Barrette

A barrette is a cast-in-place reinforced concrete column. A barrette may also be defined as a rectangular diaphragm wall element that is being used as deep foundation. The construction includes the excavation of a trench under the protection of a support fluid (i.e. a thixotropic slurry such as bentonite mud for example).

Construction sequence

Installation process in accordance with the requirements of NBN EN1538:2008

1. In advance construction of temporary guide walls and filling of the guiding trench with the thixotropic stabilizing fluid

2. Excavation through the supporting fluid suspension by means of a soil cutting grasp (clam shell grab) or hydrofraise, up to the required installation depth.

In order to avoid instability of the trench, the level of the bentonite slurry in the excavation trench should at any moment be maintained at least 2 m above the highest water level in the full height trench, especially in the case of an artesian aquifer.

3. Desanding or replacement of the bentonite suspension, placement of the reinforcing cage and concreting with one of more tremie pipes, depending on the size of the barrette

4. As concreting progresses the concreting tube(s) is (are) pulled up gradually, during which the bottom is always be kept sufficiently deep in the already poured concrete, to avoid each interruption in the continuous concrete flow.

5. Completed barrette

Construction chart

Fields of application

• Complimentary to / in combination with diaphragm wall construction

• Due to their massive size, barrettes facilitate “top down” construction (achieved by extending the barrette to ground surface level or, if the barrette sections are considered too large in the context of parking arrangements, plunge columns of smaller section can be placed into the barrettes).

• Concrete backfilling can be terminated at any depth (the execution of non-concreted lengths is possible).

• Very high bearing capacity: at great installation depths, barrettes resist large concentrated vertical loads; they carry off significant horizontal stress and bending moments better than circular piles of the same section.

• The bottom of the excavation can penetrate resistant or rocky soil layers to accommodate high loadings

Environmental impact

• Vibration-free execution

• Removal of soil and disposal of bentonite

• Low noise pollution

• Removal of debris, demolition of old foundations and backfill with stabilized sand

• A wide working platform for the bentonite production process and a designated storage space of a minimum of 300 m² are necessary.

• A stable, dry and flat work platform is required.

Technical specifications

• Various shapes including I (rectangular – single element or multiple elements in a panel), X (cross-shaped), T (tee), H (H shaped), L or polygonal circuit element

• Length to be determined by reference to the body of the grab (2.85 m or 3.40 m) and on the basis of the trench stability calculation (depending on the groundwater level, the grain size distribution of the soil, the proximity of loads, the type of the bentonite, ...)

• Width: 0.60 m, 0.80 m, 1.00 m, 1.20 m and 1.50 m

• Allowable bearing capacity from 5.000 kN upwards to 50.000 kN

• The excavation of barrettes under the protection of a bentonite suspension occurs by means of various tools, chosen in relation to the nature of the soil: hydrofraise, mechanical or hydraulic grab.

Related projects for this technique

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