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Cost-effective coastal defence

Belgian coastal anchorage specialist Braet recently pioneered an application of directional drilling which could greatly reduce the cost of underpinning weakened sea walls.

Carried out with directional drilling equipment manufacturer Vermeer, the project involved injecting grout into holes eroded in sand beneath the foundations of a section of Oostende's 2.5km long promenade.

The basically simple solution involved using directional drilling to install perforated pipes into the voids, through which grout would be pumped.

But because the drives were blind (normal directional drilling involves pulling pipe back through a pilot bore), the system was adapted by creating an oversize pilot bore and pushing the grout pipe in from the original entry point using drill rods.

After a feasibility trial, three 22m bores were drilled under the seawall at an angle of 35 to intercept the target void 2m below low tide level.

Drilling itself proved difficult because of the wet sandy conditions, but Vermeer found it could stabilise the drilled bore using a heavy bentonite mud.

Once the bores were successfully drilled an 80mm diameter tube was pushed into each and up to 6m3 of grout pumped into the void, providing a new foundation for the wall and preventing further erosion.

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