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FEAT OF CLAY

INNOVATION

An innovative drilling technique was needed to install steel piles in sensitive clay during foundation work for a new bridge in Norway.

As part of the overall improvement of the E6 highway, between Oslo and Gothenburg, a new bridge is being built over the river at Sarpsborg to double the traffic capacity crossing the water.

To achieve this, the existing 30 year old two lane carriageway structure - elevated on a series of reinforced concrete columns at its north end - will carry vehicles north, while the new structure will take traffic south.

The contract for the new bridge, about 2.5km long, is reported to be worth £180M.

As the Norwegian State Roads Authority (NSRA) wanted the new structure to mirror the existing one, engineers set about designing a number of bases to carry the new supporting columns.

Concerns over the sensitivity of the clays beneath the site meant the NSRA and its engineer Multiconsult needed an innovative method of drilling the steel tube piles that would protect the existing structure and nearby houses. Main contractor Bilfi nger Berger appointed specialist geotechnical contractor Hallingdal Bergboring to carry out the work.

The clay was laid down under marine conditions and the upper region has become dessicated and reasonably fi rm to a depth of 2m to 3m, but becomes increasingly soft with depth. Also present in the tricky strata are quick clays which can become thixotropic and wash away when fl uids are introduced.

The clays sit on a thin deposit of weathered moraine, less than 1m, which is underlain by granite.

To create a base for the bridge, columns are being formed to support concrete pads. Eight raking piles installed at 15infinity from the vertical will support each pad with two at each of the four corners.

By the time the foundations are complete, eight new pads will be in place, each supporting two columns. Rig crews formed the piles - using a Klemm 807-4 drill rig - by drilling 324mm diameter steel tubes with 7mm thick walls through the clays and penetrating a minimum of 1m into competent rock. They then drilled a 2m socket into the rock with the hole fl ushed clean in preparation for the installation of reinforcement and concrete.

HandF Drilling Supplies sales director Kevin Mallin explains that the normal requirement for a job like this is to use two techniques: one to drill into the clays using reverse circulation and another to install pile casings such as Symmetrix and drill those into rock. But the innovation on this project was that both were done in one operation while protecting the integrity of the sensitive clays. To do this HandF designed the reverse circulation mud system, working with Swedish Symmetrix dealer Geomek.

'Due to the highly sensitive nature of the clays, the proximity of the existing bridge structure and nearby buildings, a closed loop drilling system was used to drill the casing down to the weathered moraine, ' says Mallin. This meant the drilling fl uid - used to suspend the cuttings and take them from the borehole while providing a coating to prevent water from the fl uid mixture getting into the clay - was contained and recycled.

The additional need to extend the casing into the competent rock made it necessary to use an overburden drilling technique - the rig crew could advance the casing at the same time as drilling the hole in a single operation. Therefore the decision was made to use a Symmetrix Overburden System for the rock and a reverse circulation mud system for the clays, making it easier to control the fl uids that were under less fl uid injection pressure than if using standard circulation.

Welders attached the Symmetrix ring bit to the fi rst length of casing and then a modifi ed drag bit was locked in, which permitted the casing to be rotated down to the rock. Rotating the casing prevented skin friction from jamming it once it encountered the moraine.

The drilling fl uid was a special polymer blend, designed to stop water losses in the clays and to ensure the clean fl ushing of the hole while drilling. It was introduced at a low pressure through a casing swivel and extracted through a reverse circulation swivel by a vacuum pump. A pump circulated the drilling fl uid to and from a closed tank, which allowed the the fl uid to be monitored and the cuttings to be returned from the hole.

This closed circuit system protected the sensitive clays from being washed out and the polymer prevented saturation of the clays.

Once Hallingdal reached the rock it withdrew the drag bit and drill string and installed the Symmetrix pilot bit and hammer that was locked into the ring bit. The casing was then drilled into the rock.

After the casing reached its required depth, site workers removed the Symmetrix pilot bit and drill rods and used a conventional down the hole hammer method to drill a 2m rock socket. The hole was then fl ushed clean, ready for Hallingdal to complete the hole with reinforcement and concrete.

Drilling operations began in midSeptember and were on track to complete by mid-November.

Mallin says: 'The combination of traditional rotary drilling, modern polymer technologies, innovative use of Selwood pumps and the ability to drill the casing into competent rock in a single operation ensuring that no disturbance to the existing bridge occurred, really was a major success.'

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