Franki Foundations puts vertical shores back in the spotlight

Franki Foundations is putting the spotlight back on the entirely manual technique of vertical shores. The deep foundations specialist is seeing an increasing number of requests for work in situations where it is impossible to use regular foundations rigs, for example on cramped sites squeezed between two obstacles. This is currently the case for the RER at Watermael-Boitsfort, where an existing covered railway trench needs to be fitted with emergency exits.

Etienne Delheusy, Project Manager at Franki Foundations: "At Watermael-Boitsfort, we are working as a subcontractor for Socogetra, a general contractor from the BESIX group, near to the point where the railway tracks pass under the Chaussée de la Hulpe. Initially, some of the emergency exits were to be constructed using soilmix or secant piles. However, the permissible load on the cut-and-cover made it impossible to use machinery. The customer therefore opted for vertical shores".

Imagine the scene: at the bottom of the excavation, workers dig into the ground with shovels to fill a 70-litre bin. A colleague then lifts the bin using a winch, then empties it into a wheelbarrow to remove the excavated earth... In the excavation, the workers use a trowel to cut the ground into squares to accommodate and place a pre-slab that has been lowered into the excavation using the winch. Shoring is placed between the pre-slabs, and work continues at greater depths.

At Watermael-Boitsfort, the maximum depth reached is 17 metres, with an excavation width of 1.10 metres, which corresponds to around 1,300 m³ of excavated earth. All the contractor has to do then is dig up the volume between the shore and the walls of the cut-and-cover to create an opening.

Etienne Delheusy: "The vertical shoring excavation technique is used on many sites where it is impossible to proceed in any other way, often in rail and underground environments. We're busy at Hasselt station, for example. In Brussels, some sites easily reach 1,000 m³ of lined excavations. Underpinning and underground gallery work are also among our specialities. We're there every time! ■