References

Turkwel dam

KENYA, 1991

The Turkwell Gorge Hydroelectric Project (grout curtain cut-off and drainage) consists mainly of a double-curvature arch dam, 7 km of water tunnels, an access galleries and an underground power house.


The TURKWELL Gorge Hydroelectric Project consists mainly of:
- a double-curvature arch dam, 150 m high and 160 m along its crest. It contains 160,000 m3 of concrete,
- 7 km of water tunnels and access galleries,
- an underground power house with an installed capacity of 106 MW.
The dam is located on the River Suam in the North West Kenya All the geotechnical works related to the project were carried out by BACHY.
The rock in the region of Turkwel is a very homogeneous granite that has suffered little weathered.
It has a very high unconfined compressive strength (> 200 MPa) and is very abrasive. Two main faults, known as M1C and M2C, cross the two abutments and were treated from the drainage galleries.

Rockhead consolidation
The cleaned surface of the rock foundation under the dam was first consolidated as and when the level of the dam under construction gave acces to the face to be treated. Contact grouting of the rock/concrete interface was also performed as part of this phase of the works.



The grout curtain was installed from the drainage galleries at five different levels down to a depth of 85 m below the foundation of the dam and extending to 125 m on both sides of the centre point of the dam. This was a monolinear curtain with a basic interval of 5 m between the primary holes. Where significant grout takes were encountered, secondary and, in some instances, tertiary holes were also required. After water testing, the drill holes were injected with a stable bentonite/cement grout with a water/cement ratio of 1. The bentonite content of the mix was steadily increased until injection refusal was achieved at predetermined refusal pressures between 1 MPa and 2.5 MPa depending on stage depth. The Lugeon Tests indicated that the permeability of the foundation was generally very low. Only 3.5% of the stages tested gave a Lugeon Value of greater than 5 and the average grout take was only 7.8 kgf of cement per metre of hole.


Drainage curtain
The drainage curtain is located just down stream of the grout curtain cut-off. The drainage holes are spaced at an average 2 m apart and actually interconnect the five separate drainage gallery levels.

Barrage de Turkwel. Kenya

A199.pdf


Techniques

Grouting

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).

Ouvrages

Dams

The sealing and stability of dams generally requires substantial grouting, sealing and drainage works. The techniques of boring, injection and drilling, which are the original activities of Soletanche Bachy, are now complemented by other procedures, including diaphragm cut offs, either in concrete, plastic concrete or slurry, and jet grouted cutoffs.


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