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.
This framing was cut from 2 by 12-inch lumber. The lagging was inch thick, and surfaced on one side.
391. 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 COTT strueted of reinforced concrete. The details are illustrated in FiA, 2W.
The trench excavations were principally through water-bearing gravel, the gravel ranging from coarse to fine. Some rock was en countered in the trench excavations. It was a granite-gneiss of irregular fracture, and cost, with labor at 17 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? parts of aggregate, grad ed to secure a dense concrete. Care was used in placing the concrete, and very smooth surfaces were secured. Plastering of the was avoided. Any voids were grou ted 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 4S hours before the concreting of the arch section was begun. The lagging was s 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 arc readily removable, and the semicircular arch above is hinged at the soffit and is collapsible. The first cost of these forms has averaged $1S.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, 100S), 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 groundwater 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 sec ond.
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 cents, sub-foremen 30 cents, and general foremen 30 cents per hour.
Normal Cost per Linear Foot of 53 by 54=lnch Reinforced=Concrete Sewer Steel reinforcement, 171- lbs. S .43 Making and placing reinforcement cages .11 Wooden interior forms, cost, maintenance, and depreciation .12 Wooden exterior forms, cost, maintenance, and depreciation .05 Operation of forms . In Coating oil .01 Mixing concrete .30 Placing concrete. .27 Screeding and finishing invert .08 Storage, handling, and cartage of cement .0S 0.4S2 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 $0.02 per cubic yard.
392. Bronx Sewer, New York. In Fig. 211 is shown a section of one of the branch sewers that is being constructed in the Borough of the Bronx, New York City. A large part of this sewer is located in a salt marsh where water and unstable soil make construction work very difficult. The general elevation of the marsh is 1.5 feet above mean high water. In constructing this sewer in the marsh, it is necessary to construct a pile foundation to support the sewer The foundation is capped with reinforced concrete; and then the sewer, as shown in the section, is constructed on the pile foundation. The concrete for this work is composed of 1 part Portland cement, parts sand, and 5 parts trap rock. The rock was crushed to pass a 1-inch screen. "Ransome" twisted bars were used for the reinforce ment in this work.