Underwater, around the clock

The Liefkenshoekspoortunnel is the biggest construction project ever supplied by Holcim Beton Belgium. By this time the railway tunnel near Antwerp is completed, around 400,000 cubic meters of ready-mix concrete will have been produced and delivered to the site.

The project to build a new railway tunnel under a sea-freight canal and a tidal river to the north of
Antwerp started in 2008, and is projected to be completed by 2012. Part of the tunnel will pass under the Kanaaldok, a sea-freight terminal. Holcim Beton Belgium (HBB) was contracted to supply high-performance products produced to exacting standards after listening carefully to the client’s detailed requirements for two very different concrete types. About 50,000 m3 of concrete needed to be poured in two weekends to stabilize and then to reinforce the bottom of the dock.

Formulated for success
The first product requirement led to the development in-house of a specially formulated product which can be pumped long distances, used in an underwater environment, and that offers low resistance after six months to ease the boring of the tunnel afterwards. This “low resistance” or “low strength” concrete was used to replace a thick layer of soft sludge on the canal bed where the planned tunnel will run just a few meters below. Part of the underside of this soft concrete layer will be drilled away by the boring machine, because of the relatively shallow depth of the tunnel at this point. The concrete was developed, tested and produced by HBB, specifically for the project.

A long weekend in May 2010
A crucial phase of the construction spanned four days and three nights in May 2010, with 12,508 m3 of concrete from the two Merksem and Kieldrecht HBB plants being delivered and poured by around 60 mixers, each making 40 round trips over the long weekend. The average production rate per plant was 80 m3 per hour, with a peak of over 100 m3 per hour. 

Seven shifts of 100 staff drove the pour onward, with a passionate commitment to the job and the client evident at the site throughout the weekend. Practically the whole company was involved in this massive operation, and the immediate feedback from the client was extremely positive. 

More concrete delivered under water 
The second major pour at the beginning of October involved 16,000 m3 of steel-fiber reinforced concrete used to create a very strong cap over the layer of softer concrete put in place earlier in the year. This cap is designed to resist the pressure of the bore head during construction and, after completion, the potential impact and weight of a large sunken vessel resting on it, in order to protect the integrity of the tunnel below. The custom concrete design was developed during several weeks of tests and features an incredibly high fiber density of more than 150,000 fibers/m3 (33 kg/m3). The steel fibers used to strengthen this concrete are just 0.75 millimeters thick and six centimeters long, formed with small hooks at each end to promote their bonding.

For this phase of the job, all hands were on deck once again to produce and then deliver the concrete to the underwater site, with the pour requiring 880 mixer round trips over another four days of intensive activity. It was another challenge of scale which was met by all involved. The coordination of the delivery and pouring of this special product was carried out without the slightest incident, again to the great satisfaction of the customer.