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4.2 Inspection of the old pipeline
and determination of its state
As in any trenchless replacement operation,
the pipeline to be replaced was subjected to a
thorough examination.
Even while the pits were being dug, the sections
of pipeline which were cut out in the course of
this were examined for their degree of fouling,
the properties of their material and their residual
load-bearing capacity. This latter one was
particularly significant for the subsequent procedure.
Whether the pipeline needed further
strengthening before the replacement depended
on it. The pipeline which had been cut open was
also subjected to a visual inspection, the path
followed by it was documented and the positions
of valves and other installed items were
established from drawings. All these had to be
removed as otherwise there would have been an
uncontrolled rise in the tractive force.
4.3 Preparation of the old pipeline
It was found that the load-bearing capacity
was not in fact high enough for the intended
replacement technique. It was very likely that
the walls of the pipes would collapse as the
pipeline was being pressed out and would
therefore make continued pressing impossible.
The installing company therefore decided to
undertake retrospective strengthening.
For this purpose, a DN 150 PVC pipe for connecting
to sewers was slid concentrically into
the old pipe on spacers. The annular gap around
7 cm wide which was created in this way was
then grouted with a special mortar. This mortar
was notable for its good flowability, and its
high capacity for swelling and resistance to
compression. The curing time of the mortar was
five days. The nominal size of the PVC pipe was
firstly intended to produce as large an annular
gap as possible and hence as large as possible
a cross-section for the concrete. Secondly the
140 mm diameter traction string had to fit into
it.
4.4 Installation of the machinery
After the preparatory work on the old pipeline,
the installation of the machinery could begin. A
rail-type supporting structure on which the soil
removal equipment would slide freely and the
new pipes could be assembled was installed in
the launch pit (Fig. 8).
Fig. 8:
The launch or assembly pit
The traction machine and its front frame were
lowered into the target pit. They were both
braced against the shoring formed by the
concrete thrust block already cast. The front
frame served more than one purpose. Firstly the
bursting mandrel for splitting open the old pipeline
could be accommodated there and secondly
the space was needed for pulling the new pipeline
into the target pit. The traction string was
slid into the old pipeline, i.e. into the PVC pipe.
The press-pull head, followed by the system for
Fig. 9:
Assembly of the first pipe to the reamer
DUCTILE IRON PIPE SYSTEMS 55