Geology Reference
In-Depth Information
One of the most severe design situations is in high-pressure water
supply tunnels associated with hydropower constructions where for
some operational periods the tunnel carries water under high pressure,
but at other times the same tunnels are empty and have to withstand
signi
cant external water and rock pressures.
The main concerns with pressure tunnels are:
-
Potential damage by hydraulic fracturing (formation of new fractures)
or jacking (opening of existing fractures) within the rock mass, and
-
Stability, durability and low maintenance.
To avoid hydraulic fracturing, an empirical rule is sometimes used:
D
γ R
γ W >
:
ð
;
Þ
1
25
Haimson
1992
H
Where
W is unit weight of water, D is
rock overburden at tunnel location and H is the water head. However,
it is important to recognise that this formula only considers vertical
in situ stress. Horizontal stress can be very low in some situations, for
example, close to valley sides, and this will control the risk of hydraulic
fracture or jacking if water from the tunnel can reach the excavated
rock surface at suf
γ
R is unit weight of rock and
γ
ciently high pressure.
ning rock stress, vertical and/or horizontal, is too low,
fully welded continuous steel liners are generally used to prevent the high-
pressure water from reaching the rock mass. Concrete liners may be used
in competent rock but might crack under high internal water pressure if
the con
Where the con
ning stresses are too low. In such cases, there is a risk of leakage
to surrounding ground (with a risk of causing landslides in some situa-
tions) and/or water
flow into other underground openings. Haimson
(1992) presents examples of schemes where the importance of stress
conditions and the correct choice of lining only became evident late in
the design process, with
. An important task of
the engineering geologist is to ensure that the in situ stress conditions
along the route of a pressure tunnel are evaluated fully and reported to
the design team, preferably at an early stage in project planning.
In certain situations, typically in low pressure headrace tunnels, a
concrete liner can be designed with drainage holes to relieve water
pressure on the tunnel lining. Consolidation grouting is usually carried
out around the tunnel to reduce leakage out of or into the tunnel
(depending on the relative internal and external water pressures).
Unlined tunnels can be used in good rock conditions and with favour-
able in situ stresses, but there may be higher maintenance requirements
and the need to construct rock traps to catch any fallen debris. The
proper design of hydraulic pressure tunnels is particularly important as
the consequences of failure are usually very severe and costly to repair.
A comprehensive summary of the principal design and construction
considerations is presented by Benson (1989).
'
unpleasant consequences
'
 
Search WWH ::




Custom Search