Civil Engineering Reference
In-Depth Information
fabrication facility adjacent to the tunnel site may be unacceptable either in
an urban situation where the activities would be too intrusive or in a coun-
tryside location where such activity would be out of place.
In costing an immersed tunnel scheme, it is necessary to consider both
the permanent works and the main temporary works. These are typically
• Fabrication facility for building the elements
• Cost of building the elements
• Dredging the trench and dealing with the resulting material
• Transporting and placing the elements
• Placing the element foundations
• Backilling the trench around the tunnel
• Ballast exchange and placing internal ballast concrete
• Internal inishings (M&E, cosmetic)
• Cut and cover approach tunnels
• Open approach ramp structures
• Service and ventilation buildings
• Preliminaries and contingency
In developing the initial estimate, the effect of optimism bias needs to be
considered. This is a factor increasingly being used on major infrastructure
projects to allow for the fact that project outturn costs have historically
been greater than the original project estimates, but this is different from
contingency, which can be modeled mathematically according to the level
of risk or uncertainty that is assigned. There is a demonstrable body of evi-
dence to show that optimism bias exists and in some countries government
advice is available on how project sponsors should make explicit, empiri-
cally based adjustments to their estimates. This applies, incidentally, not
only to a project's costs, but also to the estimate of the construction period
and to the estimate of the project's benefits.
For example, U.K. government guidance is that for bridge and tunnel
links the median uplift on the estimated capital expenditure could be as
high as 55% if the promoters want to be sure that the risk of cost overrun is
no more than a 20% probability. If the promoter could accept a 50% prob-
ability of the outturn cost exceeding the estimate, then the uplift to account
for optimism bias would drop to about 23%. Such figures are constantly
under review as the database of outturn scheme costs gets larger over time,
so the estimator should seek the latest advice before applying such figures.
Optimism bias has resulted from the planning and procedural processes
involved in major infrastructure schemes and the strategies adopted by
the various parties involved in those processes. Only a few of the parties
involved had a direct interest in avoiding optimism bias and it was often
in their interests not to avoid it so that costs would be underestimated or
the benefits overstated and the progress of the scheme would not be halted.
