1 E. J. McCaustland, Trans. Assoc. C. E., Cornell University, 1902.
669. The bottom of the trench being cut to form, the concrete for the invert was laid directly on the sub-grade, tamped in layers carried up until the invert occupied about one hundred forty degrees of arc. The form of the inner face of the invert was maintained by template, grade stakes being set 12 1/2 feet apart along the trench. The remainder of the sewer was laid on centers resting on the invert. The ribs for this centering were made in a complete circle, of three thicknesses of one by twelve inch boards nailed together and cut to a true circle. Ribs were placed four feet center to center, and covered with lagging two inches thick and three inches wide, planed to radial joints. The strips of lagging were held in place at each end of a section by a 3/16 by 2 inch iron band passing over all of the strips, and turned in at the ends, forming a hook in which rested the lower lagging strip, the other strips being supported by this one. The lower part of each rib rested on the invert, the upper portion being cut to a diameter four inches less (that is, smaller by twice the thickness of the lagging). While the trench was near enough to the outside of the sewer ring not to measurably increase the amount of concrete over and above the desired thickness, the trench served as the outside form. Above this point, planks were inserted and braced to the sides of the trench. From the haunches to the crown the exterior was finished with a template.
When completed, the exterior form planks were removed, and a light covering of earth placed on the surface to protect it from drying too rapidly. This was especially necessary in this case on account of the kind of cement used. The centers were removed usually after forty-eight hours, by swinging the ribs about the vertical diameter and removing the lagging. As soon as the centering was removed, the inner surface was plas-ered with a mortar composed of three parts lake sand to one part cement.
The company furnished the materials used in the sewer ring and manholes, and delivered it on the work, while the contractor furnished all tools and labor to dig the trenches, complete sewer and manholes, and do the back filling. The contract prices per foot are given by Mr. McCaustland, the resident engineer, as follows: — 36-inch sewer in trench averaging 11 feet deep, 3,340 feet, at $2.30.
From the data given we have computed the approximate quantities of concrete per foot of sewer, and assuming the Cost of the materials for a cubic yard at $3.00, we obtain the following approximate costs: —
Estimated Total, Cost per Foot.
Approximate Cubic Yards Concrete.
Approximate Cost Concrete.
Contract Price per Foot.
36 in. 42 " 48 " 84 " 90 "
11 feet. 14 " 17 " 22 " 24 "
$ .85 .97 1.41 2.79 2.97
$2.30 3.00 3.57 5.91 6.68
$3.15 3.97 4.98 8.70 9.65
In the construction of a thirty inch sewer at Medford, Mass.,1 Mr. William Gavin Taylor made use of a very convenient form. The lower 240 degrees of the sewer was of concrete, the upper 120 degrees being of brick. To construct the concrete portion as a monolith, the forms were constructed in lengths of ten feet, separating on a vertical line into two halves. The two halves were connected by clamps, and held at the proper distance apart by dog irons in the end ribs of each form. After smearing the forms as usual, the concrete was deposited and rammed. When it had partially set, the dog irons were removed and turn-buckles used to slowly pull the two halves together. This method prevented the green concrete being broken, although the concrete extended up on the sides thirty degrees above the horizontal diameter.
672. The centers used for the brick arch were also ingeniously arranged, and since they might have been used for a concrete arch they may be described here. These centers were also in ten foot lengths. The ribs, of two inch plank, were:
1 Abstract from Annual Report of City Engineer, Engineering Record, Not. 7, 1903.
1.25 cu. yds. excavation and back fill, at $.59.......$0.74
.15 cu. yd. concrete, at $6.70 .............1.00
.037 cu. yd. brick masonry, at $12.05...........44
Cost of linear foot, exclusive of manholes, estimated at . . $2.18 The total cost per linear foot is given as........$2.39
In connection with the construction of the New York Rapid Transit Railway, some of the sewers were built of concrete. This work was done with exceptional care, and on a large scale, and it was found that the concrete sewers Cost one-third less than similar sewers of brick.
The method of construction of one section may be described as follows:1 The forms for the invert of the straight lengths of sewer were twelve feet in length, consisting of a strong framework covered with closely matched lagging, planed smooth and spaced two feet centers, with lagging 7/8 inch thick by 1 1/2 inch wide, with one bevel edge to make a tight upper surface. The rear end of each center was supported by wedges securely fastened to the outer end of the preceding section, the forward end being supported by a screw jack.
1 Engineering News, March 6, 1902.
After turning the arch, these centers were removed by the aid of a special truck the axles of which were bent at such an angle as to make the cast iron wheels fit the concrete invert. The axle of a roller was first fastened to the outer rib of the center to be removed; the truck was then run back a foot or so under the center and the screw jack supporting the forward end of the center released. This allowed the forward end to drop a short distance, the roller resting on the running board of the truck. The latter was then pulled into the sewer far enough to let the roller run off the end of the truck and lock itself. The truck being then pulled out of the sewer toward the finished end, drew the center away from the wedges supporting the rear end, allowing the form to drop on the truck and be wheeled out of the sewer. By this method the centers were successfully removed without injuring the concrete.
From data given, the Cost of this sewer — about sixteen hundred feet in length — is approximately as follows, labor costing twenty-five cents an hour: — greased with machine oil. After the trench was prepared, concrete was placed and rammed until the top of the concrete was within about one-half inch of the flow line of the invert. To accomplish this, a straight edge was used, bearing on the finished invert in the rear and a template secured to the trench timbering just ahead of the section under construction.
The invert centers were then placed, resting on the finished invert at the rear and on a solid foundation accurately set to grade at the forward end. Mortar composed of equal parts Portland cement and sand was then tamped between the invert form and the bottom concrete already laid. When the flow line had been thus accurately formed, the center was braced and vertical planking set to form the outside of the walls. The concrete was then rammed in place.
Joists of two inch by four inch scantling laid along the center of the top of each side wall of the invert section, formed, when removed, a mortise into which the fresh concrete of the arch section was rammed to form a bond. Similar mortises were also made in the forward end of each section as built. After twenty-four hours or more the forms were removed, and a thin cement wash was applied to the interior, sufficient only to fill any slight imperfections in the surface.
The arch centers, similar in construction to the forms for the invert, were put in place and plastered with one inch of rich Portland mortar. Concrete was then placed sufficient to make the arch eight inches thick, the outside of the walls being formed by inclined boards braced to the trench, and the top of the extrados was formed by hand.
Two novel types of centering have been devised, in which the surface next the concrete is of steel. In one of these 1 the forms are in sections about three feet long. Two of the pieces of steel are of a width suitable to reach from the bottom of the sewer to just above the spring line of the arch, while a third piece forms the arch center. The strips are bent at an acute angle at the sides, thus projecting into the sewer along an element of the surface where the plates join ; the two sides of adjacent plates, which flare away from each other, are then connected by a continuous U-shaped clip of steel slipped on from the end of a three foot section, and the intervening space in the clip filled with clay or melted paraffin. The form is assembled outside the trench, and after the paraffin is in place, the center may be handled. When the sewer is completed, the paraffin is melted by a suitable heater, or the clay is washed out, and the form may be collapsed and removed.
1 Engineering Record, Jan. 9, 1904.
676. In the other form 1 the steel plates are in continuous strips about six inches wide and are applied by setting up the wooden form on an improvised axis, revolving the form and wrapping the steel sheet about it as it is revolved. The wooden form is in two parts, upper and lower, firmly connected while in use, but the two parts may be made to approach each other by driving out the wedges between them. After the winding, the center, with its sheet steel jacket, is lowered into the trench. When the concrete is completed, the form is collapsed and removed, leaving the spiral of steel in place to support the concrete until the latter is well set. The steel is then removed by simply pulling on one end. As it comes away from the concrete it is wound into a coil, and is then ready to be rewound on the wooden form. Both of the above styles have been patented.