[Illustration: PLATE XXI, FIG. 2.--VENTILATING COLUMN AND MANHOLE, OUTFALL SEWER.] The general route of the sewer is very direct, long straight lines of several kilometers being possible, and these were joined by curves of approximately 30 m. radius. The gradient of the sewer invert is 0.2% (1 in 500) which is approximately the general fall of the ground northward from Monterrey. The total quantity of excavation was as follows: No. 1, soft earth 8,960 cu. m. No. 2, sillar 18,492 " " No. 3, conglomerate rock 9,822 " " ------ Total 37,274 cu. m. The contract prices for this excavation were: for No. 1, 32 cents; No. 2, 85 cents; and No. 3, 2.17 pesos per cu. m. All the excavation was in perfectly dry ground. Where the sewer was partly out of the ground it had a foundation of concrete, 1.75 m. wide, from 15 to 23 cm. thick, below the bottom of the brickwork, and carried up to the springing of the arch, and a well-tamped embankment, with slopes of 1-1/2 to 1, to protect the sewer to a height of 30 cm. (12 in.) above the arch. For 342 m. at the Monterrey end of the line, the sewer was constructed in tunnel, from, the open end and from two intermediate shafts. The tunnel throughout was in sillar, and the contract price for excavation was 24.50 pesos per lin. m. This work was done without timbering of any kind, except at the shaft lengths. Plate XXII shows the lining of the tunnel, which was of concrete with a brick invert. At four places the sewer passes under main railway tracks, which at these points were carried on steel girders supported on concrete abutments, the sewer being carried under the tracks in the ordinary way. _Bridges._--At three points the sewer was carried over arroyos on reinforced concrete girders. No. 1, at Station 5,600, consisted of four 10-m. spans; No. 2, at Station 8,365, over the Estanscia Arroyo, consisted of nine 10-m. spans; and No. 3, at Station 8,960, over the Topo Chico Arroyo, consisted of three 10-m. spans. One of these bridges is shown on Plate XXIII. They were designed as two parallel continuous girders with connecting top and bottom slabs. The concrete for the girders was a 1:2-1/2:3-1/2 mixture, the sand being from the crusher and the rock gauged to pass a 19-mm. (3/4-in.) screen. The inside was rendered with a coat of 1:1 cement mortar, 7 mm. thick, for water-tightness. [Illustration: PLATE XXIII, FIG. 1.--FORMS FOR MAIN GIRDERS, ESTANSCIA BRIDGE, OUTFALL SEWER.] [Illustration: PLATE XXIII, FIG. 2.--VIEW OF ESTANSCIA BRIDGE, COMPLETED.]