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Draining Experience

Repairing a failed slope above boggy land next to the London to Exeter railway line required some clever planning and nifty plant. Gemma Goldfingle reports from Devon.

In January 2009 a landslip sent tonnes of debris tumbling down a steep cutting on to the tracks of the London to Exeter line.

“A huge amount of material slipped down the steep slope. The site was relatively unknown to us; no previous problems had been identified so we weren’t actively monitoring it,” admits Network Rail Route geotechnical engineer for Kent, Sussex, Wessex and Anglia Derek Butcher.

Swift action was required to get resulting speed restrictions imposed on the section at Seaton Junction in Devon lifted.

Emergency works began in earnest, but it soon became clear the job was going to be tricky.

“Initially we brought in diggers to remove debris but they could not move through the boggy ground. We had hoped to drain the land within days but unfortunately it wasn’t going to be that easy,” says Butcher.

Even though the failure occurred on land outside Network Rail’s boundary, the immediate risk of a landslide onto the line led to the ­client deciding to stabilise the ­slope.

Speed restrictions on the bi-directional line were causing 40 minute delays each day, resulting in compensation costs to the train operator. 

“One of the main challenges for contractors is finding a way to access the site,”

The steep 100m long section of embankment has a slope of 1:3 increasing in steeper sections to 1:2. While exploring the area surrounding the slope, engineers saw evidence of dilapidated drains.

It was clear that the weak, sodden ground had been affected by more than just heavy winter rain.

Springs were found across the site and surrounding land, which is only a stone’s throw away from the River Umborne.

Bam Nuttall was brought in to carry out the £3.2M remediation job.

Sister company Bam Ritchies drilled 20 boreholes across the site with Panda Probing and inclinometers to determine the geology. In addition, pumps were applied to the boreholes in an attempt to drain the site, but to little effect.

The silty Upper Greensand Formation, which has a low permeability, overlies cohesive Branscombe Mudstone.

The geological sequence gives rise to a spring line at the interface between the two strata.

Pore water pressure reduced the integrity of overlying material and resulted in the wash out of the softened uncemented Upper Greensand on the face, triggering slips.

DSC01238

“The site has a potent mixture of dangerous geology, hydrology and topography,” said consultant for the job Arup’s associate director Saaed Mojabi. “Greensand Formation is highly sensitive to moisture, it turns to mush. The dip in the strata along this stretch makes sure the moisture gets into the ground. You find land only metres away that is much stronger.”

Site investigation showed the slips were shallow.

It was clear that improved drainage was needed to better manage the spring and surface water.

Arup’s solution was to cut off the spring water at the top of the slope and install gravel filled counterfort trenches for drainage and stability.

This sounds simple enough until you consider no machine seemed capable of moving across the marshy terrain.

“One of the main challenges for contractors is finding a way to access the site,” says Bam Nuttall site agent Steve Barton. “Both finding a way to get plant onto the slope and making sure it can operate were our key issues.”

Barton and his team scoured fields surrounding the hillside.

A suitable route was found through adjacent land, which was much more stable than the failed section. Access was gained through a small gap in a hedge.

The contractor set up a compound at the top of this pathway, about 120m from the site, and only 1.6km away from the A35, to allow deliveries to be made relatively easily.

“I was slightly sceptical about the Spider’s capability at first. I had never used one before and it is a small machine for a relatively big job. However, within 10 minutes of seeing it in action it was clear it could do the job,”

From this base, materials were delivered and transferred into a dumper which made its way to site.

A tracked dumper with a 360° rotating body was being used to prevent damage to the land caused by turning the truck.

There were concerns as to whether a standard long reach excavator could be used and whether the weak ground would support it.

In the end, access prevented the use of long-reach machines.

“The slope is divided into two sections, the lower being much steeper - around 40° - than the 15° upper section. For the lower slope we considered creating a zig-zag route for plant to travel uphill, however excavating that much material would be a huge job. Instead we decided to create an intermediate access track and explore what plant was available,” says Barton.

Constructing a geogrid reinforced access track as a base, along with lightweight aluminium mats enabled a conventional 9t excavator to be used on the shallow upper slope, however an alternative was needed for the lower section.

Bam opted to use Euromach’s “Spider” excavator, so-called because of the eight legs the machine uses to allow it to walk down steep slopes.

The legs prevent the rig getting stuck in boggy ground.

IMG_0109

Teeth on the limbs grip the ground and enable the 5t machine to work on slopes of up to 45° without an anchor.

“I was slightly sceptical about the Spider’s capability at first. I had never used one before and it is a small machine for a relatively big job. However, within 10 minutes of seeing it in action it was clear it could do the job,” says Barton.

“The operator is critical for the Spider to work effectively. It is difficult to operate all eight legs and reach arm as there are various pedals and controlling buttons. Luckily we found an experienced supplier in Wales who had lots of experience working for the Forestry Commission in inaccessible places. He was critical to the job.”

Bam Nuttall’s next task was to excavate a cut off trench to stop the spring water at source.

Aluminium mats were set up as a platform for the 9t excavator to dig a 2m deep, 900mm wide trench where the spring meets the slope face.

This collects ponded water, avoiding saturation of top material.

The trench diverts this water to the upper of two terrace levels constructed to house interceptor drains.

These drains feed directly into the trackside drainage.

Slip2

Slip1

Any additional water on site is managed with a mass of 600mm counterforts spaced at 6m centres to depths of 3m.

Perforated pipework is installed vertically into the excavation which is then filled with 40mm to 70mm gravel - beneficial for drainage and reinforcement.

The 225mm pipe takes collected water to the lower terrace area and then transfers it to new slope drainage at the toe of the cutting.

“It was quite remarkable how quickly the site dried up when the cut-off was in place,” remarks Barton.

“Despite this, the ground was still incredibly weak. We even had a small slip while digging out the counterforts. Prefabricated trench boxes were used to prevent the material collapsing into the excavation.”

Along with drainage capacity the counterfort system also acts as a shear key, locking the unstable land into intact ground, bringing greater stability.

The drains’ rodding eyes are topped with large concrete caps to prevent them getting lost in vegetation when it grows back, post-construction.

Slip3

Gentle reprofiling was also carried out to manage surface water by installing a series of shallow gravel filled grips to facilitate downhill water flow.

Advantage was also taken of the trackside location to remove the 1,500t of spoil produced on site.

The Bam Nuttall team worked during daytime hours for a week to stockpile excavated material and then took possession of the line at night the following week to remove it by road and rail vehicles.

The 10-month project was coming to a close when GE visited the site in late April.

Hydroseeding was completed and the first sprigs of green were peaking through the cutting face, which will soon become a new part of the landscape to rail passengers travelling for a South West retreat.

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