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IN THE FAST LANE

SITE INVESTIGATION - Widening of London's orbital motorway means round the clock work for the site investigation team if they are to finish the job on time.

May Gurney Geotechnical's first contract in its site investigation framework contract for major highways in the south of England is a baptism of fire. As part of its one year extendable commission for the Highways Agency, it concentrates on sections of London's M25 orbital motorway, clockwise and anticlockwise, between junctions 27 and 30 on the east side.

The work forms part of the Agency's £1.3M site investigation for the planned widening of the M25 from three lanes to four. The data from the investigation will assist future earthwork design to accommodate additional carriageways along the motorway. Site works began - overseen by consultant Parsons Brinckerhoff - in early September and should complete by the end of this month.

But May Gurney is working to a critical deadline to prevent hindering the rolling schedule for traffic management.

'It's a busy road and it's a dangerous road. Careful programming is the key to the success of the contract, ' says SI manager John Lawrence. 'We're working round the clock to fit the investigation into a 12 week programme.

'While the majority of the fieldwork will be carried out during normal working hours, additional teams are being employed throughout the night to move plant from previously drilled positions to new positions so as not to slow down the drilling process and accommodate traffic management requirements.' This will see excavators and drilling equipment lifted over crash barriers during the night rather than during the working day because of the nearby live carriageway. 'There is some scope to drill at night if we need to, ' adds Lawrence.

Along this section are a number of large embankments and cuttings that require detailed slope stability analysis. Standpipe piezometers will monitor the ground over a period of about 12 months to determine its hydrological nature with extensive soil laboratory testing being done to aid geotechnical design.

May Gurney will sink 37 cable percussive boreholes up to 20m deep on its £400,000 slice of the scheme.

These pass through strata that is a combination of made ground where there are embankments and weathered London Clay over unweathered London Clay. There are also some terrace gravels as the work gets closer to Dartford and the River Thames.

Boreholes are drilled off the hard shoulder so the existing cuttings and embankments can be examined, and scaffolding is being erected at 18 locations to position the rigs safely.

The boreholes will provide geotechnical, environmental and chemical data as well as allowing for water and gas monitoring points.

A further 100 trial pits and 50 window sample holes, using Dando Terrier rigs, will supplement data from the deeper cable percussive boreholes. One hollow stem auger hole is to be sunk using the company's recently acquired Beretta T41 hydraulic drilling rig.

Offices and a compound have been set up in Chelmsford and Harlow to service the contract.

'Logging the exploratory holes on site and directly inputting borehole data in to our Geodasy system at our Chelmsford office speeds up data transfer directly to our client's representative, ' says May Gurney senior geotechnical engineer Ben Armstrong.

This section of the M25 encroaches on areas known to contain landfill material so decontamination facilities, including emergency showers and clean areas will be provided to ensure safe drilling practices in accordance with British Drilling Association (BDA) recommendations when working on or near so called 'red' sites.

In addition to embankment and cutting investigation, further work is being carried out on the hard shoulder to examine the make up of the existing road construction. This will comprise 24 pavement trial pits and 82 concrete cores.

Following the exploratory element of the investigation, selected samples will be tested for geotechnical, geoenvironmental and material properties. The first draft of the report, prior to completion of all the monitoring works, is due for submission early in 2006.

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