mohring engels.indd - Keramo Steinzeug

2. The advantages of micro-tunnelling
Pipe-jacking systems are distinguished by a high level of
mechanisation and therefore require substantially less
manual work than conventional open sewer construction.
From the fact that thrust boring is possible with
concentrated rather than linear construction sites as
with open methods derive the many advantages, with in
some cases substantial economic and environmentrelevant
consequences.
The surface is generally disturbed only at the starting,
target and intermediate shafts or by-shafts. Breaking-up
of the road surface and subsequent restoration with the
associated disruption of traffic is thereby minimised.
Previous relaying of other pipes or underground installations
along or close to intersections with open utility
trenches for sewer construction can be reduced. The
starting shaft, normally covered by the thrust-boring
container, guarantees that work is noise-free and independent
of the weather.
Because of local topography there are often hardly any
differences in level between drainage areas; some
sewers must then be laid at greater depths. With open
construction greater depth is thus accompanied not only
by costly trench sheeting but also by substantial soil
excavation. In towns and densely-populated built-up
areas the excavated soil can seldom be accommodated
in the immediate vicinity of the construction sites, which
may give rise to long transport distances with multiple
loading and unloading. Frequently, too, the excavated
soil cannot be put back again if the requisite degree of
compaction in the road base cannot be attained with it.
In many cases neither the broken-up roadway material
nor the excavated soil may be removed for use at the
contractor’s discretion; both may constitute building
waste which must then be conveyed to listed landfill
sites. Appropriate tests with evidence of suitability must
be performed.
Construction of utility trenches is very expensive anyway,
and is made even more so by the aforementioned
requirements. Calculation of the cost shares for constructing
12,252 metres of DN 200 and DN 250 sewers
in 14 construction projects in Berlin-Heiligensee and
Tegelort in 1983 and 1984 at a figure of about DM 9.2
million (excl. VAT) showed that, taken together, lining
such trenches, excavating and transporting the soil, any
necessary soil replacement, landfill charges incurred,
backfilling and compacting the trench and removing the
sheeting made up some 39% of the construction costs.
About another 31% must go on breaking up and eventually
restoring the road (Fig. 2). This means that,
cumulatively, some 70% of the costs incurred in open
construction have nothing to do with sewer installation
proper and are thus economically questionable.
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39 %
6 %
63 %
Micro-tunnelling Page 7
Fig. 2: Cost shares for DN 200 and DN 250 domestic/industrial
sewer construction by open method
3 % Site equipment
8 %
14 %
31 %
7 %
31 % Breaking up and restoring road
8 %
3 %
12 %
39 % Sheeting, bridges, excavating and replacing
soil, backfilling
8 % Dewatering
7 % Manholes
12 % Sewers, junctions and laterals
Fig. 3: Cost shares for DN 200 and DN 250 domestic/industrial
sewer construction by enclosed method
9 % Site equipment
8 % Breaking up and restoring road
9 %
14 % Sheeting, bridges, excavating and replacing
soil, backfilling
6 % Dewatering
63 % Sewers, junctions and laterals