Consolidation of the Lawe riverbanks (Pas-de-Calais, France, 2014). © BRGM - Eric Locatelli

Consolidation of the Lawe riverbanks

As well as mine safety, the DPSM also handles safety engineering in other sectors. This includes riverbank consolidation work, as in the case of the Lawe River, which flows through the centre of the former Pas-de-Calais coal basin.

Depressions  formed  by  subsidence are  one of  the main consequences of mining in the Nord-Pas-de-Calais coal basin, where coal extraction was systematically followed by blasting the mine cavities rather than infilling. The resulting subsidence has formed localised depressions on the surface, sometimes several metres in diameter, which are liable to modify natural water flows.

"Hurpinoise"riverbank consolidationmethod: hammering bolts into a welded mesh. © BRGM - Eric Locatelli

To prevent flooding from the  many rivers flowing through the  area,  the mining companies had  to build dykes or even divert the rivers,  as in the  case of Bruay-la-Buissière, where subsidence has produced depressions some 10 m in width close to the  Lawe, a small tributary of the  Escaut River. Before safety work  could begin on  an  upstream dyke  protecting more than 1,000 inhabitants, the  BRGM first  had to consolidate the  Lawe riverbanks.

Riverbanks on the verge of collapse, dykes breached...

The Lawe is typical of rivers affected by past mining activities. Coal mining during the 19th and 20th centuries in the Nord-Pas-de-Calais coal basin had weakened the  subsoil. The Bruay-la-Buissière concession is one example among many: mining of its 103 coal seams, at depths  of 100 to 1,200  metres, has caused subsidence on  the surface to form depressions up to 10 metres across. Due  to these significant changes to  the topography, decisions were made to adapt the  flow of the  river  running through the centre of the  zone,  by diverting its course and  building artificial embankments, dykes and  levees.

But the  problems continued: riverbanks on the  verge  of collapse, dykes breached, and even floods, as in 1999, when several hundred houses were flooded.

As part of its surveillance mission, the  DPSM is already responsible for  managing the dyke  on  the left  bank of the  Lawe, which has  needed repairs on many occasions to ensure its stability over time and  to comply with new standards (raising of the dyke,  design and follow-up of a series of cofferdams, analyses of the  regulations,  etc.). Given  the  consequences of mining on the  downstream banks, the  BRGM also conducted, as the  State-delegated project manager, consolidation work involving shotcreting and  strapping retaining walls, reprofiling banks and installing sheet pile walls.

In 2014, the State commissioned the BRGM to  handle engineering work  to reinforce unstable sections along very steep and  narrow parts of the  river  bank.

Testing the pull resistance of the bolts. © BRGM - Eric Locatelli

Social  responsibility and sustainable development

This civil engineering work, entirely performed from the river bed itself and in strict compliance with environmental obligations, supplemented the efforts of the previous mine owner to remedy the  effects of the  mine closures and thus ensure the safety of people exposed to flooding. The aim  was to ensure that in the  event of a riverbank collapse, the dyke  upstream would not be submerged. The work was undertaken in several sectors of Bruay-la-Buissière, along 800 m of riverbank altogether and on both sides.

Ecological landscaping of the consolidated riverbank. © BRGM - Christian Bocquillon

The first  task  was to fell riverside trees and  build access ramps to  the river bed.  This  was  followed by  overall consolidation using rockfill with anchoring bolts over a geotextile and  a welded mesh, with hammered angle irons (“Hurpinoise” method) and shotcrete. Two zones needed special treatment: re-profiling of the  riverbank with a reed bed planted at the foot in one case, and installation of a sheet pile  wall to protect a house. Although the  engineering work  itself  was fairly  routine, it had  to be performed along a small river (5 metres in  width) that narrows sharply at certain points into gorges some 15 metres in depth.

Projects such as these are a matter of social  responsibility  and directly linked to the principles of sustainable development. This DPSM project exemplifies the BRGM’s approach by taking the  different environmental, social, economic and  technical aspects into account.

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