A complex geotechnical system is helping bridge the England-Scotland motorway gap. Alexandra Wynne nds out how it works.
About three-quarters of the £175M cost of plugging the 9km motorway gap between the M6 and the Scottish border is concentrated on a section crossing a railway and river in Cumbria.
Engineers encountered multiple geotechnical challenges to design replacements for a viaduct crossing the West Coast Main Line and a bridge over the River Esk. The Highways Agency awarded Carillion a £130M design and build contract to improve the section of road between Carlisle and Guards Mill that carries 44,000 vehicles each day.
'It is almost like two separate projects, ' says Carillion M6 chief engineer Jonathan Kirby. South of Mossband, upgrading the A74 dual carriageway to a three-lane motorway involves a simple method of digging out and replacing weak material - typically to a depth of 1m - along the existing road prole.
But variable ground conditions at Mossband proved more of a challenge. Carillion design consultant Capita Symonds (supported by designer Grontmij) came up with a complicated system using band drains, vibro concrete columns, sectional fl ight auger piles and geogrids for an embankment designed to support the new viaduct.
Over the River Esk, the new bridge will be supported by 1.5m diameter monopiles founded on Sherwood Sandstone at between 15m and 30m depths. But the ground profile deteriorates rapidly to the north of the bridge where ground levels need to be raised by up to 8m for the new viaduct. 'The rock head slopes off just where we didn't need it to and is replaced by bad ground, ' says Kirby.
This comprises between 6m and 9m of alluvial material, overlying up to 4m of dense sand and gravels above silty clays. Instead of the competent ground by the River Esk, rock here is so weathered that it resembles sand more than rock.
Carillion Piling is using a bespoke system to install sectional flight auger (SFA) piles under each embankment in two rows in the area closest to the railway line. Network Rail height restrictions forced the team to rule out using a 32m high continuous fl ght auger (CFA) piling rig capable of installing piles at 30m depths.
But a contact at plant manufacturer Bauer offered Kirby a daring alternative - a new prototype SFA rig that operates on the same principle as a CFA rig but has the advantage of needing less room to manoeuvre. The auger comes in sections that can be built up to reach piling depth.
As a result, rig operators are using two Bauer rigs based on its prototype to drill 7.5m and 10m long auger sections into the ground. The rig is detached, and an extension auger clamped on, before being reattached to the rig. This process is repeated until the required depth is reached. Operators can then pump concrete through the hollow stem as the auger is retracted.
SFA piling is slower than the CFA method because of the pause needed every time a section of auger is attached or detached. But Kirby says the SFA rig is as powerful as the CFA rig and enabled the installation of 500mm diameter piles founded on sandstone at up to 30m depths.
Site workers switch to a second piling method further away from the railway. Pennine is carrying out the work on the £1.5M subcontract to install 4300 vibro concrete columns under both embankments down to the sand and gravels. It is using three of its Stratacaster rigs to do this.
Each rig has a poker that is vibrated into the ground, displacing the soil as it goes to reach 6m to 9m depths.
Once there the poker is withdrawn, a cap on the end of the concreting shaft along its length is opened and concrete is pumped in. Each column has a 450mm diameter and load bearing capacity of 1000kN.
Kirby says on their own these columns offer no real support and are like 'nails hammered into jelly'.
But when the embankment reinforced with geogrids is built on top, the piles and geogrids will work in tandem to create a load transfer platform beneath the road.
The soft ground below the new embankment is expected to consolidate under increased pressure. To accelerate the process, the design team opted to insert a number of band drains. Subcontracter Cofra installed 18,600 Mebra-drain 7007 band drains in January and February, typically in 7m lengths into alluvial material. Once in place, layers of embankment fill add pressure to the area and force water through the drain's external lter layer and upwards through the polypropylene channel inside. Water runs through the stone ll and is released out of the side of the embankment.
About 3m thick bands of ll will be laid in three stages and each builtup section followed by a monitoring period of between two and eight weeks - depending on how quickly the ground consolidates. Kirby says most of this is likely to take place in the clays and overall settlement rate is expected to reach a maximum of 1.5m. Once monitoring periods are over, band drains remain in the ground but are dormant as consolidation is completed.
Embankments are up to 10m high and formed on a Terram 2000 geotextile separator layer. Site workers will install Huesker Fortrac 1200 geogrids laterally above the piles and columns with Fortrac 150 geogrids running lengthways.
Inside the slope will be up to 12m lengths of Fortrac geogrids, separated by up to 600mm of class 1A imported ll. Stronger grids, typically Fortrac 110/25 or 80/25, will be placed towards the base of the slope Towards the top of the embankment Fortrac 150 or 55/25 grids are suf cient. A geomat will be pinned to the face of the slope to stop ll material falling away.
Viaduct embankments on either side of the railway are designed at a steep angle to reduce costs and minimise land take. Kirby says a 1:2 gradient would require extra piling and additional imported ll. A steeper gradient of 1:1 reinforced with geogrids proved cheaper and created the added bene t of saving 10m at the base width of each embankment.
The old bridge and viaduct are expected to be demolished once the new structures are open. In the meantime, to continue the upgrade to a three-lane motorway with hard shoulder (widening the route by up to 6.7m), the new road moves away from the existing centre line. The old bridge and viaduct will remain in use while their replacements are built.
Highways Agency project manager Ziad el-Balbisi says this was a major challenge. 'We had to make sure traf c continues to ow throughout the work, ' he says.
'And it's an absolute no-no to stop the trains along the West Coast Main Line - it costs about £300 per minute if you do.' In addition to upgrade work, the contract makes provision for an all-purpose carriageway running parallel along the west side of the new road for local and non-motorway trafc.
Work on site began last July and the new road is scheduled to open in December 2008.