In this article we will discuss about:- 1. Classification of Earth Dams 2. Causes of Failure of Earth Dams 3. Essential Requirements for the Safe Design 4. Stability Analysis.
Classification of Earth Dams:
Earth dams may be classified in two ways:
1. Classification Based Upon the Method of Construction:
Under this classification earthen dams can be:
(i) Rolled fill earth dams and
(ii) Hydraulic fill earth dams.
(i) Rolled-Fill Earth Dams:
In the construction of such dams successive layers of moistened earth are laid one after the other. Each layer is laid after the layer of earth previously laid is adequately compacted. Sheeps foot roller is mostly used for carrying out the process of compaction. Bulldozers, scrapers, draglines, like heavy earth moving machinery is used for the purpose of earth work. Moisture content during compaction is maintained at the level of optimum moisture content. Proper slope of the dam is maintained during construction.
(ii) Hydraulic Fill Dams:
In this method of construction, excavation, transportation, and placing of the earth is done by hydraulic methods. Outer edges of the embankment are maintained slightly higher than the middle part of the dam. The mixture of excavated materials with water, is pumped and discharged at the outer edges of the dam.
The mixture of excavated material and water consists of coarse material and fine material. When this mixture is discharged near the outer edges, the coarser material of the mixture would settle at the edges while finer material settles at the centre. No compaction is required in this construction.
The process is continued till dam of required height is constructed. In this dam coarser material lies near outer edges which is not very impervious. The finer material deposited in the central part of the dam consists of clay mostly and thus provides imperviousness to the dam.
2. Classification Based Upon the Section of the Dam:
According to this classification the dams can be of the following three types:
(i) Homogeneous Earth Dams.
This dam is made from a single material. It is suitable for low heights only. Purely homogeneous section can be modified a little by constructing rock toe at the downstream lower end of the dam and also by putting horizontal filter drains. Both these measures control the seepage and thus enable to construct much steeper slopes of the dam.
These measures also keep the phreatic line of seeping water, well within the body of the dam. The homogeneous dams are made from impervious or semi-impervious soils. This is essential to put a barrier against the flow of seeping water. Upstream slope of the dam is generally kept flat so as to reduce the path of seeping water and to counter act the effect of sudden draw downs.
(ii) Zoned Earth Dam:
This dam is made by using more than one material. In this case, the central part of the dam, which is known as core is made from impervious material. Considerably more pervious material is used on both the sides of core. The dam also consists of a rock toe, a system of horizontal drains, and sometimes even system of inclined filters to carry out proper drainage of seeping water from the dam.
If, at certain place, a variety of soils is available the dam should always be of zoned type. Impervious material should be used for core, while pervious soils at flanks of core. U/S pervious soil provides stability against rapid drawdown, while that on D/S side acts as a drain to control the seepage line. Central core checks the seepage.
(iii) Diaphragm Type Earth Dam:
This is a sort of combination of both the earth dams. The bulk of the embankment is made from pervious soil, but a thin diaphragm of impermeable material is provided at the central part of the dam to check die seepage. The impervious diaphragm may be made of impervious clayey soil, cement concrete, masonry or of any other impervious material.
It can be located somewhere in the central part of the dam or it may be put on the upstream face of the dam also. The difference in zoned and diaphragm type earth dams is due to thickness of impervious core or diaphragm only. If thickness of impervious core is less than 10 m or less than the height of the dam above that level the dam is known as diaphragm type, otherwise it would be zoned earth dam.
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 earth dams may be divided into three categories as follows:
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 are 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 weeks. 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 known as 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. When such slides of upstream face take place, the pore pressure along the surface of slide gets dissipated to a large extent. This aspect does not give birth to the process of further sloughing and sliding and hence chances of failure due to sliding are very small.
(iii) Down Stream Slope Slide:
The chances of downstream 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 inch 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 seems 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 to wage 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:
(a) The core may develop cracks and may lead to leakage and then to ultimate failure.
(b) Dam itself or its foundation may settle down due to shaking action. This may lead to over topping of the dam.
(c) Sliding of hill tops into the reservoir and causing rise in reservoir level. This phenomenon may again cause over topping of the dam.
(d) 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.
(ix) 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 somehow 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.
(iv) Gullying:
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:
(i) Piping:
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 through the body of the dam causes following detrimental effects:
(a) Seeping water develops erosive force which dislodges soil particles from soil structure and cause rearrangement of void between larger grains.
(b) The seeping flow, along with associated differential pore pressure can lift soil particles, causing boiling.
(c) Internal erosion of the soil starts from tine 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.
(d) 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.
(ii) Sloughing:
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 downstream 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 known as sloughing which may also be named as ravelling.
Essential Requirements for the Safe Design of the Earth Dam:
If an earth dam has to be safe and stable during construction and in its subsequent operation, it must be designed considering following points:
1. The earth dam should never be allowed to be over topped during heavy flood. This object can be achieved by providing spillways of adequate capacity.
2. Sufficient free board should be provided.
3. Seepage line should remain buried sufficiently on the D/S face when reservoir is full. This will prevent occurrence of sloughing of the D/S face.
4. The dam portion laying D/S of the impervious core should be properly drained.
5. The U/S and D/S slopes should be such that they are not damaged by pore pressure during and immediately after construction.
6. The U/S face of the dam should remain stable during rapid draw downs.
7. U/S face should be protected against wave erosion and D/S face against the erosion due to heavy down pours and strong winds. For this rip rap, revetment or pitching is down on U/S face and counter booms and turfs are developed on D/S face.
8. U/S and D/S slopes of the dam should be flat. This will ensure safety of the dam against shear failure of the foundation.
9. In order to prevent internal erosion of the soil the seepage flow through the dam and foundation should be suitably controlled.
10. Free passage of water, wither through holes made by burrowing animals or by piping should be allowed.
11. In areas, frequently subjected to earth quakes, the dam should be designed considering earthquake resistant measures.
Stability Analysis of Earth Dams:
There are numerous methods of analysing the slope stability of the earth embankments. The most commonly used method is Swedish slip circle method devised in 1922.
In this method, the curved slip surface is taken to be an arc of a circle. There will be a number of such likely slip circles but we are interested to pick out the most dangerous or critical slip circle. The critical slip circle is such a circle along which the soil had the least shear resistance. The centre of this circle is located by trial and error. In the analysis by this method, the soil slope is considered as made of one type of soil only.
(i) Cohesive Soils:
Let ABC be any one slip circle with O as its centre. Let centroid of area ABCD of the soil lies at G and let x be the horizontal distance of point G from point O. The position of G can be obtained by any method used for irregular planes.
Let W be the weight of the soil mass ABCD and length one metre. For cohesive soils ϕ = 0 and hence its shearing resistance is entirely dependent on cohesion only. The resistance is same along the entire surface ABC.
The moment (Mc) causing slip along surface ABC is –
Mc = W X x kg.m
The maximum resisting moment (Mr) is the moment developed by cohesive shear resistance along surface ABC multiplied by radius R.
Mr = L x 1 x C x R kg.m
= LCR kg.m
Where L = Length of arc ABC
C = unit cohesion in kg/ m2
R= Radius of slip circle in metres
L = (2πRδ / 360) x CR
Where δ = the angle of arc ABC, in radians and centre point O
Factor of Safety (FS) against sliding = Mr / Mc
Various other slip circle can similarly be drawn and factor of safety is found for all of them. The slip circle giving the minimum factor of safety is considered as the critical slip circle. The critical slip circle is located as follows.
The values of factor of safety as worked out for different slip circles, say four, are plotted as shown in Fig. 12.28 and a curve is drawn passing through the toes of the ordinates representing the four values of the factors of safety corresponding to four slip circles. The lowest point on this curve is noted and a vertical line is drawn through it, meeting the top of soil mass at a point say Z. A and Z mark the two points through which the critical slip circle will pass.
(ii) C-ϕ soil:
Such soils do not have constant shear strength. The shear strength of such soils which is due to cohesion and internal friction, changes from point to point along the periphery of slip circle. The Coulomb’s equation S = C + p tan ϕ is used to find out the shear strength of such soils, where C = a constant along the slip circle but pn the pressure normal to the slip circle due to weight of the soil which varies from point to point along the slip circle. The method is as follows –
The whole soil mass enclosed in ABCD is divided into vertical strips or slices as shown in Fig. 12.29. All the slices are of equal width and thickness but different in height. Let us study say 6th strip shown in Fig. 12.29 –
Take one running metre of the strip, its stability is practically due to two forces only, viz. its weight W6 and its sheering strength S6 along the curved surface of length l6 metres at the bottom of the strip. The weight W6 has two components one normal and other tangential to the curved bottom of the strip. Let normal component be denoted by Wn6 and tangential component by Wt6.
The Coulomb’s equation for the strip will take the form of –
S6 = (c.l6 + Wn6 tan ϕ) kg.
Where c.l6 = cohesive strength of strip per metre length
Wn6 tan ϕ = frictional strength per metre length of the strip
Let R be the radius of this circle, the moment of this shear strength would be S6 x R kg.m
Moment of shear strength (Mr) due to entire length of slip circle ABC
General form of equation will be –
The weight of each strip one metre long is proportional to its area which can be found approximately by trapezoidal formula and accurately by a planimeter. Normal and tangential components for each strip can be found graphically by drawing the triangle for forces for each strip.
The factor of safety for several slip circles can be found out and the circle giving minimum F.S. is taken as the critical slip circle. It should be noted that tangential components of weights of some strips near the toe of the slope would be actually resisting the slipping tendency and as such they should be adopted with opposite sign.
Stability of U/S Slope:
The U/S slope of the dam is subjected to most adverse condition during sudden draw down. During steady seepage, the seepage pressure acts inwards the dam from the slope and thus tends to increase the stability of the U/S slope. Thus steady seepage conditions do not represent the critical state.
When water is suddenly drawn away from the U/S slope by emptying the reservoir, the hydrostatic force acting along the U/S slopes is completely removed. Since the drainage from the soil is not as rapid as the draw down, the weight of drainable water held in the soil tends to help a sliding failure, as hydrostatic pressure is absent to counter act this tendency. The stability of the U/S face can be analysed by developing a flow net.
The factor of safety is calculated by following formula –
Where c, L, ϕ, Wn and Wt are the same as given in cϕ soil. ΣU is the total uplift pore pressure on the slip surface.