Dam

It is the structure constructed across the river for retaining the water behind it.

Classification of the dam Classification based upon the function – Storage dam- This type of dam is usually of much height, thus stores the large amount of the water behind it and also creates the much the potential head at the dam location. Excessive water during the rainy season is stored and utilized when there is the shortage of flow in the river during the dry season. Diversion dam/weir- This type of dam is used for diverting the flow for the regular supply of the water at the conveyance channel (Open channel/tunnel) through the intake structure. They are usually of not of the much height and may store small volume of flow behind it. Detention dam- Such dams are considered for the storage of the floodwater in order to prevent downstream from flood disaster. Usually the dam is kept in unfilled condition prior to the occurrence of the flood at the rainy season and after the cessation of flood, water is allowed at downstream with the controlled way. Debris dam (Sabo Dam) These dams are constructed usually at the high grade river for the bed erosion control purpose. Because of the barrier, river bed materials like soil, gravel and boulders are retained behind the dam and prevents the river bed for further degradation. Coffer dam These are the dam, usually constructed to provide the dry working space by enclosing the river flow. These are not of the much height and are of the temporary nature. Earthen cofferdams are usually encompassed inside the dam body. Sometimes cofferdam may also be required to be erected depending upon the downstream flow level. Water retained behind the cofferdam is usually diverted through the diversion tunnel/channel to the downstream.

Classification based upon the construction materials -

- Concrete dam - Earthen dam - Masonry dam

o Stone masonry o Brick masonry

- Steel dam

- Timber dam-usually of the temporary nature - Rubber dam-usually for the small head regulation and operated by

blowing and flattening it. Classification based upon the structural load transfer mode- Gravity dam- In such dams, total loads acting on it is resisted by the self-weight of the dam.

Arch Dam-In such dams, part of the load acting on the dam is transferred to the side support (abutments) and part of the load to the foundation. Buttress dam In such dams total load on it is resisted by buttress (support) underneath the inclined slabs.

Arch dam

End support (Abutment)

Load transfer to support

G

Classification based upon the hydraulic design. Overflow dam- In such dams water is allowed to flow over the top of the dam. Besides, some of the parts of the dam may be the overflow dam and rest as non-overflow. The overflow section is termed as the spillway. Non-overflow dam – Embankment dams (made by filling of embankment materials i.e. soils, gravels, stones) are generally designed to have its top most elevation (crest) above the maximum level of the incoming flow. If the water overtops such dams, it is very susceptible for failure due to the weight reduction by buoyant forces and breaching leading the sweeping of its body materials. Such dams are called non-overflow dams. Classification based upon the head- Usually the hydropower projects are of the following types

o Runoff the river plant-The head on such plant is usually available by virtue of topographical condition of the project area. Powerhouse is usually located far behind from the intake structure.

o Valley dam plant-In such plant head is created by virtue of the height of the dam and powerhouse is usually located immediate downstream of the dam.

If the head thus created is, <15 meter-It is the low head plant 15-50 meter-It is the medium head plant. >50 meter-It is the high head plant

Selection of the type of the dam

According to the topographical condition of the site

Inclined slabs

Buttress Bracing or struts

Bracing or struts

A ASection at A-A

For the V-shape valley and strong rock abutments at beds and sides, then Arch dam is highly preferable For the U-shape valley and strong rock foundation at the bottom, then Gravity dam is highly preferable. For the wide valley and with alluvial stratum on the foundation layer, Embankment dam (earth fill/ rock fill) dam are considered. In case of unavailability of the sound foundation and also local earthen/rock materials are not found in abundant quantity at nearby, usually the concrete Buttress dam is preferred. According to the geological condition of the foundation In case of the rock foundation, any types of the dam could be constructed. Rock foundations have the high bearing capacity and minimum seepage loss. If fractures and faults exist, then they could be treated with the suitable engineering measures such as grouting, consolidation. If the fractured rock is of the shallow depth, then they could be wholly removed. Because of the low bearing capacity of the alluvial foundation (i.e. of soil, coarse sand, gravel), massive concrete gravity dams are technically discarded. Earthen/rock fill embankment dams are usually preferred in such situation. However Buttress dam or low height gravity could be designed with right technical design. Seepage flow is the measure concern for the alluvial foundation and cutoff walls, grouting are of the standard technical practice in such case. In case of the fine sand and silt foundation, earthen embankment dams are only the alternative. Low bearing capacity, excessive settlement, seepage and liquefaction failure are the major technical challenges in such foundation. Clay materials are the worst foundation case in the dam engineering practice. Low bearing capacity, excessive settlement due to long term consolidation are the major challenges in such foundation. However, low height embankment dams could be constructed with the proper foundation consolidation. According to the availability of the materials Availability of the suitable construction materials at nearby of the project side will greatly reduce the overall cost of the project. If the materials like clay, soil, gravels, rocks are available in abundant quantity, embankment dams are most suitable. In case of the concrete gravity dam availability of the crushed stones, coarse aggregates and sand at nearby will greatly reduced the project cost. According to the Climatic condition In sever climatic condition, due to the alternative thawing and freezing condition, may arise the development of the cracks on the concrete structures. So the thin membrane concrete structures such as Arch dam, Buttress dam should be avoided. In region of the much rain occurrence, it will be difficult to control the optimum moisture content and also may cause the erosion of the dam body, so the earthen embankment dam are not usually preferred even though construction materials are available abundantly. According to the Seismic condition

Non rigid dams such as earth dam, rock fill dam are most suitable than that of the massive concrete dam at the highly seismic zone. However with appropriate seismic design consideration any type of dam could be selected. According to the diversion condition If the diversion of the flow is not available and if the construction of the structure is required to be done with the stepwise blocking of the river section, then embankment dams are not considered due to the requirement of overflow condition during the stepwise construction. According to the environmental and ecological condition Aesthetic requirements, quick ecological and environmental adoption will influence on the selection of the particular types of dam. Earthen dams are more aesthetic, quick ecological adoption than that of the concrete dam. According to the life expectancy Concrete dams are more long-lasting than that of the other materials.

Loads on dam 1. Self weight of the dam-It is the weight of the dam due to the action of the

gravity and always acts downward direction through the CG of the body. The self weight of the dam depends upon the specific weight of the construction materials.

2. Hydrostatic pressure-It is the pressure exerted by the fluid on the dam body and always acts perpendicular to the contact surface. The resultant pressure acts through the center of pressure.

3. Uplift pressure- It is the pressure exerted by the fluid on the dam surface from the underneath and always acts towards the upward direction. The water enters in to the dam body through the pores, cracks, fissures and causes the uplift pressure. The uplift force will reduce the effective weight of the dam and depends upon the contact surface area where it acts. The magnitude of the uplift pressure will depend upon the types of the foundation, drainage provision, provision of the cutoff walls, grouting etc

4. Earthquake pressure-It is the pressure exerted on the dam body due to the acceleration wave of the earthquake. The wave acceleration direction may be vertical, horizontal as well as in inclined direction too.

5. Wind pressure-It is the pressure exerted by the wind on the exposed surface of the dam surface. The wind pressure depends upon the intensity of the wind velocity.

6. Wave pressure-It is the pressure exerted by the action of the wave on the dam surface. The wave force depends upon the height of the wave. The height of the wave in the reservoir depends upon the velocity of the wind and the fetching length(distance from the dam up to the end of backwater curve)

Dam

Water level at reservoir

7. Silt pressure-It is pressure exerted by the silt accumulated behind the dam and could be evaluated through the Rankine’s active earth pressure theory.

8. Ice pressure-It is the pressure exerted by the ice on the dam surface. It depends upon the ice thickness, temperature fluctuation and restrain of the resisting force.

Materials and construction of the dams Concrete dam Dam constructed by the admixture of the cement, sand and the aggregates are called the concrete dam .It has the great use in the engineering field because of its high strength, relatively low imperviousness, easiness in mechanized production, gaining of strength in short time and flexibility in casting. In hydro technical construction also various grades of concrete are used depending upon the physico-chemical properties of the water, its mechanical action, different loading condition as well as serviceability condition of the structure. Hydraulic concrete i.e. concrete to be used in hydro technical construction should have special properties such as impermeability, high strength, high density, water resistance, abrasion resistance, cavitaion résistance, crack resistance etc. The strength of the concrete depends upon its proportion, character of the aggregates water cement ratio, its age and hardening condition. Usually direct compression test are performed of the standard concrete cubes (15 cm x15 cm x15 cm) and tensile strength test are performed indirectly through the flexural test of the plain cement concrete beam specimens of 70 cm long and 15 cm x 15 cm in cross section. These are tested on 60 cm span with two point load application.

Grade designation

Characteristics compressive strength(28 days)

Flexural tensile strength or modulus of rupture of

concrete(7 days) M10 10 N/mm2 1.7 N/mm2 M15 15 N/mm2 2.1 N/mm2 M20 20N/mm2 2.4 N/mm2 M25 25 N/mm2 2.7 N/mm2 M30 30 N/mm2 3.0 N/mm2 M35 35 N/mm2 3.2 N/mm2 M40 40 N/mm2 3.4 N/mm2

Density and impermeability of the concrete is interrelated with each other but the imperviousness also depends upon the crack formation. By impermeability the concrete are divided in to following grades, Grade designation Pressure of water,

atmospheric pressure Seepage gradient not more than

B2 2 6 B4 4 8 B6 6 10 B8 8 12

The number in the table indicates the pressure of the water, that samples of the concrete withstand without allowing the water to percolate through them at seepage gradient ranging from 6 to12, respectively. Water resistance of concrete (corrosion resistance) The saturated concrete or concrete exposed to the water are susceptible to the chemical reaction depending upon the types of the cement used. Corrosion action may be occurred due to the dissolving of free lime Ca(OH)2 (soluble in water)content of the concrete by water. Mineral acids (carbonic acid CO2+H2O=H2CO3 reacting with the Ca(OH)2 results in formation of Ca(HCO3)2 which is soluble in water) contain in the water may cause the formation of soluble salts of these acids. The soluble salts are carried away by the seepage water causing gradual decrease in the density of the body causing the decrease in the strength and increasing permeability of the concrete. It will lead to the deep zone freezing causing susceptibility of deep crack formation. Corrosion effect may be tackled by selecting the suitable mineralogical content of the cement. Abrasion resistance of the concrete The surface of the concrete structure bearing the high velocity of flow as well as two phase flow (water, sediment) is prone to the abrasion. Once the erosion due to the

abrasion of concrete starts, it will further cause continuous aggravation and lead the structure to unusable or failure stage. The more the grade of the concrete the more will be the abrasion resistance. Cavitaion resistance of the concrete The concrete surface disposed to the high velocity and vacuum pressure is prone to the cavitaion. If the flow at the pressure zone reaches to the vapor pressure, air bubbles will be formed .These bubbles will be taken to the high pressure zone by the flow and will be collapsed. It will cause the flow of fluid towards the free space zone, which will cause the high dynamic pressure. Such types of repeated high pressure application to the surface of the concrete will cause the fatigue failure. Concrete of high grade (M35, M 40, M 50) are abrasion resistance and also the smaller size of the aggregates(not exceeding 30-40 mm) with W/C ratio 0.40-0.45 are much abrasion resistive. Cracking of the concrete Cracking of the concrete structure is due to the unequal settlement, excessive loading condition and also due to the temperature stress cause by the liberation of heat at the initial stage of the construction due to the hydration of the cement and also the change in the ambient temperature. If the stress developed by the uneven temperature development (much at the inner body and decreasing towards the surface) is restrained to release, it will cause the development of compressive as well as tensile stress inside the concrete structure. The tensile stress thus developed will cause the formation of cracks. The heat generation of the concrete due to the hydration of cement may be reduced by the introduction of low heat cement, laying of pre -cooled concrete mix, by adding some active substances to mix ( e.g. fly ash, volcanic tuff or other chemical dilutents), providing the cooling system inside the concrete structure or by providing the sufficient time for cooling for laid concrete before laying the next layer. For this purpose the concrete is laid with the thin layer 0.5 m at the bottom zone and up to 3 m at upper zone with the sectionional joints in longitudinal as well as in transverse direction. Laying of concrete Concrete is laid in the layer with the sectional joints. The transverse joint are make water tightness with the introduction of the metallic, bituminous or the rubber seals. The laid concrete is compacted with the vibrator. Before laying the next layer, the previous layer is thoroughly made clean with the application of brushes, jets of water or wind blower. Immediate before the placing of the fresh concrete, about 1.5 cm of cement slurry is applied on the surface of the previously laid concrete. Earthen dam These are the dams constructed by the earthen materials such as clay, soil, gravel, boulders, stone etc so they are not rigid in structure. Dams could be made of from the same materials and termed as the homogeneous dams or the combination of different materials terming non-homogeneous dams. These dams are constructed layer-by-layer. These dams are usually preferred when the foundation is not suitable for the concrete dam, when there is not much availability of the costly and scare materials

such as cement and when there is abundant availability of the earthen materials in vicinity. These dams are usually of trapezoidal shape in section and could be built to large heights. Such dams have the large width at the base, so with compare to other types of dams, could resist earthquake impact of relatively high magnitude. Method of construction

1. Rolled- filled earthen dam- In this technique, compaction of the filled soil is done with the rolling equipments (sheep foot, pneumatic, plain roller, and vibrator) with the maximum of the thickness of the layer 15 to 45 cm. Compaction is done at the optimum moisture contain of the given soil established through the laboratory taste and field control. Before laying of the new layer, previous layer is cleaned for any foreign materials and surface is made rough for the proper bonding of the laid layers. Construction materials are transported up to the site through the dumper or the conveyer and spreading of the material is done through the bull dozer or the scraper for the required thickness of the layer. 2. Hydraulic filled earthen dam- In this method, water is used for transportation of the dam material during the laying. Slurry or mud of the embankment materials is prepared at the borrow pit [with 1(soil):7(water) to 1:10]and they are transported to the site though the pipes or flumes. They are spread in the layers through the perforated pipes located at the outside edge of the dam. The coarse material (more pervious) is deposited near the pipe exit and fine materials (impervious) are carried to the center of the dam body. This will cause the zoned section of the dam with impervious core at the center. No rolling and compaction is needed in this technique. The main disadvantage of this method is that there is excessive pore water pressure inside the dam body, which will cause the decrease of the shear strength of the soil. This method is now almost obsolete.

3. Semi hydraulic filled earthen dam In this method outside the core is constructed with the dry method and only the inside core is advanced through the hydraulic fill technique.

Dry method Distribution pipe

i

Hydraulic fill

Impervious Core

Dry method Distribution pipe

Impervious Core

Homogeneous dam are constructed when there is abundant quantity of relatively impervious earthen materials. If the earthen materials are available but not much of impervious character then in such case body of the dam is constructed with such materials and to prevent the seepage of the water through the body curtain of impervious material are placed at the center or in inclined position inside the dam body. If this is of the rigid material(R.C.C)then it is called the diaphragm type and if it is if impervious earthen materials(such as clay silt)then it is termed as the core type.

Rock fill dam

Dam body if formed with the rock fragments available from the nearby quarry, excavation site or through the naturally available materials like talus, boulders, cobbles and coarse gravels. Igneous and metamorphic rocks are much suitable for the dam body. The rock fragments could be from the small piece to the size up to 3 meters and should be of the well graded, round shape ( slab shape rocks, may form the bridge action leading to large voids, should be discarded to use). Load on the rock fill material is transferred to the adjacent materials through their point of contact and mass stability of the dam is maintained through the frictional resistance. Because of its high density it has the high strength against the sliding failure and due to its flexibility much seismic resistance than that of the gravity dam could sustain. As the load on the dam is transferred through the point of contacts of the rock materials, the rock mass will get the uniform pressure if well graded rocks are placed. Small point of load transfer may lead to the crushing of the rocks due to the high concentration of the pressure, which will cause the settlement of the dam body.

Homogeneous dam

Dam with central diaphragm

Dam with central impervious core

Dam with inclined core

Method of the rock placement 1. Placing rocks in the layer of 1-3 m with the compaction by vibrators 5-10 ton

in mass 2. Placing rocks in the layer without compaction but applying forced water jet

(for 1 m3 of rock 2- 4 m3 of water). Forced jet will sluice the small size rocks in to the larger one and also lubricates and softens the surface of the rocks.

3. Gravels and pebbles are placed in the thin layer 0.5 to 1 m and compacted either by the pneumatic roller of mass up to 100 ton or vibrating roller up to10-12 ton.

Key element Free board-It is the minimum distance to be provided, so that water should not be overtopped the dam during the flood wave, tidal wave and earthquake wave action. Berms-It is the horizontal space provided at the dam body breaking the continues slope of the embankment. It is for the increasing the width of the dam section, to minimize the erosion due to rain water and passage for the regular inspection of the dam. Impervious core-It is to stop the seepage of the water through the dam body. Usually constructed by the impervious clay, silt, concrete and other impervious materials. Inverted filter-It is transitional layer between the parts of the dam having the materials of fine and coarse grain size. If these two parts were placed together without the transitional zone, there are chances of washing away of fine materials in to the coarse material due to the seepage force of flow. Pheratic line-This is the upper surface of the seepage flow from the dam body. Slope protection-Upstream slope of the dam is susceptible of the wave action of flow.These wave forces may cause the erosion of the surface materials.In order to prevent from such action ,usually the upstream slope is protected by the concrete slabs or stones riprap up to the level below than the minimum reservoir level. Downstream slope could be protected by turfing or stone riprap.

Downstream Water Level

Pheratic line

Impervious coreInverted filter

PerviousImpervious

Pervious

Concrete base as grouting cap

Grouting injectionGrouting curtain (cement, bentonite clay)

Minimum Reservoir

Maximum ReservoirFree boardSlope protection

Rock toe

Berm1 1

θ

Fseepage

W

F friction θ

θ

Concrete base- It is the foundation for the impervious core wall as well as provide the base for the injection of the grouting materials for checking the excessive seepage underneath the dam. Excessive seepage flow may cause the washing way of the finer particles and these processes may continue further so that there will be formation of the pipe like channel (Piping action of seepage) .Due to the excessive piping action, embankment may settlement down. Embankment slope(m:1) -It is m times horizontal to the 1 time vertical distance of the slope of the embankment. This slope is usually fixed according to the height of the dam and also with the angle of internal friction of embankment materials. The slope should be equal or flatter than this angle. Generally slopes are usually around 2: 1 to 3:1 depending upon the characteristics of the embankment materials. Rock toe-This is the portion of the dam at its toe, consisting of the rock embankments. It is where the seepage line of the dam ends and also protects the dam surface from the erosion action of the downstream water level. Piezometric gradient i= ∆hf / ∆s = sinθ ∆hf - head loss due to friction along distance ∆S

∆S-elementary distance Taking the elementary volume of the soil at the downstream slope of the embankment near to the surface of the exit of the Pheratic line. Let’s consider the forces acting on this elementary volume-

∆hf

∆s θ

Unit volume of elementary soil Pheratic line

(Seepage force acting per unit volume)F seepage = γ water i = γ water sinθ Submerged weight of soil per unit folume (W)= γ sub (Shear resistance per unit area note-upper area is exposed to air so do not contribute for the shear resistance) F friction per unit area= f γ sub cosθ , where f coefficient of friction If , γ water sinθ +γ sub sinθ (Sliding force) > f γ sub cos θ (Resisting force) γ water sinθ/ cos θ +γ sub sinθ /cos θ > f γ sub tanθ >[ f γ sub / (γ water + γ sub)], If the angle of the downstream slope of the embankment is less than the above value then this elementary unit volume of soil will may washed-out due to the seepage force. So in order to protect from the seepage force and also from the downstream flow wave action, rock fill toe dam is generally introduced. Similarly when the seepage force of flow beneath the dam is excessive than the resisting force, there also occurs the washing out of the small particles and consequently form the pipe like flow of particles. This is called the piping action and excessive of this may cause the settlements of foundation. As we know the gradient of flow is maximum when the flow emerges to the downstream side (termed as the exit gradient), so this is the critical place of starting of the piping action. Failure mode of earthen embankment dam

1. Hydraulic failure a) Overtopping failure-If the flow takes place above the crest level of the

embankment dam due to the various causes such as inadequate flood discharge spillway design ,insufficient free board consideration etc then the dam body is susceptible of erosion by the over flowed water.

b) Wave erosion- The wave action due to the wind and tides will cause the erosion of the dam material at the upstream face, if the dam material could not sustain wave velocity. It will lead to washing out of the dam material or overturning of the protection slab.

c) Toe erosion- Similar to the wave action at the upstream of the dam, downstream dam slope is also susceptible for erosion by the downstream water. It could be checked either by providing the riprap protection or by providing the rock toe drainage.

d) Surface erosion of the D/S slope-Heavy intensity of the rainfall may cause the erosion of the surface of the dam and may form the several gullies along its slope direction. These gullies may further erode to larger size, if rainfall

commence for the long duration causing serious damage to the dam. Turfing or providing contour drainage for arresting the high velocity flow at bearms may lessen the severity of the erosion.

2. Seepage failure a) Piping failure-if the exit gradient of the seepage flow is more than the critical

gradient i.e. if the seepage force is much than the resisting force (submerged weight of the soil) of the soil, then soil particles are susceptible for the dislocation. The soil particle at the surface is much vulnerable for the dislocation as there is no overburden support from above. As the surface particles are dislocated, then process will be even accelerated towards the upstream direction because seepage gradient will further increase continuously. This will cause the pipe like flow inside the dam body or in the foundation and dam body may settle down.

b) Sloughing-If the seepage line exists at the downstream face of the dam, the portion of the toe of the dam below the exit point will always in the wet condition. It will cause the reduction of the stability of the slope and small size sliding may occur. The repetition of wetting and sliding will be continued further and ultimately dam may lead to failure. This phenomenon is termed as sloughing.

3. Structural failure a) Failure due to the pore water pressure-If the pore water presenting in the

embankment body is draining slowly(rapid rise of layer during construction, no drainage provision for the pore water escape)then much of the stress will be beard by the water itself and effective overburden stress on the soil will be less. These pore water pressure will cause the extra laetrile load and may cause the failure of the embankment slope. This will also cause the reduction in the shear strength of the soil. In experience, it is found that pore water pressure at the central part of the dam is nearly equal to the overburden pressure due to the weight of the soil above it. It is particularly dangerous in case of the earthquake condition if occurred during the dam construction period.

b) Sudden drawdown on the upstream face-The sudden drawdown of the reservoir will cause the seepage force acting along the sliding direction causing increase of the driving force for slope failure.

c) Downstream slope failure-When the reservoir is at maximum level and if the steady stage seepage is at maximum rate, then the downstream slope are more vulnerable to slide due to seepage force acting in the direction of driving force for the sliding.

d) Foundation slide-The dam body as a whole may slide, if the foundation is from the silt or the soft soil. The slow consolidation process and expansion of clay soils due to the saturation will decrease the shear strength of the foundation soil.

e) Failure by spreading- When the earthen dam is located above the stratified deposit that contains layer of silt clay, the fail of the dam with spreading of the fill materials may happen.

f) Due to the Earthquake o Cracks at impervious layer may cause the piping and leakage problem o Excessive settlement due to earthquake may cause the overtopping of

the dam o Rise of the wave due to earthquake could cause the overtopping o The hydrodynamic pressure could lead to the shear failure o The inertia force caused by self weight of dam may cause the shear

failure o Earthquake pressure could cause the liquefaction of the soil mass at

the dam body or at foundation having the excessive pore water pressure.

o Earthquake may cause the sliding of the surrounding mass in to the reservoir causing rise of water level and wave action leading to overtopping.

o g) Failure due to the leaching-water may leach the soluble salts e.g. iron oxide,

calcium carbonate, presence in the foundation or at the abutments, causing formation of the large cavity leading to the excessive settlement of the dam. These soluble salts may also clog the filter of the drainage material causing decrease in its effectiveness.

The location of the Phreatic line is important because,

1. Its location inside the dam body will help in finding the seepage discharge through the dam and value of the pore water pressure at any point inside the dam body.

2. It is required to check for the particular designed dam body, that Phreatic line does not emerge at downstream face of the dam. It will ensure the possible failure of the downstream dam surface because of softening or sloughing process.

3. This line demarks the zone of the dry(moist) and the saturated soil which is necessary in the calculation of the unit weight of the soil mass for the stability analysis.

Design features

o Top width of dam - B=H/5+3 very low dams - B=0.55 H0.5+H/5 H<30 m - B=1.65(H+1.5)0.33 H> 30 m - B=1.67 H0.5 -

B should not be less than 2.0 m and maximum width will also be governed by the traffic volume requirement.

o Wave height-as similar to concrete dam Settlement consideration 2% for height of dam < 30 m in case of earthquake prone area. Settlement consideration 3% for height of dam >30 m in case of earthquake prone area. o U/S and D/S slope

H of dam Min Free board Min Top Width U/S slope mU:1 D/S slope mD:1 4.5 m 1.5 1.8 2 1.5 4.5-7.5 m 1.5-1.8 1.85 2.5 1.75 7.5-15 m 1.85 2.5 3 2 15-22.5 m 2.1 3.0 3 2

H max

t bottom t top =2-3 m t bottom = 1.25 H max / I kr

t top

x

o Central core of dam - Shear strength of the core is less than the dam material, hence thinner

core is preferable but there is possibility of piping failure. - Width of core at top is usually of 2.0- 3.0 m - Width of the core at the bottom should be

Maximum size of the core Side slope (x-0.5):1

I kr (Critical Gradient) Soil type

Central Core , Inclined core Body Foundation Clay 12 8-12 0.8-0.7 Loam 8 4-1.5 0.4-0.35 Sandy loam 2 2-1 0.35-0.32

drawn be could parabola base and x. theof valuedifferent with thefound be couldy of valuedifferent equation above theFrom

yxy2y

,Finally)2(yxy2xxy

,sidebothon)1(equationSquaringbbhy

y of ordinate thehaving b distanceknown with theobtained be couldy theof distance The

)1(yxxy

PRDistancePFDistance

2oo

2oo

222

22o

0

o22

+=

++=+

−+=

+=+

=

-True parabolic line, F-Focus of parabola, -Corrected portion of the phreatic line, -Directrix of parabola, -Horizontal drain

Analytic method

Seepage discharge from the dam

The surface FC is termed as the discharge face of the dam from which the seepage discharge is emerged though the drainage structure.

From Darcy’s law,

v = k i, where k is the permeability of the soil and i is the hydraulic gradient.

Seepage discharge (q) per unit width of the dam is ,

1

1

4

2

3

A B

F

2

43

MN

x

y

P R

yo b

h

C

point. focus at the flow theof area is y whereky

yxy2y2yxy22

1k

ydx

)yxy2(dk

)1y(dxdykq

o

o

2ooo2

oo

2oo

=

++

=

+=

×=

q = k i A, Area of flow for unit length

Let us take any point of the phreatic line,

The hydraulic gradient at any point is dy/dx

With the known equation (2) of the parabola we could find the hydraulic gradient at any point.

Grout - A mixture of water and cement or a chemical solution that is forced by pumping into foundation rocks or joints in a dam to prevent seepage and to increase strength.

Grout blanket - A grouted zone in the shallow portion of a foundation which has been treated to improve its strength and reduce its permeability.

Grout cap - A cap, usually consisting of concrete, through which grouting operations of foundations are performed.

Grout curtain - A zone in bedrock beneath a dam and parallel to its length that has been injected with grout to stop or reduce seepage beneath a dam.

Grout trench - A trench excavated to enable construction of a grout cap.

Grout veil - The same as a grout curtain.

Grouting - The operation whereby grout is injected under pressure into openings in a dam or in its foundations.

Under seepage control All dams on earth foundations are subject to under seepage. Seepage control is necessary to prevent excessive uplift pressures and piping through the foundation. Generally, siltation of the reservoir with time will tend to diminish under seepage. Conversely, the use of some under seepage control methods, such as relief wells and toe drains, may

increase the quantity of under seepage. The methods of control of under seepage in dam foundations are horizontal drains, cutoffs (compacted backfill trenches, slurry walls, and concrete walls),upstream impervious blankets, downstream seepage berms, relief wells, and trench drains.

Cutoffs

Complete versus partial cutoff. When the dam foundation consists of a relatively thick deposit of pervious alluvium, it must be decided whether to make a complete cutoff or allow a certain amount of under seepage to occur under controlled conditions. It is necessary for a cutoff to penetrate a homogeneous isotropic foundation at least 95 percent of the full depth before there is any appreciable reduction in seepage beneath a dam. The effectiveness of the partial cutoff in reducing the quantity of seepage decreases as the ratio of the width of the dam to the depth of penetration of the cutoff increases. Partial cutoffs are effective only when they extend down into an intermediate stratum of lower permeability. Compacted backfill trench. The most positive method for control of under seepage consists of excavating a trench beneath the impervious zone of the embankment through pervious foundation strata and backfilling it with compacted impervious material. The compacted backfill trench is the only method for control of under seepage which provides a full-scale exploration trench that allows the designer to see the actual natural conditions and to adjust the design accordingly, permits treatment of exposed bedrock as necessary, provides access for installation of filters to control seepage and prevent piping of soil at interfaces, and allows high quality backfilling operations to be carried out. When constructing a complete cutoff, the trench must fully penetrate the pervious foundation and be carried a short distance into unweathered and relatively impermeable foundation soil or rock. To ensure an adequate seepage cutoff, the width of the base of the cutoff should be at least one-fourth the maximum difference between the reservoir and tail water elevations but not less than 6.0 m. If the gradation of the impervious backfill is such that the pervious foundation material does not provide protection against piping, an intervening filter layer between the impervious backfill and the foundation material is required on the downstream side of the cutoff trench. The cutoff trench excavation must be kept dry to permit proper placement and compaction of the impervious backfill. Dewatering systems of well points or deep wells are generally required during excavation and backfill operations when below groundwater levels. Because construction of an open cutoff trench with dewatering is a costly procedure, the trend has been toward use of the slurry trench cutoff. Slurry trench. When the cost of dewatering and /or the depth of the pervious foundation render the compacted backfill trench too costly and/or impractical, the slurry trench cutoff may be a viable method for control of under seepage. Using this method, a trench is excavated through the pervious foundation using a sodium betonies clay and water slurry to support

the sides. The slurry-filled trench is backfilled by displacing the slurry with a backfill material that contains enough fines (material passing the No. 200 sieve) to make the cutoff relatively impervious but sufficient coarse particles to minimize settlement of the trench forming the soil-bentonite cutoff. Alternatively, cement may be introduced into the slurry-filled trench which is left to set or harden forming a cement bentonite cutoff. The slurry trench cutoff is not recommended when boulders, talus blocks on buried slopes, or open jointed rock exist in the foundation due to difficulties in excavating through the rock and slurry loss through the open joints. When a slurry trench is relied upon for seepage control, the initial filling of the reservoir must be controlled and piezometers located both upstream and downstream of the cutoff must be read to determine if the slurry trench is performing as planned. If the cutoff is ineffective, remedial seepage control measures must be installed prior to further raising of the reservoir pool. Normally, the slurry trench should be located under or near the upstream toe of the dam. An upstream location provides access for future treatment provided the reservoir could be drawn down and facilitates stage construction by permitting placement of a downstream shell followed by an upstream core tied into the slurry trench. For stability analysis, a soil-bentonite slurry trench cutoff should be considered to have zero shear strength and exert only a hydrostatic force to resist failure of the embankment. Concrete wall When the depth of the pervious foundation is excessive (>45 m) and/or the foundation contains cobbles, boulders, or cavernous limestone, the concrete cutoff wall may be an effective method for control of under seepage. Using this method, a cast-in-place Continuous concrete wall is constructed by tremie (from the bottom) placement of concrete in a bentonite-slurry supported trench. Since the wall in its simpler structural form is a rigid diaphragm, earthquakes could cause its rupture; therefore, concrete cutoff walls should not be used at a site where strong earthquake shocks are likely. The deepest concrete cutoff wall to date was constructed at Manicouagan 3 Dam in Quebec, Canada, in 1972, where two parallel concrete walls, 0.6 m thick and 3.0m apart, extended 130m deep. The tremie system consists of a hopper, tremie pipe, and a crane or other lifting equipment to support the apparatus. The hopper should be funnel shaped and have a minimum capacity of 0.5 cu m. The size of the tremie pipe depends upon the size of aggregate used in the concrete mix. For 3/4-in. maximum diameter coarse aggregate, a 10-in.-diam tremie pipe should be used. Concrete is added to the hopper at a uniform rate to minimize free fall to the surface in the pipe and obtain a continuous flow. The tremie apparatus is lifted during placement at a rate that will maintain the bottom of the pipe submerged in fresh concrete at all times and produce the flattest surface slope of concrete that can practically be achieved. A sufficient number of tremies should be provided so that the concrete does not have to flow horizontally from a tremie more than3.0 m As soon as practical, core borings should be taken in selected panels through the center of the cutoff wall to observe the quality of the final project. Unacceptable zones of concrete such as honeycombed zones, segregated zones, or uncemented zones found within the cored panels or elements should be repaired or removed and replaced

Upstream impervious blanket When a complete cutoff is too costly, an upstream impervious blanket tied into the impervious core of the dam may be used to minimize under seepage. Upstream impervious blankets should not be used when the reservoir head exceeds 30 m because the hydraulic gradient acting across the blanket may result in piping and serious leakage. The thickness of the blanket varies from 1-3 m. Downstream under seepage control measures (relief wells or toe trench drains) are generally required for use with upstream blankets to control under seepage and/or prevent excessive uplift pressures and piping through the foundation. Usually the length of the upstream blanket is about 5 times the head when combined with the D/S relief well. Effectiveness of upstream impervious blankets depends upon their length, thickness, and vertical permeability, and on the stratification and permeability of soils on which they are placed. Grouting. Grouting of rock foundations is used to control seepage. Seepage in rock foundations occurs through cracks and joints, and effectiveness of grouting depends on the nature of the jointing (crack width, spacing, filling, etc.) as well as on the grout mixtures, equipment, and procedures. A grout curtain is constructed beneath the impervious zone of an earth or rock-fill dam by drilling grout holes and injecting a grout mix. A grout curtain consisting of a single line of holes cannot be depended upon to form a reliable seepage barrier; therefore, a minimum of three lines of grout holes should be used in a rock foundation. Through a study of foundation conditions revealed by geologic investigations, the engineer and geologist can establish the location of the grout curtain in plan, the depths of the grout holes, and grouting procedures. Careful study of grouting requirements is necessary when the foundation is crossed by faults, particularly when the shear zone of a fault consists of badly crushed and fractured rock. It is desirable to seal off such zones by area (consolidation) grouting. When such a fault crosses the proposed dam axis, it may be advisable to excavate along the fault and pour a wedge-shaped concrete cap in which grout pipes are placed so that the fault zone can be grouted at depth between the upstream and downstream toes of the dam.

Basic requirement of the earthen dams are the-

- Imperviousness (less seepage from the body and beneath the dam of the dam)

- No piping action - Slope stability under different loading condition

As the embankment dam is not of the rigid structure, there are risks of sliding of its body due to the action of water pressure, self-weight of the embankment materials and due to the seepage forces etc. Usually these failures are happening on the circular surface and terms as the slip circle fail. Usually both the upstream and downstream slope of the dams are checked for the slip failure for the different loading condition.

Possible slip failure surface

τA

B C

D

X0

R

O

G

Sudden drowdown may cause the excessive seepage force to act as the driving force for the sliding to occur. Dam slope should be safe against the adverse combination of these loading conditions against the slip failure. If it is found not be in safe position usually the slope is flattened and again the stability against the slip failure is checked.

Intake It is the structure constructed to obtain the required quantity of water from the river or the reservoir for the different engineering purposes such as irrigation, power generation, water supply etc Basic consideration during the design of good intake:

o Hydraulic and structural consideration - Vortex formation, sediment exclusion, head loss consideration, structural stability and strength to carry out various operations above it o Operational consideration -diversion of required quantity of flow with the gate operation, trash handling, and maintenance provision o Environmental consideration fish diversion systems

Types of the intake

o Surface intake –canal intake, drop intake

o Sub surface intake-pressure intake from the reservoir-side slope intake, tower intake, dam intake, power or forebay intake

Control requirement for the good intake design

Vortex free flow Vortex formation at the front of intake causes,

o non-uniform flow conditions o Introduce air into the flow, with unfavorable results on the turbines: vibration,

cavitation, unbalanced loads, etc. o Increase head losses and decrease efficiency o Draw trash into the intake

The criteria to avoid vorticity are not well defined, and there is not a single formula that adequately takes into consideration the possible factors affecting it. According to the ASCE Committee on Hydropower Intakes, disturbances, which introduce non-uniform velocity, can initiate vortices. These include:

o Asymmetrical approach conditions o Inadequate submergence o Abrupt changes in flow direction cause separation and eddy formation o Approach velocities greater than 0.65 m/sec (Froude Number)

Lack of sufficient submergence and asymmetrical approach seem to be the commonest causes of vortex formation. An asymmetric approach is more prone to vortex formation than a symmetrical one. Placing inlet of the penstock deep enough from the water surface cause the vortex formation unlikely. For symmetrical approach

s/(Vd 0.5) >0.3

For Lateral approach

s/(Vd 0.5)>0.4

where s is the depth of submergence and V is velocity of flow in to the conduit

Froude number V/(gd)0.5< 0.5

Sufficient aeration

o Air vent is necessary to release the air formation behind the gate of pressure intake, which if accumulated will decrease the discharge carrying capacity of the conduit.

o When the flow is stopped at the intake and if the penstock is required to be

drained out, then in such case negative pressure (suction pressure) will be developed at the pipe and may cause the collapse of the pipe.

Location of surface intake The main requirement of the surface intake is to trap the required quantity of flow with the minimum of the sediment in to it. Due to the secondary current formation at the curve reach of the river, the quantity of sediment entering in to the intake depends upon the location of the intake either on convex or the concave side of the river. Intake at the convex side provides the condition of less sediment entering in to the intake.

Design of trash rack

o Discharge through the intake is already fixed. o Width of the intake is selected. o With the permissible velocity of flow at the trash rack, height of the intake is

fixed.

Permissible velocity is usually taken to be 1-1.5 m/s so as the decrease the head loss at the entrance and easy in the trash cleaning.

Inclined trash rack is easy to clean and little resistance to flow i.e. head loss is decreased.

Sediment accumulation

Relatively clear water

Vb

hf

Φ

Kr 2.4 1.8 1.6 0.8

t b

Φ

= sin

g2V

btKhf

2b

34

r

For deep intake with no cleaning provision of the trashes, velocity is decreased up to 0.4 m/s A formula due to Kirchmer for Trash rack loss Main components of canal intake

1. Weir –it is for the raising of the water level of the river to provide the continuous supply of water in to the intake.

2. Under sluice-It is the structure usually located at the bottom of the intake to bypass the accumulated sediments at its front to the downstream of weir. Sluicing is performed through the diversion weir and intake sill is kept at higher elevation to stop the entry of bed load.

3. Gates-it is provided to control the entry of the flow in to the channel as well as stop the flow when deem necessary (during repair and maintenance, flood)

4. Screen or Trashrack-It is provided to stop the entry of unnecessary materials (wood, leaves as well as large bed loads) in to the channel

5. Scrapper- It is provided for periodic cleaning of the materials collected at the screen

6. skimming wall- It is provided to stop the entry of floating materials.

Derivation canal

Under sluice gallery

Under sluice gallery gate

Service gate

Groove for repairing gate

Canal intake

A

A

Section at A-A

Skimming wall

Under sluicing gallery

Screen

Scraper for screen cleaning

Crane Groove for upper repairing gate

Weir

7. Crane- It is provided for lifting of the gates when necessary. The drop intake is essentially a canal built in the streambed, stretching across it and covered by a trash rack with a slope greater than the streambed slope. The trashrack bars are oriented in the direction of the stream flow., placing the bars as cantilevers to avoid the accumulation of small stones carried by the water

The power intake is a variant of the conventional intake, usually located at the end of a power canal. Because it has to supply water to a pressure conduit i.e. the penstock- its hydraulic requirements are more tough than that those of a canal intake. In small hydropower schemes, even in high head ones, water intakes are horizontal, followed by a curve to an inclined or vertical penstock. The design depends on whether the horizontal intake is a component of a high head or a low head scheme. In low head schemes a good hydraulic design - often more costly than a less efficient one- makes sense, because the head loss through the intake is comparatively large related to the gross head. In high head schemes, the value of the energy lost in the intake will be small relatively to the total head and the cost of increasing the intake size to provide a lower intake velocity and a better profile may not be justified. In a power intake several components need consideration:

o Approach walls to the trashrack is designed to minimize flow separation and head losses

o Transition from rectangular cross section to a circular one to meet the entrance to the penstock

o Piers to support mechanical equipment including trashracks, and service gates o Guide vanes to distribute flow uniformly o Vortex suppression device

Sub surface Intakes – For such intakes the entering of the sediment is not the major concern. The major concern is the smooth entrance of the flow in to the intake with the minimum of the head loss and the vortex formation

A A Section at A-A

Cylindrical gate

Screen

Foundation

Gate maneuvering mechanism

Tower intake

Crane

Screen

Scraper

Aeration vent

Hydraulic lift

Service gate Emergency gate

Storage space for gates

Side slope intake

Desanding tank During the collection of water through the intake, it is not possible to extract only the clear water. Certain percentage of bed as well as suspended load will also enter along with the water. If the sediment particles along with the water enter in to the turbine, it will cause the damage to the turbine due to the abrasion. So it is necessary to supply as much as clear water in to the turbine or only the allowable size of the sediment is accepted. In order to exclude the sediment particles that are harmful to the turbine, Desanding tank is generally constructed along the derivation channel. Deasnder tank has the wider size and has the much depth than that of derivation channel, so velocity will be greatly reduced causing settling of the incoming sediment. Width and length of the desanding tank is fixed such that minimum required size of the sediment is settled down along its length. Desanding tank could be with number of compartments, so that flushing of the accumulated sediment could be carried out without stopping the operation of plant.

Hydraulic lift

Emergency gate

Crane

Screen

Service gate

Aeration pipe

Intake at the dam body

5

1. Derivation canal 2. Repairing gate 3. Trash rack 4. Fore bay 5. Air vent 6. Emergency cum repairing gate 7. Penstock

1 2

6 4

3 7

Fore bay The water conveyed by the conveyance channel is uniformly distributed to the penstocks through the pond known as the Fore bay. This pond could be created through the excavation, masonry wall or the embankment. Transition from the conveyance channel to the forebay should be accomplished through the gradual variation. This pond stores some quantity of flow which could be utilized for the regulation of hydro plant (hourly or daily, but not much for the longer duration), uniform flow supply during the startup of the turbine and also the minimize the hydraulic hammering action in the penstock pipe due to the sudden closer of the valve. As the surface area of forebay is much than the conveyance channel, velocity will be greatly reduced and sediment particles, if escaped from the desander may also settle in this pond. Where the plants are located at the base of the dam, reservoir itself acts as the forebay.

Surge tank

Pressure headrace tunnel

Reservoir

Transient water level duringsurge

Penstock pipe

Static water level

Surge tank If the flow is conveyed through the pressure headrace tunnel, then a tank is provided near by the powerhouse at the entrance of the penstock pipes for the release of any water hammer development due to the sudden closer of the valve. This tank is called the surge tank. Water hammer thus developed has the high surge pressure due to the sudden momentum change of flow by the closer of the valve and may cause the burst of the penstock pipe, if this surge is not countered by any means. Either we have to increase the thickness of the penstock pipe or we could provide the open place to release the surge developed. The surge thus developed at the penstock pipe is compensated by the surge tank through the oscillation of its water surface. This oscillation comes to the rest by the friction resistance. Besides this function, surge tank also serves as the storage tank. Stored water could be utilized ,when plant has to switch to the increased load and similarly receive the flow when there is decrease in load.

AT – Cross section area of the headrace tunnel L - Length of the headrace tunnel AST – Cross section area of the surge tank QT – discharge at the headrace tunnel QST - discharge at the surge tank after the closer of valve Q P – discharge at the penstock pipe Lets z represents the fluctuation of the water surface due to the surge. Assuming that pressure difference at the section I-I and II-II caused by the frictional head loss is not much when compare to the surge pressure. Mass of water between the section I-I and II-II. m = γ/g (AT L)

Writing the momentum equation between the section I-I and II-II, Total forces = (P II-II AT + AT ρ g z)- PI AT Rate of change of momentum = m dV/dt Assuming that pressure difference at the section I-I and II-II caused by the frictional head loss is not much when compare to the surge pressure

So, m dV/dt = AT γ z γ/ g AT L dV/ dt = AT γ z dV/ dt = -(g/L ). Z By the continuity equation, QT = QST + Q P

Penstock pipe

Static water level

L II

II I

I

AT AST

z

Up surge level

Down surge level

Differentiating with respect to the time, dQT / dt = d QST / dt + d Q P / dt After the gate closure leading to surge, d Q P / dt =0 AT dV/dt = AST d2z/dt2 (v = dz/dt, dv/dt= d2z/dt2)

Substituting the value of dV/dt at the above equation AST d2z/dt2 - AT (g/L ). z =0 d2z/dt2 -( AT / AST). (g/L ). z =0 It is the differential equation governing the water level fluctuation in the surge tank and it has the general solution as, z = C1sin [AT.g/ AST.L]0.5 .t + C2 cos [AT.g/ AST.L]0.5 .t

with the initial condition when t =0 , z =0 gives C 2 =0 so, z = C1sin [AT.g/ AST.L]0.5 .t ………………….(A) Differentiating with respect to time, dz/dt = C1sin [AT.g/ AST.L]0.5 .t

= C1 [AT.g/ AST.L]0.5 cos[AT.g/ AST.L]0.5 .t ……………..(B) When the t=0, then all the incoming flow QT will enter in to the surge tank, A ST dz/dtt=0 = QT dz/dtt=0 = QT / A ST from equation (B) dz/dtt=0 = C1 [AT.g / AST.L]0.5 Putting , dz/dtt=0 = QST / A T in above equation C1 [AT.g / AST.L]0.5 = QT / A ST C1 = QT / A ST [AST.L /AT.g]0.5 Putting the value of C1 in equation (A),we will get the final equation of the fluctuation of flow at the surge tank,

z = (QT / AST).[AST.L /AT.g]0.5 sin [AT.g/ AST.L]0.5 .t Fluctuation of z is attained maximum when, sin [AT.g/ AST.L]0.5 .t is maximum i.e. is equal to 1. Z max = QT / A sT[AST.L /AT.g]0.5 So, [AT.g/ AST. L]0.5 .t = π/2 Time period for 1 oscillation is, τ = 4. t = 4. π/2. [AST.L /AT.g]0.5 .t = 2 π[AST.L /AT.g]0.5 We could see that the oscillation occurs with sinusoidal function. Due to the friction resistance of the headrace tunnel and the surge tank, this fluctuation gets dampened and water at the surge tank comes to the rest. In our basic equation we have not considered the friction resistance. In actual practice there is always the friction force, so the maximum of z thus calculated will not reach that value. Following are the empirical formulae for the calculation of z with the consideration of friction force, Z upsurge /z max = 1-2/3 P0+1/9 P0

2

Where, hf =friction head loss P0 = hf / z max

Z down surge / z max = -1+ 2 P0

D. Thoma has provided the empirical relationship for the calculation of minimum area of the surge tank,

AST min ≥ V2/2g [(L .AT)/{hf0 (H-hf0)}] Where, V-velocity of flow at the headrace tunnel L-length of headrace tunnel AT-area of headrace tunnel hf0-Total head loss from reservoir to the turbine H-Static head available between the reservoir level and the powerhouse

Diameter of the penstock

Ann

ualc

ost

Cost of the material

Cost from energy lost

Total cost

Economic diameter of the penstock

Penstock It is the conduit that carries the flow from forebay to the power house.In case of the pressure headrace tunnel of the flow derivation; it is the conduit between the Surge tank and the powerhouse. It could be made from the different materials e.g. wood, asbestos cement, reinforced cement concrete, steel etc. and among them frequently using are the steel and RCC materials.

Thickness of the still penstock pipe

Where, t = wall thickness in cm P = water pressure in kg/cm2 R= Internal radius of penstock pipe in cm S=Design stress of steel in kg/cm2 η= Efficiency of joint ,about 85% 0.15 cm = extra thickness allowance for the corrosion

Economic diameter of the penstock

As we know that frictional head loss will increase with the increase of the velocity of flow and decrease of the diameter of the penstock. The increase of the frictional head loss is the lost of the useful head for the generation of the electricity. The practice for decreasing the head loss is to use the bigger diameter penstock pipe with together decreasing the velocity inside it. But the increase of the size of the penstock will cause the increase in the expense of the material due to the increase in volume. So the selection of the diameter of the pipe should be done through the hydraulic analysis with considering the energy loss due to the head loss.

For this analysis, several diameters of the pipes are selected and its cost in terms of annual expenditure is calculated. Consequently, the annual energy loss cost is also calculated for these diameters. Both of these costs are added and minimum of it corresponds to the economical diameter of the penstock.

Tunnels

15.06.0

+−

=PS

RPtη

Tunnels are the structure constructed for the conveyance of flow or for the transportation purpose. Usually in hydro technical practice, tunnels are used for the conveyance of flow. Depending upon the flow condition inside the tunnel, it could be either pressure tunnel or the non-pressurized tunnel. Usually the tunnel conveying the flow for the power generation is the pressurized type, where as tailrace tunnel or the diversion tunnel could be of the non pressure types. Flow diversion tunnels are usually made of the free flow type (i.e. non pressurized flow) in order to achieve the maximum discharge through it. Geometric shape of the non pressure tunnel

Shape of tunnel Coefficient of strength, f

R1/b R2/b R3/b

A ≥8 0.71 0.1-0.15 - B 8>f>4 0.5 0.1-0.15 - C 4≥f≥2 0.25 0.2-0.25 1-0.9 D F<2 0.5 0.1-0.15 1-1.5

C) Egg shape tunnel

D)Horse shoe type tunnel

B)Semi circular type roof tunnel with vertical wall

R1

R2 R2

R1

R3 R3

R2 R2

b

A)Flat arch roof tunnel with vertical wall

h R1

b

R2 R2

Rock hardness coefficient (f) Sound Basalt, Quartz 20 Granite 15-10 Dolomite, Marble 8-5 Weak sedimentary rock 4-1.5 For above mentioned non pressure tunnel, the ratio of b/h is taken to be 1.0 and if the depth of water in the tunnel is more than the 0.3 h ,then b/h>1.0 could be taken Usually the pressurized tunnel is made of the circular shape. Strength of the rock, f = σ /100 , where σ compressive strength of the rock in kgf/cm2 Loads on the tunnel

1. Earth pressure 2. Reaction force due to the elastic deformation of the rock 3. Over burden pressure 4. Hydrostatic pressure 5. Earthquake pressure

Depending upon the strength of the rock, the horizontal pressure on the wall of the tunnel is different. For high strength rock, vertical pressure exist with no lateral pressure and only the reactive stress due to the elastic deformation of the medium. For the low strength rock, there is passive earth pressure too exist. The vertical pressure depends upon the height of the arch(hq) that could withstand the rock without the support by lateral transfer of the load due to the internal frictional resistance. The greater the hardness of the rock less will be the height of the arch. Loose rock will need greater height of the arch.

hq

H

b

450-φ/2

bq

gqzn

gqxn

450-φ/2-it is the angle at which the soil wedge will slide due to the passive earth pressure. The height of the arch is, hq = bq/2f where bq = b+ 2 H tan(450- φ/2) where φ = angle of internal friction Calculation of rock pressure on the tunnel

1. If f < 4 and if depth of the tunnel from the ground surface (hqz) is less than the height of the arch, then vertical pressure could be defined as

gqzn = γ rock x hqz 2. If f < 4 and if depth of the tunnel from the ground surface (hqz) is more than the

twice the height of the arch, then vertical pressure could be defined as gqzn= β x γ rock x hq

where β could be taken equal to 0.7 if bq≤5.5 m and could be taken equal to 1.0 if bq ≥7.5 m. The value between the limit could be taken through the linear interpolation. 3. If f < 4 ,then lateral pressure could be defined as

gqxn= γ rock x (hq+ 0.5H) tan2(450+φ/2)

4. If f ≥ 4 ,then vertical pressure could be defined as gqzn= β x γ rock x hq1

where hq1= depth of fractured zone=Ka x b Coefficient Ka considers the hardness of the rock(f) and degree of fracture(crack formation) If Mq = number of cracks in 1 meter length of rock

Degree of fracture Ka Hardness coefficient less

fractured,Mq≤1.5 fractured,1.5<Mq<5.0

Highly fractured, 5≤Mq≤15

4 0.2 0.3 0.4 5-9 0.1 0.2 0.3 10 and greater 0.05 0.1 0.15

5. If f ≥ 4 and Mq <5.0 and for tunnel of height less than 6.0 m, lateral pressure

could be ignored. 6. Pressure from underneath on the sill level of the tunnel could be developed for

rock with f<4.0, if the lateral stability of the bearing rock is disturbed due to the vertical load on the tunnel or from the load above the sill level

Because of the applied external rock pressure, hydrostatic pressure, hydrostatic pressure from inside the tunnel and self weight, bending stress as well as compressive stress is developed in the tunnel .Due to that reason , tunnel lining cross section will be deformed and elastic reaction may be developed

δ×=σ kelastic

ratio sPoisson'ν tunnelofwidth b

rock of elasticity modulus E Where,ν)(1 b 0.5

Ek

ratio sPoisson'ν tunnel theof radiusr

rock of elasticity modulus E Where,ν)r(1

Ek

==

+=

==

+=

Structural analysis The external force will cause the deflection of the tunnel membrane, so there is development of elastic reaction force form their side. Calculation of deformation of the tunnel membrane could be done through the known value of bending moment and the compressive force. With the known value of the deformation, the elastic reaction could be estimated with the following relationship, Where, δ is the deformation k (kN/m3)-elastic resistance of the rock ,which could be established through in situ test or from the Galerkin’s relationship, For the approximate calculation value of k for f ranging from 1.5-20, following empirical relationship could be used. k0= 50α f kgf/cm2

k=k0 (100 / re)

X1 X1

X2 X2 X3

X3

Pressure Tunnel

Filling grouting

Drainage pipe

Pipe For grouting

re is the radius of tunnel where, α=crack intensity coefficient α=0.8 highly fractured rock i.e. Mq=5-15 α=1 fracture rock i.e. Mq=1.5-5 α=1.2 less fractured rock i.e. Mq<1.5 f=hardness coefficient of rock Hydraulic design of the Tunnel

o Manning’s equation, n varies with the material o Max velocity- 4-5 m/s but even up to 7 m/s

Internal RCC -10 cm If metal 12 mm Shotcrete-6 cm ( for the smoothening) Shotcrete with Reinforcement -6-12 cm Tunnel Hydro technical tunnel could be divided in to two groups, non-pressure tunnel (Spillway tunnel, diversion tunnel, tailrace tunnel etc)- in which flow takes place with the free surface exposed to the atmosphere and pressure tunnel (Headrace tunnel)-in which flow takes place with the pressure. Usually pressure tunnel is designed of the circular section and non-pressure tunnel of other shapes too.The minimum diameter of the tunnel is fixed with consideration of the transportation, excavation and hauling during the tunneling and should be greater then 2.0m in case of circular section and in case of other shapes should be greater 1.9-2.7 meter in width and 2.1-2.7 meter in height.

Hole for pressure grouting

Concrete lining

Reinforcement

R.C.C lining

The design of the tunnel depends upon the physical characteristics of the rocks through which it is to be aligned. Among them the main characteristic is the rock strength -which is expressed as in hardness coefficient (f) for the different types of rock. For the high strength rock and without its defective laying (fault, cracks), it may not be necessary to do lining but in order to decrease the friction usually concrete lining of 15-30 cm is preferred. Usually in case of the overburden pressure, it is necessary to do the concrete lining to takes the part of the load on it. Usually the lining is done in two layers. External layer is constructed of the concert of 25-45 cm thickness taking the rock pressure and working in the compression condition. Thin internal layer of RCC is constructed, which takes the pressure of the flow and works in the tension condition. Internal layer could also be laid on the reinforcement mesh above the concrete lining with the Shotcrete (mixture of cement and sand laid in high pressure). In case of high pressure, internal lining could also be done with the steel plate. Beneath the tunnel usually drainage is provided to release the underground water pressure on the concrete lining as well as minimize the seepage flow through it. During the concrete lining it is practically not possible to place the concrete firmly on the upper portion of the tunnel. So after the concreting, cement grouting is done by injecting the cement with very high pressure through the drill hole provided at the concrete lining. Cement grouting will also help in filling the cracks if it exists on the surrounding rocks due to different causes (Blasting, natural fault etc)