As a reminder: the City of Paris Sanitation Division (SAP) entrusted the Impluvium consortium (comprising Soletanche Bachy France and Bessac) to build a tank to store and return water (the Austerlitz tank), as well as two structures to recover water from stormwater drains on either side of the Seine (the Valhubert and Tournaire shafts).
The work began in August 2020 and is set to take 44 months, the objective being to finish before May 2024. The project includes:
- A storage tank, 50m in diameter and with a capacity of 46,000m3, built under the shelter of a 1.2m thick diaphragm wall sunk to a depth of 62m.
- Two water intake structures on either side of the Seine, the first comprising a 0.8m-thick and 41m-deep diaphragm wall and, the second, a supported trench protected by jet grouting columns. The jet grouting work also provides a connection to the existing Buffon stormwater overflow.
- A DN 2500mm collector, sunk by Bessac with a microtunnelling machine from a starting shaft positioned alongside the tank. The 11m diameter shaft is lined with a 1m thick diaphragm wall.
One of the major technical points of the project is the slab-anchoring of the storage tank. This is subject to significant uplift pressure of nearly 300 kPa, which is insufficiently balanced by the weight of the hollow structure. Added to this uplift pressure is the difficulty of a slab positioned about 15m above expansive clay. The slab anchors are therefore subject to negative friction at that height and must be set deeper in the chalky bedrock.
The solution initially planned in the design consisted of building 236 micropiles and 20 barrettes more than 90m deep. They were also due to act as prefoundations during the works phase, the earthworks being cast-in-situ after construction of the cover slab. The contract included conducting an O-Cell test to optimise the barrette length.
The work on the main tank therefore progressed as follows: the diaphragm wall work on the tank, the dimensioning of which did not take account of the test results, was carried out over four months. During this period, the concrete of the test barrette increased in strength (one month), the load test was conducted over two to three days and the results were subject to an analysis report (three to four weeks).
The dimensioning of the foundations was then updated and approved by the various parties over the course of the remaining two months. Immediately after completion of the diaphragm wall of the tank, the barrettes could be progressively bored and concreted in parallel with the wall panels of the entry shaft during a second three-month work phase.