Figs. 208 and 209 illustrate sections of the walls of the pure water basin and the 50-foot circular tanks which have been partly described in Part I under the heading of Waterproofing.

The pure water basin was 100 feet by 200 feet, and 13 feet deep, giving a capacity of 1,500,000 gallons. The counterforts are spaced 12 feet 6 inches center to center, and are 12 inches thick, except every fourth one, which was made 18 inches thick. The 18-inch counterforts were constructed as two counterforts 9 inches thick, as the vertical joints in the walls were made at this point; that is, the concrete between the centers of two of the 18-inch counterforts was placed in one day. On the two ends and one side of the basin, the counterforts were constructed on the exterior of the basin to support about 10 feet of earth. But on one side it would have been necessary to remove rock 6 to 8 feet in thickness to make room for the counterforts, had they been constructed on the exterior of the basin. Therefore they were constructed inside of the basin. If both faces of the vertical wall had been reinforced, the same as the one shown, then the wall would have been able to resist an outward or inward pressure, and the "piers" would act as counterforts or buttresses, depending on whether they were in tension or in compression.

The concrete used consisted of 1 part Portland cement, 3 parts sand, and 5 parts crushed stone. The stone was graded in size from 1/4-inch as the minimum to 3/4-inch as the maximum size. Square-sectioned deformed bars were used as the steel reinforcement. The forms were constructed in units so that they could be put up and taken down quickly.

The size and spacing of the bars in the walls of the circular tanks, are shown in Fig. 209. The framework of the forms to which the lagging was fastened was cut to the desired curve at a planing mill.

Fig. 208. Section of Water Basin Wall.

Fig. 208. Section of Water Basin Wall.

Fig. 209. Section of Tank.

Fig. 209. Section of Tank.

This framing was cut from 2 by 12-inch lumber. The lagging was 7/8- inch thick, and surfaced on one side.

39-1. Main Intercepting Sewer. In the development of sewage purification work at Waterbury, Conn., the construction of a main intercepting sewer was a necessity. This sewer is three miles long. It is of horseshoe shape, 4 feet 6 inches by 4 feet 5 inches, and is constructed of reinforced concrete. The details are illustrated in Fig 210.

The trench excavations were principally through water-bearing gravel, the gravel ranging from coarse to fine. Some rock was encountered in the trench excavations. It was a granite-gneiss of irregular fracture, and cost, with labor at 17 1/2 cents per hour, about $2.00 per cubic yard to remove it. Much trench work has varied in depth from 20 to 26 feet. Owing to the varying conditions, it was necessary to vary the sewer section somewhat. Frequently the footing course was extended. However, the section shown in the figure is the normal section.

The concrete was mixed very wet, and poured into practically water-tight forms. The proportions used were 1 part Atlas Portland cement to 7 1/2 parts of aggregate, graded to secure a dense concrete. Care was used in placing the concrete, and very smooth surfaces were secured. Plastering of the surfaces was avoided. Any voids were grouted or pointed, and smoothed with a wooden float. Expanded metal and square twisted bars were used in different parts of the work. In Fig. 210, the size and spacing of the bars are shown. The bars were bent to their required shape before they were lowered into the excavation.

The forms in general were constructed as shown in the figure. The inverted section was built as the first operation; and after the surface was thoroughly troweled, the section was allowed to set 36 to 48 hours before the concreting of the arch section was begun. The lagging was § inch thick, with tongued-and-grooved radial joints, and toe-nailed to the 2-inch plank ribs. The exterior curve was planed and scraped to a true surface. The vertical sides of the inner forms are readily removable, and the semicircular arch above is hinged at the soffit and is collapsible. The first cost of these forms has averaged

Fig. 210. intercepting Sewer at Waterbury, Conn.

Fig. 210. intercepting Sewer at Waterbury, Conn.

$18.00 for 10 feet of length; and the cost of the forms per foot of sewer built, including first cost and maintenance, averaged 10 cents. Petrolene, a crude petroleum, was found very effective in preventing the concrete from adhering to the forms.

A mile and a-half of the sewer has been completed (May, 1908), and is in use, all of which has been constructed in water-bearing soil; and the greater part of it has been 4 to 12 feet below the ground-water level. The interior surface in this length subjected to percolation is 118,-000 square feet. The total seepage from this area has been less than 0.03 cubic foot per second.

Cost records kept under the several contracts and assembled into a composite form, show what is considered to be the normal cost of this section under the local conditions. Common labor averaged 17 1/2 cents, sub-foremen 30 cents, and general foremen 50 cents per hour.

Normal Cost per Linear Foot of 53 by 54-Inch Reinforced-Concrete Sewer

Fig. 211. Section of Bronx Sewer, New York.

Fig. 211. Section of Bronx Sewer, New York.

Steel reinforcement, 17 1/8 lbs..

$ .43

Making and placing reinforcement cages..

14

Wooden interior forms, cost, maintenance, and depreciation.

12

Wooden exterior forms, cost, maintenance, and depreciation.

05

Operation of forms...

16

Coating oil...

01

Mixing concrete....

30

Placing concrete...

27

Screeding and finishing invert....

08

Storage, handling and cartage of cement..

08

0.482 bbl

cement at $1.53.....

74

0.17 cu

yd. sand at $0.50.....

09

0.435 cu. yd. broken stone at $1.10....

47

Finishing interior surface...

01

Sprinkling and wetting completed work...

02

Total cost per linear foot....

. $2.97

This is equivalent to a cost of $9.02 per cubic yard.