Deep foundations works for a household waste treatment plant.
The Isséane underground plant for sorting and incinerating household waste produced by Issy-les- Moulineaux required major deep foundation works for its construction. A 400 x 100m excavation 31m deep had to be created in a dense and sensitive urban environment. The super structure has a height of 21m.
The foundation works, supervised by Solétanche Bachy, took 15 months to complete and included the construction of more than 80,000m2 of diaphragm walls varying in thickness between 600 to 1,500mm to a depth of 50m, 400 foundation piles, 35,000m2 of cut-off grouted down to a depth of 71m, 231 temporary ground anchors, watertight grouted blankets in the chalk, watertight jet grouted slabs and dewatering to a maximum depth of 31m with flows of approximately 250m3/hour to dry out the excavation, that is to say to between 25 and 30m below the level of the River Seine.
The project was successful because it was based on our ability to master several fundamental points:
- Controlling the water in the aquifer. It was important that we only pump out reasonable amounts from each of the major excavations including the excavation for the plant which had to be cleared to a depth of 32m and withstand a hydrostatic head of 27m. The site’s service water requirements were tapped off during the pumping process.
- Controlling the vertical alignment of the structures. The walls and pre-cast columns will only tolerate very minor non-vertical alignment compared with the plant’s equipment.
The targets were achieved thanks to the deployment of the latest technical developments involving electronic control equipment mounted on the drilling tools.
- Controlling the environment. Foundation works had to be carried out within the available time bands: 24 hour a day, 5 days a week. A preliminary acoustic study was produced and recordings made throughout the duration of the works in order to control noise levels.
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A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.
A ground anchor is a load transfer system designed to transfer the forces applied to it to a competent stratum. An anchor is said to be temporary if it has a lifespan of under two years and permanent if the lifespan is over two years.
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.
A pile is a structural element driven into the soil for transferring loads and prevent deformation. Its slenderness ratio is not limited.
Pile shafts can be uniform and rectilinear, telescopic and belled out.
Piles can be installed either separately or in groups. They can also form a retaining wall, a mixed curtain wall, contiguous piles, secant piles and composite curtain walls, such as Berlin walls and similar. Piles are also used as precast beams to be placed in the structure of the building they support.
Suitable foundations are required for all building and civil engineering structures to ensure that they perform within the settlement criteria established in the design of the structure. Special foundations are used where shallow footings do not provide adequate support for a structure.
Major urban excavations are one of the specialist activities of Soletanche Bachy. Such excavations are required for basements to buildings that are usually part of the foundations of the structure and also used for under-street car parks, cut and cover tunnels for roads, rail, metro and storm water tank...
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