Gallery in Concrete Gravity Dam

Read about all types of galleries that are provided in a concrete gravity dam with their function.

1. m anishh_29 manishkhandelwal76 Manish Khandelwal Gallery in Concrete Gravity Dam

2. 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.

3. 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: Preparing the surface Grouting the foundation

4. 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.

5. Grouting the foundation The foundation grouting can be divided into ( a) Consolidation Grouting (b) Curtain Grouting 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.

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

7. 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.

8. 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.

9. 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.

10. This is how entry would look like in a gallery!!

11. Types of galleries in Dams 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 . 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 .

12. 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.

13. Let’s see how it happens!!

14. All the 3 galleries in a single dam!!

15. Galleries serve the various purposes which are as under: They intercept and drain off the water seeping through the dam body. Porous holes are provided in the gallery for collecting this water. They collect water seeping through the foundation and hence decreases the uplift pressure on dam. Drainage holes are provided in gallery for this purpose. 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. 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. They provide enough space for carrying pipes during artificial cooling of concrete . 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. 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.

16. 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.

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