The first step in constructing the buildings of Anse du Portier 2/3
Conditions in which the piles are made
There are two main areas to consider:
- Near the caissons, in the 25-metre zone directly behind them, the piles supporting the two basements and the buildings above them must pass through sand and then limestone aggregate before reaching the substratum. The greatest depth, 60 metres on average, is found in this area.
Limestone aggregate that was deposited at the back of the caissons to completely block their position on the backfill. The piles must pass through this extremely dense layer. Photo © SAM L’Anse du Portier
- Beyond this 25-metre zone, the depths to be reached vary up to a minimum of -17 metres. The composition of the strata to be drilled through is sandier.
As a result, the design and size of the piles are not identical over the entire area. They vary according to:
- the weight and shape of the structures they are supporting,
- the nature of the soil in which they will be fitted.
Five rotary drilling rigs are necessary to make the deep foundations.
By using different drill heads, they can dig five different diameters: 1.8m, 1.5m, 1.2m, 1m, and 0.9m.
On the left, the imposing mass of two drilling rigs (in yellow) on the construction site, seen from the sea. On the right, the head of a drilling rig. Photos © SAM L’Anse du Portier
Making the piles
As the drilling rig progresses, metal tubes are installed in the hole drilled in order to prevent landslides inside the hole and limit the risk of introducing foreign bodies that could alter the quality of the injected concrete.
A metal tube before insertion. Photo © SAM L’Anse du Portier
During drilling, the drilling rig stores the material extracted in what is called the “bucket”. When the bucket is full, the corer is raised, emptied and then lowered again to continue drilling. The material brought to the surface is taken away by earth-moving equipment for reuse on site or for disposal.
When the desired depth is reached, the concrete is introduced into the metal casing. Beforehand, the casing is equipped with reinforcement cages.
A reinforcement cage. Photo © SOGEFON
The casting is continuous. This operation can take up to 10 hours for the widest and deepest piles; on average, 5 to 7 hours will be required.
Depending on the nature of the different layers of material to drill through, the casing is either temporary or permanent.
In the first case, the metal pipe is used as a formwork. It is removed when the concrete is injected. This is the procedure used for the areas near the caissons where the piles pass through the thickest layer of the backfill.
In the second case, the sandier areas, the tube is retained and envelops the pile.
The connection of the piles to the supported structures is done by means of another concrete structure called the pile head.
It connects the deep foundation with the structure it supports by providing a perfect base.
The piles will not be a visible part of the buildings but controlling their quality is essential given the importance they play in their solidity. The controls focus on the quality of the concrete, which was poured “blind”.
Control by calculations
This is the comparison between the theoretical volume of concrete required to make the piles and the volume actually injected. Two curves are drawn: the theoretical curve and the actual curve. The second must always be above the first. If the curves show that the volume of concrete used is less than the volume predicted by the calculations, it is an indication that a void has been left somewhere.
Conversely, if more concrete than expected is used, it is proof that it has filled the voids in the ground.
Control by sonic probing
All piles are equipped at their centre with metal tubes into which electric transceivers can be lowered. They allow sonic auscultations to be carried out. Ultrasounds are emitted which are translated into curves on computers. A defect on the curve corresponds to a defect in the concrete. In the event of an anomaly, an X-ray of the pile is taken to find the defective section. A coring is done to analyse the nature of the defect and then to carry out work to restore the concrete.
All piles located in particular areas (heavy load distributions) are systematically tested. In other areas, one pile out of six is checked.