Constructing watertight cutoffs using jet grouting and grouting.
Stage 5 of the Teesta Dam is part of the works carried out on the hydraulic development of the River Teesta in India. Works on the water intake include a 95m high concrete gravity dam, a river diversion tunnel and two cofferdams isolating the 50m deep excavation into highly permeable alluvia. Solétanche Bachy was responsible for constructing the watertight cutoffs in the two cofferdams.
The method selected to meet the requirements of the project consisted of a combination of jet grouting and grouting in order to form a cut off through sand containing large boulders:
- Construction of a line of jet grouted columns with “as drilled” survey for all boreholes. Based on this survey, column spacing and diameter was adjusted on a case-by-case basis in order to achieve a minimum overlap of 0.50m between two adjacent columns,
- Tubes à manchette were used to grout bentonite cement and silica gel into the sand outside the secant jet column cut off.,
- The drillholes were extended into the underlying rock to form a single line grouted cut off. The holes were grouted in descending stages.
In order to drill throug boulders of all sizes and to adhere as closely as possible to the theoretical hole alignment,, the rod/tube roto-percussion method was the adopted. The as-drilled survey was carried out after withdrawing the drill string and the borehole was then filled with grout. The jet tool could then re-drill the same trajectory without having to drill through hard rock. The columns were constructed using the double jet technique (grout column surrounded by air). The drilling for the grouting was carried out using a similar method: the tube a manchette was inserted into the borhole and the sleeve grout placed as the tube was withdrawn. This drilling method produced as drilled survey results showing an average misalignment of less than 1.6% at a depth of 40 m.
For 4 months, the site worked 6 days out of 7 and 24 hours a day. It involved 42 expatriates and 170 employees and workmen recruited locally, 7 drilling rigs, 3 jet grouting pumps, 3 automatic grout batching plants and 12 the capacity to grout at 12 locations simultaneously.
• 8,500m2 of watertight cutoff,
• 420 jet grouting columns (or 14,500m3 of jet grouting),
• 360 grouting drillholes ( 1,100m3 of bentonite-cement grout and 900m3 of silica gel injected)
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).
Jet grouting is a construction process that uses a high-pressure jet of fluid (generally 20 – 40 MPa) to break up and loosen the soil at depth in a borehole and to mix it with a self-hardening grout to form columns, panels and other structures in the ground. The parameters for the jet-grouting process and the desired final strength of the treated soil depend on a number of characteristics, such as the soil type, the technique used and the objective to be reached. In granular soils, the high-pressure jet breaks up the grains through erosion, while in a cohesive soil, such as clay, the jet breaks the mass up into small particles. High pressure is needed to produce the kinetic energy required for the jet through a small-diameter nozzle. Waste material from the process (a mix of soil, water and binder) is recovered at the surface before being taken away for disposal.
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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