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MANCHESTER EASES ITS BELT

GEOTECHNICS OF TRANSPORT - Vibro stone and concrete columns are improving ground for widening on a busy part of Manchester's outer ring road.

The stretch of the M60, Manchester's outer ring road, between junctions 5 and 8, is notorious for its traffic congestion. Here, on the southern side of the city, the motorway has only two lanes in each direction carrying both long distance and local traffic - and much more than it was designed for.

Work to widen the 7.4km section by one lane in each direction began late last year. This includes construction of embankments, slip roads, bridges and abutments, all on a busy live motorway.

Between junctions 6 and 7 the road runs mainly on embankment, up to 10m high near the junctions, built on the western edge of the Mersey Valley flood plain. Here widening is concentrated mainly on the clockwise side of the motorway.

The main contractor, an Amec-Alfred McAlpine joint venture, aims to minimise lane closures and disturbance to traffic.

'However, because of the complexities of the scheme, with many phases of traffic management, it is not always possible to allow enough time for the new embankments to settle, ' says Pennine Vibropiling principal geotechnical engineer Colin Serridge.

'This meant using ground improvement on parts of the site to provide a stable earthworks platform and for the finished road.'

Pennine has designed and is installing more than 8,000 dry bottom feed vibro stone columns and 1,300 vibro concrete columns, as well as prefabricated vertical band drains, on a subcontract worth about £1M.Ground improvement is principally in two zones, centred around junction 7 and between junctions 6 and 7.

The aim is to provide settlement control beneath new embankments built over weak made ground and alluvium, and to create transition zones and interfacing with piled motorway bridge abutments and existing embankments.

Concrete columns, working with a load transfer platform, are designed to transfer embankment loads to competent glacial deposits below the alluvial soils and limit differential settlement in a 10m zone between the piled bridge abutments and embankments supported on stone columns.

The stone columns are designed to reduce the compressibility of the weak fill and alluvial soils and improve overall stability.

'A critical factor, particularly for the stone columns, is the time available for consolidation settlements to take place, ' Serridge says.

'This is typically less than three months, to allow fast track staged embankment construction, controlled by monitoring.' The Highways Agency has stipulated that settlement on the finished road should not exceed 25mm.

The River Mersey flood plain is a sequence of weak alluvium, underlain by fluvio-glacial and glacial deposits consisting of glacial sand/ gravel and glacial till. These overlie Triassic bedrock, either mudstone, siltstone or sandstone.

Groundwater is between 2.5m and 5m below the base of the embankment.

'The weak alluvial soils are frequently blanketed by a veneer of fill, mainly waste from historic tipping, inert fill and embankment fill from construction of the motorway, ' Serridge says.

The upper surface of the cohesive alluvium is generally between 1.5m and 5m below the base of the embankments.

'Because the glacigenic deposits are generally competent, ground improvement is mainly treating the weak fill and the weak alluvium.

In the areas being treated, these are about 4m to 5m thick and are predominantly soft, very silty clays and very clayey silt, ' Serridge explains.

Additional site investigation boreholes and static cone penetration testing, incorporating pore pressure dissipation tests, were undertaken to help refine design parameters, particularly in the alluvium.

Serridge says the aim of vibro stone columns is to provide a composite 'ground structure' of insitu soil and stone columns. 'The stone columns act as vertical reinforcement and the ground has lower compressibility and increased bearing capacity.

'In loose granular soils, the vibratory action of the poker also densifies material immediately surrounding the column, improving the geotechnical properties of the soil.

'A further advantage is that the columns provide very efficient drainage paths for pore water pressure dissipation, which also accelerates settlements in the treated zone under surcharge loading, ' he says.

The new embankments extend out over the slope of the existing motorway embankment and for some distance beyond. Because of the gradient of the existing embankments, stone columns have to be installed on two levels.

Some preboring, particularly from the higher working levels, is needed to get through the competent embankment fill and into the alluvium.

As well as working platforms, granular drainage blankets with geotextile separators are being installed to allow excess pore water to dissipate during construction.

The concrete columns are installed from a lower level because a geogrid reinforced granular load transfer platform has to be laid afterward. Serridge says the platform spreads the embankment load on to the columns.

Temporary sheet piling next to the motorway was installed to allow construction of the platform while maintaining the integrity of the embankment.

Spacing and length of the stone columns is a function of the embankment loads, including whether ground improvement was carried out from a higher or lower level, the geotechnical properties of the soil profile and the settlement requirements, Serridge explains.

'For the lower working platforms, calculated design loads, including allowance for traffic loading, typically range from 176kN/m 2to236kN/m 2, with the 6.3m to 9m long stone columns installed on a 1.5m to 1.8m triangular grid.'

For the higher working platform levels, design loads are typically between 126kN/m 2and 168kN/m 2.Columns are between 9m and 13m long and installed on a 1.8m to 2.1m triangular grid.

Loads for the slip road are lower, between 50kN/m 2and 80kN/m2, with 9m stone columns on a 1.7m to 2.1m triangular grid. These are installed from a single level.

Design of the concrete columns is based on the same criteria. Design loads range from 168kN/m 2to 236kN/m 2, with 7.3m to 10.5m long columns installed on a 1.95m to 2.2m square grid. Working loads are between 600kN and 900kN.

For the slip road, the 9.5m long concrete columns are installed on a 2.5m square grid, with design load of 80kN/m 2.Installation procedures are being close monitored and recorded using state-of-the-art rigs and verification testing.

Testing includes plate load tests on individul columns and 'zone load' tests on groups of columns.

Dynamic testing is being carried out on selected vibro concrete columns to predict the elastic/ immediate settlement behaviour under design loading. Integrity is checked randomly by sonic integrity testing.

Pennine started work on site late last year and is due to finish early in 2005. The new road is due to open in spring 2006. The Highways Agency hopes the widened artery will not only improve traffic flow on the motorway but benefit surrounding local roads too.

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