CAUSES OF FAILURE OF EARTH DAMS
- One of the investigation reports on past failures of earthen dams states that 40% failures were due to hydraulic failures and 30% each due to seepage and structural failures.
- The causes of failure of earthen dams may be divided into three categories as follows:
- Structural failures.
- Hydraulic failures.
- Seepage failures.
1. Structural Failures.
Causes of structural failure of the earthen dams are the following.
(i) Failure due to pore pressure.
- The drainage capacity of impervious compressible soils is very slow.
- When earth dams are made from such soils, excessive pore pressures are developed in the soil, during and immediately after construction of the dam.
- The amount of pore pressure is dependent upon the permeability of the soil.
- Lesser the permeability greater is the pore pressure.
- If the permeability of the soil being used in the construction of the dam is very low, it is possible that there may be no substantial drop in pore pressure in the central part of the dam, by the time the construction of the dam finishes.
- The amount of pore pressure, equal to weight of the soil has been noticed above the point where pressure is being measured.
- The pore pressure thus may cause failure of the slope and may even impair the stability of the dam.
- Hence construction stage is more critical when soil being used for construction is almost impervious.
- It has been further noticed that pore pressures cause two types of construction slides which differ in speed and magnitude of the movement.
- The first type of slide is uniform and slow and continues for a period of about two weaks. Failure of North Ridge Dam in Canada had been attributed to this cause.
- The second type of slide is sudden and rapid.
- The most of this sliding is completed within few minutes.
- The failure of Marshal Creek Dam in USA is the example of such a failure.
- It is stated that this movement took 15 to 20 minutes only.
(ii) Sudden drawdown on the upstream face.
- When reservoir is emptied suddenly, the pressure due to water suddenly vanishes from the water face.
- But this action being sudden, the saturated soil up to which water was filled before emptying, does not get time to release water so as to develop equilibrium conditions.
- This phenomenon is know sudden drawdown.
- Due to drawdown the hydrostatic pressure due to reservoir water is removed, without leaving any counteraction against the pressure due to water held in the soil.
- This unbalanced outward pressure due to water held in the soil of upstream face causes upstream face to slide. Such a slide of upstream face normally does not cause failure of the dam, but soil thus slided may block the outlets of conduits and cause difficulties.
- This aspect does not give birth to the process of further sloughing and sliding and hence chances of failure.
(iii) Down stream slope slide.
- The chances of down stream slope sliding are maximum when reservoir is full and rate of percolation is at its maximum rate.
- The earthen dam can easily fail by this reason.
- The slides that may occur on D/S side of the dam may be deep slides or shallow slides.
- Shallow slides occur in sandy soils only and they never exceed 1 to 2 in in depth in direction normal to the slope.
- Deep slides take place in clayey dam and clayey foundations.
- When a deep slide has taken place there is no relief to pore pressure and the unstable vertical slide scrap, left standing often sloughs or slides.
- This process of slides keeps on repeating until dam reaches the point of breach.
- One strong wave of water will now cause breach and failure of the dam.
(iv) Foundation slide.
- The dam can slide as a whole if it is founded on fine silt or soft soil.
- Presence of soft weak clayey seem or that of silt and sand both can cause failure of foundation.
- Expansion of soil on saturation may cause lifting of the slopes and thus may cause failure of foundation.
(v) Failure by spreading.
- When dam is constructed on stratified deposits containing layers of soft clay, the failure by spreading of the fill may take place.
(vi) Slope protections failure.
- The slopes of the dam are generally protected by pitching or rip rap.
- Pitching or rip rap is a protective layer of large boulders.
- This layer is generally laid over a layer of gravel or filter blanket.
- Due to repeated striking of water waves to the riprap at level of the reservoir, the rip rap may be dislocated.
- This exposes the embankment towage erosion.
- The failure of the rip rap may ultimately lead to failure of the dam.
(vii) Failure due to holes caused by burrowing animals.
- The burrowing animals may dig holes through the dam or through foundation and may cause failure of the dam by piping.
- Normally such animals do not dig in moist soils i.e. below the seepage line.
- However, if water level in the dam is very low for a number of years, the burrowing animals may honey comb the dry upper part of the dam.
- As soon as water level rises, the water may escape through the honey combed holes and may cause failure of the dam.
(viii) Failure due to earthquake.
Following may be causes of failure of earthen dam due to the effect of earthquakes.
- The core may develop cracks and may lead toleakage and then to ultimate failure.
- Dam itself or its foundation may settle down due to shaking action.
- This may lead toover topping of the dam.
- Sliding of hill tops into the reservoir and causing rise in reservoir level.
- This phenomenon may again cause over topping of the dam.
- Horizontal component of the acceleration force due to earthquake may cause shear slide of the appreciable portion of the slope of the dam and may cause dam failure.
- Failure due to soluble material in dam.
- If some material which is soluble in water, is present in the dam or in its foundation, it may cause failure of the dam.
- Such materials are washed away with time and may cause settlement of the dam and thus failure.
- Such materials if get deposited in the filters, designed for drainage purpose, may cause clogging of the filters and this may ultimately lead to dam failure, as drainage of the dam will not be proper in that case.
2. Hydraulic Failures.
- Hydraulic failure of the dam may take place due to following reasons.
(i) Over topping.
- If amount of flood has been under estimated, the capacity of the spillways will not be adequate.
- This may lead to the rise in water level in the reservoir above the maximum estimated level.
- This may ultimately lead to the over topping of the dam.
(ii) Wave erosion.
- If U/S face is not properly protected by rip-rap, the wave erosion of the upstream force will occur.
- Roller motion in the soil is set and thus easily gets scooped out when wave returns.
- If this action of wave erosion is not checked in time, it may lead to scooping out whole of the free board soil and may ultimately cause failure of the dam.
- Waves can also cause u/s slips and consequent failures.
(iii) Toe erosion.
Toe erosion of the dam may occur if water on the D/S side of the dam is some how coming contact with the Toe of the dam.
The failure due to this reason can be avoided by providing a thick riprap on D/S Toe of the damp up to a height slightly above the tail water level.
- Gullying failure is due to heavy rainfall.
- Gully formation can be avoided by turfing, or making counter berms on D/S slope of the dam.
- Good system of drainage from d/s side of the dam helps a great deal in providing this failure.
3. Seepage Failures.
- Piping and sloughing area the two seepage failures of the earth dam.
- Both these phenomena of failures are as follows.
- Piping is nothing but process of progressive erosion of concentrated leaks.
- The piping trouble can be in the body of the dam or in the foundation of the dam.
- The water seeping though the body of the dam causes following detrimental effects.
- Seeping water develops erosive force which dislodges soil particles from soil structure and cause rearrangement of void between larger grains.
- The seeping flow, along with associated differential pore pressure can lift soil particles, causing boiling.
- Internal erosion of the soil starts from the point of exit and slowly progresses backwards.
- As concentration of leak goes on increasing with more and more movement backwards a conduit or pipe is formed through the soil. Formation of this conduit is known as piping.
- The internal pressures in the soil water reduce internal friction and thus weaken the soil mass.
- The piping trouble can easily occur if soil around the concrete pipe is pervious and is not properly compacted.
- Piping may also take place if there is poor bond or compaction between the dam fill and the foundation or abutment.
- The phenomenon of sloughing is very closely related to the process of piping.
- When reservoir is full, the Toe of the dam is generally saturated.
- The Toe may erode producing a small slide.
- This slide makes down stream face more steep which again becomes saturated by seepage and slides again.
- This again causes still more steepness of D/S face.
- This process of seepage and sliding continues till the dam section is rendered too thin to withstand any water pressure and thus cause failure of the dam.
- This process of repeated saturation of the Toe and then its sliding is know sloughing which may also be named as ravelling.