Table 13 gives the particulars of the average distributed cost of laying the 75.6 km. of sewers.
TABLE 13.—Average Cost, Per Linear Meter, for 75.6 Km. of Sewers,
for Materials and Labor Complete.
| Kind of Sewer. | Internal Diameter of Sewers. | Cost of materials including 10 cm. (4 in.) branches every 41⁄2 m. Pesos. | Earthwork and Labor: | Total cost of sewer complete per linear meter. | |||
| cm. | in. | Average depth of sewer m. | Cost of excavation, including back filling, removing surplus, etc. Pesos. | Cost of labor in laying (including hauling, etc.). Pesos. | |||
| Fire clay | 24.3 | 8 | 2.00 | 2.10 | 3.46 | 0.21 | 5.67 |
| " | 25.4 | 10 | 2.78 | 2.25 | 3.97 | 0.2625 | 7.0125 |
| " | 30.5 | 12 | 3.64 | 2.50 | 4.705 | 0.305 | 8.65 |
| " | 38.1 | 15 | 6.14 | 2.75 | 5.50 | 0.4375 | 12.0775 |
| " | 45.7 | 18 | 8.80 | 3.00 | 6.745 | 0.645 | 16.19 |
| " | 50.8 | 20 | 11.30 | 3.50 | 8.275 | 0.815 | 20.39 |
| Concrete | 55.9 | 22 | 5.93 | 3.50 | 9.19 | 1.325 | 16.445 |
| " | 61.0 | 25 | 7.30 | 3.75 | 11.245 | 1.685 | 20.23 |
| One brick thick on concrete foundations | 68.6 | 27 | 7.17 | 3.75 | 11.735 | 3.93 | 22.835 |
| 76.2 | 30 | 7.925 | 4.00 | 14.53 | 4.515 | 26.97 | |
Fig. 17.—Sketch Showing Disconnecting Trap On House Drain.
The house connections are chiefly of 10-cm. (4-in.) pipes, laid on a minimum gradient of 21⁄2%, from oblique branches on the sewer to siphon intercepting traps near the house, as shown by Fig. 17. From this trap a 10-cm. fire-clay inspection pipe is carried up and capped at the sidewalk level with a cast-iron box having a locked cover. From this inspection pipe a branch is connected to a cast-iron fresh-air inlet, in most cases set in the wall of the house, the inlet being 30 cm. above the level of the pavement.
Effect of the Flood on Sewers.—The flood of August 27th and 28th, 1909, partly destroyed one of the main collectors, which was laid along the banks of the river and encased in concrete. This has now been relaid farther north, and out of the way of any future floods. The total length of the new sewers replacing those damaged amounts to 1200 m., and they vary in internal diameter from 20 to 55.9 cm. (8 to 22 in.).
Main Outfall Sewer.
The direction of the main outfall sewer was determined after a thorough study of all the available land lying to the north and northeast of the city, as it was the intention of the Company to utilize
for irrigation purposes the sewage and any surplus waters that might be developed. The best available site was found to be about 12 km. north of the city, a little northwest of the village of San Nicolas de los Garzas, as shown on [Plate II]. The long length of outfall required was justified by the cheap cost of the land and its excellent character for sewage irrigation. The sewer was designed for a capacity of 90,000,000 liters a day (36.76 cu. ft. per sec.) in order to allow for conveying surplus waters as well as sewage.