About erosion mechanism
About erosion mechanism
(OP)
Is this the right erosion mechanism explination ? Please advise:
The differential settlement along the base of a highly compacted sand dam will result in a vertical crack breaking up the dam. If the internal compressive stress is not high enough to squeeze the crack shut, it will remain open and water will ingress into it from the surface or from the neighboring saturated zones. The walls of the crack will most likely be under tension and if the dam cannot resist this tension (through its unsaturation-induced suction) and/or if the water flowing into the crack exerts pressure that renders the effective stress vanished, erosion will take place.
The differential settlement along the base of a highly compacted sand dam will result in a vertical crack breaking up the dam. If the internal compressive stress is not high enough to squeeze the crack shut, it will remain open and water will ingress into it from the surface or from the neighboring saturated zones. The walls of the crack will most likely be under tension and if the dam cannot resist this tension (through its unsaturation-induced suction) and/or if the water flowing into the crack exerts pressure that renders the effective stress vanished, erosion will take place.





RE: About erosion mechanism
first, dams are rarely built of sand. If so, they usually have a clay core zone which limits the amount of water flowing through cracks.
I suppose differential settlement could create a vertical crack, however subgrade collapse or settlement more often creates a horizontal crack at the base of the dam (usually on the wet side). Differential settlement also causes low spots along the crest of the dam which are prone to overtopping and then failure.
I'm not sure there is any "internal compressive stress" which is "squeezing" cracks shut in a sand dam. What mechanism could create such a force in a sand embankment? A clay core would expand to fill cracks.
There are a couple of typical failure mechanisms for flow through a crack in a dam.
1) Internal erosion is when flow through cracks / along outlet conduits or abutments erodes the material internally and creates a "pipe".
2) High phreatic pressure exiting from the toe of the dam carries away fine materials. Given enough time, this can also create a "pipe" which would allow the reservoir to drain and cause a dam failure.
In order to cause failure of the dam, both of these mechanisms require sufficient hydraulic head to cause the erosion. They also require that the embankment soil can form a stable "pipe" to allow the sediment transport to progress. This second requirement may not be present in a sand dam unless there is sufficient clay mixed in with the sand to provide the necessary cohesion to allow the pipes to form. In fact, clean sand filters are often used as crack stoppers in flood control dams. The sand is installed in a central filter through the embankment or in a filter diaphram around a conduit penetrating the dam to prevent erosion from progressing through the dam and causing failure.
RE: About erosion mechanism
-I do not know really the consequence of this huge settlement and how can be fixed (references will be appreciated)..?
-Regarding "internal compressive stress": with the progress of construction the dam will be raised resulting in compressive pressure on its underlying portion (which starts to act as foundation).I meant that such compressive stress may be large enough to push the walls of the cracks toward each other (and thus close the crack)
- When it comes to settlement outcome, I thought that that compromised freeboard that might cause the overtopping is mainly due to the overall total settlement rather than the differential settlement that causes piping/ erosion concerns. Please confirm
I should admit that my experience on this subject is less than humble and any suggestion, references, comment will be helpful
RE: About erosion mechanism
I'm not sure about this topic, just wondering. . . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: About erosion mechanism
Loss of freeboard due to settlement is common. However, differential settlement causing just a portion of the embankment to settle is the worse case and more likely to cause failure during overtopping. The overtopping flow is concentrated at one location in this case producing higher velocity flow and more likely to cause erosional failure than if the entire embankment overtops.
RE: About erosion mechanism
But if they occur during operation, how one can repair or account for them
RE: About erosion mechanism
To prevent overtopping, simply add material to the top of the dam to increase the freeboard. Or, add an auxilliary spillway.
Or build another dam downstream to augment the original dam.
RE: About erosion mechanism
For a water-retaining structure, one would require a filter 99% of the time. (Maybe only 98%.)
RE: About erosion mechanism
RE: About erosion mechanism
The 2% is reserved for low-hazard dams, in some cases homogeneous dams with flat slopes where there are no suitable materials available for zoning within a very long haul distance, temporary cofferdams, etc.
RE: About erosion mechanism
Again, for overtopping failure, the spillway is theoretically passing the majority of the PMF before the failure occurs, so the downstream channel is already seeing large flows. The wave created by the breach may cause only an incremental increase in the flood elevation downstream.