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4. Selection criteria for jacking
systems
As soon as different systems are available on the market
the question of which it is advisable to use for which
purpose arises. There is no single answer to this
question since all systems have both advantages and
disadvantages, the scope for use is wide and the constraints
in problem definitions are often very complex.
It is important that the promoter/client provides potential
contractors with not only a comprehensively documented
unambiguous plan but also, specifically, the most
exact possible and sufficient data on the geology and
soil mechanics of the line to be followed by the sewer
and on the groundwater conditions. The decision on the
jacking system to be used should – for purposes of
rational spreading of risks – ultimately be left to the
bidder, i.e. the subsequent construction company.
Capital outlay for thrust-bore pipe-jacking systems is
lower. They need less space and personnel, yielding
time saving of up to 35% over shield jacking systems for
setting-up the site.
The following two tables analysing advance rates
attained, including set-up time, yield further information.
The first comparison shows a distinct decline in average
advance rates for thrust-bore pipe-jacking systems as
nominal size increases. For > DN 500 they drop to about
66% / 71% of the rates attained for DN 250.
In contrast, with shield pipe jacking, i.e. with hydraulic
conveying, advance rates including set-up time are
found to be more-or-less independent of nominal size.
BWB: thrust-bore pipe jacking, average
advance per 8 hours in metres
Advance Advance
with set-up time
DN 250 9,93 6,08
DN 300 8,61 5,20
DN 400 8,32 4,91
DN 500 8,46 5,00
>DN 500 6,58 4,34
The analyses relate to approximately 167,000 metres’
total advance up to the end of 1994 in Berlin. It should
be noted that the figures naturally reflect all local
constraints, but also the skill and motivation of the personnel
on site and the whole ethos of the executing
company.
Micro-tunnelling Page 15
BWB: Shield pipe jacking, average advance
per 8 hours in metres
Advance Advance
with set-up time
DN 250 10,83 6,19
DN 300 9,82 5,41
DN 400 11,22 6,52
DN 500 11,91 6,69
DN 600 10,82 6,30
DN 800 11,01 6,00
DN 1000 11,65 6,71
DN 1200 11,99 7,23
The trend towards distinctly higher rates with shield pipe
jacking systems at nominal sizes DN 400 is however
conspicuous. This is partly due to the fact that at larger
nominal sizes continuous transport of soil by flush-conveying
is clearly superior to that using buckets. Moreover,
the higher extraction and comminution rate has
particularly beneficial effects where the ground is difficult.
The constantly direct drive of the cutting head, the
smaller losses compared with auger drive and the possibility
thus afforded of driving greater lengths also make
themselves felt in favour of auger systems.
Selection of a jacking system with the particular
extracting tools needed is however most strongly
influenced by the geological conditions and groundwater
level. See in this connection a publication by W. Becker
(Berlin) on ”Scope and limits of micro-tunnelling taking
extraction tools into account” (offprint from the journal
”Tiefbau” , Vol. 7/1996, obtainable from Steinzeug
GmbH, Max-Planck-Strasse 6, 50858 Cologne). Taking
as its basis the soil classifications of the General Technical
Contract Terms (ATV), DIN 18319, VOB Part C, this
article makes recommendations on selecting suitable
jacking systems.
5. Micro-tunnelling applications
With the jacking systems available on the market construction
orders for all nominal sizes used in sewage
systems for both extension and renewal purposes are
possible. Oval cross-sections too, which are enjoying
something of a renaissance in waste-water engineering
by virtue of their hydraulic advantages, can be jacked
underground. Their external profile must of course have
a circular cross-section, and precise adherence to the
pipeline invert must be ensured. This requires roll-free
jacking, which can be achieved if adjacent pipes are
joined with shearing pins and machines with flush-conveying
and thus cutting heads rotating clockwise and
anti-clockwise are used.