Civil Engineering Reference
In-Depth Information
21.2.4 Contract, Technology and Problems
Contract
In the framework of the contract the client provided
- all available information on the geological, geotechnical and hydrogeological condi-
tions of the Hallandsas project,
- the requirements on geometry and technical equipment of the tunnels,
- design criteria, such as the required lifetime, durability requirements, minimum con-
crete and steel classes, load cases and safety factors,
- technical specifications for NATM heading, TBM heading, the tunnel lining, grout-
ing and other works.
On this basis, the contractor had to do the complete design and construction on their
own responsibility. This also included the selection of construction method. Selected
aspects of the design had to be reviewed and accepted by the owner.
TBM and segmental lining
On this basis, the consortium Skanska-Vinci decided for a TBM heading with a shielded
machine and a watertight segmental lining. Owing to the strict requirements regarding
water inflow it was not possible to drive the two tunnel tubes with two TBMs in parallel.
Instead, one TBM was used, excavating one tunnel tube after the other.
The TBM was equipped with a cutterhead with an excavation diameter of 10.7 m, de-
signed for the excavation of hard and abrasive rock as well as soil-like material. It can
be operated in open mode using a belt conveyor system. Alternatively, closed mode
operation with a slurry circuit was possible. In addition, the TBM was equipped with
installations for advanced drilling and grouting.
The segmental lining consists of eight pre-cast concrete segments with a width of 2.2 m
each. The lining is 54 cm thick and has an internal diameter of 9 m.
Problems encountered during construction
The average advance rate, which was achieved during construction in the year 2006,
amounted to about 2 to 3 m/day, which is far lower than the expected average advance
rates for open and closed mode without pre-treatment. This caused large delays and
cost increases. The small advance rate was mainly due to the following two major prob-
lems encountered during heading:
1. Instabilities of the temporary face were repeatedly observed. The discontinuities
formed blocks which were not stable, resulting in large overbreaks. The tunnel con-
tour along the shield skin generally remained stable and no major overbreaks were
observed in this area. Under the conditions prevailing at the temporary face, the
TBM acted rather like a stone crusher with the consequence of extreme wear of
the cutting tools, mucking system and other machine components. The wear was
highest in closed mode operation but was also observed in open mode. Owing to
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