31-03-2012, 03:33 PM
GRAVITY DAMS
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 nonoverflow
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 (Wc) 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.
(ii) Reservoir and Tail-water Loads (Ww , Ww′, W1, and W1′)
These are obtained from tail-water curves and range of water surface elevations in reservoir
obtained from reservoir operation studies. These studies are based on operating and hydrologic
536 IRRIGATION AND WATER RESOURCES ENGINEERING
data such as reservoir capacity, storage allocations, stream flow records, flood hydrographs,
and reservoir releases for all purposes. In case of low overflow dams, the dynamic effect of the
velocity of approach may be significant and should, therefore, be considered. If gates or other
control features are used on the crest, they are treated as part of the dam so far as the application
of water pressure is concerned.
Earthquake
Gravity dams are elastic structures which may be excited to resonate by seismic disturbances.
Such dams should be designed so that they remain elastic when subjected to the design
earthquake. The design earthquake should be determined considering (i) historical records of
earthquakes to obtain frequency of occurrence versus magnitude, (ii) useful life of the dam,
and (iii) statistical approach to determine probable occurrence of earthquakes of various
magnitudes during the life of the dam.
CAUSES OF FAILURE OF A GRAVITY DAM
A gravity dam may fail on account of overturning. For a gravity dam to be safe against
overturning, the dimensions of the dam should be such that the resultant of all the forces
intersects the base of the dam within its middle-third portion. Consider any horizontal section
(including the base) of a gravity dam and the resultant of all the forces acting on the dam
above the section. If the line of action of this resultant passes outside the downstream edge of
the section, the dam would overturn. However, the section of a gravity dam is such that the
line of action of the resultant force is within the upstream and downstream edges of the section
and overturning would never results.