Gallery in Concrete Gravity Dam

manishh_29 manishkhandelwal76 Manish Khandelwal
Gallery in Concrete
Gravity Dam

 What is called Axis of dam?
The line of the upstream
edge of the top (or crown)
of the dam. The axis of the
dam in plan is also called
the base line of the dam.

 Foundation Treatment for Gravity Dams
 The material underlying the base of a dam i.e. the foundation of the dam must be
strong enough and capable to withstand the foundation pressure exerted on it
under various conditions of loading in dry as well as wet conditions.
 Most of the failure of the dams have occurred because of the failure of the
underlying strata. Hence foundation treatment is necessary in the case of
concrete gravity dam.
 The foundation treatment commonly adopted for all foundation can be divided
into two steps:
1. Preparing the surface
2. Grouting the foundation

1. Preparing the Surface
 The surface preparation consist removing the
entire loose soil till a sound bed rock is
exposed. The excavation should be carried out
in such a way that the underlying rock is not
damaged.
 The final surface obtained above is stepped, so
as to increase the frictional resistance of dam
against sliding.
 If faults, seams or shattered rock zones are
detected in the exploratory geological
investigations, special steps and remedies
must be taken to ensure their removal.

2. Grouting the foundation
The foundation grouting can be divided into
(a) Consolidation Grouting
(b) Curtain Grouting
a) Consolidation Grouting
 The entire foundation of the dam is consolidated by grouting.
 For this purpose, shallow holes are drilled through the foundation
rock. The depths of these holes generally vary between 10 to 15 m.
They are situated at about 5 to 20 m apart.
 After the holes have been drilled, mixture of cement and water
called grout is forced into the holes at low pressure of about 30 to
40 N/ cm2.
 This is accomplished before any concreting for the dam section is
laid. This low pressure grouting will result in a general
consolidation of the foundations.
 These low pressure grout will later serve the purpose of a cut-off
against leakage of high pressure grout. Which is to be used after
some concreting of the dam has taken place.

Consolidation grouting- for obtaining continuous & well
compacted solid rock mass in foundation of dam (before
concreting starts)

b) Curtain Grouting
 It helps in forming the principal barrier or a curtain against the
seepage through the foundations and reduce the uplift pressure.
 Increased seepage path will decrease the value of hydraulic
gradient.
 To accomplish this high pressure grouting, relatively deeper
holes are drilled near the heel of the dam.
 The spacing of the holes may vary from 1.2 to 1.5 m. Holes are
first of all drilled and grouted at about 10 to 12 m apart and then
the intermediate holes are drilled and grouted.
 After the holes have been drilled, a mixture of cement and water
is forced into holes under high pressure.
 The grouting pressure may be kept as high as possible without
lifting the foundation strata.
 This grouting is generally carried out only after some portion of
the dam section has been laid.
 This grouting have to be accomplished from the foundation
gallery.

 Temperature control in Concrete Dam
 Control of concrete temperature to reduce the temperature cracks is one of the main tasks in the concrete
dam construction.
 Water cooling is the most effective temperature control measure of concrete dam.
 Water pipes are embedded throughout the body of dam and ice water will be circulated through these pipes
once concreting is done.
 Galleries provide enough space for carrying pipes during artificial cooling of concrete.

 What is called Gallery in gravity dam?
 Galleries are the horizontal or sloping openings
or passages left in the body of the dam.
 They may run longitudinally (parallel to dam
axis) or traversely (normal to the dam axis) &
are provided at various elevations.
 All the galleries are interconnected by steeply
sloping passages or by vertical shafts fitted with
stairs or mechanical lifts.
 Dam gallery can be constructed in various
shapes as shown in figure.

 This is how entry would look like in a gallery!!

 Types of galleries in Dams
1. Foundation Gallery:
 A gallery provided in a dam may serve one particular purpose or more than one purpose.
 A gallery provided near the rock foundation, serves to drain off the water which percolates through the
foundations. This gallery is called a foundation gallery or a drainage gallery.
 It runs longitudinally and is quite near to the upstream face of the dam. Drain holes are drilled from the
floors of this gallery after the foundation grouting has been completed. Seepages is collected through
these drain holes.
 Besides draining off seepage water, it may be helpful for drilling and grouting of the foundations when
this cannot be done from the surface of the dam.
2. Inspection Gallery:
 The water which seeps through the body of the dam is collected by means of a system of galleries
provided at various elevations and interconnected by vertical shafts. All these galleries besides draining
off seepage water, provide access to the interior of the dam and are therefore called inspection gallery.

3. Aeration Gallery:
 It will be provided in very few dams where changes of
negative pressure generation is there due to spilling water
over the spillway.
 Sometime the spilling water may flow in such a way that it
losses contact with spillway surface and due to which
separated water start exerting suction (negative) pressure
over spillway surface. Bubble formation will take place in
water. These bubbles will strike the surface due to which
cavitation will start and hence surface erosion will take
place.
 To avoid this negative pressure generation we provide a
gallery in which aeration ducts will be provided which will
supply air to the spilling water and hence cavitation will
be avoided on spillway surface.

 Let’s see how it happens!!

All the 3 galleries in a single dam!!

 Galleries serve the various purposes which are as under:
1. They intercept and drain off the water seeping through the dam body. Porous holes are provided
in the gallery for collecting this water.
2. They collect water seeping through the foundation and hence decreases the uplift pressure on
dam. Drainage holes are provided in gallery for this purpose.
3. Uplift pressure-meter is provided inside the foundation gallery for measurement of uplift pressure
acting on dam. Uplift pressure instrument-UPI holes are provided for this purpose.
4. They provide access to dam interior for observing and controlling the behavior of the dam.
Ventilation holes are provided inside the galleries for ventilation purpose while inspection of dam.
Hume pipes will be carrying air through air inlets provided on dam surface.
5. They provide enough space for carrying pipes during artificial cooling of concrete.
6. They provide access to under-sluice gate by housing its electrical and mechanical control. All
these under-sluice gates can be easily controlled by men from inside the dam itself.
7. They provide space for drilling and grouting of the foundations when it cannot be done from the
surface of the dam. Generally the foundation gallery is used for this purpose.

 Reinforcement for Gallery
 The provision of a gallery in a dam
body, changes the normal pattern of
stresses in the body of the dam.
Stress concentration may therefore
occur at corners and hence in order to
minimize this stress concentrations,
the corners must be rounded
smoothly.
 Tension and compression zones may
be worked out and proper
reinforcements are provided to
counteract them.

manishh_29 manishkhandelwal76 Manish Khandelwal

Gallery in Concrete Gravity Dam

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Gallery in Concrete Gravity Dam