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Finding the missing link

Geotechnics Carlisle to Guards Mill

A complex geotechnical system is helping bridge the England-Scotland motorway gap. Alexandra Wynne reports.

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 ncountered 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 42,000 vehicles each day.

fiIt is almost like two separate projects, fl 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 pro-le.

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 vibro concrete columns, sectional 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 sandstone at between 15m and 30m depths. But the ground pro-le deteriorates rapidly to the north of the bridge where ground levels need to be raised by up to 8m for the new viaduct.

fiThe rock head slopes off just where we didn't need it to and is replaced by bad ground, fl 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, the rock here is so weathered that it resembles sand more than rock.

Carillion Piling is using a bespoke system to install sectional ight 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 ight auger (CFA) piling rig capable of install piles at the 30m depths required.

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 the required 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 re-attached 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 that the SFA rig is just as powerful as the CFA and has 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. Here, subcontractor Pennine is installing 4,300 vibro concrete columns with a 450mm diameter and load bearing capacity of 1,000kN. These are shorter than SFA piles and found in the sand and gravels, typically at 7.5m.

Kirby says on their own these columns offer no real support and are just like finails hammered into jellyfl. 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 embankments are up to 10m high and formed on a Terram 2000 geotextile separator layer. Site workers will install up to 12m lengths of Huesker 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 35/20 or 55/25 grids are suf-cient.

The bulk -ll within the centre of the embankment will not be reinforced and can be comprised of Class 2 locally won material.

A geomat will be pinned to the face of the slope to stop the -ll material falling away.

The 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 to be cheaper and created the added bene-t of saving 10m at the base width of each embankment.

In addition to this 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 traf-c. Work on site began last July and the entire scheme is scheduled for completion in December 2008.

Safety Facts AFR (2006): Safety innovation: Behavioural Based Safety is being championed on the project.

More commonly used in the US, Carillion hopes to encourage individuals to take more responsibility for their own and others' health and safety. Volunteers conduct safety audits and make individual workers aware if they demonstrate any high risk behaviour. The objective is to keep accidents down to zero.

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