Environmental Engineering Reference
In-Depth Information
(c) The drains within the body of the dam should be cored or formed and at least 150 mm
diameter;
(d) The drains drilled into the foundation from a lower gallery should be a minimum of
75 mm and preferably 100 mm diameter and spacing should not exceed 3 m;
(e) Provision should be made for the drains to be serviced, e.g. by re-drilling foundation
drains from a gallery.
The authors emphasise that it is essential that the drains penetrate well below the level
of any potential failure surface and that they are oriented to intersect the defect system.
Hence for horizontally bedded rock with a sub-vertical joint system, it is essential that the
holes be drilled inclined to the vertical to intersect the joints. The depth should be deter-
mined from the geology of the foundation and the geometry of the dam, but it is unlikely
that drains which do not penetrate at least 4 m below a potential failure surface would be
effective. It would be wiser for them to be at least 6 m below the potential failure surface.
The drains should be constructed after the dam is constructed and the grouting of the
grout curtain, (which is usually drilled and grouted from the drainage gallery and located
upstream of the drain holes), is completed. At this time the stresses and deformations
induced by the dam are present and the grouting cannot block the drainage system.
As pointed out by FERC (2000) the grout curtain may retard flows initially but the degree
of uplift reduction is difficult to predict and may reduce with time as the cement in the grout
is leached. Grouting alone (without drains) should not be considered sufficient justification
to assume a reduced uplift.
The question of the effectiveness of drains within the dam is one that is often asked.
Under normal operating conditions when in most dams there is compressive effective
stress over the whole section, any seepage water that did reach the drains through the nat-
ural porosity of the concrete would cause the outlet at the drain to be sealed with calcium
carbonate. The reaction here involves the seepage water picking up excess lime (CaO)
from the concrete's matrix to form Ca(OH)
2
which combines with CO
2
in the air at the
exit to give CaCO
3
. If the seepage's exit velocity is small enough, the CaCO
3
comes out of
solution to create the seal. The drains therefore would be ineffective in reducing internal
pore pressures. The same outcome would occur if a small crack extended to the drain, as
the exit velocity of the seepage would be very small. Only for substantial cracks would the
seepage water discharge into the drains continuously. Even then, over time CaCO
3
would
probably came out of solution as the water trickled down the drain hole, a situation that
can be seen in many concrete dams. The authors believe that drains within the dam wall
should be ignored in the design of a new dam or assessment of an old dam, unless calcu-
lation show that a crack or cracks would extend to the drains and only then for short-
term loading conditions.
It is therefore suggested that within the dam's concrete, uplift across any section should
be assured as varying linearly between the pressure at the upstream face and zero or tail-
water level at the downstream face (drain ignored in this case). This is consistent with
modern guidelines. For a crack-free dam, such an assumption may be conservative. Some
expect the pressure to fall rapidly within a short distance of the upstream face of the dam.
US Corps of Engineers (1995) suggest that a 50% reduction in pressure across the section
would be acceptable to account for the likely sudden pressure drop at the upstream face,
but the warning is given that such an assumption would apply to crack-free concrete.
Given it is difficult to prove no cracks, the preferred approach is to assume linear pressure
as described above.
The appearance of CaCO
3
on the downstream face of a dam, at, say, a construction
joint, could be interpreted as a crack with full head applied up to the blockage, in this
case, the downstream face. Even a formed drain in the concrete might not help as there
could be a similar CaCO
3
seal here. This particular condition seems to have been ignored
