Contents preface (VII) introduction 1—37

16.3. 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

GRAVITY DAMS

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(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. But, if the line of action of the resultant passes sufficiently outside the middle-third of the horizontal section, it may cause crushing of the downstream edge of the section. This would reduce the effective width and, hence, the sliding resistance of the section and may cause the resultant to pass outside the dam section. Further, when the resultant passes downstream of the middle-third of the horizontal section, it induces tensile stresses at the upstream edge of the section. These tensile stresses may cause cracks in the dam section which would result in increased uplift pressure. The stabilising forces would, thus, be reduced. It follows, therefore, that before a gravity dam overturns bodily, other types of failures, such as crushing of toe material, sliding, cracking of the material due to tension, and increase in uplift may occur. A gravity dam is considered safe against overturning if the criteria of: (i) no tension on the upstream face, (ii) adequate resistance against sliding, and (iii) suitable quality and sufficient strength of concrete/masonry of dam and its foundation are satisfied.

Concrete and masonry are relatively weak in tension and as such the design of a gravity dam should ensure that there are no tensile stresses anywhere in the dam section. In very high gravity dams, however, if it becomes difficult to ensure such a condition, one may allow small tensile stresses not exceeding 50 N/cm² under the most adverse condition of loading.
The horizontal forces acting on a dam above any horizontal plane may cause failure of the dam due to sliding if these driving forces are more than the resistance to sliding on the plane. The resistance to sliding is due to the frictional resistance and shearing strength of the material along the plane under consideration. The shear-friction factor of safety, F_s, which is a measure of stability against sliding or shearing, can be expressed as follows:

F = CA + Σ Wµ

(16.8)

^s ΣH
where, C = unit cohesion,
A = area of the plane considered (A can be replaced by the width of the plane, if one considers unit length of the dam),
ΣW = sum of all vertical forces acting on the plane, µ = coefficient of internal friction, and
ΣH = sum of driving shear forces i.e., resultant horizontal forces.
The shear-friction factor of safety can be used to determine the stability against sliding or shearing at any horizontal section within a dam, its contact with the foundation or through the foundation along any plane of weakness. The minimum allowable values of F _s for gravity dam are 3.0, 2.0, and 1.0 for the usual, unusual, and extreme loading combinations, respectively
(3). The value of F_s for any plane of weakness within the foundation should not be less than 4.0, 2.7, and 1.3 for the usual, unusual, and extreme loading combinations, respectively (3).
Generally, the acceptable factor of safety against overturning and shear for normal or usual loading condition is taken as 2.0. The corresponding value for extreme loading condition is 1.25. The acceptable value of the sliding factor (= the ratio of the sum of the horizontal forces and the sum of the vertical forces) is the ratio of the coefficient of static friction and the chosen factor of safety.

542 IRRIGATION AND WATER RESOURCES ENGINEERING
The maximum allowable compressive stress for concrete in a gravity dam should be less than the specified compressive strength of the concrete divided by 3.0, 2.0, and 1.0 for usual, unusual, and extreme load combinations, respectively. The compressive stress should not exceed 1035 N/cm² and 1550 N/cm² for usual and unusual load combinations, respectively (3).
The maximum allowable compressive stress in the foundation should be less than the compressive strength of the foundation divided by 4.0, 2.7, and 1.3 for usual, unusual, and extreme load combinations, respectively. These values of factor of safety are higher than those for concrete so as to provide for uncertainties in estimating the foundation properties.