Tunneling(3)
III]
TUNNELING THROUGH FOLDED STRATA:-
1) The folded rocks are
always under a considerable stresses and the stresses are realsed whenever any
kinds of excavation is done through them.
2) The folds are disturb by some external
stresses this may results movements in the rocks.
3) It is therefore advice that the highly
folded site should be avoided as far as possible for tunneling some time if
engineer is compelled to adopt such site then to study following two cases.
Case
I- Tunneling through Articlinal folds –
The rocks in limbs are always
better in a physical condition than those in axial region. The rocks at crest
and trough of the folds are in broken fragmented condition. Due to tentional
forces, that develops during the folding.
A tunnel in folded rocks should be aligned
through the limbs rather than along the axis of the Articlinal fold.
Case II- Tunneling
through the synclinal fold :-
Tunneling through syncline fold
will be create several hazardous. Syncline form excellent reservoir of ground
water when there is a aquifer sandwiched between two impervious beds. The
tensile cracks are developed at crest a trough during the folding. At trough of
synclines roof fall may takes place if support is removed. This trouble will
also along the axis of syncline.
Therefore syncline most unfavorable
conditions for tunneling and should be avoided by changing the alignment.
IV]
TUNNELING THROUGH A FAULT ZONE:-
1) Fault is defined as
relative movement of beds along the fault plane due to tensional forces.
2) Fragmented rock
resulting by crushing and fracturing along fault zones. The rock will be weak
and unstable due to fragmented condition.
3) This open zones will
provide excellent passage way for ground water.
4) The fragmented rock
will be decomposed because of the constant presence of water and will be
further weakened.
5) Excavation will
therefore be difficult because of roof fall, sideslips and ground water.
6) If a fault present
it is most serious problem. The movement along the fault crossing the tunnel
has dis-located the tunnel. Avoid the structure.
7) A small in active
fault are not dangerous. The weak and crushed rock along the fault zone should
be excavated and back filled with cement concrete.
8) Examples :- i) Koyna
3rd stage tail race tunnel the entire cover of about 15 m collapse
along a fault zone. And both side of fault is jointed compact basalt also came down
along with fault material.
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