Sewerage and Sewage Treatment

by Harold E. Babbitt

The sewer section may be built as a monolith, in two parts, or in three parts. In casting the sewer as a monolith the complete full round inside form is fixed in place by concrete blocks and wires. The full round outside form is completed as far as possible without interfering too much with the placing and tamping of the concrete. The concrete is poured from the top, being kept at the same height on each side of the form, and tamped while being poured. The remaining panels of the outside form are placed in position as the concrete rises to them. An opening is left at the top of the outside arch forms which is of such a width that the concrete will stand without support. The casting of sewers as a monolith is difficult and is usually undesirable because of the uncertainty of the quality of the work. It has the advantage, however, of eliminating longitudinal working joints in the sewers which may allow the entrance of water or act as a line of weakness.

Fig. 130.—Construction Joints for Concrete Sewers.

If the sewer is to be cast in two sections the invert is poured to the springing line or higher. A triangular or rectangular timber is set in the top of the wet concrete as shown in Fig. 130. When the concrete has set the timber is removed and the groove thus left forms a working joint with the arch. After the invert concrete has set, the arch centering is placed and the arch is completed. This is the most common method for the construction of medium-sized circular sewers.

Large sewers with relatively flat bottoms are poured in two or three sections. First the invert is poured without forms and is shaped with a screed. About 6 inches of vertical wall is poured at the same time. This acts as a support for the side-wall forms. The side walls reach to the springing line of the arch and are poured after the invert has set. At the third pouring the arch is completed. The sewer shown in Fig. 1 is being poured in two steps, as the side walls are so low that they are poured at the same time as the invert. A transverse working joint similar to one of the types used in Fig. 130 is set between each day’s work.

The length of the form used and the capacity of the plant should be adjusted so that one complete unit of invert, side wall, or arch can be poured in one operation. The forms are left in place until the concrete has set. Invert and side-wall forms are generally left in position for at least two days, and in cold weather longer. The arch forms are left in place for double this time. For example if 20 feet of invert and arch can be poured in a day, 60 feet of invert form and 100 feet of arch form will be required. As the forms are released they must be moved forward through those in place. For this reason collapsible or demountable forms are necessary and steel forms are advantageous. Wooden arch forms are sometimes dismantled and carried forward in sections, but are preferably designed to collapse as shown in Fig. 131, so that they can be pulled through on rollers or a carriage.

189. Construction in Tunnels.—In tunnels the invert and side walls are constructed in the same manner as for open cut work. The tunneling, which acts as the outside form, is concreted permanently in place. The concreting of a tunnel by hand is shown in Fig. 132. If the work is to be done by hand the concrete is thrown in between the ribs of the arch centering and behind the plates or lagging, which are set in advance of the rising concrete. The lagging plates are 5 feet long which makes it possible to throw the concrete in place at the arch, and to tamp it in place from the end. A bulkhead and a well-greased joint timber are placed in position as the concrete rises.

Fig. 131.—Section through a Collapsible Wood Form.

Pneumatic transmission of concrete is also used for filling the arch forms as well as the side walls and invert forms. In using this method the mixer may be placed at the surface or at the bottom of the shaft or other convenient permanent location which may be some distance from the form. The mixture is discharged into a pipe line through which it is blown by air to the forms. The starting pressure of about 80 pounds per square inch can be reduced after flow has commenced. In constructing the St. Louis Water Works tunnel the compressor equipment for moving the concrete had a capacity of 1,600 cubic feet per minute at a pressure of 110 pounds. The tunnel is horseshoe shaped, 8 feet in height and with walls varying from 9 to 20 inches in thickness. The extreme travel of the concrete was 1,100 feet in an 8 inch pipe. The amount of air consumed at 110 pounds varied from 1.2 to 1.7 cubic feet of free air per linear foot of pipe. By the time the batch had been discharged the pressure had reduced to 25 to 40 pounds, depending on the length of the pipe. It is reported that a 6–inch pipe line would probably have given better results.