The town of Wiehl, located a short drive to the East of Cologne, Germany, has a number of attractions to offer including a publicly accessible stalactite cave with an 868 m long tunnel system, a 13.4 km length panorama trail, the so-called ‘Beer Trail‘, the ‘Axle, Wheel and Carriage’ museum and a wildlife park with a nature trail.
The town had another temporary attraction to offer at the end of May 2020 with the renewal of a 275 m length combined sewer pipeline under rather difficult conditions which included:
- The course of the pipeline to be renewed was on a steep slope
- It was situated 7 m beneath a country road
- It was embedded in solid and fragmented rock
- The slope was difficult
- The was extremely difficult to access with impassable terrain
- There was 4 to 5 m excess cover
The combined sewer was built about 50 years ago with reinforced concrete pipes in nominal diameters of 600 and 700 mm, but cracks, fractures, and tree root intrusion lead to more and more to leakage. A conventional sanitation operation was out of the question, as on the one hand, the cross-section had to be upsized and on the other hand, the static load on the damaged reinforced concrete pipes was no longer guaranteed, due to the excess cover, oppressive and difficult slope and the country road just above the slope.
The municipal works department in Wiehl commissioned the Schumacher Planning Agency to plan and produce a tender for the renewal of the problematic pipe section, which due to the complicated framework conditions, would only be possible, using static pipe-bursting equipment with the closed, trenchless construction method. The course of the pipeline on the steep slope and the extremely poor accessibility alone definitely did not allow the open trench method without huge cost. This does not also take into account the need to re-locate the country road by 150 m, which would have been unavoidable using an open trench method. The contract was awarded to Alfes & Sons GmbH from Wenden, which has mastered many a difficult project over the years using pipe-bursting methods, including in the Oberbergisch district in Wiehl.
The result of two prior dynamic probes hardly provided much room for optimism. The entire pipe route, over a length of 275 m, was completely embedded in the natural existing rock and it was a complete mystery how much compressible soil was available next to the pipe path. Therefore, room for soil displacement was probably as good as zero. This also meant that application of the pipe bursting method was on thin ice as, despite all the advantages, there was quite a big risk involved! In the absence of an affordable alternative and with the confidence of the responsible employees at the specialists Alfes & Sons, the Stadtwerke Wiehl decided to use pipe bursting after all.
There is a solution for everything! In Wiehl one part of the solution meant, a powerful, resilient and reliable machine had to be applied, in this case a GRUNDOBURST 2500G. The second part of the solution was an adapted dimensioning of the installation lengths. It made sense to carry out the first installation length over 45 m and the second over 100 m. The third length, over 130 m, was planned to be the last, as it also seemed to be the most critical. The 300 m length of PP-HM installation pipes ordered for this task, were welded together on site to form three separate sections.
The excavation pits on the slope, which were up to 5.5 m deep, were clad with SBH Mega-sheeting panels, which even had to be secured with rock anchors at the deepest point. Likewise, the 25 ton chain excavator was secured during the excavation with the aid of rock anchors to prevent it from falling into the approximately 20 m deep and 60° steep abyss.
Then work started. The GRUNDOBURST 2500G, with its maximum pulling force of 2,550 kN, was brought into position, together with the magazine with the Quicklock-burst rods. Not exactly smooth, but nonetheless pretty even, the bore rig pushed the burst rods into the old pipeline. Here the old reinforced concrete pipes had a diameter of ND 700. Breaking open and displacing the old pipes worked quite well, as did the expansion to 800 mm with four welded-on blade strips and the pulling in of the new pipe length. The renewal of the first two installation lengths had already proved to be a success.
Then, on the last 130 m installation length, which ran at a depth of 7 m, things started to get tougher and the sweat started to flow, for man and machine. The old pipe narrowed down to ND 600 in a manhole and the bore path no longer ran in a straight line. It bent in the flow path some 10° to the right and then ran in the final third in a slight curve to the left. Here all the power reserves of the GRUNDOBURST 2500G were required. Shortly after the 10° bend the pulling force increased considerably until, after only a few burst rods, pipe pulling came to a standstill. About 30 m before the end of the pull-in process a weld seam tore and split the pipe length into two sections of equal length. Work was brought to a halt immediately in order to locate the exact point of the tear. In doing so, the crew discovered that an approximately 6 m long section was missing at the first manhole passed through, but the bore channel was still distinctive in the entire cross-section.
Problem identified, problem eliminated. The next day the team closed this ‘gap’ from the manhole, by installing short pipe sections.
Parallel to this the crew dug an additional excavation pit at the known location of the bursting head, which now no longer had to be excavated on the steep slope and was therefore sufficiently dimensioned to a depth of 3.5 m.
Now the remaining 30 m of the old pipe could be completed with the pipe bursting method, by pulling in short pipes.