Brick and Block Sewers
183. The Invert.—In good firm ground the excavation is cut to the shape of the sewer and the bricks are laid directly on the ground, being embedded in a thick layer of mortar. After the foundation has been prepared and before the bricks are laid, two wooden templates, called profiles, are prepared, similar to that shown in Fig. 126, to conform to the shape of the inside and outside of the sewer. Each course of bricks is represented by a row of nails in the profile and each nail corresponds to a joint in the row. The two profiles are set true to line and grade. A cord is stretched tightly between the two lowest nails on opposite templates and a row of bricks is laid. The bricks are laid radially and on edge with their long dimension parallel to the axis of the sewer and with one edge just touching the string. As each one or two or three rows are completed the guide line is moved up to the next nails. When the bricks are laid on the ground all but large depressions are filled in with tamped sand or mortar by the masons. Approximately the same number of rows of bricks is kept completed on either side of the center line. The succeeding courses follow within three to five rows of each other, the only bond between courses being the mortar joint. This is called row lock bond and with few exceptions has been used on all brick sewers in the United States. As the sides of the sewer become higher during the construction, platforms must be built for the masons. These platforms are built of wood and rest directly on the green brickwork. They should be designed to spread the load as much as possible. The brickwork of the invert is continued up in this way to the springing line. As soon as one section is completed one profile is moved 10 to 20 feet ahead along the trench according to the standard length of sections, and set in position. The line is then strung from it to nails driven or pushed into the cement joints of the last completed section. Between work done on separate days the bricks are racked back in courses to provide a satisfactory bond.
Fig. 126.—Profile for Brick Sewers.
In ground too soft to support the brickwork directly a cradle is prepared by placing profiles in position in the sewer and nailing 2–inch planks to these profiles, first firmly tamping earth under the planks. The bricks are laid in this cradle in a manner similar to that explained for sewers with a firm foundation. In still softer ground it may be necessary to construct a concrete cradle to support the bricks.
184. The Arch.—The arch centering consists of a wooden form made up of wooden ribs as shown in Fig. 127. The center conforms to the shape of the inside of the arch with allowance for the thickness of the lagging. The lagging is nailed on the ribs in straight strips parallel to the axis of the sewer. The center is supported on triangular struts resting against the sides and on the bottom of the sewer and is lifted into position by wedges driven between it and the support. The centers may be placed immediately after the completion of the invert, or a day or two may be allowed to pass to give the invert an opportunity to set. After the centers are fixed in place the arch brick are carried up evenly on each side and are pounded firmly into place. The center is usually, but not always “struck” immediately, and the arch brick are cleaned and pointed up from the inside. The outside is covered with a layer of ¼ to ¾ of an inch of cement mortar and may be backfilled to the top of the arch in order to maintain the moisture of the mortar during setting and to press the bricks of the arch together firmly. The centers are sometimes made collapsible so that they can be carried or rolled through the finished brickwork to the advanced position. In “striking” the centers the wedges are removed and the wings folded in.
Fig. 127.—Centering for Brick Sewer.
In tunneling, the invert of the sewer is constructed in the same fashion as for open cut work. The arch centering is made in short sections and the bricks are put in position by reaching in over the end of the centering. All of the timbering of the tunnel is removed except the poling boards or lagging against which the bricks or mortar are tightly pressed, the boards being bricked in permanently.
185. Block Sewers.—Sewers made of unit blocks of concrete or vitrified clay are constructed in a similar manner to brick sewers. Fig. 128 shows the construction of a block sewer at Clinton, Iowa. In this sewer there are two rings; an inside one of solid blocks and an outside one of hollow blocks. Block sewers do not demand the skill in construction that is demanded by brick sewers, as the blocks are so cast that the joints are radial, whereas only experienced masons can lay bricks radially.
Fig. 128.—Segmental Block Sewer at Clinton, Iowa.
186. Organization.—The number of men employed on a brick or block sewer is proportioned according to the size of the sewer and the working conditions. The number of men working on different tasks usually bears the same ratio to the number of masons employed, regardless of the size of the work. These proportions are shown for different jobs, in Table 66.
| TABLE 66 | ||||||
|---|---|---|---|---|---|---|
| Organizations for the Construction of Brick and Block Sewers | ||||||
| Type of Work | General Ratio on Basis of Four Brick Layers | 15–foot, 5–ring Brick, Chicago | 66–inch Circular Brick, Gary | 84–inch Circular Brick, Gary | 84– to 108–inch Sewer Brick in Detroit Tunnel | 42–inch Lock-Joint Tile Block |
| Foreman | 1 | 1 | 1 | 1 | 1 | 1 |
| Brick layers | 4 | 12 | 6 | 6 | 5 | 2 |
| Helpers | 2 | 11 | 3 | 3 | 1 | |
| Scaffold men | 2 | 21 | 3 | |||
| Brick tossers | 2 | 7 | 15 | 2 | ||
| Brick carriers | 2 | 2 | 2 | |||
| Cement mixers | 2 | 6 | 6 | 5 | 1 | |
| Cement carriers | 2 | 10 | 8 | |||
| Form setters | 1 | 3 | 3 | |||
| Laborers | 1 | 8 | 19 | 3 | 14 | 7 |
| Source of Information | Municipal Engineering, Vol. 54, p. 228 | H. P. Gillette, Handbook of Cost Data | ||||
187. Rate of Progress.—In a general way it can be assumed that the laying of 1,000 bricks will require 3⅓ hours of the time of one mason, 10 man-hours for helpers and laborers, 2 barrels of cement, 0.6 cubic yard of sand, and about 10 feet board measure of centering. One thousand bricks will make about 2 cubic yards of brickwork. To the costs, as estimated on the basis of materials and labor, must be added about 15 per cent for overhead and an additional amount for the contractor’s profit. The number of bricks required in various size sewers is shown in Table 67. A mason can lay more bricks per hour in a large sewer than in a small one as there is a smaller percentage of face work, there is more room to work, and it is easier to lay the bricks radially. The number of bricks laid and the rate of progress on various jobs are shown in Table 68.
| TABLE 67 | ||||
|---|---|---|---|---|
| Brick Masonry in Circular Sewers. Cubic Yards per Linear Foot | ||||
| (From H. P. Gillette) | ||||
| Diameter, Feet and Inches | One Ring (4½ Inches) | Two Ring (9 Inches) | Three ring (13½ Inches) | |
| 2 | 0 | 0.103 | 0.240 | |
| 2 | 6 | 0.125 | 0.280 | |
| 3 | 0 | 0.147 | 0.327 | |
| 3 | 6 | 0.169 | 0.371 | |
| 4 | 0 | 0.191 | 0.415 | |
| 4 | 6 | 0.213 | 0.458 | |
| 5 | 0 | 0.234 | 0.501 | 0.802 |
| 5 | 6 | 0.256 | 0.545 | 0.867 |
| 6 | 0 | 0.278 | 0.589 | 0.933 |
| 6 | 6 | 0.633 | 1.000 | |
| 7 | 0 | 0.677 | 1.063 | |
| 7 | 6 | 0.720 | 1.128 | |
| 8 | 0 | 0.763 | 1.193 | |
| 8 | 6 | 0.807 | 1.260 | |
| 9 | 0 | 0.851 | 1.325 | |
| 9 | 6 | 0.895 | 1.390 | |
| 10 | 0 | 0.938 | 1.456 | |