Tihange Nuclear Power Station
BELGIUM, 1988 1989
Installing four water inlet and outlet ducts under a nuclear plant cooling canal.
These works aimed to make watertight and consolidate soil under the nuclear plant's cooling canal in order to sink four 3.80-m diameter ducts. These ducts carry water in and out of the plant towards a new atmospheric cooler. Canal subsoil is made up of sandy-gravelly alluvium with rocks. The thinness of ground between the upper duct generator and the bottom of the canal required strengthening the roof, via freezing.
Works were performed in 3 phases:
- Building slurry cut-off walls.
- Grouting alluvium, via ring-grouped boring from right and left banks around the canal. Cement-bentonite slurry and MICROSOL slurry were used.
- Grouted ground was frozen to a thickness of 1 m, a length of 18 m and width of 6.5 m, successively for each duct.
Freezing is performed by using liquid nitrogen in horizontal borings. For each sinking, 2 borings equipped with temperature gauges enable to check that the freezing criteria is maintained.
These temperatures are recorded above ground, via a multichannel device.
A soil cover, set at the bottom of the canal by a diver and filled with slurry, enabled to limit loss of frigories, despite high water flow (70 m3/s) and high temperatures (26° C). Nitrogen consumption was, however, two to three times higher than normal.
Ground freezing works on the principle of freezing the water in the soil pores, thus rendering the soil impermeable and of greater strength.
Groundwater is one of the most complex issues to be resolved in geotechnics. During the start phase of a project, the proper techniques have to be chosen for building any cut-off walls that might be required, pumping (wells, filter points, etc.) and monitoring (piezometers) resources.
Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).
A retaining shell is a slim shell structure used during vertical excavations. Unlike retaining walls, the weight of a retaining shell has little effect on its capacity to balance the pressures.
In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.
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