The outlet channel from the tanks leads to a measuring chamber, 3 m. square, as shown on Plate XXIX. This chamber is fitted with penstocks, 1.83 m. wide, and measuring weirs. From this chamber the sewage is delivered to two main irrigation ditches, which distribute the sewage in two directions, one northward and the other to the western extremity of the lands.
_Construction of Tanks._--The excavation for the tanks was in soft earth for a depth of 1-1/2 m.; the lower depths were in a firm foundation of sillar and calcareous clay. The total excavation in the tanks, channels, etc., was 8,335 cu. m., and the actual cost was 45-3/4 cents per cu. m. To facilitate the construction, about six-tenths of the concrete beams were cast as single monoliths and placed in position by sliding them across the tanks on temporary timbers. The remainder of the beams, the roof, and the slab were placed in position in the ordinary way with timber forms. The total quantity of concrete placed was 1,360 cu. m. A 1:2-1/2:4-1/2 concrete was used for the walls, channels, etc., and a 1:2:3 mixture for the roof slab and beams.
Table 14 gives the average cost per cubic meter for all the concrete work.
TABLE 14.--AVERAGE COST PER CUBIC METER FOR CONCRETE IN TANKS.
+-----------------------------------------+-----------+-----------+ | | Pesos per | Pesos per | | | cubic | cubic | | | meter. | meter. | +-----------------------------------------+-----------+-----------+ | LABOR : | | | | Mixing and placing | 5.20 | | | Carpenter work in forms, framing, etc. | 4.20 | | | | _____ | | | Total labor cost | | 9.40 | | | | | | MATERIALS : | | | | Screened gravel | 4.04 | | | Sand (from neighboring arroyo) | 4.98 | | | Cement (including hauling) | 15.19 | | | Lumber, nails, and other supplies | 1.90 | 26.11 | +-----------------------------------------+-----------+-----------+ | Total cost of concrete per cubic meter 35.51 | +-----------------------------------------------------------------+
_Sludge-pit._--The sludge-pit, used when cleaning out the tanks, is carried 639 m. northward, far enough to get the available fall to drain the bottom of the detritus chambers and liquefying tanks. The drainage pipe was formed of 56-cm. (22-in.) concrete tubes. The sludge-pit is merely an excavation in the earth 20 m. square and 2 m. deep, the sides having a slope of 1-1/2 to 1. An overflow drains the pit to an irrigation ditch, the solid matter being allowed to settle and the liquid to drain off. From time to time it is proposed to dig out the solids and plow them into the land.