Posted by
Wanze on Monday, December 28, 2009 10:27:16 PM
Dr.-Ing. Gregor Nieder — Dec 21, 2009
Despite
the fast-paced development of the technology of pilot tube
microtunneling in the past 10 years, these techniques can only be used
in displaceable grounds. Therefore the conventional ground displacing
pilot drilling technology cannot be used in dense material (SPT values
of standard penetration tests > 35) and obviously in rock.
With the development of a patented steering technology called “Front
Steer,” Bohrtec has for the first time overcome the application
restrictions and enabled applications in very dense strata with SPT
values > 35 and in moderately strong rock with strengths of up to 20
MPa. This is accomplished with the help of a guided auger boring system
that excavates the ground as it advances. The expanded possibilities
offered by this application does not affect the economic advantages of
the pilot tube method – also known as guided boring – because the
system with the Front Steer retains the advantages of quick and easy
set-up of the equipment on site and simple operation.
Boring with the Front Steer
Just like the conventional ground displacing methods – the Front Steer
system uses the proven optical guidance system consisting of theodolite
with CCTV camera, monitor and LED target board. As shown in Figure 1,
the ground/rock is continuously excavated by the cuttinghead and then
transported to the starting shaft by augers with a hollow center for
the optical path. The steering pipe uses the ground reaction force for
steering. The machine operator can steer remotely by tilting the
steering pipe in either manual or automatic mode and the control panel
shows the respective steering position of the head.
Pipe Eating in Berlin
Although
the Front Steer technology was developed for non-displaceable soil and
weak rock, the system had a special challenge on the first project in
Berlin. In the Berlin district of Wilmersdorf, an existing sewer 175-mm
pipe needed to be replaced by a new 300-mm sewer on the same alignment.
From the beginning, Berlin Water Service plans included on-line
replacement by trenchless construction using the pipe eating method
because the 60-m long line passed below six mature trees with trunk
diameters of 300 to 350 mm.
When the Berlin company Frisch & Faust, which already had different
Bohrtec pilot drilling systems, received information about this
project, it found this to be the perfect occasion to use the new guided
auger boring method with Front Steer, which excavates the ground as it
advances.
The company already had the BM 500 optical measuring system and the
appropriate steel casings with hollow augers for the job. Only the
Front Steer and the operation and control panel had to be provided. The
six house connections on this length of sewer were diverted in advance.
When doing this it was found to be necessary to remove the house
connection branches before beginning the drilling works because they
were all fixed to the old sewer with three metal clamps. Due to water
pipes, gas pipes and electrical lines across the starting shaft the pit
was supported with wood sheeting with a concrete thrust wall. One week
before beginning of the drilling works the existing sewer was grouted.
After completion of all preliminary works, the Bohrtec team arrived
with the Front Steer equipment and after a few hours had successfully
drilled the first few meters. From the beginning, the crew had no
difficulty controlling the steering head even with the different soil
conditions, which included a sand/gravel bed below the old pipe, stone
bedding and soils of the classes LBM 2/LBM 3, as well as the grout in
the old line. These challenges, however, did not lead to any problems
with regard to the soil clearing or steering.
Already
after a short induction period with the new steering system, John
Adams, an experienced machine operator with Frisch & Faust, was
able to steer the head independently and without any problems due to
the simple operational handling. The team of Frisch & Faust,
well-trained in many conventional pilot tube projects, achieved an
advance rate of about 4 m per hour, including all coupling and other
activities on its first day. At the end of day, 25 m of 419-mm OD steel
casings had already been driven successfully, even though the drilling
only begun late in the morning.
After two days, 54 m of casing were driven without incident. Although
the installation of the steel casings could have been completed on this
second day, work was stopped short before reaching the target shaft in
order to demonstrate the Front Steer driving into the target shaft and
to explain the operation to owner representatives the next morning.
After reaching the target shaft, the steering head was uncoupled and
the team started to push in the 300-mm clay jacking pipes.
It
took only four days to complete the 60 m drilling length to the
satisfaction of all parties involved and the pipe eating process with
Front Steer proved to be a success.
Breaking New Ground
Inspired by the successful use of the Front Steer on the Misdroyer
Street project, Frisch & Faust prepared an alternative proposal for
the Berliner Strasse/Treskow Allee project. An existing 200-mm clay
sewer pipe had been in operation for several years but because of
further development an additional connection to main sewer became
necessary.
One section of this replacement sewer, which passed beneath the rails
of a tramway, was planned to be performed in trenchless construction
while open trench was intended for the installation of the rest of the
pipe run. The original plan involved leaving the abandoned and
backfilled sewer in the ground since laying the new pipe on the same
alignment would not have been possible without disturbing the rails.
As an alternative proposal, Frisch & Faust proposed the on-line
replacement of the existing sewer by pipe eating with the Front Steer
system. The essential advantage of this proposal was the fact that the
risk of meeting unknown obstacles was minimized by following the
existing line.
After
approval from Berline Water Service, the preliminary works were done
and the Bohrtec Front Steer was used a second time for the on-line
replacement of an existing sewer. After a smooth beginning of the
drilling works, after 5 m there was a dramatic increase of the pushing
force and the cutting wheel torque. It was presumed that a
sheet pile
had been struck since according to the construction plan there had been
some
sheet piles at this position of the old line which should have
been cut down to a depth of about 3 m, but this was obviously not the
case. Despite the risk of drilling head damage, it was decided to
continue boring because the cost of disturbing the rails and the
consequential cost for the rail replacement bus service would have been
much higher. The
sheet pile remains were passed despite much wear and
tear of the cutting tools and the full length of 28 m was completed
successfully. The choice of the Front Steer proved to be a lucky choice
especially with regard to the sheet pile. If a conventional ground
displacing drilling system had been used – as originally planned – the
drilling would have had to be stopped when the
sheet pile was
encountered, thus inevitably requiring the rails to be disturbed and
the provision of a special bus service.
Conclusion
Although the described projects of trenchless pipe replacement using
the pipe eating method to remove the old line do not really represent
the planned standard use of the new Front Steer system, it has proved
to be effective for guided auger boring even under these extreme
conditions.
With the development of the Front Steer system, Bohrtec has extended
the range of application of the well proven and economical pilot tube
technique successfully used for small diameters and short lengths into
non-displaceable ground (SPT > 35) and rock with strengths of up to
20 MPa.
from:News|trenchlessonline