The sewer to be renewed is located directly at the rear of a higher regional court building in Hülchrather Street. As an alternative to microtunnelling, the city council’s sewage department wanted to test pipe bursting as a method to replace two sections of 76 m and 66 m pipe, in the residential and shopping district.

Pipe bursting was chosen as it can be a cost-saving possibility to renew damaged sewers without having to dig trenches and also minimises the impact on local available infrastructure.

In addition to addressing leakages from the joints of the old sewer, the main reason for the replacement was to improve the sewer’s hydraulic capacity.

A special challenge

The plan was to replace the concrete oval profile pipe 350/450, situated at a depth of 4 m, with a circular PP-HM Pipe with an outside diameter (OD) of 560 mm. Bursting oval profile pipes and simultaneously upsizing the cross-section is not carried out very often with the bursting method, which made this project a special challenge.

The construction contract was awarded to Friedrich Wassermann and Weitz and Co, who sub-contracted ALFES and Sons GmbH in Olpe.

The required pulling rig was a Tracto-Technik Grundoburst, which is available in four performance classes from 40–250 tonne pulling and thrust forces.

The QuickLock bursting rod, which is initially pushed ahead into the old pipe and then pulled back together with the bursting tool expander, and attached to the new pipe, is a special characteristic of the machine. The QuickLock’s no-screwing function is an advantage because the single rods are linked together, giving pulling and thrust stability and saving time.

Overcoming compact sandy ground conditions

In Section 1, the soil inspection carried out in advance showed loose, sandy soil and the best conditions for the displacement work was to expand the bore hole from 450 mm to 610 mm. The bursting tools were configured accordingly and a Grundoburst 1250G, with 125 tonne pulling force, was applied.

After only a few metres, the work had to be stopped because the pulling forces for the displacement work were insufficient due to the extremely compact layered sandy ground in the immediate surroundings of the pipe.

In the end, work continued with a Grundoburst 2500G instead, with 250 tonne pulling force, and the first section was completed successfully after only three working days.

250 tonne pulling force

The experience gained from the first section made Section 2 easier, and the Grundoburst 2500G was applied from the beginning.

Accordingly, a large machine installation pit, an intermediate pit – for the five house connections with an emergency disposal unit – and a pipe installation pit were set up according to the regulations and secured with sheet piles.

The traffic and driveways were only slightly affected in selected areas.

Pushing the rod

After the installation of the rig, the bursting rods were pushed through the old pipe towards the pipe installation pit. Each 225 kg QuickLock bursting rod was attached and dismantled with the help of an excavator.

The rod pushing process took approximately three hours. The installation of the 1.6 m long blade, the expander, with a diameter of more than 610 mm, and the connection of the first pipe inside the pipe installation pit required approximately two hours of set-up time.

In many cases, marginal conditions – such as lack of space, depth of the pipe or groundwater – only allow for the installation of single pipes. The new pipe in this case was a 3 m, 560 x 40 mm Schöngen polypropylene pipe with a multi-latch welding connection.

Inside the expander there was a connection for a smaller latch-on rod, which runs through the new pipe and is connected to the hydraulically-operated Burstfix tensioning device at the end of the pipe string. Tension is necessary for the pipes during the installation to guarantee a tight connection for the pipe string.

The multi-latch-welded connection combines the well-known multi-latch technology with inductive welding technology, to minimise the usually long cooling time. Due to the rigid support and pulling elements inside the connection, the pipes can withstand pressure as well as pulling forces immediately after the inductive welding process.

The necessary press-on pressure for the welded pipe is sustained by the latches. Welding of each single pipe was carried out in only eight minutes, and installation can be carried out without long cooling times.

A bit of pipe improv

The pipe installation itself took approximately one hour for each pipe, including the connection and welding time. Some improvisation was required when driving the bursting tools and pipe string into the machine pit.

Normally the disassembly is carried out within the area of the extension frame, which also operates as an abutment. In this case, there was not sufficient space inside a standard extension frame, so two T-beams, each with a length of 3 m, were placed between the pit wall and the Grundoburst in its place, so that the blade and the expander, as well as the pipe string could be pulled in far enough for the disassembly.

After these first experiences with the renewal of oval profiles using the pipe bursting method, the technology has proven to be an interesting and economical alternative to other methods.