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Renewal of a combined sewer pipe with GRUNDOBURST 2500G

19.10.20 - The town of Wiehl, located in the Oberbergisch District is not exactly large in terms of area and population, but nevertheless has quite a few interesting sights to offer. For instance, a public 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, which presents the 5.500 year history of the automobile industry and a wildlife park with a 1,8 km long nature trail.

This combined sewer was built about 50 years ago with reinforced concrete pipes with the nominal widths 600 and 700 mm, but now a camera inspection reported a series of cracks, fractures and tree ingrowth. A conventional sanitation 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 of the damaged reinforced concrete pipes was no longer guaranteed, due to the excess cover, oppressive slope and the country road just above the slope.

Trenchless, was the only way

Stadtwerke Wiehl commissioned the local Schumacher Planning Agency (pbs) with the planning and tendering for the renewal of the problematic 275 m length pipe section. For those responsible at pbs, who were very familiar with similar defined requirements, the complicated framework conditions allowed only one solution. The renewal of the pipeline with the closed, trenchless installation method, applying the static pipe bursting method (destroying the old pipe with a burst head and penetrating it radially into the surrounding soil, while simultaneously pulling in the new pipe) and upsizing the diameter by one or two (respectively), nominal sizes. The course of the pipeline on the steep slope and the extremely poor accessibility alone definitely did not allow the open trench method without causing a cost explosion. Not to mention the need to relocate the country road by 150 m, which would have been unavoidable using the open trench method.

The Stadtwerke themselves had witnessed many positive experiences with the trenchless renewal of pipelines in the past and therefore highly appreciate its advantages. In recent years numerous large combined installations have been completed to their satisfaction with this method, so they endorsed the decision of the planning office (especially for this task) to apply the trenchless construction method, using the pipe bursting method, to be more precise. The contract was awarded to Alfes & Sons GmbH from Wenden, who have mastered many a difficult project over the years with the pipe-bursting method, even in this Oberbergisch region in Wiehl.

Beware, there is a risk

The result of two prior dynamic probes, to determine the penetrability of the ground, as a basis for the subsoil assessment, hardly provided much room for optimism. The complete pipe path over 275 m was completely embedded in the existing rock and it was a complete mystery, how much modifiable soil was available close to the pipe path. How much room would there be for soil displacement? The answer was probably: zero.  This also meant that the application of pipe bursting was standing 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 and also the technical advice given by managing director Gerhold Alfes, the Stadtwerke Wiehl decided to go ahead and use pipe bursting.
 

Intelligent planning

Gerhold Alfes would not be such a successful entrepreneur, if he did not balance out all the eventualities for such a difficult construction project. On the day the renewal of the combined sewer was to start he was on site personally and explains: “Of course, I was fully aware that this would not be a piece of cake and we would need to keep our focus on the massive rock, in which the pipes were embedded, as well as the steep slope beneath the country lane. However, there is always a solution to everything. Here in Wiehl one part of the solution is, a powerful, resilient and reliable machine – and that is the GRUNDOBURST 2500G. The second part of the solution is an adapted dimensioning of the installation lengths. It makes sense to carry out the first installation length over 45m and the second over 100 m. The third installation length, over 130 m, was planned to be last, as it also seemed to be extremely critical.
 

Perfect Preparations

 
The 300 m length PP-HM installation pipes 730 x 35 mm (long pipes) from SIMONA AG, ordered for this task, were welded together on site to form three separate sections and ready for installation. What proved to be more difficult, was creating the excavation pits in depths of up to 5,5 m on the slopes; a real challenge for those responsible. The steep slopes had to be stabilised to, not only guarantee the safety of the operators, but also to prevent the country lane from sinking lower. “We even have to be prepared for such extreme situations”, Gerhold Alfes explained, “and in this case we clad the excavation pits with SBH Mega-sheeting, which even had to be secured with rock anchors at the steepest 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 approx. 20 m deep and 60° steep abyss.”
 

First and second installations are okay

And off they went: The GRUNDOBURST 2500G with its maximum pulling force of 2.550 kN, the strongest among the strong, was brought into position, together with the magazine and 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 also pulling in the new pipe length. The renewal of the first two installation lengths had already proved to be a success.
 
Then, in the final 130 m installation length, which ran at a depth of 7 m, things start to get sweaty – for man and machine. The old pipe narrows down to ND 600 inside a manhole and the bore path no longer runs in a straight line. It bends in the flow direction, first by 10° to the right and then runs in the final third in a slight curve to the left. Now the Alfes team had to expect higher forces, which meant all the power reserves of the GRUNDOBURST 2500G were required.
 
 

Big problems on the third section

Shortly after the 10°-bend the pulling force started to increase considerably – yes, even alarmingly, until after only a few burst rods, pipe pulling came to a standstill. About 30 m before the end of the pulling-in process a weld seam tore and split the pipe into two equal lengths. Work was brought to a halt immediately in order to locate the exact point of the tear. In doing so, they discovered that an approximately 6 m long section was missing at the first manhole they passed through, but the bore canal 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 they dug an additional excavation pit at the current location of the bursting head, which no longer had to be created in the steep slope and was therefore sufficiently dimensioned with 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.
 
 

All’s well that ends well

They made it, everybody had given everything. The Stadtwerke Wiehl, as the client with their trust in the pipe bursting method and in the people who used it to execute the difficult task involved, the planning agency with their foresighted cost calculation and their confidence in a secure order procurement, as well as the executing company Alfes + Sons GmbH, with their many years of know-how, their real risk assessment and finally their tried and tested problem-solution management. At the end of the day, the GRUNDOBURST 2500G, with its enormous power, which really went to the limit of its capabilities, but never left any doubt about the successful outcome of this pipe renewal task, was what really made the difference. Chapeau for man and machine!
 
 
 
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