Within the scope of what is called the Kombiloesung (combined solution), Karlsruhe's centre is being converted to a pedestrian zone without any railway tracks. For this purpose, a 2.4 km long, single-tube city tunnel is being built underneath Kaiser Road with a 1 km long southern branch leading towards the Congress Centre. The major project also includes the construction of a new road tunnel between Karlstor and Mendelssohn Square.
Since most of the subterranean stations are located in the Upper Rhine Rift in layers that are unstable or not watertight, impermeable excavation enclosures consisting of diaphragm or bored pile walls had to be built in several sections due to the high ground water levels.
Strand anchors with a total length of nearly 24 km
To tie back the different retaining walls, DSI had previously supplied approximately 100 t of strand anchors. Recently, DSI received a follow-up order from the subcontractor Stump Spezialtiefbau GmbH. At the Ettlinger Gate, DYWIDAG strand anchors were also used to tie back the walls. In total, 457 semi-permanent, 8 strand strand anchors with a total length of 11,422 m were installed to stabilize the walls against ground water pressure. DSI also supplied 572 temporary, 5-6 strand strand anchors that were installed at a total length of 12,511 m. Furthermore, DSI supplied 150 load cells to monitor the forces acting on the anchors.
The high ground water levels and several obstructions in the ground such as steel sheet piles posed a special challenge.