Environmental Engineering Reference
In-Depth Information
The likely effects of this are:
- Possible drying shrinkage cracks in the sides of the trench;
- Different moduli of the compacted backfill compared to the original dam leading
to cracking and differential settlement;
- Potential for poor compaction of the backfill in confined spaces.
Note that it is the continuity of the potential defect caused by a conduit and the exca-
vation through the dam which is so critical to the initiation of piping.
(b)
Continuation and progression
. The conduit facilitates this by providing a “side” or in
the case of a circular outer shape, near its invert, a “roof” to the potential erosion
hole which will not collapse. Hence the likelihood of erosion developing beyond the
initiation is greater than without a conduit.
(c)
Breach
. The conduit itself does not cause a breach mechanism which is mostly con-
trolled by such factors as overall zoning. Flow from the conduit may facilitate
removal of eroded or slumped material and therefore contribute in a secondary way.
13.5.3
Flow out of the conduit
(a)
Initiation
. The conditions conducive to flow out of the conduit (if it is cracked, or has
open joints) are as for piping into the conduit;
In addition, the conduit must flow under pressure, either because it fills and pres-
surises under some conditions, e.g. large flood flows for a spillway outlet, or if the
conduit is an outlet with the control valves on the downstream side;
(b)
Continuation and progression
. The flow out of the conduit leads to erosion of the soil
along the conduit which may then lead to a piping failure mechanism or to increased
pore pressures in the downstream part of the dam, leading to slope instability;
(c)
Breach.
The breach mechanism may either be as for erosion along the pipe or by slope
instability, leading to loss of freeboard.
13.5.4
Conclusions
Sherard (1973) concluded that where conduits must be placed through earth dams:
-
“It is particularly important that the embankment adjacent to the conduit be placed at
a relatively high water content and not be a soil susceptible to piping;
-
Even in small, homogeneous dams where no chimney drain is installed it is advisable to
provide a drain and filter around the conduit at its downstream end for the purpose of
intercepting concentrated leaks which follow the conduit;
-
In cases where the soil foundation is thick and compressible, it is not desirable to exca-
vate a trench under the conduit and fill it with compacted earth
.
”
Sherard and Dunnigan (1985) indicate that, because of concerns relating to compaction
of soil adjacent to cutoff collars and the increased confidence in the use of filters to con-
trol erosion, they recommend that:
- No seepage cutoff collars be provided;
- Concrete surfaces surrounding conduits should be smooth and the sides should be sloped
at 1H:8V or 1H:10 V so that earthfill can be compacted directly against the concrete;
-A filter be provided to surround the downstream position of the conduit, i.e. under-
neath as well as on both sides and the top so that all potential leakage travelling along
the concrete-earth core interface exits in a controlled manner.
At the ASDSO-FEMA 2000 workshop in Denver on issues, solutions and research needs
related to seepage through dams, 29 participants, including senior dams engineers from the
