Civil Engineering Reference
In-Depth Information
As important as the founding strata themselves are any rapid changes in
strata under the tunnel. This will usually result in increased differential settle-
ment between the tunnel elements and consequent increased shear transfer
between the elements. In severe cases, ground treatment or special short tunnel
elements may be needed to overcome this. If modest settlements are expected,
then the segmental form of construction has benefits, but if large differential
settlement is expected, then monolithic elements are more commonly used.
The dredged material has to be disposed of somewhere, and this is an
important consideration in the early planning stages. Ideally, as much of
the material dredged from the bed as possible will be reused, either directly
in the tunnel project as backfilling material or on an associated project
that has a requirement for such material. If no such outlet is available, then
consideration can be given to using the material as reclamation material
for such things as nature reserves, which can make use of such low-grade
material. In ports or urban areas, the bed material may be contaminated
and require special treatment, which may be very costly.
The most important early activity in relation to all of these issues is mak-
ing sure enough good data is available to make decisions about what the
founding conditions are and what construction activities will be necessary.
If ground investigation data is already available for the area, this is a good
start, but for most immersed tunnel projects, some data can often be found
for the tunnel approaches, whereas there is often little or no data for the
marine section. It will be important to instigate ground investigation as
soon as possible in the project cycle so that there is adequate data to com-
mence outline design. Planning and costing this work is important, includ-
ing determining when the investigations are best carried out to manage risk.
Typically, a limited investigation is carried out early by the project owners
with the intention of gaining enough data to have confidence in the solution,
but the detailed investigations are left to the detailed design or design and
construct stage of the project, when data gathering can be more targeted.
STRUCTURAL OPTIONS
The appropriate form of tunnel construction will be determined from an
options study that considers a number of factors, including its suitability in
meeting the employer's requirements, the possible methods of construction,
the location and facilities required for construction, and the construction
program, construction risk, and cost. At the early planning stage, it may
be possible to keep options open and allow the contractor the freedom to
choose the form of tunnel. If this is done, then the main factors governing
the choice are likely to become material costs and the contractor's pref-
erence for the structural form based on the experience of available labor
resources.
