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Folds signify bends and curvatures and a lot of strain energy stored in the rocks. Their influence on design and construction of tunnels is important from at least three angles:
Firstly, folding of rocks introduces considerable variation and uncertainty in a sequence of rocks so that entirely unexpected rocks might be encountered along any given direction.
This situation becomes especially serious when folding is not recognized properly in preliminary or detailed surveys due either to its being localized or to misinterpretation.
Secondly, folding of rocks introduces peculiar rock pressures.
In anticlinal fold, loads of rocks at the crest are transferred by arch action to a great extent on to the limbs which may be highly strained
These conditions are reversed when the folds are of synclinal types. In such cases, rocks of core regions are greatly strained.
Again, the axial regions of folds, anticlinal or synclinal, having suffered the maximum bending are more often heavily fractured.
The alignment of a tunnel passing through a folded region has to take these aspects in full consideration.
When excavations are made in folded rocks, the strain energy is likely to be released immediately, soon after or quite late to tunnelling operations, very often causing the dreaded rock bursts.
Thirdly, folded rocks are often best storehouses for artesian water and also ideal as aquifers.
When encountered during tunnelling unexpectedly, these could create uncontrollable situations.
The shattered axial regions being full of secondary joint systems are highly permeable.
As such very effective drainage measure are often required to be in readiness when excavations are to pass through folded zones.
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