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| Geological photographs from around the world | ||||||||
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In an ideal world we would build tunnels, bridges and buildings only in areas with the right geology, for example
where the strata are horizontal, the rock is competent, the beds are massive, the jointing minimal and faults absent.
More likely than not, the place where we want to build them is not in fact the most suitable, and various engineering
techniques are needed to overcome the geological problems.
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On the other hand, the best rock for building on is not necessarily the best for tunnelling. What are the properties
of the best tunnelling rock, in terms of competence, permeability, strength and uniformity? Even in good tunnelling
rock, there are other problems to overcome during construction when solid rock originally under pressure is replaced
by a hole which is at atmospheric pressure. Occasionally, a sharp crack is heard during tunnelling - what happens to
the rock to cause this percussion effect? Is there any equivalent natural effect on a body of rock when the confining
pressure is reduced during erosion?
The photograph shows the wall of a tunnel (the red arrow is about 10cms long). Does it illustrate any of the points mentioned above? |
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In some tunnels, the rock is strong enough not to need lining or supporting. Other tunnels use rock bolts, shotcrete,
cast concrete linings to prevent rock falls.
Areas subject to high rainfall, seismic activity and avalanches need other precautions. |
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If a building or bridge is constructed with its foundations on strong bedrock, it will last a long time. If the ground
is weak, unconsolidated, jointed or faulted, then an artificial "rock" such as a concrete raft is needed.
Alternatively, the weak rock is by-passed by piles made of reinforced concrete, extending down to the solid bedrock.
In some cases, old mine workings, buried valleys or limestone caves are, perhaps unexpectedly, encountered, adding to
the cost of the foundations.
The photograph shows the bridge at Millau, France. |
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Geopix Study Topic "Civil Engineering" describes the
- geological factors which influence construction;
- terms used to describe the properties of rocks;
- methods used to prevent rock falls;
- best geological conditions for, and problems often encountered during, the construction of tunnels;
- survey methods used to investigate the suitability of potential sites prior to building bridges;
- the engineering techniques used to improve the properties of otherwise unsuitable sites, illustrated by examples;
and includes a Case Study: the Channel Tunnel and Seward Tunnel, Alaska
To order the "Applied Geology" Study Guide on CD, or the "Civil Engineering" Study Topic by download,
please visit the Geopix on-line store
The "Civil Engineering" Study Topic includes the following pages:
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