Drinking water culvert with ductile cast iron pipes under the Nežárka river
The Chotýany-Zlukov drinking water supply pipeline is one of the most important transport pipelines of the Southbohemian region. Previously, it crossed the River Nežárka as an above-ground pipeline, suspended from a steel bridge, but now, for various reasons, it had to be laid under the river with the construction of a culvert.
Data on the culvert
As a 71 m long pipeline of DN 600 ductile iron pipes, the culvert was pulled using the trenchless horizontal directional drilling (HDD) technique. The total length of the drilling, including the pilot bores, was 113 m. The curve in the culvert benefitted from the flexibility of the socket joints of 2 degrees in each case, which with this installation technique, the high flexibility of the socket joints means that, at the same time, they can also withstand the high tractive forces which occur while the pipes are being pulled in by the drilling rig. For the DN 600 ductile iron pipes with BLS®restrained push-in joints used, tractive forces of up to 1,525 kN are permissible. The external diameter of the joints is 742 mm; the positive-locking joint with welding bead and locking segments withstands an operating pressure PFA = 32 bar.
With the use of trenchless technologies for example, it is necessary to select a pipe system with high-quality mechanical external protection and also strong corrosion protection. Therefore, ductile iron pipe with fibre-reinforced cement mortar coating were used as this protects the pipe against mechanical damage when it is being pulled through in the trenchless technique. The sockets of all the joints are protected with a rubber or thermoshrink sleeve and a sheet metal cone, which prevents the rubber sleeve from being stripped away and grinding damage being caused to the socket.
Data on the HDD process
The pipe string was pulled in with a special pull-in head to fit the shape of the socket. It is longitudinally positive-locked by means of locking segments to the welding bead of the first pipe.
An important parameter which has a considerable influence on the success of the horizontal directional drilling technique is the geological composition and condition of the subsoil at the installation site. Here the basic conditions were complicated: on the starting side there were deposits of loose river sediment as far as the slightly weathered paragneiss at a depth of around 3 m and then hard granite. On the opposite, target bank, which lies approx. 2.5 m lower, there were also deposits of loose river sediment, but to a considerably greater depth. The profile beneath this is similar to that on the starting side. The groundwater level is at a depth of approx. 1.2 m. According to the planning, from the starting side the drilling would be through the hard rock strata; at about halfway across, the drilling would be through the sandy river sediment.
Driving, widening and pipe pulling in 12 days
With a horizontal drilling rig (with a maximum pulling force of 150 kN) the pilot bore was driven with a roller bit and then widened to 300 mm in a second drilling process. Using the considerably larger drilling machine the bore was widened in stages to 400 mm, 500 mm, 700 mm and finally to 950 mm.
In order to be able to pull in a DN 600 ductile iron pipe with an outside socket diameter of 742 mm, an opening of approx. 950 mm is necessary. The volume of a borehole with a diameter of 950 mm and a length of 71 m corresponds to 50 m3. So that the pipe string can be drawn through the borehole, this volume of earth has to be transported to the starting and target pits in each case! That means: The borehole has to be cleared as far as possible during each individual widening process.
During the stage-by-stage widening of the borehole to the diameters of 500 mm, 700 mm and 950 mm, in total around 350 m3 bentonite was used for the drilling fluid, which could then be sent to a recycling plant for reuse.
For the pipe pull-in, the pipes were preassembled and successfully pulled through in one piece at the same time as the final widening stage without problems.
A premiere under difficult framework conditions
By the construction of a culvert under the Nežárka, the essential drinking water pipeline could be protected from flooding, vandalism and other disrupting influences and the undisturbed supply of an entire region could be ensured. It is not without pride that the responsible members of the association regard the fact that this was the first project in the Czech Republic in which ductile iron pipes have been installed using the horizontal directional drilling technique to drill under a river – and this under quite difficult conditions.