CONSTRUCTION OF MASONRY DAMS Foundations.—Masonry dams are ordinarily applicable only t-0 situations where foundations of solid rock may be obtained. Careful examinations of the character of the rock should always be made to considerable depths below the foundation in order to make sure that no seams or strata of porous materials exist, which might cause slipping of the foundation when subjected to pressure of water behind the clam.
Nearly all of the failures of masonry dams which have been recorded have been due to defective foundations, causing settlements through washing out the foundation materials, or sliding of base of dam, and foundation on seams or soft strata through which water under pressure found its way.
Where the depth to solid rock is considerable and the rock or gravel near the surface is of a character to give substantial support to the structure, masonry dams may sometimes be used without carrying the base of the dam into the solid rock. In such cases, curtain walls at the heel of the clam should be carried down to the rock to shut off leakage and possible washing of the foundation.
When rock is seamed or fissured, it may frequently be made tight by grouting, which is clone by drilling into it and forcing grout (usually of neat cement and water) under pressure into the fissures until the cracks become sufficiently filled to force grout to the surface through adjacent drill boles.
When a high clam is to be constructed, the geological structure of the valley should be studied, and core drill borings made over the site of the dam so as to determine fully the stability of the foundation and the probability of leakage around or under the dam.
placing of the foundations of a clans is usually the most diffi cult part of the work of construction. Commonly it is necessary* to divert the water of the stream to be damned, and seepage water must be handled in making the excavations and placing the masonry. The methods used in such work are described in Mr. Chester W. Smith's "Construction of Masonry Dams," New York, 1915.
147. Masonry for Dams.—Several types of masonry are some times used in dams of massive construction.
Ream rubble masonry has commonly been employed, in which the large stones are set in mortar beds, and the vertical joints filled with mortar and small stones carefully placed by masons. The stones are put into place by derricks and must be held and lowered so as to seat evenly upon the mortar bed, being set with careful attention to bond, so that no continuous joints exist in any direction. The complete filling of all joints is important.
Cyclopean masonry, in which the large rubble stones are set in beds of concrete and the ,joints filled with soft concrete, has recently been used to considerable extent. The joints are made thicker than mortar joints, and the labor required in setting the stones and making good joints is much less than in the ordinary rubble.
Rubble concrete is masonry in which rubble stones are distributed through a mass of concrete. In some eases, boulders of considerable size are used in such work. This differs from cyclopean masonry mainly in the smaller amount of large stone used and larger quantity of concrete. It uses more cement, but is more rapidly constructed and requires less hand labor.
Plain concrete is now frequently used, without large stone, for massive work, as well as for the dams of thin sections. The plant required is less, as derricks are needed for handling the heavy stone, and usually more rapid progress is possible in placing the concrete. The nature and location of materials and character of labor available determine the relative costs of the different methods of construction. The rapidity of construction is usually greater as the quantity of large stone becomes less.
148. Overflow Dams.—When water is to flow over the top of a dam or spillway, the section must be modified to provide fer the pass ing of the water with the least disturbance possible, and to take into account the additional head of water above the dam.
If the water falls freely over the clam, its crest should be given such form as to eliminate the possibility of causing a vacuum behind the sheet of falling water. The effect of the impact of the falling water must also be taken into account, and provision made for pro tecting the toe of the dam against erosion, which is frequently done by providing a water cushion into which the stream may fall.
In overflow weirs of considerable height, the downstream face of the darn is given approximately the form of the curve that the water would take in falling freely over the weir under maximum head. The water may then follow the surface of the clam, and, by reversing the curve in the lower part of the section, be turned to horizontal direc tion at the toe of the darn. In designing such a section, the weight of the water on the downstream face is neglected, the pressure on the upstream face being taken as that clue to the full head at greatest expected flood. Special attention should also be given to the possi bility of uplift or of scouring at the toe.