ENGINEERING GEOLOGICAL STUDIES for DAM CONSTRUCTION

1. PRELIMINARY STUDY

1. Evaluation of the data having at archives of Universities,......etc2. Field investigation for limited time (PRELIMINARY Study)3. Some maps in small scale, for example 1/25,000 or 1/50,0004. Some hydraulic data about

a. Basinb. Precipitation areac. Runoff, maximum discharge {Q=R/t (m3/s)}

5. Some approach to the reservoir area, dam site and type of dam and height of dam...etc

6. Photogeological studies7. A preliminary report

2. PRELIMINARY STUDIES at the RESERVOIR AREA and DAM SITE

1. Dam site investigations1. Location of dam axis2. Location of diversion tunnel3. Location of spillway4. Location of powerhouse...etc

2. Geological studies3. Underground investigations

1. Boreholes2. Investigation galleries

4. Surveying for materials1. Field surveying2. Laboratory tests

5. Slope stability investigations6. Earthquake hazard & risk analysis7. Environmental studies8. Leakage possibilities from reservoir area9. Leakage possibilities from dam site10. Erosion, sedimentation & siltation

FACTORS AFFECTING to the DAM TYPE SELECTION

Topography Geology

Bearing capacity of the underlying soil Foundation settlements Permeability of the foundation soil

Material availability Spillway position Earthquakes Safety Height Aesthetic view Qualified labour Cost

FACTORS AFFECTING the PLACE of the DAM AXIS

Topography Geology Materials Spillway location availability Sediments in the flowing water Water quality Expropriation costs Earthquake possibility Downstream water rights

ENVIRONMENTAL IMPACTS OF DAMS

ENVIRONMENTAL IMPACTS of CONSTRUCTION PHASE of DAMS

River pollution Erosion Loss of aesthetic view Air pollution Noise pollution Dust

ENVIRONMENTAL IMPACTS of RESERVOIRS

Loss of land Habitat Destruction :

The area that is covered by the reservoir is destroyed, killing whatever habitat existed there beforehand.

Loss of archeological and histrorical places Loss of special geological formations Aesthetic view reduction Sedimentation Change in river flow regime and flood effects Reservoir induced seismicity Change in climate and plant species

EFFECTS of DAMS on WATER QUALITY

Change in temperature Turbidity Dissolved gases in the water

Water discharged from the spillway contains 110-120% saturated nitrogen. This amount may be destructive for fish life.

Eutrophication It means increase in vegetation. If plants exist in water,

quality of that water gets worse.

SOME DAMS from SPACE

CONCRETE GRAVITY DAMS

CONCRETE GRAVITY DAMS

A concrete gravity dam is the solid concrete structure, which is designed, and shaped so that its weight is sufficient to ensure stability against the effects of all imposed forces.

If the dam is properly designed, a solid concrete dam will require little maintenance.

Gravity dams are classified with reference to their height. Dams up to 100 feet high are generally considered as low dams, dams from 100 to 300 feet high are classified as medium-height dams, and dams over 300 feet high as high dams.

SELECTION OF DAM SITE

Various considerations which taken into selection of gravity dam are:

•A narrow gorge, opening upstream, at the dam site.•Strong rock foundation to safely withstand static and dynamic forces including earthquakes.

•Strong and watertight abutments.•Stable side slopes of abutments.•Suitable location for power house and spillway•Availability of construction materials nearby.•Accessibility by rail and road. •Availability of electric supply for construction.

FORCES ACTING ON DAM

(i) Dead load. (ii) Reservoir and tail water loads.(iii) Uplift pressure.(iv) Earthquake forces.(v) Earth and silt pressures.(vi) Wind pressure.(vii) Ice pressure.(viii) Wave Pressure(ix) Foundation Reaction

It comprises the major resisting force. It includes weight of the con crete or masonry or both plus that of appurtenances such as weight of gates and bridges.

Dead Load

Reservoir and Tail Water Loads 

Earthquake Forces

The earthquake causes acceleration to the foundation of the dam. The acceleration is

• Horizontal accelerationThe horizontal seismic coefficient is taken as 0.03 to 0.24 at the top of the dam reduced linearly to zero at the base. Horizontal acceleration equals to 0.1 g to 0.15 g is sufficient for high dams in seismic zones. Horizontal acceleration causes inertia force and hydrodynamic pressure.

Inertia forcesInertia force = W Where, W = weight of damg = acceleration due to gravity =earthquake intensity (acce. Coeff.)= earthquake acceleration/gravity acc.

)

hy2(

hy)

hy2(

hy

2Cm C

Hydrodynamic forces

Pe = C W h

Where, 

w = unit weight of waterh = total depth of reservoir (reservoir level-base level of dam) y = the vertical distance from reservoir surface to elevation in question,Cm = maximum value of pressure coefficien

Moment of this force about the center of gravity of section, at a depth y from the

water surface is given byM= 0.299 Pe y2 (Kg m/m)

•Vertical acceleration Vertical acceleration may act either downward or

upwards. Acting downward.

When acceleration is acting downward, the foundation tends to move downward from the body of the dam, thereby reducing the effective weight and stability of the dam. It represents worst condition for design purposes.

Reduced weight of dam material = Wc (1-)Reduced weight of water = W (1-)

When the acceleration is acting vertical it tends to lift the foundation upwards, i.e. closer towards the dam body thereby increasing effective weight of dam and the stress is increased.

Increased weight of dam material = Wc (1+)Increased weight of water = W (1+)

Acting upwards.

Earth and Silt PressuresEarth pressures have a minor effect on the stability of the dam

and are ignored. Silt gets deposited against the upstream face of the dam. The

horizontal silt and water pressure is assumed to be equivalent to that of a fluid weighing 1360 kg/m3. Vertical silt and water determined as if silt and water together have a density of 1925 kg/m3.

Wind PressureWind loads on dams are ignored. However, if considered, wind pressure is taken as 100 to 150 Kg/m2 for the dam area exposed to wind.Ice Pressure Ice pressure is taken as 25000 Kg/m2 for the dam

area contact with ice.

Wave Pressure

km 32 Ffor , F271.0763.0 FV 0.0322 h height wave 4w

km 32 Ffor , FV 0.0322 h height wave w

total wave force (Pw)  Pw = 2 hw

2 ton/m acting at 0.375 hw above reservoir level Where, hw = wave height (m)

F = fetch of reservoir (km)V = wind velocity on water surface (km/hr)