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Nailed extension Environmental pressures during construction of the M6 motorway extension into Scotland demanded some technically and visually impressive slope engineering.

The M6 extension into Scotland, one of the largest design, build, finance and operate projects yet awarded, involves construction of 28km of new motorway north of the Scottish border.

The northern section posed some highly challenging engineering and environmental problems especially where the route runs through the narrow Evan River glaciated valley. Here motorway construction is squeezed between the West Coast Main Line high speed railway and the steep hill slope of Tinny Bank.

Gibb was commissioned by the construction-maintenance joint venture of Amey, Sir Robert McAlpine, Taylor Woodrow and Barr, to carry out design for this northern section. This includes the 18km of motorway plus an additional 9km length of a near parallel-running new A-class road, designated as the all purpose road (APR) under the contract (see Figure 1).

Following a period of post-tender value engineering, when the detail design was developed in close collaboration with specialist subcontractors, the construction of the northern section began in summer 1997.

Environmental aspects prevailed in the final engineering solution for the 1km long Tinny Bank section. The aim was to make the motorway appear as 'natural' as possible.

Achieving this has required major slope strengthening work in the variable glacial tills

and shattered bedrock. The northbound carriageway will be sup- ported on a 40 reinforced earth embank- ment, while the southbound carriageway is formed in a cutting, again with a face slope of 40, and up to 20m deep.

After completion of the engineering works, extensive planting and seeding on the slopes is proposed on lower and upper Tinny Bank slopes, which will reduce significantly the visual impact of the road (Figure 2).

Gibb's optimum solution to achieve a stable upper slope consists of an extensive grid of drilled and grouted soil nails (Figure 3). Soil nails have been drilled on 3m by 3m grid to a maximum depth of 12m. The soil nail head detail has been developed to minimise long term maintenance requirements, and Geogrid is fixed across the slope to ensure surface stability.

After soil nail installation, soil slopes are covered by topsoil, held by a retention mat and seeded to provide a green finish. Pre-dug 300mm by 300mm by 300mm deep pits, on 1m by 1m grid, backfilled with topsoil beneath the geogrid are provided for tree planting.

Throughout the project Gibb has applied the observational method and risk assessment to the design of the nailing. This has been based on 'most probable' ground conditions combined with the use of observational tech- niques during construction and a detailed probability assessment. The observational technique has been embodied in a construction monitoring pro-gramme with simple and robust criteria which enable the nail lengths to be optimised as work proceeds.

Pre-construction soil nail pull-out tests, which monitored the performance of 10 nails loaded to failure, deter- mined site specific correlations between bond strength, bond length and stress condit- ions for different soil types.

From these, and working to procedures set down in construction control charts, monitoring was carried out during installation to provide an indication of variations in the expected behaviour.

'Close collaboration of the design and construction teams with the specialist subcontractor was established to promote progressive design modifications and increase productivity,' explains Gibb's Liljana Spasicgril.

The risk assessment was carried out for various soil nail design parameters in the form of a sophisticated parametric study. This enabled the joint venture contractors to determine the cost implications which would result from changes to the nailing design.

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