Chapter 7 Dams

8/3/2019 Chapter 7 Dams

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Chapter 7 Dams and Dam Sites;

Tunnels and sites Earth Works;

Reservoir Engineering Geology

DAMS

what is dam?Purpose of dam?

Discuss and come with solution?



Dams

Dam is a solid barrier constructed at a suitablelocation across a river valley to store flowing water.

Dams are made from a variety of materials such as

rock, steel and wood Storage of water is utilized for following objectives:

Hydropower

Irrigation

Water for domestic consumption Drought and flood control

For navigational facilities

Other additional utilization is to develop fisheries



Structure of Dam

Heel

Gallery

Toe

Spillway

(inside dam)

Crest

NWL

Normal

water level

MWL

Max. level

Free boardSluice way

Up stream Down stream



Heel: contact with the ground on the upstream side

Toe: contact on the downstream side

Abutment: Sides of the valley on which the structure of the dam rest

Galleries: small rooms like structure left within the dam for checking

operations.

Diversion tunnel: Tunnels are constructed for diverting water before theconstruction of dam. This helps in keeping the river bed dry.

Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side

Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.



TYPES OF DAMS

Gravity Dams: These dams are heavy

and massive wall-like

structures of concrete

in which the wholeweight acts vertically

downwards

Reservoir

Force

As the entire load is transmitted on the small area of foundation, suchdams are constructed where rocks are competent and stable.



Bhakra Dam is the highest

Concrete Gravity dam in

Asia and Second Highest

in the world.

The construction of this

project was started in the

year 1948 and was

completed in 1963 .

It is 740 ft. high above the deepest foundation as straight concrete dam being more than three

times the height of Qutab Minar.

Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30feet)

Bhakra Dam is t he hig hest Concrete Gravity dam in Asia and Second Hig hest in t he world.



Buttress Dam:

Buttress Dam ± Is a

gravity dam reinforced by

structural supports

Buttress - a support that

transmits a force from a

roof or wall to another

supporting structure

This type of structure can be considered even if the foundation

rocks are little weaker



These type of dams areconcrete or masonry dams

which are curved or convex

upstream in plan

This shape helps to transmitthe major part of the water load

to the abutments

Arch dams are built across

narrow, dee p river gorges, but now in recent years t hey have

been considered even for little

wider valleys.

Arch Dams:



Earth Dams:

They are trapezoidal inshape

Earth dams areconstructed where thefoundation or theunderlying material or

rocks are weak to supportthe masonry dam or wherethe suitable competentrocks are at greater depth.

Earthen dams arerelatively smaller in height

and broad at the base They are mainly built with

clay, sand and gravel,hence they are also knownas Earth fill dam or Rock fill dam



Cont..



Cont... The geological services required for the engineering

of a large dam are in the following areas; The safety of the dam on its foundations;

The watertightness of the reservoir basin;

The availability of natural materials for itsconstruction

The engineering geologist is a key member of an

engineering team, since he will ensure the feasibilityof the project, continuing through the design stage

and terminating only when construction has either

proved that geological conditions revealed are in

conformit with the remises ado ted in desi n, or



he has made possible proper evaluation of any

conditions not foreseen in the earlier stages. The safety, viability and cost of a dam are all

dependent upon geology. Most rocks have

adequate strength but their weakness is in the

orientation and dip of discontinuities relative to the

loading from the dam, as well as the infilling

material in, and depth of, weathering in such

discontinuities.

It is necessary to investigate both the regional

geology and the specific local geology to ensure a

global picture is developed.



Geology Terminology

Bedding planes - The planes marking thetermination of one sedimentary deposit and the

beginning of another; they usually constitute a

weakness along which the rock tends to break. Foliation - In rocks that have been subjected to heat

and deforming pressures during regional

metamorphism, some new materials such as

muscovite and biotite mica, talc and chlorite may be

formed by recrystallisation. These new minerals are

arranged in parallel layers of flat or elongated

crystals - the property of foliation.



Joints - These are fractures along which no

movement has occured. All rocks are jointed to

some extent and weathering occurs in these joints.They offer pathways for water, any clay infilling

offering little resistance to sliding.

Faults - These are fractures along which movement

has occured. They may range from rather

inconspicious zones hundreds of metres wide and

many kilometres long. The movement may have

formed a zone that is so crushed and chemicallyaltered as to be unable to support any weight. The

presence of faults may be recognised from such

physical features as;



± Offset of beds, dykes or veins;

± Slickensides;

± Gouge; ± Brecciation or crushing;

± Topographic features like escarpments, linear trenches or

sag valleys.

Gabbros, Andesites, Dolerite and Basalt: These

types of rock cannot be trusted for dams and

reservoirs. Porphyritic rocks need careful grouting.

What is grouting ? Discuss



Grout

Grout is a liquid, either a uniform chemical

substance or an aqueous suspension of solidsthat is injected into rocks or unconsolidated

materials through specially drilled boreholes to

improve bulk physical properties and/or toeliminate seepage of groud water.

There are three basic types;

Portland cement-base slurries Chemical Grouting solutions

Organic resins, including epoxy resins.

Portland based are the most widely used.



Cont..

In dam foundations three kinds of grouting

programs are identified:

Shallow blanket or consolidation grouting over

critical portions

Curtain grouting from a gallery or concrete

grout cap

Off pattern, special purpose grouting to

improve strength



Amphibolites

Gneiss, mica schists and associated

rocks are considered to be satisfactoryfor sustaining bearing pressure and for

water-tightness. However, gneiss and

particularly mica schist are less

favourable due to the mica which may

facilitate slipping.



Cont... Where gneiss and mica schists are associated, a very weak zone of disintegrated

rock may be found at the junction of these two rocks.

Metamorphic Rocks: Metamorphic and intrusive

igneous rocks are to an extent unpredictable. The

usual types of dams constructed are gravity, buttress

and rock fill. Grouting of the foundations isgenerally essential. Limestone dam sites vary widely

in their suitability. Thickly bedded horizontally

lying lime stones which are relatively free from

solution cavities afford excellent dam sites. On the

other hand, thin bedded, highly folded, or cavernous

lime stones are likely to present serious foundation

or abutment problems involving bearing capacity



water tightness.

Concrete dams have been constructed on

Jurassic limestone at

Castillon , where slips

and leakage problems

have occurred.



Sand stone These have been surmounted by an extensive

grouting scheme. Generally sandstones do notdeteriorate rapidly on exposure to the surface

with the exception of shaly sandstone. As a

foundation rock sandstone is not susceptible to plastic deformation, even with poorly cemented

sandstones. However, sandstones are

susceptible to erosion due to the scouring and

plucking action from the overflow of dams andso have to be adequately protected by suitable

hydraulic structures.



Sandstones are frequently interbedded with shales.

The sandstone-shale contact may allow seepage of

water and may cause potential sliding. Severeuplift pressures may also develop beneath beds of

shale in a dam due to the swelling characteristics

of shales.

Clays

Clay formations are often thick and massive and

are frequently associated with thin seams of

sandstone or limestone. Earth dams or rockfilldams are usually constructed on clay foundations

because clays lack the load bearing properties

necessary to support concrete dams.



Cont.. Gravel, Sands and Boulder Clays

Gravels, sands and boulder clay of glacial origin are

notoriously variable in composition both laterally

and vertically. As a result dam sites in glaciated

areas are among the most difficult to appraise on the

basis of surface evidence. Generally, earth dams areconstructed in areas of glacial deposits.

Crushing Strength

In general the compressive load from a dam on to itsfoundations will not exceed 10 MPa. The strength of

a rock will depend upon its - Quality ,The degree of

weathering ,Presence of micro-cracks



Cont.. The strength of a rock mass will depend upon -

The number of cracks and joints

The nature of their infilling material

Whether there are any rock-to-rock contactsacross the joints

Planarity and continuity of seams and foliations



Rock strength

Rock type Strength (MPa)

Siltstone 24-120

Greywacke 20-30

Shale 35-110

Sandstone 40-200

Limestone 50-240

Dolomite 50-150

Granite 90-230

Basalt 200-350

Dolerite 240-320

Gneiss 80-330



Cont...

When the crest chord-height ratio is under 3

and the rock is capable of withstanding high

pressures, not being able to fail by shearing,

thin arch or thin cupola dams are the most

successful and the most economical. Soundness of the foundation is of

paramount importance for all arch



Cont.. When the crest chord-height ratio is under 3 and

the rock is capable of withstanding high pressures, not being able to fail by shearing, thin

arch or thin cupola dams are the most successful

and the most economical. Soundness of the foundation is of paramount

importance for all arch

Dams in narrow valleys Narrow valleys have a chord-height ratio of

between 3 and 6. Gravity arch dams are

normally constructed in narrow valleys

rovidin that the foundations are suitable.



Cont..

If the narrow valley is filled with permeable

and compressible material, for examplefrom a glacial origin, the dam engineer has

two choices:

To increase the depth of excavation to bedrock

If the depth of material is economically

unfeasible to remove, then redesigning thedam to an earthfill or rockfill design may be

the only option.

More and more thick arch dams with a



thickness of less than the gravity sectionwill be constructed in the future as more

confidence is gained in:

The reliability of new models confirm and

even supplant the mathematical analyses.

The experience of strengthening weak

foundations to carry heavier unit pressures

which are to be sustained compared with thegravity section.



Geologic Conditions Promoting Foundation Failure

Brittle, fractured sandstones rest on a weak shale layer dipping

upstream.

Horizontally layered limestone rest on a weak shale layer which

extends downstream to a steep slope in the valley floor.

Fractured crystalline rocks lie above a flat fault containing sheared,

gougy materials of very low strength.

Intersecting strong conjugate joints have attitudes that promote easy

mass shear dislocations.

Sedimentary rocks dipping downstream are intersected by a fault

dipping upstream and containing materials of low strength.

Folded rocks containing thin, weak layers of shale present a potentialfor foundation failure.

eservo r



eservo r

Reservoirs

What is reservoirs? Discuss for 5min.

Its Purpose ?



Cont..

There are a range of factors that influence the

feasibility and economics of a proposed reservoir

site. The most important of these is generally the

location of the dam. After that, consideration must be given to the run-off characteristics of the

catchment area, the watertightness of the proposed

reservoir basin, the stability of the valley sides, the

likely rate of sedimentation in the new reservoir, the

quality of the water and, if it is to be a very large

reservoir, the possibility of associated seismic

activity. Once these factors have been assessed, they



cont...

must be weighed against the present landuse and social factors. The maximum

elevation to which the water in a reservoir

basin rises during ordinary operating

conditions is referred to as the top water or normal pool level. For most reservoirs, this

is fixed by the top of the spillway.

Conversely, minimum pool level is thelowest elevation to which the water is

drawn under normal conditions, this being

determined by the lowest outlet.

C



Cont..

Between these two levels, the storage volume is

termed the useful storage, whereas the water below the minimum pool level, because it

cannot be drawn upon, is the dead storage.

During floods, the water level may rise above

top water level but this surcharge cannot be

retained since it is above the elevation of the

spillway.



the most important aspect of storage in

reservoir design is the relationship betweencapacity and yield. The yield is the quantity

of water that a reservoir can supply at any

given time. The maximum possible yield

equals the mean inflow less evaporation and

seepage loss. In any consideration of yield,

the maximum quantity of water that can be

supplied during a criticaldry period (i.e. during the lowest natural flow

on record) is of prime importance and is

defined as the safe yield.

I i i f R i Si



Investigation of Reservoir Sites consideration must be given to the amount of

rainfall, run-off, infiltration and

evapotranspiration that occurs in the

catchment area, as well as to the geological

conditions. The Leakage from a reservoir

takes the form of sudden increases in streamflow downstream of the dam site with boils in

the river and the appearance of springs on the

valley sides. It may be associated with major defects in the geological structure, such as

solution channels, fault zones or buried

channels through which large and essentially

localized flows take place.



Problematic foundation materials

In foundations in unconsolidated material excavation of natural

deposits may reveal inadequate localized or widespread foundation

materials that require special treatment or total removal. Unacceptableor inadequate materials rich in organic substances such as topsoil,

swamp muck or peat, loose deposits of sand or silt, talus

accumulations and plastic, active, sensitive, or swelling clays.

Poor foundation conditions in rocks are associated with close

fracturing, weathering or hydrothermal alteration, or poorly induratedsedimentary rocks.

Pressures Associated with Dams and Reservoirs

Construction of a dam and filling of the reservoir behind it

create load stresses on the floor and sides of a valley that did

not exist previously



Seepage - Introduction

Seepage under an embankment is much moredangerous than that for a concrete dam, since

embankments are usually built on soft material

which is liable to be scoured out and it is also

vulnerable to influx of water; whereas a concrete

dam is usually built on rock which is not worn away

so rapidly by the scouring action of water; and even

then a defective dam will not necessarily beendangered by passage of water through it or even

under it.



Basic seepage problems Stored water behind dams, gives rise to

three basic seepage problems, which canlead to difficulties and in serious cases to

total failure:

Piping occurs when water picks up soil particles and moves them through

unprotected exits, developing unseen

channels or pipes through a dam or its

foundation.

Heave or slope failures caused by seepage

forces.

Excessive loss of water.

C



Con.. Three basic methods for controlling seepage are:

Use of filters to prevent piping and heave Seepage reduction

Drainage

Settlement



Settlement All structures undergo some settlement,

regardless of their construction or of the quality

of their foundations. Structures made of soil or founded on soil settle so much that their

performance is affected and their safety is

compromised. Concrete dams are almost always based on

strong rock foundations where settlement of the

dam is kept to a minimum otherwise the dams

would crack leading to serious structural faults.

Embankment dams can be founded on soft

compressible materials and are able to withstand

large settlements.

Causes o sett ement



Causes o sett ement Bearing capacity failure or instability,

including landslides. Failure or deflection of the foundation

structure.

E

lastic or distortion of the soil (E

xpansivesoils) or rock.

Consolidation (compression) of the soil or

rock.

Shrinkage due to desiccation.

Change in density due to shock or vibration.

Chemical alteration of constituents,

C t



Cont... Underground erosion.

Collapse of underground openings such ascaves or mines.

Structural collapse due to weakening of

cementation upon saturation

Total settlement

Tilting

Distortion

Settlement is of two types



Settlement is of two typesfirst µimmediate settlement (short term)¶, which results from the

elasticdeformation of the ground (using the word elastic in a very

general sense)

Where, s = plate settlement; B = width of foundation and S =

settlement of foundation.

and then

µconsolidation settlement¶ (long term)which is the result of the

long-term compaction or consolidationof the ground under the load imposed by the structure.

Settlement of each layer may then be calculated

from the formula:

Sc= mv zH



where mv= coefficient of volume compressibility for

the layer (m2N±1); H = thickness of the layer (m);

z= average effective vertical stress imposed on the

layer as a result of the foundation loading (N m±2)

and Sc= consolidation settlement (m).

The total settlement (St) at any time t may beexpressed by the formula:

St= Si+ U Sc

where U is the degree of consolidation (given as a percentage) at time t,



1 Whi h f h f ll i d di i



1.Which one of the following ground conditions are

recommended for dam sites?

A. sand stone with shale interbeded B.Cavernous lime

stone C. mica schist D. all E. none

2. Which types of dam will be used when the foundation

rocks are little weaker?

3. Which types of dam is used at the narrow valley George?4. What is the d/f b/n Abutment and spillways?

5. What are advantage and disadvantages of dam?

6. What are the Basic seepage problems ?7. All structures undergo some settlement, regardless of their

construction or of the quality of their foundations(True or

False.



8.What are geological services required for the

engineering of a large dam ?

9.

Tunnels and Tunnelling



Tunnels and Tunnelling

What is tunnelling?

Under ground path/space?

Why tunnelling?

Chalenges in tunnelling?

Supports in tunnelling



Geology is the most important factor that

determines the nature, form and cost of a

tunnel. For example, the route, design and

construction of a tunnel are largely dependent

on geological considerations. Estimating the

cost of tunnel construction, particularly in areasof geological complexity, is uncertain.

Prior to tunnel construction, the subsurface

geology is explored by means of pits, adits(drifts), drilling and pilot tunnels. Exploration

adits driven before tunnelling proper

commences are not usually resorted to unless a

particular section appears to be especially

D t d l f t i t i t



Dangerous or a great deal of uncertainty exists.

Core drilling aids the interpretation of geological

features already identified at the surface.

A pilot tunnel is probably the best method of

exploring tunnel locations and should be used if a

major-sized tunnel is to be constructed in ground

that is known to have critical geological conditions.It also drains the rock ahead of the main excavation.

If the inflow of water is excessive, the rock can be

grouted from the pilot tunnel before the mainexcavation reaches the water-bearing zone.

ont



ont... Geophysical investigations can give valuable

assistance in determination of subsurface conditions,

especially in areas in which the solid geology is

poorly exposed. Seismic refraction has been used in

measuring depths of overburden in the portal areas

of tunnels, in locating faults, weathered zones or buried channels, and in estimating rock quality.

Seismic testing also can be used to investigate the

topography of a river bed and the interface between

the alluvium and bedrock when tunnels are

excavated beneath rivers.



Geological and geotechnical investigations

provide early information on the tunnel

feasibility and on the ground characteristics to be used for design.

They mainly refer to the developments of

geological models and the increased use of geophysical methods in underground



DESIGN METHODS

Three main issues to be addressed for the designof soft ground tunnels:

- stability of the opening during construction,

with particular attention to tunnel facestability;

-Evaluation of the ground movements induced

by tunnelling and of the incidence of shallow

underground works on surface settlements;

- design of the tunnel liner system to be

installed to ensure the short and long term

stability of the structure.

Pro ems ur ng tunne ng



Pro ems ur ng tunne ng Surface Settlements-it has its own mathematical

design Horizontal Surface Displacements-similar

Inflow into Tunnels

With regard to groundwater, planning andconstruction of a tunnel must take account of the

following:

amounts and locations of water inflow into the

tunnel during construction;

effect on surface structures of the possible

withdrawal of support by water table lowering

durin tunnel construction

Cont



Cont.. affect of the interaction between water in the

finished tunnel, if it is to be an aqueduct,andthe prevailing external hydrogeological regime;

and effect of hydrostatic pressure on the tunnel

lining.



Cont... Resistivity techniques have proved useful in

locating water tables and buried faults, particularly those that are saturated. Resistivity

logs of drillholes are used in lateral correlation

of layered materials of different resistivitiesand in the detection of permeable rocks.

Ground probing radar offers the possibility of

exploring large volumes of rock for anomalies

in a short time and at low cost, in advance of

major subsurface excavations.

Geological Conditions and



Geological Conditions and

Tunnelling

Large planar surfaces form most of the roof ina formation that is not inclined at a high angle

and strikes more or less parallel to the axis of a

tunnel. In tunnels in which jointed strata dipinto the side at 30 or more, the up-dip side

may be unstable. Joints that are parallel to the

axis of a tunnel and that dip at more than 45

may prove especially treacherous, leading toslabbing of the walls and fallouts from the

roof. The effect of joint orientation in relation

to the axis of a tunnel is given in Table below

Cont



Cont..effect of joint strike and dip orientation in tunnelling

Water in Tunnels



Water in Tunnels

Correct estimation of the water inflow into a

projected tunnel is of vital importance, asinflow influences the construction

programme (Cripps et al., 1989). One of the

principal problems. created by water entering a tunnel is that of

face stability. Secondary problems include

removal of excessively wet muck and the

placement of a precision-fitted primary

lining or of ribs.

Cont



Cont..

The value of the maximum inflow is required and

so are the distribution of inflow along the tunnelsection and the changes of flow with time. The

greatest groundwater hazard in underground work

is the presence of unexpected water-bearingzones, and therefore, whenever possible, the

position of hydro geological boundaries should be

located. Obviously, the location of the water

table, and its possible fluctuations, are of major consequence.



Most of the serious difficulties encountered

during tunnelling operations are directly or

indirectly caused by the percolation of water towards the tunnel. As a consequence,

most of the techniques for improving

ground conditions are directed towards itscontrol. This may be achieved by using

drainage, compressed air, grouting or

freezing techniques.

ases n unne s



Naturally occurring gas can occupy the pore

spaces and voids in rock. This gas may be under

pressure, and there have been occasions whengas under pressure has burst into underground

workings, causing the rock to fail with explosive

force (Bell and Jermy, 2002). Wherever possiblethe likelihood of gas hazards should be noted

during the geological survey, but this is one of

the most difficult tunnel hazards to predict. If the

flow of gas appears to be fairly continuous, thenthe entrance to the flow may be sealed with

concrete. Often, the supply of gas is exhausted

quickly, but cases have been reported where it

C t



Cont.. continued for up to 3 weeks.

Many gases are dangerous. For example, methane,CH4, which may be encountered in Coal Measures,

is lighter than air and can readily migrate from its

point of origin. Not only ismethane toxic, it also iscombustible and highly explosive when 5±15% is

mixed with air.Carbon dioxide, CO2, and carbon

monoxide, CO, are both toxic.



Temperatures in Tunnels

Temperatures in tunnels are not usually of

concern unless the tunnel is more than 170

mbelow the surface. When rock is exposed

by excavation, the amount of heat liberateddepends on the virgin rock temperature,

VRT; the thermal properties of the rock; the

length of time of exposure; the area, size

and shape of exposed rock; the wetness of

rock; the air flow rate; the dry bulb

temperature; and humidity of the air.

Cont



Cont.. In deep tunnels, high temperatures can make

work more difficult. Indeed, high temperatures

and rock pressures place limits on the depth of

tunnelling. The moisture content of the air in

tunnels is always high and, in saturated air, the

efficiency of labour declines when thetemperature exceeds 25C, dropping to almost

zero when the temperature reaches 35C.

Conditions can be improved by increasedventilation, by water spraying or by using

refrigerated air. Air refrigeration is essential

when the virgin rock temperature exceeds 40C.

Cont



Cont..

The rate of increase in rock temperature with

depth depends on the geothermal gradient that,in turn, is inversely proportional to the thermal

conductivity, k, of the material involved:



Read for your self !!

Excavation of tunnel and design and its

roof

Chapter 9- Construction materials

Chapter 7 Dams

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Transcript of Chapter 7 Dams