Twin-pack ductile iron pipes go round the bend

1 The problem

In the south-east of Berlin, in the Altglienicke/ Grünau district, two new pressure pipelines needed to be installed in the context of a reorganisation of the sewage system. After a careful comparison of the options available, Berliner Wasserbetriebe decided in favour of the shortest route for the pipeline. However, this was one which crosses a railway track and a seven-lane main road, meaning that open-trench installation was excluded from the start.

Installation using the HDD process was also excluded, both for space reasons and on account of the railway crossing. So this left a section of protective conduit to be driven for the trenchless installation of the new pipes (Figure 1). However, because of the railway bridge abutments, the protective conduit had to be jacked in at a great depth, otherwise driving in a straight line would not have been possible.

2 The solution

Under these difficult conditions, it was decided to run a DN 1600 reinforced concrete pipe in a curved line passing the abutments of the bridge at a depth of 13 m, at a sufficient and permissible distance from these, by means of manned compressed-air shield tunnelling over a length of 215 m. The two DN 500 and DN 600 utility pipes were then to be drawn into this “protective conduit”. As a pressure pipeline, this piping system had to be locked against longitudinal forces and also capable of bending because of the curve in its route. In addition, the pipes had to be laid one on top of the other so that they have the same angular deflections and lengths. Because of the depth of the shafts, the pipes could not be too long and they had to be easy to assemble (Figure 2).

Ductile iron pipes meet this requirements profile 100 %. Therefore, in each case 216 m of DN 500 and DN 600 sewage pipes to EN 598 [1] with BLS^® restrained push-in joints and wall thickness class K 9 were used.

3 Implementation

The following conditions need to be met if a challenging construction project like this is to be completed successfully:

• precise planning taking account of all influencing factors,
• the performance of a conscientious approval process with the participation of all utility companies as well as constant and prompt communication with these and all other parties involved,
• exact tendering processes for materials and processes and the contractors providing them,
• selection of a competent and experienced construction company taking account of the corresponding references and economic criteria in the context of a specified schedule,
• consistent supervision of construction measures and the conditions under which they are performed, including acceptance and documentation.

All a matter of course in day-to-day business for Berliner Wasserbetriebe. With an eye to efficiency and performance, even with particular and special construction projects, the Berlin construction company Beton & Rohrbau 2.0 GmbH, with its convincing references, was awarded the contract.

3.1 Manned shield tunnelling of the section of protective conduit

The work began with the sinking of the launch and target shafts to a depth of 14 m, entirely in the groundwater, using bored pile walls in watertight concrete (Figure 3). The bored pile walls had to be watertight in order to withstand the enormous pressure of earth and water and to absorb the forces of the pipe-jacking machine. For static reasons, an elliptical layout was selected for the launch and assembly pits, with the longer axis in the direction of driving.

This layout was advantageous both for setting up the pipe-jacking machine and for assembling the sewage pipes. Finally, the bottoms were concreted so as to be watertight.

To support all the work, a crane runway was erected above the launch and assembly shafts with which all machines, pipes and accessories could be lowered into the pit.

After assembling the machine for the manned shield driving, the bored pile wall was opened up and sealed in the direction of driving.

Continuously manned pipe-jacking is indispensable when it comes to driving a controlled curve deep in the groundwater, especially when inclusions in the soil (boulders) dating back to the ice-age are to be reckoned with.

With manned pipe-jacking there are two workers in a spoil chamber behind the tunnelling shield who control the driving. The soil extracted is conveyed to the surface by piping systems. Obstacles such as large stones and boulders are removed by hand and larger chunks are broken up in situ. To prevent groundwater penetration the spoil chamber is kept pressurised. The men are working under this increased air pressure and therefore need to pass through the airlock installed in the machine before and after work (Figure 4). Just like divers, they have to spend a certain amount of time in the airlock to allow for pressure equalisation.

Without this pressurisation, the section being tunnelled and the launch pit would be flooded. Between the reinforced concrete jacking pipes and the bored pile wall, some complex sealing is required. The shield machine and the airlock are pushed forward from the launching pit by the jacking pipe segments as the tunnelling progresses.

The climax is always reaching the target pit accurately and opening up the bored pile wall. This worked out here perfectly.

3.2 Installation of the ductile iron pipes

In order to be able to pull in both pipes one on top of the other, the following parameters needed to be taken into account:

• Regardless of the process (pushing or pulling in) the horizontal position of the pipe string in a curve must be ensured so that the pipes are not drawn out of the curve or pushed into the curve.
• When subsequently sealing the protective con- duit, the stability of the vertical position must also be considered.
• The preferred direction of installation for restrained pipes is always pulling because in this way the push-in joints are already extended and locked during the pulling-in operation.

Therefore, a U-rail was positioned in the bottom of the inside radius of the casing pipe section along which the pipe clamping frames fitted with rollers were then run. The pipe string was also supported on rollers on the clamps along the springing line of the protective conduit so that it could not tip over during the pulling-in process. Pipe clamps on the frames took up the pipes and ensured their smooth insertion. There was no connection between one frame and the next. The pulling forces were only transmitted via the restrained push-in joints of the pipes.

The BLS^® push-in joint is characterised by its ease of assembly and the high permissible tractive force, even during angled insertion. Therefore, the BLS^® push-in joint has proved to be of great value for all installation processes and high-pressure applications.

This thrust resistance is achieved in the following way: the spigot end of the pipe has a welding bead applied around the circumference of the pipe in the factory, which is firmly and deeply anchored in the wall of the pipe by its root. In the socket section, there is a (thrust protection) chamber with two “windows” arranged on the front of the socket. Once spigot end and socket have been put together, the cast-iron locking segments are inserted through these windows and arranged around the circumference of the pipe. And dismantling the pipes is just as easy as assembly: after extracting the locking segments the joints can easily be released again, e.g. when dismantling fittings after pressure testing or when dismantling the pulling heads; this is where the BLS^® system has clear advantages over other systems.

Traction heads are mounted on the spigot ends (also BLS^® system) of the first pipes in each case (Figure 5). The tractive forces are taken up by a traction-head string construction. The possible tractive loads of 86 t or 120 t with a permissible angular deflection of 2° or 3° are guaranteed by the manufacturer, although with this project, with only 5 t, there was a great deal of capacity to spare and hence a high degree of safety. The traction string construction ensures the balanced transmission of the tractive forces to both pipe strings. The construction was pulled by a traction cable. In this process the DN 600 pipe was beneath the DN 500 pipe (Figure 6). It was also important to make sure that the pipe lengths were the same so that the socket joints linking them were always directly aligned. The pipes were bundled together for fixing onto the clamp construction and then assembled, extended/locked and screwed down (Figure 7).

The joints are wrapped with shrink-on sleeves because the surrounding space in the protective conduit was later to be filled/sealed.

For the closure and flange transition in the shafts, the pipe manufacturer supplied the appropriate fittings from the BLS^® system (flanged spigots and sockets, [2]) and also the traction heads.

During the assembly of the traction heads and the first pipe joints, the construction company was supported by an engineer from the pipe manufacturer, who was also present for the application of the welding beads onto pipes which were shortened on site (Figure 8).

After successfully completed pressure testing and camera inspection the shafts were partially filled with concrete; hence they also served as thrust blocks for the rising conduit arm.

4 Concluding thoughts

The ever more densely entwined infrastructure of today’s cities requires some new thinking as regards the utilisation of installation space and the application of modern construction methods if it is to satisfy all aspects of sustainability. In order for challenging construction projects to be economically justified it is always essential to aim at an operationally secure, problem-free working life which is as long as possible.

The best chain of competence consists of 

• excellence in prudent planning and prepara- tion,
• competence in performance by an experienced specialist construction company,
• experience in construction supervision and
• durable materials with high safety margins.

 

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