Causes of Failure of Earthern Dam
Causes of Failure of Earthern Dams: * Hydraullic Failure * Seepage Failure * Structural Failure Hydraullic failures: Due to , * Overtopping (design level is underestimated) * Erosion of U/S face (wave action – height) * Cracking in upper portion of dam due to frost action (additional freeboard allowance up to 1. 5 m) * Erosion of D/S face due to rain action (maintenance – berms – grass) By overtopping: the water may overtop the dam ,if the design flood is under-estimated or if the spillway is of insufficient capacity or if the spillway gates are not properly operated.
Sufficient freeboard should, therefore ,be provided as an additional safety measure. Erosion of upstream face: The waves developed near the top water surface due to the winds, try to notch-out the soilfrom the u/s face and may even, sometimes, cause the slip of the u/s slope. U/s stone pitching or riprap should ,therefore, be provided to avoid such failures. Cracking due to frost action: Frost in the upper portion of the dam may cause heaving and cracking of the soil with dangerous seepage and consequent failure. An addition freeboard allowance upto a maximum of say 1. m should therefore, be provided for dams in areas of low temperatures. Erosion of Down stream(D/S) face by gully formation: Heavy rains falling directly over the d/s face and the erosive action of the moving water, may lead to the formation of gullies on the downstream face, ultimately leading to the dam failure . This can be avoinded by proper maintenance, filling the cuts from time to time especially during rainy season, by grassing the slopes and providing proper berms at suitable heights so that water has not to flow for considerable distances.
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The proper drainage arrangement are made for the removal of rain water collected on the horizontal berms. So the provision of berms may be helpful to reduce runoff and decrease the velocity. Seepage failure: Due to, * Uncontrolled seepage (causes scour through DS wet zone – needs adequate filters) * Piping (through dam foundation – either prevention or control of percolation…may cause dam subsidence) All earth dams have seepage resulting from water permeating slowly through the dam and its foundation. Seepage must be controlled in both velocity and quantity.
If uncontrolled, it can progressively erode soil from the embankment or its foundation, resulting in rapid failure of the dam. Erosion of the soil begins at the downstream side of the embankment, either in the dam proper or the foundation, progressively works toward the reservoir, and eventually develops a direct connection to the reservoir. This phenomenon is known as “piping. ” Piping action can be recognized by an increased seepage flow rate, the discharge of muddy or discolored water, sinkholes on or near the embankment, or a whirlpool in the reservoir.
Once a whirlpool (eddy) is observed on the reservoir surface, complete failure of the dam will probably follow in a matter of minutes. As with overtopping, fully developed piping is virtually impossible to control and will likely cause failure. Piping through the foundation Sometimes, when highly permeable cavities or fissures or strata of coarse sand gravel are present in the foundation of the dam water may start seeping at a huge rate through them. This concentrated flow at a high gradient may erode the soil.
This leads to increased flow of water and soil, ultimately resulting in a rush of water and soil there by creating hollows below foundation . Piping through the dam body When the concentrated flow channels get developed in the body of the dam, soil may be removed in the same manner as was explained in foundation piping, leading to the formation of hollows in the dam body, and subsequent subsidence of the dam. These flow channels may develop due to faulty construction, insufficient compaction, cracks developed in embankment due to foundation settlement, shrinkage cracks,animal burrows ,etc .
All these causes can be removed by better construction and better maintenance of the data embankments. Piping through the dam body, generally get developed near the pipe conduits passing through the dam body. Contact seepage along the outer side of conduits may either develop into piping , or seepage through leaks in the conduits may develop into piping. This can be avoided by thoroughly and properly compacting the soils near the outlet conduits and by preventing the possibilities of leakage through conduits and by preventing the formation of cracks in the conduits, but preventing the formation of cracks in the conduits.
These cracks in the conduits are caused by differential settlement and by overloading from the embankement. Sloughing of D/S Toe This process is somewhat similar to piping , this process starts when downstream toe becomes saturated and get eroded, producing a small slump or a miniature slide . The miniature slide leaves a relatively steep face which becomes saturated by seepage from the reservoir and slump again , forming a more unstable surface.
The process continues till the remaining portion of the dam is too thin to withstand the horizontal water pressure, leadig to the sudden failure of the dam. Structural failure : Due to , * Foundation slide by soft soil (fine silt – soft clay, …all dam body slides on foundation) * Slide of slopes (U/S or DS slopes) Structural failures can occur in either the embankment or the appurtenances. Structural failure of a spillway, lake drain, or other appurtenance may lead to failure of the embankment. Cracking, settlement, and slides are the more common signs of structural failure of embankments.
Large cracks in either an appurtenance or the embankment, major settlement, and major slides will require emergency measures to ensure safety, especially if these problems occur suddenly. If this type of situation occurs, the lake level should be lowered, the appropriate state and local authorities notified, and professional advice sought. If the observer is uncertain as to the seriousness of the problem, the Division of Soil and Water Resources should be contacted immediately. The three types of failure previously described are often interrelated in a complex manner.
For example, uncontrolled seepage may weaken the soil and lead to a structural failure. A structural failure may shorten the seepage path and lead to a piping failure. Surface erosion may result in structural failure. Minor defects such as cracks in the embankment may be the first visual sign of a major problem which could lead to failure of the structure. The seriousness of all deficiencies should be evaluated by someone experienced in dam design and construction. A qualified professional engineer can recommend appropriate permanent remedial measures. Precautions of eathern dam: 1. Filling earth is of sufficiently low permeable soil 2. Provision of spillway + outlets to avoid overtopping 3. Provision of sufficient freeboard 4. Seepage line remains inside DS face of dam 5. No possibility of free flow from US to DS 6. US face should be protected against wave action 7. DS face should be protected against rains 8. Provision of filter drains to drain parts DS of impervious core 9. Stable US and DS slopes under worst loading conditions 10. Counteraction due to consolidation (about 3% of dam height)