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SLOPE ENGINEERING - Soil nails and a high strength steel mesh are preventing failure of a railway cutting in south west England. Max Soudain reports.

The Bristol and Exeter Railway was built by Isambard Kingdom Brunel between 1836 and 1844.While the line did not require any particularly difficult engineering, the geology of the area has begun to throw up challenges for modern engineers.

The Flax Bourton cutting, 5km south west of Bristol, has seen a number of slips, surface slides and track flooding.

Excavated in Red Marl mudstone in the bottom of a valley, the 3km long cutting is up to 16m deep, with slopes varying from 30infinity to 50infinity.

There have been two or three slips a year, typically after prolonged periods of heavy rain, which has resulted in extensive remedial works.

In 2002 Network Rail carried out a quantitative risk assessment through its Great Western Zone Earthworks and Structures Partnership (GWESPA) with Alfred McAlpine Capital Projects (AMCP), to identify areas of the cutting most at risk. This allowed it to draw up an asset management strategy for upgrading and improving stability of the slopes.

Site investigations were then carried out, resulting in an outline scheme involving construction of deep drains at the crest and on the face of the cutting.

But experience from excavating and regrading the slopes during emergency works led to some concerns over the logistics of implementing this solution. AMCP was keen to look at alternatives that could bring benefits in terms of both constructability and cost.

Ritchies, the geotechnical division of contractor Edmund Nuttall, proposed an alternative solution based on a design and build basis.

With its consultant Donaldson Associates it developed proposals using soil nails and anchors and high tensile steel wire mesh to stabilise the cutting slopes.

Design was based on detailed assessment of the type of failures that had occurred and the ground conditions, focusing on construction methods, access constraints and minimising cost and risk, says Ritchies design development manager Richard Garland.

A further ground investigation revealed most of the problems arise from relatively shallow slips, up to 1.5m deep, in a highly weathered 'mantle' on the slope face. This is susceptible to sloughing and wash-out, where water, associated with limestone bands in the mudstone, seeps out of the slope face.

'Having identified the main problem, our focus was to develop a scheme that could be built safely while minimising the risks associated with the ground and groundwater conditions and the difficult access in the deeper parts of the cutting, ' Garland explains.

Once the outline proposal and initial costs had been presented to GWESPA and Network Rail, Ritchies and Donaldson Associates carried out detail design of the scheme under a design and construct agreement with AMCP.

The current phase of work covers about 900m of the south east slope of the cutting on the downline (Bristol to Plymouth) side.

Ritchies' contract for this part of the project is worth 'in excess of £1M' says Ritchies business development manager David Gibson.

Two solutions are being used depending on the level of risk to the railway - renewal and containment.

Where cutting slope stability is considered critical, the slopes are being 'renewed'with full stabilisation comprising a grid of soil nails installed in the slope and with a semi-rigid slope facing of high tensile steel wire Tecco mesh from Swiss firm Geobrugg Systems.

Although used on similar schemes in mainland Europe for some time, Tecco mesh use is relatively recent in the UK, Garland says. The high strength mesh provides active support to the slope, which is better than other meshes normally used, although it is more expensive, Gibson says. He adds that the 17,000m 2of the mesh being used at Flax Burton represents the largest use of the product in the UK.

At top of the slope, vertical 'dowels' - 25mm diameter, 4m long Dywidag Gewi bars - are installed at 2.5m intervals.

Rows of 25mm diameter Gewi bar 'nails' 7m to 8m long, installed at an average of 10infinity to the horizontal, are placed 4.6m apart horizontally and between 4.6m and 6m vertically (depending on the topography of the slope). All are placed in 100mm diameter holes and grouted with a sulphate resistant cement for their full length.

The Tecco mesh is then placed over the full height of the slope and pinned down with diamond-shaped plates fixed to the top of the nails, which are tensioned up to 30kN to 50kN.

One nail in 20 is tested up to full working load, says Ritchies site agent Phil Howard.

Where the risk to the railway is deemed lower, the containment system is being used.

'This ensures that if a slip occurs, debris is contained on the slope and does not impact on the railway, ' Garland explains. Design considered past failures, including theoretical 'worst-case' failure volumes.

The containment system comprises a crisscross arrangement of steel cables tensioned up to 1t to hold the Tecco mesh across the top two thirds of the slope face.

The cables are attached to a row of 12.5m long Geobrugg wire rope anchors, with working loads of 500kN, installed along the top of the slope at 45infinity, and nails running down the edge and bottom of the containment area, as well as soil nails installed across the face.

The edge and bottom anchors are 32mm diameter Gewi bars, 7m long and installed at 4m spacing. Typically one row of 25mm diameter, 6m long Gewi bar ground nails are installed across the area, although in some areas up to three rows have been used, says Howard.

Garland says: 'A significant advantage of this system compared to other containment schemes used previously for rail work is that if need be, it can be upgraded to a full stabilisation scheme in the future by adding more ground nails without changing the facing.'

Both systems include drainage measures, incorporating arrays of 64mm diameter socked slotted pipe drains up to 7m long installed through the face of the slope to lower groundwater where levels are causing concern. These feed into track drainage.

Slope nails are installed in holes drilled by three of Ritchies in-house designed and built rope-supported Terrapin rigs. On the containment sections, the rigs are slung from ropes attached to the top row of anchors but in the renewal sections, special 8.5m long temporary vertical anchors have been installed.

Work to improve drilling rigs, equipment and working methods and safety has resulted in a fourth generation of the Terrapin rigs, with improved drilling heads and rigging arrangements as well as soft formation bits to cope with the weathered mudstone and limestone bands, Garland explains.

Howard adds: 'The increased power from the new rigs has improved outputs from the start of the job.' Output is matching and in some cases exceeding expectations. Each is drilling about four holes a day - while drilling is relatively fast, he explains, it can take up to two hours to move between positions.

Ritchies began work on this phase last October under a partnering arrangement with AMCP as principal contractor and Network Rail as client and planning supervisor.

Work is being carried out under a target cost contract, with firms incentivised to achieve savings through 'further innovation and co-operative working' Garland says. This includes the design, which is being reviewed throughout construction as more ground condition information and experience is gained.

'The investigations, design development and value engineering took around a year to complete, ' he says. 'We've been able to engineer significant cost savings and reduce overall risk compared to the original scheme.' One of the biggest benefits to Network Rail of the system, he says, is reduced maintenance costs, as well as an associated drop in the cost of emergency works and uninterrupted running of the railway.

Ritchies' work is due to finish in May. Further work is planned elsewhere in the cutting over the next few years, Gibson says.

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