Buderus Manual on Trenchless Installation of Ductile Cast ... - Duktus

Trenchless replacement of gas and water pipelines
3.3 Requirements for the new pipe
The new pipe and its joints have to withstand high tractive forces and still have to have
large safety margins when they do so. The pipe has to have rugged outside protection
because it cannot be guaranteed that there will not be any debris, stones or fragments
in the zone where the pipeline is situated. It is important for the traction head to be able
to be fitted and removed quickly and straightforwardly when it is being recovered, even
when the weather is very bad. The pipe and its joints have to be long-lived and resistant
to tree roots. Because the paths followed by many pipelines are not exactly straight, it is
vital for the sockets to be able to accommodate angular deflections. In inner-city areas,
launch pits are a maximum of 7 metres in length so the ratio between the size of the
launch pit and the number of joints to be made is very good. Because a combination of
trenchless and open-trench techniques is often used, the types of pipe used have to be
compatible and there has to be a complete range of fittings available, including for use in
the intermediate pits.
<strong>Buderus</strong> ductile cast iron pipes with cement mortar coatings and BLS ® joints have all
these essential prerequisites. They are thus the ideal pipes for the trenchless replacement
of pipelines. What is more, there are fittings for performing leak tests which are very
easy to use and which are easily fitted and removed. The requirements in respect of
permitted tractive forces, angular deflections and minimum radiuses of curves which
have to be met under the DVGW rules and the internal rules of Berliner Wasserbetriebe
are given in Tables 2.1 and 2.2 in section 2.
When the technique is being used in sandy soils, the external shape of the socket, with its
shoulder-like transition to the main body of the pipe, has been found to be very successful.
Where, in soils containing non-cohesive material, there is a transition to material of this
kind individual pebbles tend to roll under the main body of the pipe during the pulling-in
process, and the entire pipe string thus undergoes an upward movement as it is pulled
in. The result may be excessively thin cover over the pipe and even pushing-up of the
pavement at the surface. For special cases
of this kind, <strong>Buderus</strong> has developed the
ZMU-PLUS pipe on which the transition
between the main body and the socket
is filled in with additional layers of the
cement mortar coating so that the upward
movement described above will not occur
(Figs. 3.1 and 3.17).
2
Fig. 3.16 A stack of DN 300 ZMU-PLUS pipes