Civil Engineering Reference
In-Depth Information
and required transverse stiffening beams. This made the structure quite
expensive compared to a conventional approach ramp structure but was
necessary because of the circumstances of the project site.
When the road or railway alignment in the tunnel approaches has risen
from the tunnel to a certain elevation, it is possible to switch from a struc-
tural solution to an earthworks and ground membrane solution. This
requires a watertight, tough flexible membrane that is buried in the earth-
works around the approach ramp that prevents groundwater intrusion into
the tunnel approaches. It can pass beneath the ramp structure, but this is
unusual as it would require additional excavation and dewatering to install.
More commonly, it attaches to the sides of the ramp structure and extends
into the earthworks to the side. This allows the height of the structural
retaining walls to be kept lower than the level of groundwater table, as they
are no longer the means of providing watertightness (Figure 8.4). A ground
membrane will still be subject to uplift pressures and has to be checked
against prescribed factors of safety to prevent it displacing.
This solution was used on the Øresund Tunnel, which had a very wide
approach ramp because of the combined road and railway. A structural
solution for such a width would have been very expensive, so a membrane
solution was adopted. The membrane needs to be sufficiently low beneath
the road to allow a full depth of pavement construction and to enable road
drainage to be installed. Similarly for a railway, it must allow enough depth
for the railway foundation and ballasting as well as track drainage and
installation of service ducts and draw pits. Some leakage of the membrane
was experienced on the Øresund Tunnel project after construction that led
to some ongoing maintenance costs through increased pumping from the
tunnel drainage sumps. Overall, it was still probably the most cost-effective
solution. Nevertheless, it illustrates the difficulty with this approach, in
that it is highly workmanship dependent. The membrane must be tough
and yet workable in order to lay. On-site welding is required, and this is
potentially a source of leakages. Care must then be taken in trafficking the
membrane by foot and by machine once a sufficient layer of fill is placed
over it. Sand layers are needed above and below the membrane to minimize
the possibility of puncture.
INTERFACE WITH IMMERSED TUNNEL
One of the first issues to decide at the preliminary design stage is the loca-
tion of the junction between the immersed part of the tunnel and the in situ
approach structures. The immersed tunnel method requires that the tunnel
element is floated into position, but this in itself would not rule out dredging
a channel into the riverbank and extending the immersed part of the tun-
nel under the bank. The main governing criterion though is the water level.
