Rapid bridge replacement IS a construction technique that has been gaining momentum in recent years. In seeking to reduce the time that critical roads are out of action, municipalities and highway operators have been maximising off-site construction. For bridges this means building the new structure and installing it as a unit, rather than building it in-situ.

Moving bridges has been made possible thanks in part to development of self-propelled modular transporters (SPMTs). These lifting devices carry huge loads along a series of axle lines that can be adjusted depending on the size and weight of the required load. Each transporter consists of anything between four and eight axle lines along with its own power and control systems. But crucially the units can be stacked together both longitudinally and horizontally to create a transportation system suitable for loads of any size or shape. Lifting two tunnels of up to 3,600t in Warsaw then, was a straightforward task for Mammoet’s SPMTs. The firm was brought in by Austrian contractor Strabag to develop a new solution for installing tunnels beneath a railway in the city centre. “Warsaw Municipal Road Building Project Authority as a developer wanted to connect the city centre and the Targówek district in Warsaw,” explains Adam Malik, project engineer for Strabag. “The contract where the tunnels were built is a section of Swietokrzyska route, with its length about 3km.”

But to provide the connection to the Targówek district meant tunnelling underneath the existing live railway and the client did not want to undertake a lengthy possession that could have meant closing the railway lines for months. “The original idea was to close it and somehow redirect all those lines to other railway roads but this would be very difficult,” explains Edvinas Ivanauskas, general manager of Mammoet Baltic. What is more the rail lines were not only for local connections but international trains. “We partnered up with Strabag and they asked us to look at the problem in a different way. They said you have to do it in a couple of days and all in all we have a week or two maximum closure of the railway,” he says.

Mammoet therefore came back with a solution based on constructing the two required tunnels as independent structures close to the final location and then moving them into place. “As the first structure was 3,600t we could do it in a couple of ways, we can do it in a skidding way where you put it on a big railway track then push it with special hydraulic systems, or we can put on multi-wheeler SPMT and we can drive it and position it to 10mm precision,” he says.

Due to the ground conditions, which included a high water table, and restricted city centre area it was decided that skidding would be too time consuming and require too much ground preparation work to undertake. The SPMTs would be faster. “Using our method to pick up from the original place and drive it to the final point took us around an hour and a half. That makes a very big difference,” says Ivanauskas.

But before work could start on site Strabag had to be convinced that this new method was worth the investment. “The client asked us to make a test so they could see it because they have never built it before so we asked them to come and see a big structure that we were driving around in Holland. For them and for us everything was working as planned and this convinced the client to take a new approach,” he says explaining that this itself was a huge achievement as it required a completely new way of thinking.

Following the test, the client decided to move ahead with the new approach. “This Mammoet technology in comparison to other options enabled us to reduce temporary works like sheet pile walls, temporary viaducts, and temporary concrete slabs. Reducing impediments into railway traffic was also very important to get acceptance by the railway authority,” says Strabag’s Malik.

Explaining that the ground itself was strong and cohesive Ivanauskas says that the preparation for the tunnel insertion was simply to excavate the hole beneath the railway and install a concrete support framework for the tunnel to sit on. “There was no special preparation required. It was built on special supports which we could drive under.”

The tunnel sections were sitting on a prefabricated base which held the tunnel up at a level which allowed the SPMT to drive beneath it. Hydraulic jacks were then used to lift the tunnel from the base before the SPMT drove it over to the new location. The SPMTs were operated using remote controls. “The tunnel was not straight and we had to position the whole thing a couple of degrees,” says Ivanauskas. “We did this move and positioned it exactly, for the client they had never seen that.” Careful consideration had to be given to the structural integrity of the concrete which was at risk of cracking. “We also had to be very careful with this. It is easy to crack concrete, steel has much different bending moment to concrete. It can crack quickly so there are big engineering calculations needed for this,” Ivanauskas says.

The first, and largest of the two tunnels was installed in June 2016. This tunnel was 80m long and 9m high. The second 3,200t, 70m long tunnel was installed a few months later in September. Constructing each tunnel took a number of days rather than months. For example, the second installation from excavation to installation to backfilling and reopening the railway took a total of 18 days and the client says the method proved very useful. “Technology to transport civil engineering constructions with SPMT trailers appears as a very useful method to minimalise impediments into the public traffic and gives opportunity to reduce time and costs of works,” says Malik