The rear drain terminates at a weir box (Z) outside of the down-stream slope at a distance of 500 ft. from the axis of the dam. This drain branches at the down-stream side of the central trench, (Y), one branch being carried up the hillside to high-water level (W) at the North end of the dam, and the other to the same elevation at the South end (X).
[Fig. 5] shows how these drains were constructed. After the removal of all surface soil and loose rock, a trench 5 to 10 ft. wide was cut into the solid rock, the depth of cutting varying with the character of the bed rock. Upon the floor of this trench a small open drain was made by notching the bed rock and by means of selected stones of suitable size and hardness. The stringers and cap-stones were carefully selected and laid, so that no undue settlement or displacement might occur by reason of the superincumbent weight of the dam. All crevices were carefully filled with spawls and the whole overlaid 18 ins. in depth with broken stone 1 to 3 ins. in diameter. Upon this layer of broken stone and fine gravel was deposited choice clay puddle, thoroughly wetted and compacted, refilling the trenches.
FIG. 7.–VIEW OF NORTH TRENCH, TABEAUD DAM.
FIG. 8.–VIEW OF SOUTH TRENCH, TABEAUD DAM.
FIG. 9.–VIEW OF MAIN CENTRAL DRAIN, TABEAUD DAM.
These drains served a useful purpose during construction, in drying off the surface of the dam after rains. The saturation of the outer slope of the dam by water creeping along the line of contact should thus be prevented, and the integrity or freedom from saturation of the down-stream half should be preserved. It is believed that the puddle overlying these rock drains will effectually prevent any water from entering the body of the embankment by upward pressure and that the drains will thus forever act as efficient safeguards.
The main drain was extended, temporarily during construction, from the central trench ([Fig. 4]), to the up-stream toe of the dam. This was cut 5 or 6 ft. deep into solid rock, below the general level of the stripped surface. [Fig. 6] is reproduced from a photograph of this trench. An iron pipe 2 ins. in diameter was imbedded in Portland cement mortar and concrete, and laid near the bottom of the trench.