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Better by half

A novel solution dealt with both known and undiagnosed problems in a quay wall in Brittany.

Acombination of jet grouted quarter and half columns has provided an effective remediation solution for two quay walls in northern Brittany.

The walls, at the port of Legue near the pretty coastal town of Saint Brieuc, were showing signs of settlement only a couple of years after they were finished. Sand fill was washing out and both walls needed sealing to make them more watertight.

Keller Fondations Speciales, the French division of the international geotechnical contractor, was brought in to remediate the structures, made up of vertical steel H-sections at 3m centres with 400mm thick concrete panels in between.

Keller first considered using 1m diameter jet grouted columns to fix the sand and create a low permeability block behind the walls' face. However, after discussion with the client, it proposed a novel combination of large diameter jet grouted quarter and half columns.

Keller's Francois-Xavier Bailly says: 'We were able to produce half, but bigger, columns which provided a better and more reliable solution. The use of quarter and half columns is not new but few other specialist contractors have mastered it.'

The H-beams on the two 80m long quays penetrate 1m into the hard underlying shale, while ground anchors at just over 4m spacing tie back the walls.

For Quay 1, jet grouting was needed only around the H-beams. Here Keller installed 54, 7.5m deep, 1.6m 'diameter' quarter columns, on either side of each of the 27 steel sections.

Quay 2 required a complete cut-off wall, which Keller provided using 74 secant half columns installed along the back of the wall, down to 9.5m. This enabled Keller to form a planar and continuous jet grout treated zone right up against the rear of the wall. Full columns of smaller diameter would have left wedges of untreated material from which there could have been further loss of sand.

More secant half columns provided protection along the returns at the end of both quays.

Throughout, drilling precision was critical to avoid hitting the existing ground anchors and a 500mm diameter drain running through the dredged sand behind the wall.

Grouting parameters were closely controlled to ensure the correct column diameters and depths were achieved.

The project was supervised by CETE, the local authority for Saint Brieuc. Comprehensive testing was required at all stages.

Keller's jet grouted solution also addressed a problem that came to light after construction. Investigations revealed that groundwater levels at the back of Quay 2 were much higher than those allowed for in

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