TUNNEL LOCATED IN ANTICLINE AND SYNCLINE
Folds signify beds 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.
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.
Folding of rock introduces peculiar rock pressures. In anticline folds, loads of rocks at the
crest are transferred by arch action to a great extent on to the limbs, which may be highly
strained.
A. Anticlinal – low pressure
in middle region
B. Synclinal- high pressure
in middle region
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, anticline or syncline
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 tunneling operations; very often
causing the dreaded rock bursts. Very slow release of small amounts of strain energy
might cause bulging of walls or caving in of roofs.
Folded rocks are often best storehouses for artesian water and also ideal places of
aquifers. When encountered during tunneling unexpectedly, these could create
uncontrollable situations. The shattered axial regions being full of secondary joint
systems are highly permeable. As such very effective drainage measures are often
required to be in readiness when excavations are to cross-folded zones.
TUNNELING ALONG DIP AND STRIKE JOINTS SYSTEMS
The design, stability and cost of tunnel depend not only on the type of rock but also on
the structures developed in these rocks.
DIP AND STRIKE: These two quantitative properties of rocks determine the attitude of the rocks and hence
influence the design of excavation to a great extent. Three general cases may be
considered.
Horizontal strata occur for longer tunnels, when encountered for small tunnels or for
short lengths of long tunnels, horizontally layered rocks might be considered quite
favorable. In massive rocks, that is when individual layers are very thick, and the tunnels
diameter not very large the situation is especially favorable because the layers would then
over bridge flat excavations by acting as natural beams.
A= Safe situation
B=Unsafe at Top
But when the layers are thin or fractured they cannot be depended upon as beams, in
such cases either the roof has to be modified to an arch type or has to be protected by
giving a lining.
Moderately inclined strata, such layers that are dipping at angle up to 45o may be said as
moderately inclined. The tunnel axis may be running parallel to the dip direction, at right
angles to the dip direction or inclined to both dip and strike directions. Each condition
would offer a different set of problems.
In the first situation that is, when the tunnel axis is parallel to the dip direction the layer
offer a uniformly distributed load on the excavation.
A= Dip of layers
B=Strike of la
When the tunnel axis is parallel to dip or parallel to strike or inclined to both dip and strike
directions. In vertical rocks for examples, when the tunnel axis is parallel to dip direction, the
formations stand along the sides and on the roof of the tunnel as massive girders. An apparently
favorable condition, of course, provided all the formations are inherently sound and strong when
considered individually also.
Tunneling in steeply inclined strata
A= General View
B= Part shown in enlarged section
In tunnels running parallel to strike of vertical beds, it is more than likely that a number of
bedding planes are intersected at the roof and along the arch so that natural beam or arch action
gets considerably weakened.