If the dam is constructed of concrete, the tendency to slide will be resisted by the combined effect of the shearing strength of the concrete (I 408) and of friction.
Again, the earth on the toe of the dam and also that in front of it, add somewhat to the resistance to sliding.
If the stability against sliding is computed by equation 6, either with or without omitting the vertical component of the water, the three factors as above give additional security. In view of the above, there is no probability of a dam's failing by sliding, except possibly upon the foundation.
A low dam may be founded upon the soil, but a high one should rest upon bed-rock. When the dam must be founded upon the soil, the resistance to sliding on the foundation can be increased (1) by driving a row of inclined piles in front of the dam or (2) by sinking a comparatively narrow tongue of the wall below the level of the main foundation. In the latter case, the maximum resistance attainable is equal to the weight of the dam multiplied by the co efficient of friction of masonry on the particular soil plus the weight of the soil which would be moved if the dam slid forward multiplied by the coefficient of friction of the soil upon itself. The surface along which motion of the soil should be assumed to take place will depend upon the profile of the natural surface below the dam, and may be either a horizontal line or one inclined up, according to which one of these is the line of least resistance. This neglects the'cohesion of the soil, i.e., assumes that the resistance to shear is the same as the resistance to sliding; but as the former is the greater, the assumption is on the safe side, although there is not a great deal of difference between the two.
If the dam is founded upon bed-rock, the resistance to sliding on the foundation may be greatly increased by leaving the bed rough; and, in case the rock quarries out with smooth surfaces, one or more longitudinal trenches may be excavated in the bed of the foundation, and afterwards be filled with the masonry. In building the New Croton Dam (˘ 964), two trenches 6 feet deep and 10 feet wide were excavated in the bed-rock, the surface of the foundation was thor oughly cleaned and carefully painted with a grout of neat portland cement, and then cyclopean granite rubble masonry laid in a 1 : 2 portland-cement mortar was started.
The weight of the masonry and the gravel upon the foundation of the New Croton Dam are 2.46 times the net horizontal thrust of the water; in other words, W + V = 2.46 H, or H = 0.41 (W + V). This means that if the coefficient of friction is 0.41, the dam will be on the point of sliding; and consequently, if the actual coefficient of friction is 0.75 (see Table 74, page 464), the nominal factor of safety is 0.75 _ 0.41 = 1.8. But the tendency to slide is resisted by the cohesion of the mortar and by the interlocking of the stones (˘ 929) as well as by friction, and consequently the real factor of safety is considerably more than that computed above.