Project details

Scheme: Chiltern Railways - Evergreen3

Project: Stabilisation of embankments and cuttings at Gerrards Cross and Northolt

Main contractor/client: Bam Nuttall

Ground engineering sub-contractor: Bam Ritchies

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