Slide 1
Excavation redistributes stress in the surrounding ground
Removing material for a tunnel changes how surrounding ground carries stress. Support must manage that redistribution as excavation advances.
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Strong intact rock can still contain joints that release blocks into an opening. Engineers assess the rock mass and its discontinuities, rather than relying only on the strength of a small sample.
Slide 2
The timing of excavation and support changes movement
A tunnel may be excavated in stages rather than opened to its full size at once. The timing of support affects how much ground movement develops.
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Ground that stands briefly without support may not remain stable indefinitely. Advance length, excavation order and installation time must work together as one construction method.
Slide 3
Bolts and sprayed concrete support different ground features
Rock bolts can help hold blocks together, while sprayed concrete supports the exposed surface. The appropriate combination depends on the ground conditions.
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These measures do not all carry load in the same way. Some reinforce the surrounding ground, while others provide a structural lining around the opening.
Slide 4
Groundwater adds inflow, pressure and instability
Groundwater can create inflow, pressure and soil instability during tunnelling. Controlling it may require drainage, sealing or pressure-balanced construction methods.
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Lowering groundwater can also affect the surrounding area through settlement. A solution that dries the excavation must therefore be checked for effects beyond the tunnel boundary.
Slide 5
Measurements can trigger changes to the construction plan
Surveying and instruments track movement as construction proceeds. Observed behaviour can trigger changes to support or excavation procedures.
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Monitoring needs agreed response thresholds and actions. Collecting measurements without a plan for interpreting unexpected movement does not itself make the excavation safer.
Sources
- Technical Manual for Design and Construction of Road Tunnels
Federal Highway Administration