Factor of Safety against Sliding. Although the discussion immediately in hand is the stability against overturning, it is inter esting to note that Fig. 99, page 466, affords an easy method of determining the factor of safety against sliding.
The wall can not slide horizontally, when the angle rag is less than the angle of repose, i.e., when tan rag is less than the coefficient of friction. The factor against sliding is equal to the coefficient of friction divided by tan rag, which is only a different form of the principle stated in equation 6, page 461.
It is nearly, if not quite, impossible to make masonry absolutely impermeable under a high head; but the water which forces its way through reasonably good masonry or concrete is in a capillary state and not likely to exert any considerable hydrostatic pressure. If cracks are formed, due to poor construction or to settlement or to temperature changes, which are large enough to permit water to enter them under hydrostatic condition, the area subject to such pressure is so small in comparison with the whole horizontal section of the dam that the effect may be neglected. This view seems to be sustained by experience with masonry dams, which shows that although all dams leak more or less, the water which comes through is not under any appreciable pressure.* However, there is a con siderable difference of opinion among engineers as to the possibility of making masonry water-tight or of preventing cracks and fissures which will give the water a free path into the body of the dam.
Some engineers claim that although the percolating water is not under pressure at the down-stream face, it is likely to be at the up-stream surface, and that therefore the percolating water should be assumed to be under full hydrostatic pressure at the up-stream face and decrease to zero at the down-stream face.
In support of this view reference is frequently made to some experiments conducted in 1888 f which showed that water pressure was communicated, almost undiminished, through a layer of 1 : 2 portland-cement mortar 1 foot thick. However, these experiments were made before the effect of a well-graded sand upon the density and permeability of a mortar was clearly understood; and it is probable that the mortar employed was not as good as that now ordinarily used in the construction of dams. "An examination of a few high masonry dams seems to show that the pressure of the per colating water is slight and independent of the head or of the thick ness of the masonry."* The most care should be taken with the up-stream face so that it may be the most nearly water-tight portion of the dam; and if this is done, it is not likely that water will exert hydrostatic pressure in the body of the dam, unless possibly the down-stream face becomes water-tight through the freezing of the seepage water on or near the surface. To obviate this possibility, drains are sometimes inserted in the masonry to carry away any water that may seep through the up-stream face. These drains, or weepers, consist of vertical pipes, 3 or 4 inches in diameter, having open joints or being perforated, placed 5 or 6 feet from the up-stream face and 8 or 10 feet apart, and connected to a drainage gallery in the base of the dam which is drained by a cross tunnel to the down stream face.
In view of the above, it does not seem necessary to modify the above discussion of the stability against overturning.