Dams constructed out of masonry or concrete and which rely solely on its self weight for stability fall under the nomenclature of gravity dams. Masonary dams have been in use in the past quite often but after independence, the last major masonry dam structure that was built was the Nagarjunsagar Dam on river Krishna which was built during 1958-69. Normally, coursed rubble masonry was used which was bonded together by lime concrete or cement concrete. However masonry dam is no longer being designed in our country probably due to existence of alternate easily available dam construction material and need construction technology. In fact, gravity dams are now being built of mass concrete, whose design and construction aspects would be discussed in this chapter. There are other dams built out of concrete like the Arch/Multiple Arch or Buttress type. These have however not been designed or constructed in India, except the sole one being the arch dam at Idukki on river Periyar. In India the trend for concrete dam is only of the gravity type and therefore the design other types of concrete dams have not been discussed in this course. Interested readers may know more about such dams from standard books on the subject like Engineering of Large Dams by Henry H. Thomas, Volumes I and II published by John Wiley and Sons (1976). A slightly outdated publication, Engineering of Dams, Volumes I, II and III by W P Creager, J D Justin, and J Hinds published by John Wiley and Sons (1917) has also been long considered a classic in dam engineering, though many new technologies have do not find mention here.
It is important to note that, it is not just sufficient to design a strong dam structure, but it is equally important to check the foundation as well for structural integrity. For concrete dams, the stress developed at the junction of the base becomes quite high, which the foundation has to resist. Usually concrete gravity dams are constructed across a river by excavating away the loose overburden till firm rock is encountered which is considered as the actual foundation. Nevertheless not all rocks are of the same quality; they vary with different geological materials and the process by which they have been formed over the years. For example, the hills of the Himalayan range of the mountains are considered geologically young, as well as weaker than the massif of the Deccan plateau. The quality of foundation not only affects the design, it also guides the type of dam that would be suited at a design site. Hence, discussions on the ground foundation aspects have been introduced in this lesson as well.
It may also be realized that designing a dam based on field data (like the geometry of the river valley, the foundation allowable bearing capacity .etc) is not the only part that a water resource engineer has to do. He has to get it constructed at the design site which may easily take anywhere between 5 to 10 years or even more depending on the complexity of the work and the volume and type of the structure. It may easily be appreciated that constructing a massive structure across a flowing river is no easy task. In fact tackling of the monsoon flows during the years of construction is a difficult engineering task.
Concrete gravity dam and apparent structures- basic layout
The basic shape of a concrete gravity dam is triangular in section (Figure 1a), with the top crest often widened to provide a roadway (Figure 1b).
Figure 1 : Concrete gravity dam section (a) Basic triangular shape (b) Modified shape
The increasing width of the section towards the base is logical since the water pressure also increases linearly with depth as shown in Figure 1a. In the figure, h is assumed as the depth of water and γh is the pressure at base, where γ is the unit weight of water (9810 N/m³), W is the weight of the dam body. The top portion of the dam (Figure 1b) is widened to provide space for vehicle movement.
A gravity dam should also have an appropriate spillway for releasing excess flood water of the river during monsoon months. This section looks slightly different from the other non-overflowing sections. A typical section of a spillway is shown in Figure 2.
Figure 2: Typical overflow section of a gravity dam
The flood water glides over the crest and downstream face of the spillway and meets an energy dissipating structure that helps to kill the energy of the flowing water, which otherwise would have caused erosion of the river bed on the downstream. The type of energy dissipating structure shown in Figure 2 is called the stilling basin which dissipates energy of the fast flowing water by formation of hydraulic jump at basin location. This and other types of spillway and energy dissipators are discussed in a subsequent section. Figure 3 shows the functioning of this type of spillway
Figure 3: Water (towing over a spillway
Usually, a spillway is provided with a gate, and a typical spillway section may have a radial gate as shown in Figure 4. The axis or trunnion of the gate is held to anchorages that are fixed to piers.
Figure 4. A gated spillway section
Also shown in the figure is a guide wall or training wall that is necessary to prevent the flow crossing over from one bay (controlled by a gate) to the adjacent one. Since the width of a gate is physically limited to about 20m (limited by the availability of hoisting motors), there has to be a number of bays with corresponding equal number of gates separated by guide walls in a practical dam spillway.
The upstream face of the overflowing and non-overflowing sections of a gravity dam are generally kept in one plane, which is termed as the dam axis or sometimes referred to as the dam base line (Figure 5).
Figure 5. Co-planar upstream faces of overflow and non-overflow blocks.
Since the downstream face of the dam is inclined, the plane view of a concrete gravity dam with a vertical upstream face would look like as shown in Figure 6.
Figure 6. A typical layout of a concrete gravity dam in plan.
If a concrete gravity dam is appreciably more than 20 m in length measured along the top of the dam from one bank of the river valley to the other, then it is necessary to divide the structure into blocks by providing transverse contraction joints. These joints are in vertical planes that are at the right angle to the dam axis and separated about 18-20 m. The spacing of the joints is determined by the capacity of the concreting facilities to be used and considerations of volumetric changes and attendant cracking caused by shrinkage and temperature variations. The possibilities of detrimental cracking can be greatly reduced by the selection of the proper type of the cement and by careful control of mixing and placing procedures. The contraction joints allow relieving of the thermal stresses. In plan, therefore the concrete gravity dam layout would be as shown in Figure 7, where the dam is seen to be divided into blocks separated by the contraction joints.
Figure 7. Layout of blocks for concrete gravity dams.
The base of each block of the dam is horizontal and the blocks in the centre of the dam are seen to accommodate the spillway and energy dissipators. The blocks with maximum height are usually the spillway blocks since they are located at the deepest portion of the river gorge, as shown in Figure 7. The upstream face of the dam is sometimes made inclined (Figure 8a) or kept vertical up to a certain elevation and inclined below (Figure 8b).
Figure 8. Upstream inclined face for concrete gravity dams, (a) Full face inclined; (b) Partly inclined
In plan, the dam axis may be curved as for the Indira Sagar Dam (Figure 9), but it does not provide any arch action since each block is independent being separated by a construction joint.
FIGURE 9. Indira Sagar dam curved layout
The construction joints in a concrete gravity dams provide passage through the dam which unless sealed, would permit the leakage of water from the reservoir to the downstream face of the dam. To check this leakage, water stops are installed in the joints adjacent to the upstream face (Figure 10).
Figure 10. Typical installation of a water stop near upstream face of dam
Not very long ago, different types of water – stops were being used like copper strips, asphalt grouting, etc. apart from rubber seals. However, unsatisfactory performance of the copper strips and asphalt seals and advancement in the specifications and indigenous manufacture of good quality Polyvinyl Chloride (PVC) water stops have led to the acceptance of only the PVC water stops for all future dam construction. This has been recommended by the following Bureau of Indian Standard codes:
The recommended cross section of a PVC water stop is shown in Figure 11.
Figure 11. Cross section of a PVC water stop (All dimensions are in mm)
According to the recommendations of IS 1220-2001, there needs to be more than one layer of PVC water stop at each joint between two blocks, as shown in Figure 12
Figure 12. Sectional plan through two adjacent blocks showing relative placement of PVC water stops. Note the formed trap drain for removing any leakage water
In the vertical plane, the water stop needs to continue right up to the elevation of the maximum water level plus at least 1000 mm as shown in Figures 13 and 14.
Figure 13. Water stop details near lop of non-overflow section of concrete gravity dam. The formed trap drain also extends upto the same elevation as water stops.
Figure 14. Water stop details near top of the crest of spillway (overflow) section of a concrete gravity dam.
In spite of the provision of water stops there may be leakage through the body of the dam due to the pressure of water from the upstream. In order to remove this water, vertical formed drains to trap the seeping water through the contraction joint is recommended as may be observed from Figures 12 to 13. These vertical drains convey the drainage water to a drainage gallery at some lower level within the dam body as shown in Figure 15.
Figure 15. Vertical formed drain connected to drainage gallery.
It may be noticed that the formed drain is provided not only at the transverse contraction joint between two adjacent blocks of a concrete gravity dam, they are placed at an equal interval of about 3metres centre to centre in all the dam blocks, as recommended by the Bureau of Indian Standard code IS 10135-1985” Code of practice for drainage system for gravity dams, their foundations and abutments”.
These drains are required to intercept any seeping water from the reservoir through the upstream face of the concrete dam. The vertical drains may be formed drains or may be filled with porous concrete, which is formed by mixing 1 part cement with 5 parts of 5 to 20 mm size aggregates. It is also recommended in IS 10135 that the permeability of a 200 mm thick slab of this concrete under a head of 100 mm should be such that the discharge should not be less than 30litres/min/m². It is important to note that the general mass concrete of a dam, though not as porous as the lean concrete mentioned above also seeps water but to a very lesser quantity.
The foundation drainage gallery (shown in Figure 15) is connected to all the vertical drains passing through the body of the dam (Figure 16).
Figure 16. Details of a foundation drainage gallery
This gallery, of a size large enough for a person to walk comfortably, extends throughout the length of the dam at about the same height above.
In plan the gallery is near and parallel to the axis of the dam. The dam foundation is as shown in Figure 17.
Figure 17. Alignment of drainage gallery shown in an elevation view of a concrete gravity dam
The foundation drainage gallery should have a small slope, about 1 in 1000 to drain away the collected water in the side drain up to a sump at the lowest level from where the water may be pumped out to the downstream side of the dam. As such, it is compulsorily recommended to provide a drainage gallery whose normal foundation level is more than 10metres measured from the crest level of the overflow portion of the dam. For dams with maximum height less than 10metres between the deepest foundation level and the overflow section crest level, the provision of a drainage gallery is optional.
Another important location of water seeping through is the bottom of a concrete gravity dam. This water seeps through the foundation material, like through the joints of a fractured rock upon which the dam is founded. This seepage water also causes uplift at the base of the dam and produces an upward force that must be countered by the weight of the dam apart from countering other forces discussed in the next section. Thus drainage of foundation material is an important consideration in concrete dam design.
It may be observed from the details of a foundation drainage gallery that 100mm diameter perforated steel pipes are usually provided through the floor of the gallery that penetrates into the foundation. These drainage holes are drilled once the foundation gallery base has been constructed and the foundation grouting (explained later) is completed. The size, spacing and the depth of these holes are assumed on the basis of physical characteristics of the foundation rock, foundation condition and the depth of the storage reservoir. The diameter of the hole may be kept at 75mm and the spacing of holes may be kept at 3m centre to centre. The depth of holes may be kept between 20 and 40 percent of the maximum reservoir depth and between 30 and 75 percent depth of the curtain grouting (explained later). The drainage holes of 75mm diameter are drilled through 100 mm diameter pipe embedded in concrete portion of the dam. For foundation drainage holes in soft foundation, the arrangement shown in Figure 18 may be adopted.
Figure 18. Details of foundation drainage gallery in soft foundation
Another function of the foundation gallery is to provide a space for drilling holes for providing what is called grout curtain, which is nothing but a series of holes drilled in a line deep inside the foundation and filled with pressurized cement mortar. The location of a grout hole within a foundation gallery may be seen from Figure 16. The purpose of providing these holes and injecting them with cement mortar is to create a barrier in the foundation rock at the heel of the dam (Figure 19) which will prevent leakage of water from the reservoir and thus reduce uplift pressure at the bottom of the dam.
The depth of the grout curtain holes depends upon the nature of the rock in foundation and in general, it may range from 30 to 40 percent of the head of the water on good foundation and to 70 percent of head on poor foundations.
Figure 20. Series of grout holes forming a grout curtain shown in elevation of a concrete gravity dam
Figure 20 shows a typical layout of grout holes for a concrete gravity dam shown in an elevation view.
According to IS: 11293 (Part2)-1993 “Guidelines for the design of grout curtains”, the following empirical criteria may be used as a guide:
D= (2/3) H + 8 (1)
Where D is the depth of the grout curtain in meters and H is the height of the reservoir water in meters.
The grout holes may be either vertical or inclined.
The orientation, plan and inclination of grout holes depend upon the type of joints and the other discontinuities in the foundation rock. The most common practice is to drill holes inclined towards the upstream at 5 to 10 degrees to the vertical.
Apart from the gallery at the foundation level, there could be other galleries located at intermediate levels as shown Figures 21 and 22.
Figure 21. Galleries in a non-overflow section of a typical concrete gravity dam.
Figure 22: Galleries in a overflow-sect ion of a typical concrete gravity dam
At times, gate galleries have to be provided in a dam to give access to and room or the mechanical and electrical equipment required for the operation of gates in outlet conduits, penstocks, etc. Inspection galleries are also sometimes provided to give access to the interior mass of the dam after completion. Foundation, drainage and gate galleries also serve as inspection galleries. In order to connect these galleries that are parallel to the dam axis, transverse galleries called adits are also provided in a dam. Adits providing access to the galleries from outside the dam are also called access gallery or entrance gallery.
It may be interesting to note that all spillways of dams (including gravity dams) may not be necessarily be gated. Ungated spillways have been provided in dams located in remote areas where spillway operation by manual control is difficult. For the gated spillways, radial gates are more common nowadays, though vertical lift gates have been used in some dams earlier. More details about gates and hoists have been presented in a subsequent lesson.
Apart from the openings in dams discussed earlier sluices are also provided in the body of the dam to release regulated supplies of water for a variety of purposes. Details about the types of sluices and their uses are provided separately.
Sometimes the non overflow blocks of a concrete gravity dams are used to accommodate the penstocks (large diameter pipes) which carry water from the reservoir to the powerhouse. Design of the intakes and conveyance systems for power generation have been discussed in another lesson.