Fig. 260.—Traveling Invert Form for Sewer Construction.
Mode of Operation.—In using the device described the outer mold A is first placed in the trench with its rear end at the end of the trench. An inner mold E is then suspended on the tracks of the outer mold and locked therein by the dogs H, with its rear end flush with the rear end of the outer mold. The partition K is then placed in position against the forward end of the inner mold and a jack J of any suitable form is interposed between diaphragms K and C, the jack being extended sufficiently to press diaphragm K firmly against the front end of the inner mold. The deflector I is next placed in position on the inner mold and the concrete is forced down with an iron rammer between the two molds, so as to fill completely the annular space. The deflector aids in directing the concrete into this space, as will be obvious. After the mold has been filled and the concrete compacted as much as possible, the jack is operated to separate the diaphragms K and C, and as the partition K is pressed against one end of the mass of concrete which has been laid, the opposite end of which abuts against the end of the trench, it follows that any backward movement of the diaphragm K will compress the concrete. This movement will be practically inappreciable in distance, but enough to compact thoroughly the concrete and fill any voids. The action of the jack will also push forward the diaphragm C and the outer mold A, the latter being withdrawn from beneath the inner mold and the newly laid concrete, the tracks D of the outer mold being drawn from beneath the arms f of the inner mold, leaving the latter behind resting on the freshly laid concrete. Further compression of the concrete after it has been left by the outer mold will fill the spaces between the inner mold and the surface of the trench. The outer mold is moved forward in this manner a distance equal to the length of the inner mold, and then the diaphragm K is drawn forward and another inner mold is lowered into the outer mold exactly as was the first one. The jack is then placed, the concrete deposited and the outer mold again advanced exactly as before. As the outer mold advances, the inner molds become disengaged one after another and are set ahead; in practice, enough inner molds are provided to enable the concrete to harden sufficiently to keep its position when it becomes necessary to take up successively the rearmost molds and place them ahead.
Haverhill Sewer Work.—The work at Haverhill, Mass., previously mentioned in which the form just described was used, was a 24-in. circular sewer with 6-in. walls. The outer form was 36 ins. in diameter and 6 ft. 2 ins. long; the inner form was 24 ins. in diameter and 3 ft. long. Angle B was 3 ins. and the track angles D were 1½ ins.; diaphragm K was made of two thicknesses of 3-in. plank and diaphragm C of one thickness of 3-in. plank, the other diaphragms were of 2-in. plank. The shells of the molds were of ¼-in. steel plate; the jack was an ordinary screw jack. Eight inner molds were used.
The form used at Haverhill was built by the city carpenter, the metal portions being made in a boiler shop. Its cost was not ascertained, but was, it is thought, about $75. The concrete used was a 1-3-5 stone mixture, with cement costing $2 per barrel, sand $1.50 per load of 36 cu. ft., and stone $2.50 per load of 36 cu. ft. The men were paid 25 cts. per hour. Records kept on 265 ft. of invert, or, theoretically, 19.3 cu. yds. of concrete, gave the following figures:
| Per lin. ft. | Per cu. yd. | |
| Labor, setting and moving forms, 42 hours, at 25 cts. | $0.05 | $0.67 |
| Labor, mixing, placing and wheeling concrete, 179 hours, at 25 cts. | 0.16 | 2.19 |
| —— | —— | |
| Total labor cost | $0.21 | $2.86 |
With the ordinary 1-3-5 mixture the cost of materials would run about as follows:
| Per cu. yd. | |
| Cement, 0.96 bbl., at $2. | $1.92 |
| Sand, 0.47 cu. yd., at $1.13 | 0.53 |
| Stone, 0.78 cu. yd., at $1.88 | 1.47 |
| —— | |
| Total cost materials | $3.92 |
Two men were worked in the trench, one alternately ramming the concrete into place and working the jack, and the other shaping the trench ahead and assisting in bringing the rear forms ahead.
The form described was invented by Mr. Robert R. Evans, of Haverhill, Mass., and has been patented by him.
29-FT. SEWER, ST. LOUIS, MO.—The following account of the method and cost of constructing 162 ft. of very large sewer section at St. Louis, Mo., is compiled from information furnished by Mr. Curtis Hill.