shaped section, shown in Fig. 50, was also used as a portion of the main sewer.1

Fig. 51 shows a section of a large storm-water outfall 12 to 14 feet wide by 8 feet high. The arch is 13 inches thick, backed with additional rings of brickwork at the haunches. The concrete is 8 inches thick at the bottom and 24 inches thick on the sides.

Fig. 52 shows a combined brick and concrete sewer built in lagging in very wet sandy soil. The invert was first laid between the 2-inch sheeting driven obliquely to shut off the flow of sand. The brickwork was then carried up, the concrete backing being placed between the brickwork and the lagging as the former advanced.

1 Eng. Rec., Vol. 44, p. 587.

Fig. 53 shows a section of the basket-handled arch adopted for the aqueduct from the Wachusett Dam on the Nashua River, to the Sudbury River. The aqueduct, about 9 miles long, is II feet 6 inches wide by 10 feet 5 inches high, and has a slope of 1

-11' 6

-10'6"

Fig. 53

in 2500, and an estimated capacity of 300,000,000 gallons per day. The arch has three rows of brick, cut down to one where the concrete backing is added. The entire masonry is 3 feet thick at the springing line and about 6 feet thick at the base. This backing was, however, reduced in tunnel and in rock cuts.

CHAPTER VI.

REINFORCED CONCRETE SEWERS.

THE tendency of construction is towards the use of reinforced concrete for all large sewers. It has many advantages; and the great disadvantage, the porosity, has not been emphasized sufficiently to act as a drawback. The saving of expense is very great, since the additional cost of steel does not equal the cost of the concrete saved, except for small sewers, i.e. up to 3 feet diameter. For these smaller sizes it is cheaper to increase the amount of concrete slightly and omit the reinforcement. The steel is supplied in two forms, either as a wire mesh wrapped around the pipe and buried in concrete, the size of mesh being from 3 to 6 inches, or as rods placed around the pipe at intervals of about 12 inches with longitudinal rods spaced twice that distance. The amount of metal needed, empirical entirely, is about the same in the two cases, and the more intimate association of the steel and concrete afforded by the mesh gives that form of reinforcement a decided advantage. As a guide to the amount of steel used, the table on the following page, taken from the catalogue of the Jackson Reinforced Pipe Company, is given, there being two circular bands in each two feet, and five longitudinal rods in the circumference.

The following examples of actual construction are given, where expanded metal has been used.

Fig. 54 shows a cross-section of the reinforced concrete aqueduct which was built in 1906 to supply the City of Mexico with water. This aqueduct is about 17 miles long, and is laid on a grade of 3 feet in 10,000, its capacity being estimated at about 60 cubic feet per second. The rock used was a hard

basalt, mixed in the proportion of 1: 3:3, fine screenings being

used in place of sand. The maximum width is 6 feet 8 inches, and the maximum height is 8 feet 5 inches. The thickness of the crown is 7 inches and of the base 12 inches, the haunches being thickened as shown. One layer of expanded metal was used by way of reinforcement, and was located in the section as shown in the drawing.

-6.727

-6.230

Fig. 54

A concrete sewer in Providence shown in Fig. 55 is reinforced with expanded metal. The sewer is 36, 48, and 56-inch diameter, and for the smaller size is but 4 inches thick at the crown. The expanded metal is No. 14 gauge, 4-inch mesh. A piece of the metal 18 inches wide is embedded in the invert, and then the arch form placed. The arch reinforcement is then placed so as to lap the invert metal about 6 inches. The concrete was made of 1 cement to 9 bank gravel. A portion of the invert, where the scour is greatest, is finished with a rich mixture and troweled down like a sidewalk.

During the year 1903 a reinforced concrete sewer was built in the city of Wilmington, the entire length being 7436 feet.