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A high-profile overhaul for the siaap pretreatment plant

Built nearly 160 years ago, the SIAAP pretreatment plant in Clichy-la-Garenne (Hauts-de-Seine) is the transit point for much of the wastewater in the Paris area. The gigantic project to overhaul the plant is currently ongoing and particularly includes a new 70,000m³ basin to store excess water during wet weather, the modernisation of the current pretreatment unit, and the construction of a second unit. The complex 24-month project for Sefi-Intrafor will feature diaphragm walls, grouted walls, pretreatment grouting, injected foundations and contact grouting, micropiles and even jet grouting !

Pretreatment plant for wastewater from Paris, Clichy, Levallois and Neuilly, but also flood-prevention pumping station: its modernisation, work on which is set to last until 2022, will boost the reliability of its functioning, increase its capacity, lessen the impact on the natural environment, reduce disruption, and improve staff working conditions.

Sefi-Intrafor is building, as part of a consortium with Soletanche Bachy, the special foundations of the project for theBouygues Travaux Publics / Razel-Bec  Civil Engineering consortium. Split into two sites, either side of the current plant, the project includes site A with the T10 soil stack and the new upstream grit chamber/screen rake, and site B, with the T30 storage basin.

Work began on the site in April 2017, with a phase featuring 150 sample bores (diam. 1,200 and 1,400mm), particularly by Franki Fondation, to remove existing obstacles on the route of the future wall of site A. 28 hollow auger foundation piles were also bored to up to 18 metres at the end of this phase. Pretreatment works followed, both on site A (30 borings to 34 metres) and on site B (140 borings to 55 metres), to prevent any potential losses of fluid linked to possible dissolution of gypsum during boring of the diaphragm walls.

The wall team then set up shop on sites A (cranes) and C (plant). From late July to late September, the reinforced grout wall (including metal sheets up to 18m long) of the future grit chamber / screen rake was built using a grab, with a thickness of 62cm, to depths of between 14 and 23 metres, i.e. 4,500m² of grouted walls. This temporary retaining wall will then be reinforced with a concrete sheet to create a totally smooth facing enabling proper management of the hydraulic sections and, as a result, the water flow rates. Another grouted wall was built for the penstock next to the T10 soil stack.

In early October, at the far Seine end of the rectangle formed by the grouted wall, 4 sections of diaphragm wall were installed at a depth of 24 metres in front of an existing stack. A Jet Grouting curtain (20 columns) will complete the system, to fully waterproof the box. Until mid-November, the T10 soil stack was built including a circular diaphragm wall (internal diameter: 13.6 metres, depth: 32 metres - i.e. 1,450m² of wall).

In parallel, the teams installed 500lm of micropiles (46 units at 33 metres) for the raft foundations of the grit chamber. They then continued with the injected foundations of both the grit chamber (159 borings, approx. 3,800lm) and the T10 stack, including 36 borings (1,250lm) completed at the end of December. Jet-grout plugging, for the start point of the tunnelling machine from the penstock, will start in January.

At the start of the year, pumping tests will be carried out on the grit chamber to adjust the size of the final pumping system, before it is handed over to Bouygues/Razel-Bec. From February to the end of May: the propping work will be at the same time as the earthworks. Finally, in March, then in June/July, the 2nd pile phase (35 units) will take place, including preliminary sample boring in some cases.

On site B, the T30 71-metre diameter and 70,000m3 capacity monolobe storage basin, has been under construction since mid-October. Using a Stein grab, Sefi-Intrafor carries out preliminary boring and boring until the ground is compacted, before Soletanche Bachy finalises the drilling to a depth of 54m (thickness 1.02m) using a hydromill. Sefi-Intrafor also manages the new 160-tonne / 63-metre LR1160 boom loader.

To make up for losses in speed due to very hard stretches of loam rock, marl and rough limestone, at the start of January Sefi-Intrafor sent its BC40 cutter on a MC96 carrier as well as its new D550 disassembly plant, to guarantee meeting the project schedule. Upon completion of the diaphragm wall at the end of next March, the teams will complete the injected foundations of the basin and make its 600 borings to a depth of 55 metres with 4 workshops, i.e. 33km of borings in 4 months! After the earthworks phase, 740 micropiles (length: 26m) need to be installed in the bottom of the trench. This work should extend the schedule until June 2019.

 

Owner : SIAAP

Pre-Seine consortium: Stereau (consortium head) / OTV / Bouygues TP / Razel-Bec / HB Architectes

Civil engineering sub-consortium: Bouygues Travaux Publics / Razel-Bec

Special foundations consortium: Soletanche Bachy / Sefi-Intrafor

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