Civil Engineering Reference
In-Depth Information
on the foundation construction of the structure. Often, old buildings adja-
cent to waterways are founded on shallow rafts or on timber piles, both of
which are susceptible to movement as the soil is dewatered. Timber piles
are also likely to deteriorate if they dry out. Condition surveys will be
required of any structure that could be affected by the works. This is as
much to safeguard the tunnel constructor from spurious claims that the
construction affected a particular building as it is to monitor the effects of
construction.
If the effects of dewatering are too great, then it may be necessary
to install recharge wells. These pump water into the ground around
the site, and therefore limit the effects of the extraction operation. The
design of such a groundwater recharge scheme should be carried out by
specialists.
Once the excavation has been established, the construction of the
immersed tunnel elements is straightforward. However, there is one impor-
tant geotechnical aspect that affects construction: the floor of the casting
basin. A working platform is needed on which to construct the elements.
Generally, this is provided by a 500 mm thick layer of granular material,
with the tunnel elements being constructed directly on it. The granular
layer can be accurately screeded to form the correct bottom profile for the
element. Although they may often appear straight, immersed tunnel ele-
ments are usually curved in the vertical direction and often horizontally
as well.
The granular layer also serves another purpose. The elements are finely
balanced between floating and sinking and are designed to have only a
small freeboard. When the element is being floated up in the basin, there is
a possibility that it will stick to the floor. If that happens, there is a risk that
the element may suddenly free itself and rise uncontrollably to the surface.
The granular layer under the element enables the water to flow under the
element when the basin is flooded, making sure that the uplift distributes
evenly over the underside of the element. Often, as an additional safeguard,
porous pipes are laid in the gravel under the element. If the element looks
like it is sticking, then air can be pumped through the pipes and the airflow
from the pipes will be enough to loosen the gravel and enable the element
to float.
Other treatments for the basin floor are possible. No-fines concrete
has been used, but that is expensive. Sand has also been used, contained
between a grillage of concrete beams that are cast to the correct geometry
for the underside of the element. Either the steel bottom plate or plywood
formwork can then be placed directly on to the sand. If plywood formwork
is used, it is provided with fixings so that it does not come off and flap
about during the immersion process. If that happened, the placing of the
sand foundation could be severely impeded.
