McCook reservoir - Phase 1
UNITED STATES, 2006-2008
Construction of a grout curtain using computer controlled injection.
The Mc Cook reservoir is one of the main components of an extremely extensive wastewater and stormwater collection system that serves the town of Chicago and 51 neighbouring towns. It has a completed capacity of 27 million m3. The basin’s water tightness relies on an enclosure comprising a bentonite slurry wall that descends through the overburden soils down to a depth of 18 to 22m keyed on the top of rock, supplemented by a two-line grout curtain bored into rock down to a maximum depth of 99m.
The US Army Corps of Engineers awarded Nicholson Construction, the American subsidiary of Solétanche Bachy the contract for the first section of the works. This comprised the construction of a 2,500m two-line grouted curtain. A later section of the works involving a similar grout curtain length will encircle the entire reservoir. Both lines of boreholes follow the curtain and are tilted at an angle of 15° from the vertical, in opposite directions.
Grouting was computer controlled using the Solétanche Bachy GROUT I.T.® system. This system measures, records and displays in real time a large number of grouting settings such as the amount of grout, the volume of each component in the grout mix, nominal pressure and true pressure, the Equivalent Lugeon, etc. Above all, it is used to monitor production efficiently by displaying the progress on the grouting using graphic views which can provide a summary of treatment completion by depth and along each grout line. The GROUT I.T.® system, extensively used on the Group’s sites, had been specially adapted to the specific characteristics of the American market: systematic use of rotary pumps on a closed loop (“return line” system) and injection controlled using “Equivalent Lugeon” criteria at low pressures .
Two technologies were used when putting grouting boreholes in place:
1 - The sonic drilling method used to lay piping in the superficial terrain. This technique makes use of high frequency mechanical vibrations generated by a special rotary drilling head. When the vibrations reach resonance, they fluidise the soil particles and destroy their resistance. The first few metres are core-drilled and then destructive drilling is used. This process allows us to drill through both superficial terrain and rock.
2 - The water activated drilling system used for drilling through rock. Two new drilling machines were specially adapted for use with this technique and equipped with high pressure pumps. The drilling unit included a hydraulic system, computerized supervision and a touch screen. These units used a new water pressure activated drilling system with a 3’’ 7/8 inch cutter.
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).
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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