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A. Original tunnel cross section. C. Tunnel plan view with floodgates.
B. Tunnel elevation with floodgates. D. Tunnel cross section with floodgates.
Figure 32. Floodgates
· Contractor staging area outside of tunnel. · Contractor staging area is outside of tunnel.
· Underwater construction expertise required. · Underwater construction expertise required.
Countermeasure 50: Bollards or Fenders
in the Water 5.5 Conclusion
Bollards or fenders can be constructed on the water side of When using these guidelines, one must recognize that most
a tunnel portal to create a stand-off distance and protect the mitigation countermeasures fall between two extremes. One
portal from waterborne hazards and threats, such as off- extreme is to prevent all damage at enormous cost, and the other
course ships or ships carrying explosives. See Figure 34. extreme is to spend nothing and risk enormous damage. Tun-
Constructability issues include the following: nel owners, operators, and engineers must make balanced deci-
sions in selecting countermeasures for their facilities, preferably
· Environmental issues require permits and approval from to risk an acceptable level of damage at a reasonable cost. How-
responsible agencies. ever, finding this balance becomes more complicated when con-
· Coordination with water traffic authorities. sidering possible loss of human life, which is extremely difficult
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Figure 33. Barrier walls.
Figure 34. Bollards or fenders in the water.
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if not impossible to assign a value to. Protection of human life subjective. They depend on a number of variables, including
should always receive the highest priority. tunnel length, tunnel construction type, construction materi-
While preparing budgets for tunnel-hardening counter- als, surrounding earth geology and groundwater conditions,
measures, be careful to include the costs associated with labor, available clearances, and interruption of operations. To fur-
material, equipment, protective services (i.e. flagging), outage ther explore the suitability of particular countermeasures to a
costs of highways or rail lines, and interruption of traffic and specific facility, in-house or outside experts should be used to
operations during construction. Although the relative effec- develop conceptual designs and cost estimates. Once these
tiveness and order-of-magnitude cost ratings in the counter- designs and estimates meet approval, final construction doc-
measure guides are based on many years of engineering uments--including design drawings, specifications, construc-
expertise and past project experience, the rating systems are tion cost estimates, and schedules--should be developed.
