UNDERGROUND ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 259

technical management, maintenance and operation for a period of 30 years, the
Dutch Government acting as shareholder, and the contractor known as KMW, who
designed and built the tunnel, according to a design-build contract, was quite
exceptional. The budget of about 750 million euro was exceeded by only 6%.
In many respects the construction was a pioneering project. The logistics were a
great challenge since the civil fitting out of the tunnel immediately followed the
boring process, including the 26 cross-connections, which were constructed using
ground freezing techniques under extreme conditions. A bored tunnel at this depth
had not previously been realised in partly soft and partly stiff sediments. Soil
investigations included unprecedented deep cone penetration tests reaching 65m
below sea level through the Boom clay into tertiary glauconitic sands. Soundings in
these sands showed a resistance with significant variation between 5 MPa and 35
MPa (Fig 15.9). This has a serious implication on the soil bedding of the tunnel
shield and lining as is demonstrated next.
The boring started in 1999 with two TBMs. In May 2000 in the Tertiary
glauconitic sand, serious deformations at the tail of the shields of both TBMs were
noticed and some equipment damage occurred. The theoretical free space measured
between the TBM and the tunnel lining was 85 mm. Deformation measurements
showed that at a number of spots the tail of the shield almost touched the lining.
Over a length of 4.5 to 5 m (in the longitudinal direction of the shield, total length
of 11.55 m) the boring shield of the east TBM showed severe irregular
deformations (buckling). The maximal deformation of the west shield was 40/50
mm and the maximal deformation of the east shield was 52 mm. Finally the west
TBM and the east TBM stopped near the deepest point in the glauconitic sand (Fig
15.9). The TBMs seemed almost to be lost and the project would be a disaster.
Fortunately, proper measures could be taken and boring continued. Despite these
measures the deformations of the TBM increased to a maximum of 60 mm. As
soon as the TBM went up, back into the Boom clay, these deformations decreased.
Boring in glauconitic sands was a new experience.
The stagnation had cost serious delay and additional costs reaching 35 million
euro. An investigation started to find possible causes and work out who should
finance the unexpected loss. Concerning the ground conditions and design
approach the following observations were made. The design of the tunnel shield is
based on Duddeck's method (Fig 15.10), which is a common state of the art.
Original calculations showed sufficient soil bedding support for the shield stability
(buckling risk) under the extreme high water pressures (60 m below sea level),
when adopting low soil stiffness. This was the basis for the tunnel shield design.
However, when adopting high soil stiffness, there is locally no soil bedding,
meaning that under the very high pore pressures shield deformations (buckling)
may occur. The soundings (CPT's in Fig 15.9) show that the soil stiffness is
strongly non-uniform and it is known that, particularly when there is significant silt
or clay content, it is possible for cavities to remain unsupported in sands.
Furthermore, a few case histories of tunnels constructed in loose clay-silt sands
show that radial effective stresses may become very low.