References

In the framework of the construction of a storage zone for liquefied hydrocarbons in Gabes (Tunisia), TERRASOL, together with TERRASOL TUNISIA, was entrusted with the detailed design for 6 storage spheres of 4000m3 capacity each, and more particularly for their foundations.

The storage spheres are protected with a reinforced concrete containment structure filled with sand. Each sphere, with a total weight of 105.000 kN, is lying on an annular raft of 24m diameter. Very strict settlement limitations have to be complied with (maximum absolute settlement: 15mm).
The local geology is composed of a succession of sandy and clayey layers with 3 main typical layers: fine and loose sands close to liquefiability (5 to 10 m deep); fine to medium-grained very dense sands (10 to 22 m deep); stiff clay with gypsum nodules (from 24 m deep). Soil investigations highlighted several unconfined and compressible zones (corresponding to gypsum dissolution) within the deep clay layer.

This major hazard as well as the strict settlement limitations have played an important role in the choice of the foundation system. The geotechnical design, including a combination of finite element and analytical methods, led us to adopt a piled-raft foundation solution, with short piles embedded in fine dense sands. Each sphere is thus founded on 150 CFA (Continuous Flight Auger) piles of 0.6 m diameter and 13 m long.

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