Longitudinal and transverse prestressing for Maas Delta tunnel

As the main supplier of the prestressing for the Blankenburg connection, DYWIDAG supplies enormous quantities of prestressing steel. Especially for this project, the company had to scale up considerably and even quadrupled its staff. An interview with Robert Jansen of DYWIDAG about this wide-ranging project and, in particular, about the uncommon prestressing for the Maas Delta Tunnel.

We are involved in the complete Blankenburg connection, Jansen begins. "That is, the prestressing of the Holland Tunnel, of various structures on the north side near Maassluis and at the Rozenburg interchange, and to top it all off, the prestressing for the Maas Delta Tunnel. The construction of a tunnel is in itself an unusual undertaking by Dutch standards, and even more so in this form as an immersed tunnel. Unlike 'usual', the tunnel elements are not only constructed with a transport prestressing, but also with a transverse prestressing in the floor and roof."

Cruise ship workshop

The tunnel elements of the Maas Delta Tunnel are being built by the BAAK consortium in the Damen Verolme dry dock in Rotterdam, normally a 'workshop' for large cruise ships. Jansen: "On the face of it, it looks like an enormous concrete container, but the tunnel elements are curved, which makes it quite a lot of trial and error. At some points adjustments were made to the design in order to be able to pre-tension at all, because of the limited space between the elements and the wall of the dry dock. All this, of course, in close consultation with the construction team and consortium BAAK. We even had jacks flown in from America to pre-tension some of the elements." The elements are assembled in the dry dock into two slots and then sailed out and sunk at the final location. "By mid-April, the tunnel elements have to be transported, because another ship will arrive at the dry dock for maintenance. So that means a hard deadline."

2,500 tons of steel

The design of the tunnel elements means that a transverse prestressing is required in addition to a transport prestressing (longitudinal prestressing in the floor and roof for buoyancy, sailing out and sinking). "The tunnel will soon lie on the ground like a 'snake' but it needs to be moved as a rigid mass," Jansen explains. "The moment the tunnel is immersed, the transport prestress is cut through and the soil package takes over, so to speak. Unlike usual, the design does require transverse prestressing, implemented in the floor and roof with 22 and 27 strands, respectively. All the prestressing added up to 1,300 tons of steel for the Maas Delta Tunnel alone. The entire project involves over 2,500 tons of prestressing steel, many of which had to be carried out simultaneously. So as a company we had to scale up considerably to meet the demand, both in work preparation and at the execution level. And that worked out well, without shortchanging other projects."