Civil Engineering Reference
In-Depth Information
Calculating the subsequent course
After determining the position, the next step was to calculate the subse-
quent course and to determine the sequence of the rings to be built in
(left or right). When calculating a correction course, it was not only impor-
tant to take into account the geometry of the rings, but also the (limited)
possibilities of steering the TBM. When plotting the correction course, the
TBM was put back tangentially to the theoretical alignment. The radius of
the correction curve always had to be smaller than the radius of the theo-
retical alignment because otherwise the TBM would run off the required
alignment.
Ring building: making a proposed sequence
Based on the course to be followed, the programme proposed the type
of ring (left or right) which would qualify for being put in position first.
The foreman driller could deviate from this if the situation so required.
Steering the TBM
The position of the TBM was constantly displayed on the screen. Based on
the information available to him, the operator of the TBM was able to steer
the machine in the correct direction. This was achieved by varying the pres-
sures on the thrust jacks around the circumference of the ring last built;
pressing harder on the right-hand side means bending to the left and vice
versa.
Start and finish of tunnel boring machines
Start of the boring process
The boring of the tunnel commenced from the starting shaft in Terneuzen.
Prior to this, an opening had to be made in the shaft wall. In order to prevent
soil (not water, because the starting shaft was excavated in a polder) from
getting into the shaft as a result of this at the start of the boring process, a
structure was required behind the shaft wall that would firstly hold back the
soil and secondly could be bored through.
Creating this so-called impermeable block can be done in several ways,
for example - as in the case of the starting shaft in Terneuzen - by applying
sand-cement stabilization layer by layer. The impermeable block makes it
possible to make the transition from an unloaded atmospheric situation to
a loaded situation.
TheTBM was put in position in the starting shaft on the so-called shield cra-
dle in front of the hole in the wall with the tunnel openings. A temporary
structure, the starting frame, was installed at the back, which transfers the
propulsion forces of the TBM to the surroundings at the start phase. Before
the boring started, two auxiliary rings, the so-called blind rings, were built
between the TBM and the starting frame. After checking the whole set
up, the jacks gradually extended until all the clearance had been taken up and
the boring machine slid into the supporting ring at the tunnel opening. After
the first two metres had been bored into the impermeable block, the jacks
were retracted in pairs and the next blind ring was built. As soon as the back
of the TBM had reached the supporting ring at the tunnel opening, the con-
struction of the first permanent tunnel ring could finally take place. Then an
inflatable seal fitted into the supporting ring at the tunnel opening was
inflated and filled with grout to seal the supporting ring at the tunnel open-
ing to the tunnel rings.The filling of the tail slot with grout took place inside
the impermeable block.
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