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Take To The Road

Bachy Soletanche, better known for its work on mega-projects, is undertaking a much smaller £2.5M project to improve the A46 in Gloucester. Adrian Greeman reports.

Big name Bachy Soletanche is usually known for large diameter piling and diaphragm walls.

But on a road stabilisation project in the West Country, significantly smaller scale work is to the fore.

“It is relatively unsung, but part of our bread and butter,” says Bachy marketing manager Paul Hodgson, adding that it is “an increasing part in the current economic conditions”.

The firm’s AGM division does anchors, grouting and minipiling as its initials indicate, and for a scheme on the A46 main road near Gloucester several dozen small piles are being installed to good effect to hold back sliding ground.

An assortment of others are being used to repair, or rather encase and replace, an old kingpost retaining wall which is gradually tilting away from the vertical and into the valley below.

There is a wrinkled feel to the landscape in Gloucestershire and the area’s Cotswold hills.

Virtually every slope shows “lumpy” irregular undulations, a telltale sign of ground slips, many dating back thousands of years to the last Ice Age.

Where roads and highways must pass landslips can cause difficulties and lead to constant repairs.

“Pretty much every valley around here has been mapped as landslip,” says locally experienced Atkins geotechnical engineer Jonathan Merrick, who is currently working as the project manager for Gloucester Highways, a partnership between the consultant and Gloucester County Council for managing the county roads.

“The ground is highly plastic clay and very sensitive once it has been mobilised. It might only take a small change in local conditions to trigger some movement.”

One notable 40m long section of the A46 in this rural area of the county has been gently creeping downhill for many years.

Cracks and tension damage have been patched and the road surface replaced many times, says Merrick, with recently excavated carriageway up to 2m thick and showing multiple striations from repeated resurfacing.

The old single track road was also widened in the 1920s.

Just 30m away, a 120m long retaining wall has also shown slip effects with its kingpost and concrete plank structure gradually tilting over in the ground, the rotation particularly pronounced at one end.

Patching might have continued, but in 2007 the country was deluged with the exceptional summer rains which swamped Yorkshire and the West with headline grabbing floods.

Accelerated movement was seen at the time, which meant action was needed.

“The ground is highly plastic clay and very sensitive once it has been mobilised,” says Merrick. “It might only take a small change in local conditions to trigger some movement.”

DSC03446

As an initial safety measure, the council decided to monitor the slide, with Atkins installing a variety of instruments such as tiltmeters in the kingpost wall, inclinometers variously in the road and slopes, and various boreholes and other sampling carried out.

Telemetry links all this to a website that is accessible from anywhere.

“A dry stream from uphill, which runs during rainstorms, was then redirected into a new culvert round the slip,” says Merrick.

The streamline is caused by the presence of relatively porous Oolitic limestone which forms the escarpments of the Cotswolds; it overlies less porous layers of old and younger Lias, comprising mainly clays and mudstones.

Springs occur where the boundary crops out.

The old culvert through the slip had shattered over the years, almost certainly from the creep, allowing the stream potentially to raise the water table in the slip.
The drainage reduced movement but a full solution was still needed.

This comes in two parts.

For the slip itself, says Merrick, there are to be stabilising piles in the roadway and on the slopes up and down from the carriageway.

Gloucester_piling11

“In the road there is a grid of 508mm bored holes with a 450mm diameter circular hollow section steel tube reinforcement. They form a grid pattern of four rows.”

The high-strength piles are drilled down through about 8m of clays to rockhead, the Lias mudstone, where they are keyed in for 5m.

Once in place they will act as lateral load piles.

On the slopes, a further 54 piles - known as “B” piles - will go in too, this time smaller at 300mm and with 273mm CHS inserted reinforcement.

Above the piles the road structure will be remade using a 1m deep granular fill wrapped in 200mm thick layers with a geotextile for tensile strength.

Above that will go the normal sub-base and surfacing asphalt.

Meanwhile, the old wall has to be dealt with.

This comprises the king posts and concrete slats and behind it a V-shaped wedge filled with boulder like material.

Merrick believes this was a fill added when the road was widened and its old 19th century contour-following sinuosity ameliorated.

The wall has dates on the piles declaring it to have been installed in 1926.

Since then a form of anchoring made up from concrete beams has broken down with shear movements.

The movement on the hillside with the wall is not a full slip, Merrick believes.

This is not least because there is no obvious slip circle.

“We suspend the CHS reinforcement with a clamp while the grout goes in, to ensure it passes in the centre of the tube and around the annulus.”

He thinks it is caused by repeated shrinkage and heave of the moisture-sensitive soil, dried out by nearby tree root absorption in summer and soaking up rain in winter.

“The shrinkage is vertical but on the slopes, the winter heave is perpendicular to the slope angle which has the effect of gradually inching the ground downwards,” he explains.

Over the years this has allowed the wall to rotate.

To repair or demolish the old wall would have been too risky, he says, and so it is being “encased” by a new one just in front of it using 300mm diameter bored piles, this time with universal column sections grouted in for reinforcement.

Unlike the multidirectional forces possible on the road piles, the H-piles have essentially to take vertical load for concrete pilecaps and then new concrete wall posts and slats above it.

The piles are at 2m intervals.

Before Bachy began those, a series of 108 tension piles had been installed through the old wall, in two rows with a 10 degree inclination above and a 15 degree inclination below.

P1040837

With some strongback beams at the front these 22m long “anchors” restrain further movement in the old wall.

Once the new wall is in place the 50mm steel pile reinforcement can be extended in 3m lengths to pass through the structure and restrain it with some tensioning.

Before tensioning, the gap behind the wall in front of the old structure will be filled with a foamed concrete, which will also be pumped into the mix of boulders and fill behind the old wall, filling any interstices.

When GE was on site in mid-January Bachy was working on the wall piles with a Klemm 709 rig drilling down between 9m and 11m through the clay and 5m into the rockhead below.

Once the hole is formed the reinforcement H-section is lowered in and then suspended just off the bottom using a top clamp.

The bore is filled with tremied cement grout rather than concrete, avoiding any issues with compaction of larger aggregate.

The same method is used for the bigger road piles, says AGM site engineer Andy Tucker.

“We suspend the CHS reinforcement with a clamp while the grout goes in, to ensure it passes in the centre of the tube and around the annulus.”

A steel temporary casing is used for both types pulled out during grouting with a 30t Sennebogen crane hired in for the project.

Gloucester_piling08

The road piles are done one carriageway at a time, with a single file of traffic controlled through traffic lights.

For these larger piles there is a bigger Casagrande M6A drill rig. Grouting fills the finished piles only to within 1m on the surface to allow for main contractor, Welsh firm Dawnus, to excavate the road bed and then rebuild it.

“For the smaller hillside piles, which will be installed after the road piles, we are using the Klemm again,” adds Tucker. “It will be modified slightly for that.”
Once the piling is in Dawnus can complete the road structure itself.

A final resurfacing will be due around 12 months later following initial movement in the road piles which “are designed to flex at first as they take up the load”, says Merrick.

Three of the piles have inclinometers built into them to measure this and for the longer term, telemetric monitoring, which is likely to continue over another 18 months to check that the problem has been contained by this £2.5M project.

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