"The forms for the invert were made of 2-in. rough hemlock boards cut out 4 ins. less diameter than the diameter of the sewer, except for 18 ins. at the bottom of the form which coincided with the inside form of sewer. The bottom of the sewers was laid to the bottom of this form before it was set. Then the lagging, consisting of 2×6-in.×16-ft. hemlock planed, was placed against each side of the form, one at a time, and the concrete brought to the line of this top in 6-in. layers until the whole invert was finished. These forms were set in 16-ft. sections, five to each section.
"The centers consisted of seven ribs of 2-in. hemlock upon which was nailed 1½-in. lagging, 2 ins. wide, tongued and grooved, and were 16 ft. long, non-collapsible, but had one wing on each side, 9 ins. wide, which could be wedged out to fit any inaccuracies in the invert. We used four of these centers setting two at each operation and worked from two ends. We left the centers in for 18 hours before drawing.
"The cost of the concrete on the smaller sewers was the same as are the larger sewers, but the steel metal cost less, as it was wire woven metal that cost 2½ cts. per sq. ft. It was much easier handled and cut to no waste as it came in long rolls and was very pliable.
"After training our men, which occupied about one week or 10 days, we had no difficulty in getting the concrete well placed around the metal, preserving the proper location of the latter, which, however, bore constant watching, as a careless workman would often take the temporary supporting blocks and allow the metal to rest against the wooden center, in which case the metal would show through the surface inside of the sewer. The metal was kept 2 ins. away from the inside forms and the arch. To keep it at this location we had several 2-in. wooden blocks cut which were slipped under the wire or expanded metal and as soon as some concrete was pushed under the wire at this point the block was removed.
"After the forms were removed the invert needed plastering, but the arch was practically like a smoothly plastered wall except where it joined the invert, where it frequently showed the result of too much hurry in depositing the first loads of concrete on the arch. We remedied this by requiring less concrete to be deposited at the start, thus giving the rammers time to place the material properly.
"An interesting result was obtained in the smoothness of the inside surface by using a mixture of different sized stones. When ¾-in. stones or smaller were used in the arch, the inside was honeycombed; but, where 1 to 1½-in. stones (nothing smaller) were used, the inside was perfectly smooth, and the same was true of the invert, showing that the use of larger stones is an advantage and secures more monolithic work. When the run of the crusher from 1½ to ¾-in. stones was used the work was not at all satisfactory.
"The difference in cost of mixing by hand and machine is practically nothing on this kind of work. As the moving of the machine to keep pace with the progress of the work equals the extra cost of mixing by hand when the mixing can be done close to the point where the cement is being placed."
The total cost of the sewers, including excavation, etc., was:
| Cost per lin. ft. | |
| 9¼-ft. sewer through marsh | $32.00 |
| 9¼-ft. sewer in cut averaging 24½ ft. | 24.00 |
| 6½-ft. sewer in cut averaging 12 ft. | 10.00 |
| 5-ft. sewer in cut averaging 11½ ft. | 6.70 |
SEWER WITH MONOLITHIC INVERT AND BLOCK ARCH.—The following records of construction for a sewer built at Coldwater, Mich., in 1901, are given by Mr. H. V. Gifford. The sewer had a monolithic invert and a block arch.