Contents preface (VII) introduction 1—37

EXERCISES

1. 
   Discuss the factors which influence the design of an embankment dam.
   

2. 
   What are the common causes of failure and corresponding safety measures adotped in an em-bankment dam?
   

3. 
   Describe different methods of controlling seepage through an embankment dam and its founda-tion.
   

4. 
   For the earth dam of homogeneous section with a horizontal drain as shown in Fig. 15.33, draw the top flow line and the flownet. Also estimate the discharge per metre length through the body of the dam (K = 5 × 10^–4 cm/s).
   

								7 m
										3
22		m	1	4			25	m	1

40 m
Fig. 15.33 Sketch for Exercises 15.4 and 15.5

5. 
   For the embankment section shown in Fig. 15.33, if K_x = 9 × 10^–4 cm/s and K_y = 6 × 10^–4 cm/s, estimate the discharge through the dam section per metre length.
   

534 IRRIGATION AND WATER RESOURCES ENGINEERING

6. 
   Determine the length of blanket to reduce the foundation seepage by 40 per cent for the dam section shown in Fig. 15.34.
   

20 m	Core
1.0 m	50 m
–3	6 m
Kf = 5 × 10 cm/s

Fig. 15.34 Sketch for Exercise 15.6

7. 
   Find the factor of safety against sudden drawdown for the failure surface shown in Fig. 15.35 with the following data:
   

Angle of internal friction

: For shell = 32°, for core = 20°

Cohesion

: For shell = 0, for core = 40 kN/m^2

Saturated weight

: For core material = 20 kN/m^3

For shell material = 21 kN/m^3 (not freely draining)

Assume water up to the top of dam under full reservoir condition.

8 m

B

2.5

1

1

2.5

40

m

1

1

1

Shell

Core

1

Shell

20 m

A

Failure surface

Fig. 15.35 Sketch for Exercise 15.7
REFERENCES

1. 
   Krishan, MS, Geology of India and Burma, Higginbothams (Pvt) Ltd. Madras, 456.
   

2. 
   Schuyler, JD, Reservoirs for Irrigation, John Wiley & Sons, Inc., New York, 1905.
   

3. 
   Golze, AR (Ed), Handbook of Dam Engineering, Van Nostrand Reinhold Company, New York, 1977.
   

4. 
   Sherard, JL, RJ Woodward, SF Gizienski, and WA Clevenger, Earth and Earth-Rock Dams, John Wiley & Sons, Inc. USA, 1963.
   

5. ...... Desingn of Small Dams, USBR, 1960.

6. Sharma, HD, Embankment Dams, Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1991. 7. ...... IS: 10635–1983, IS Guidelines for Freeboard Required in Embankment Dams.

8. 
   Cedergern, HR, Seepage, Drainage and Flownets, John Wiley & Sons Inc., USA, 1967.
   

8. 
   Casagrande, A, Seepage Through Dams, J. of New England Water Works Association, June 1937.
   

8. 
   Bennett, PT, The Effects of Blankets on Seepage through Porous Foundations,Trans. ASCE, Vol. III, 1946.
   

11. ...... Is: 9429–1980, IS Code of Practice for Drainage System for Earth and Rock fill Dams. 12. ...... Is 7894–1975, IS Code of Practice for Stability Analysis of Earth Dams.

13. 
    Creager, WP, JD Justin and J Hinds, Engineering for Dams, Vol. III, John Wiley & Sons, New York, 1945.
    

13. 
    Anderson, MG and KS Richards (Ed.), Slope Stability, John Wiley & Sons, 1987.
    

13. 
    Bertram, GE, Slope Protection of Earth Dams, 4th ICOLD, New Delhi, 1951.
    

16
GRAVITY DAMS
16.1. GENERAL
A gravity dam is a solid concrete or masonry structure which ensures stability against all applied loads by its weight alone without depending on arch or beam action. Such dams are usually straight in plan and approximately triangular in cross-section. Gravity dams are usually classified with reference to their structural height which is the difference in elevation between the top of the dam (i.e., the crown of the roadway, or the level of the walkway if there is no roadway) and the lowest point in the excavated foundation area, exclusive of such features as narrow fault zones (1). Gravity dams up to 100 ft (30.48 m) in height are generally considered as low dams. Dams of height between 100 ft (30.48 m) and 300 ft (91.44m) are designated as medium-height dams. Dams higher than 300 ft (91.44 m) are considered as high dams.
The downstream face of a gravity dam usually has a uniform slope which, if extended, would intersect the vertical upstream face at or near the maximum water level in the reservoir. The upper portion of the dam is made thick enough to accommodate the roadway or other required access as well as to resist the shock of floating objects in the reservoir. The upstream face of a gravity dam is usually kept vertical so that most of its weight is concentrated near the upstream face to resist effectively the tensile stresses due to the reservoir water loading. The thickness of the dam provides resistance to sliding and may, therefore, dictate the slope of the downstream face which is usually in the range of 0.7 to 0.8 (H) : 1(V). The thickness in the lower part of the dam may also be increased by an upstream batter.
When it is not feasible to locate the spillway in the abutment, it may be located on a portion of the dam in which case the section of the dam is modified at the top to accommodate the crest of the spillway and at the toe to accommodate the energy dissipator. The stability requirements of such overflow sections of gravity dams would be different from those of non-overflow gravity dams.

16.2. FORCES ON A GRAVITY DAM
The forces commonly included in the design of a gravity dam are shown in Fig. 16.1. These are as follows (2, 3, 4):
(i) Dead Load
The dead load (W_c) includes the weight of concrete and the weight of appurtenances such as piers, gates, and bridges. All the dead load is assumed to be transmitted vertically to the foundation without transfer by shear between adjacent blocks.