Franki-Cementation needs about 550m 3of bentonite on hand to support trench walls during excavation at all three of its sites.
There is not enough space at any of them for bulk storage tanks, so a central bentonite plant has been created on a platform over the Singelgracht canal between the Vijzelgracht and Ceintuurbaan Station sites.
Pipelines have been laid along canal beds, under pavements and over lock gates to deliver bentonite to each site. At 1.4km from Singelgracht to Ceintuurbaan, the contractor called in Dutch pumping specialist Eekels for reassurance that the plan was feasible.
Demand and supply are controlled from trenchside by a banksman via a control unit. This enables bentonite volume in the trench to be reduced as the colossal grabs are lowered, causing displacement, and increased as a bite of spoil is removed. Bentonite levels are kept at 1.2m or more above the water table to prevent water ingress and destabilisation of the surrounding ground.
Bentonite removed from the trenches is treated at a small onsite desanding, reservoir and pumping unit before being dispatched back to the main depot, said bentonite plant manager Nial Simmonds.
'Sand content by volume can be as great as 25%, ' he says. Sand content is reduced to about 5% before leaving site and given a second treatment to get the fines content down to 2%-3% before the bentonite is tested and recycled.