Contractor WMZ from Wittislingen installed five multi-pipe bundles, each over a length of 220 metres, beneath the Rhine as part of developments to extend the Rhine power plant Iffezheim with a fifth turbine. Works on the project began in 2009 and will be completed in 2012.

A small bore area within the undercurrent of the sluice required the pilot bore to be carried out directly from the landing stage at the waterside. The bore rig had to be set up on the other side of the Rhine at the substation for the expanding and installation process of the four-fold pipe bundle.

Preliminary work on the starting pits included securing the pits with sheet piles and installing casing pipes with some of the tasks. Approximately 30 metres of long steel pipes ND 600 had to be installed with an inclination of 42 degrees by auger boring. It was important to protect the embankments against any blowouts caused by the drilling fluid under pressure. A distance of three metres had to be held to the landing stage.

The bore path

WMZ used a Tracto-Technik Grundodrill 25N for the pilot bore. The Grundodrill 25N was positioned diagonally to the landing stage and, following the preliminary works on the starting pit, drill rods were pushed into the casing pipe

After a tour through 30 metres of steel pipe the bore head travelled approximately 190 metres in the direction of the substation. The deepest point of the bore path was approximately 8 metres below the river bed at a total depth of 23 metres.

The difficult ground conditions of sand, pebbles and rubble underneath the riverbed required that a spate magnetic field be set up for detection purposes .The cables installed for this field formed a square, which led from the starting point of the first bore, through the already installed pipes. The cables then ran parallel to the Rhine to the starting point of the following bore.

The first bore was used as a reference path which was converted with the Para-Track-Method to each of the next bores at distances of ten metres parallel to the bore. Intended path and actual path were clearly indicated with symbols for inclinations, depths and lengths on the laptop monitor. A data cable was installed which carries out continual checks and collects data. Steering orders are generated from this data cable and converted on the bore rig into the required thrust or rotation of the pilot rod. With each added rod the data cable was carefully extended by a further three metres.

The expanding process

For the following expanding processes to 280 mm, 360 mm and to 580 mm bore diameter and the pulling in of the 4-fold pipe bundle, the bore rig had to be transported to the substation on the opposite Rhine embankment and placed in position, as the pre-stretch of the pipes to be installed was only possible on the landing stage. 20 metres of egelen PE 100 pipes with a diameter of 160 mm, SDR 11 in a bundle of four, each with a 220 m length, were welded and pulled in together.

The drilling fluid was tuned optimally to the stabilisation of the bore canal and the reduction of friction during the pulling process. The penetrable structure meant that there were high demands on drilling fluid.

Pulling forces reached 12 tonnes during the installation of the 5 tonne pipe bundle. Inside one of the pipes three further protection pipes of 50 x 4.4 mm were installed for control lines, as well as the cable for the set-up of the magnetic field for the next pilot bore. A remaining three protection pipes with a diameter of 160 mm will pull in cables for the triple-phase transfer line.

Rhine power plant Iffezheim

The Rhine power plant Iffezheim will be the largest hydropower plant in Germany and one of the largest in Europe. A joint partnership of the EnBW, as well as the EDF and operators of the hydropower plant, the Rhine power plant Iffezheim is a €90 million investment which will produce 122 million kWh C0²-free, environmental friendly power annually.