It concerns a type of drilled and cast-in-place pile, the distinctive characters of which are the shape and way of drilling.

Grab-bucket or Hydrofraise type drilling tools are used. The size of these tools determines that of the piles.

The simplest piles are made with one stroke of a standard size grab-bucket (or Hydrofraise cutting drums). The sizes are :

• width : 0.52, 0.62, 0.82, 1.02, 1.22, 1.52 m
• length : 1.80, 2.20, 2.70, 3.0 m

  Starting from these dimensions, bigger or more rigid piles can be formed : bars, crosses, H shaped piles, T shaped piles...

  The methods of calculation and measurement are the same as those for drilled and cast-in-place circular piles.
  

Usual multiform strip piles

Classically, it includes 3 stages :

• drilling,
• putting the reinforcements in place,
• concreting.

  The drilling is generally done under a bentonitic slurry, like a classical diaphragm wall. Often, when the working platform is poor, simplified guide walls are used.

  After desanding the drilling slurry, the reinforcement cage is lowered into the trench.

  Concreting is done in the usual way with a tremie pipe. Depending on the size of the pile, several pipes may have to be used.
  

Due to their shape, the strip piles have several advantages :

• resistance to horizontal stress and to bending moments better than circular piles of the same section,
• easy adjustment to structures, so that one single pile is sufficient under each column or bearing unit,
• better mobilisation of lateral friction than a circular pile of the same section, because of a larger perimeter.

  The preferential application field is that of high bearing capacities : from 5 000 kN upwards.

During the accomplishment of the work, the following are checked successively :

• the quality of the slurry, the verticality and the depth of the borehole,
• the position of the reinforcements,
• the volume of concrete at each level.

  After the concrete has set, a quality control very often carried out, is to inspect the continuity of the concrete by means of a sonic control. For this, 4 or more pipes are inserted into the reinforcement. Not destructive, this type of control is generally very exact and reliable.
  

The development of pre-stressed tiebacks has permitted high capacity loading tests at relatively acceptable costs.

The example shown here deals with one of the 113 piles 2.20 X 1.33 m loaded to 1 340 t intended to hold a reservoir in the methane terminal Gaz de France in St-NAZAIRE.

The test took place on a half-size strip-pile ie. 2.20 X 0.62 m. The load was mobilised by 4 pre-stressed tie-backs 18 T 15 capable of 1 764 t at breaking point.

The maximum load applied was equal to 1.5 time the working load and the stress in the concrete was 7 MPa.

The vibrating wire cells used to monitor the distribution of the stresses show that most of the load was taken up by pile toe.