Cats, Dogs and Creep

Curing a sliding hillside in the Welsh Valleys meant dealing with the slide during the works too, reports Adrian Greeman.

Contractors, like farmers, will always complain about the rain. But main contractor Carillion and its piling subcontractor BAM Ritchie had slightly more cause to complain than normal during slope stabilisation work for a rail line at Sebastopol in the Welsh valleys this summer. It rained solidly for two months during July and August. It was, says BAM-Ritchie's senior contracts engineer Phil Howard, "just biblical".

Not only did the unprecedented downpours - the worst recorded in South Wales - thoroughly test the drilling crews' wet weather gear and Carillion's follow-on concrete team, but, more importantly, tested their logistical skills and planning. The downpours came just as the teams had a crucial six-week railway possession, with intensive round the-clock work needed to complete vital piling work alongside the rail track and construction of a concrete longitudinal capping beam, before letting the trains restart, which they did with just hours to spare.

Halting the slide
The piles and beam are to be the main element of new groundworks that are intended to halt the slow slide of the hill down the slope above the railway and into a river bed at the bottom.

Downslope creep at this point has turned into full-scale slide several times since the single-track line was installed in 1858 to carry millions of tonnes of high-grade steam coal from the valley mines down to Cardiff docks for the British Navy and the Merchant Marine.

"Among others there were major slides in 1906 and 1932," says project manager for client Network Rail Dan Tipper. "The latter taking out one of two lines in the valley at the time."

Creep in recent years has been about 150mm annually on the hillside and about 40mm at the track location, which has gradually distorted the alignment, despite routine track maintenance. "We have had to impose speed restrictions," says Tipper "with the line down to 20mph [32km/h] in 2005." Recently a lOmph [16km/h] limit has been needed on the 200m long section.

The Cardiff-Rhymney route up through Rhymney valley remains sufficiently important in the postmining era to warrant significant stabilisation works. It carries commuter traffic and is part of efforts to keep the valleys alive, economically and socially. Rail operator Arriva also has one of its main depots in the valley.

Remedial work had been attempted including the use of Grundomat driven piles in the rail cesses and dewatering installed on the hillside to help stabilise the slope. The piles helped for a while, but it is thought the dewatering had little impact.

"We did a small-scale trial for that," says Austin Weltman, geotechnical director from consultant Tony Gee & Partners which worked for Network Rail to examine the problem. The consultant has since transferred to Carillion once the company, which has a term works contract with Network Rail, was asked to take on the £7.95Mjob.

"Although we got a lot of water there was no detectable difference in the movement," says Weltman.

Water is important, however, he says. Monitoring has shown a strong correlation between heavy rainfall and an accelerated movement on the slope above the track, some two or three days after a rainfall.

Staggered piles
With dewatering ruled out, various options were looked at by Network Rail, working with the Environment Agency. A massive retaining wall below the rail line was considered, but the hill slope is steeper there and access difficult.

Toe weighting was considered the best option, but this would have meant loading the ground at the valley bottom and shifting the river over to do it.

Apart from environmental considerations, this option would be difficult to execute because of the presence of an old mine waste tip on the other side of the valley.

The eventual plan was for a staggered, twin row of piles alongside the affected track length on the lower valley side. These would be anchored back into the bedrock of the hillside at a 300 angle, through a top beam along the piles.

More work was to be done further up the slope to cut through the shear line of the slip plane and stop the movement there by installing a grid of piles to go in the hillside.

The shear surface is about 8m-10m deep, says Weltman, sometimes coinciding with the point where bedrock begins, although often above it; the rockhead varies between 10m-20m deep below glacial tills and slope debris.

Piles, a total of 178 in all, would be 16m to 23m up here into bedrock - an assortment of mudstone, siltstone and sandstone. But first came the staggered piles for the trackside. Like the grid piles, they are continuous flight auger (CFA) piles 600m in diameter with a hefty 16mm thick steel tube for reinforcement, 406mm in diameter. Piles were 12m-23m long depending on location, with deeper ones needed at one end of the site where bedrock was deeper.

Two Soilmec CF ISO machines were used to install these, working from a 6m wide platform formed along the 200m track length once Network Rail had removed its track during the six-week summer possession. Two staggered rows were installed at a 1m interval to make up 259 piles.

"The rigs are very powerful," says Howard, "with a lOt torque." The big rigs weighing 38t were chosen over smaller machines to make sure the socket could be cut into the bedrock and in case any large boulders were encountered; the area is notorious for "house-size" boulders, he says, although "we were lucky".

Even larger rigs were contemplated, but a balance had to be struck between power and the load on the ground, since everything was being done on essentially a live slide area.

The rigs were serviced by two cranes working on a second platform further up the hillside, which would be used later for the pile grid as well.

These lifted in skip loads of concrete from an agitator on the top platform, says Carillion's project manager Andrew Sorley. A local Tarmac plant was the supplier. These cranes also lifted in the steel tubes once the drills were withdrawn and these were pushed down through the fresh concrete by a mixture of gravity and vibration from 4t units suspended from them. Most piles needed the extra force for the last few metres.

Heavy rain
Getting the tubes' positions correct was critical with a lateral tolerance of 50mm and vertical tolerance of just 25mm.

The rain made extra problems here too because the slide was gently moving throughout. In the heavy rain, the top of the slide on the hill moved as much as 300mm.

"It played havoc with the setting out," says Carillion engineer Matthew McKenzie. Survey points further away had to be used constantly to check positions.

Some six to eight piles were completed each day, each with about 6m3 of concrete. Following on behind came more concreting in the nightshift as Carillion formed new sections of the capping beam, a substantial structure 1.25m deep and 1.2m wide. Within the beam the contractor had to form ducts for the anchors to pass through later on.

Anchoring work came next. But most of this could be done working from the trackside, within Vortok safety fencing and with a carefully managed posting of lookouts and controlled movement on site. Trains pass at about half-hour intervals now possession has ended.

"There were a dozen or so anchors that did have to go in during the possession because the track space narrows at the top end of the site and the rigs had to sit on the railway line space," says Howard.

Boom time
Trackside space was still narrow, however, for about 50% of the remaining distance and BAMRitchies had to use excavator-mounted drills, Atlas Copco A65 masts on Hitachi units.

Sitting on the end of the boom, these allowed the bores to be made back underneath the machines. Conventional rigs working forwards were used for the rest. "We used a Casagrande and Atlas Copco," says Howard.

Anchors were set at 30° and were between 276m and 37m long, depending on bedrock. They were made with a 250mm diameter bore with a 32mm tendon, with a bonding length at the end of about 11m. A sulphate-resistant cement grout filled the bores.

While this was under way the top platform was being bored with the CPA rigs, the pattern of piles forming a 5m by 5m grid.

Work up here was cautious and 20 or so piles were installed with small lighter rigs at the beginning of the job to allow the cranes to operate and later the bigger piling rigs finishing the job.

To guard further against potential slip movement on the hillside, the platform was also loaded with temporary fill. This left no space for the drill rigs and therefore a small section of the temporary toe loading had to be constantly shifted as the rig moved along, to create a working area.

Work has progressed swiftly and the last of the anchors were installed in late November 2008 with only site finishing and tidying to be completed by Carillion in December. And the trains are back to 50mph, their old speed limit on this curving valleyside line.

Article courtesy of Ground Engineering - Jan 2009