197. Methods.—Careful backfilling is necessary to prevent the displacement of the newly laid pipe and to avoid subsequent settlement at the surface resulting in uneven street surfaces and dangers to foundations and other structures.
The backfilling should commence as soon as the cement in the joints or in the sewer has obtained its initial set. Clay, sand, rock dust, or other fine compactible material is then packed by hand under and around the pipe and rammed with a shovel and light tamper. This method of filling is continued up to the top of the pipe. The backfill should rise evenly on both sides of the pipe and tamping should be continuous during the placing of the backfill. For the next 2 feet of depth the backfill should be placed with a shovel so as not to disturb the pipe, and should be tamped while being placed, but no tamping should be done within 6 inches of the crown of the sewer. The tamping should become progressively heavier as the depth of the backfill increases. Generally one man tamping is provided for each man shoveling.
| TABLE 70 | |||||||
|---|---|---|---|---|---|---|---|
| Division of Costs For the Construction of Concrete Sewers | |||||||
| Gillette’s Handbook of Cost Data. | |||||||
| Item | Location | ||||||
| Fond du Lac | South Bend | Wilmington | Richmond, Indiana | ||||
| Diameter in inches | 30 | 66 | 53 | 54 | 48 | 42 | |
| Shape | circular | circular | horseshoe | circular | circular | circular | |
| Plain or reinforced | plain | rein. | rein. | rein. | rein. | rein. | |
| Cubic yards per foot | 0.11 | 0.594 | 0.37 | 5″ shell | 5″ shell | 4″ shell | |
| Daily progress, feet | 47 | 24 to 36 | |||||
| Cost per foot, dollars | 1.20 | 4.40 | 2.97 | 1.35 | 1.08 | 0.91 | |
| Per cent of total cost: | |||||||
| Labor | 39.0[[103]] | 33.5 | 33.0 | 17.1 | |||
| Tools | 1.5 | 11.5 | |||||
| Sand and gravel | 12.4 | 15.5 | 18.9 | 19.3 | |||
| Lumber | 0.9 | ||||||
| Water | 0.7 | 11.5 | |||||
| Reinforcing | 0.0 | 14.5 | 22.3 | ||||
| Cement | 23.0 | 20.0 | 27.5 | 32.0[[104]] | |||
| Frost prevention | 2.0 | ||||||
| Forms | 12.5 | 8.0 | 6.1 | 9.3 | |||
| Engineering | 8.0 | ||||||
| Length of day, hours | 8 | 10 | |||||
| Year of construction | 1908 | 1906 | Pre-war conditions | ||||
Above a point 2 feet above the top of the sewer the method pursued and the care observed in backfilling will depend on the character of the backfilling material and the location of the sewer. If the sewer is in a paved street the backfill is spread in layers 6 inches thick and tamped with rammers weighing about 40 pounds with a surface of about 30 square inches. One man tamping for each man shoveling is frequently specified. If no pavement is to be laid but it is required that the finished surface shall be smooth, slightly less care need be taken and only one man tamping is specified for each two men shoveling. On paved streets a reinforced concrete slab with a bearing of at least 12 inches on the undisturbed sides of the trench may be designed to support the pavement and its loads. This is of great help in preventing the unsightly appearance and roughness due to an improperly backfilled trench. On unpaved streets the backfill is crowned over the trench to a depth of about 6 inches and then rolled smooth by a road roller. In open fields, in side ditches, or in locations where obstruction to traffic or unsightliness need not be considered, after the first 2 feet of backfill have been placed with proper care, the remainder is scraped or thrown into the trench by hand or machine, care being taken not to drop the material so far as to disturb the sewer.
If the top of the sewer, manhole, or other structure comes close to or above the surface of the ground, an earth embankment should be built at least 3 feet thick over and around the structure. The embankment should have side slopes of at least 1½ on 1 and should be tamped to a smooth and even finish.
If sheeting is to be withdrawn from the trench it should be withdrawn immediately ahead of the backfilling, and in trenches subject to caving it may be pulled as the backfilling rises.
Puddling is a process of backfilling in which the trench is filled with water before the filling material is thrown in. It avoids the necessity for tamping and can be used satisfactorily with materials that will drain well and will not shrink on drying. Sand and gravel are suitable materials for puddling, heavy clay is unsatisfactory. Puddling should not be resorted to before the first 2 feet of backfill has been carefully placed. More compact work can be obtained by tamping than with puddling.
Frozen earth, rubbish, old lumber, and similar materials should not be used where a permanent finished surface is desired as these will decompose or soften resulting in settlement. Rocks may be thrown in the backfill if not dropped too far and the earth is carefully tamped around and over them. In rock trenches fine materials such as loam, clay, sand, etc., must be provided for the backfilling of the first portion of the trench for 2 feet over the top of the pipe. More clay can generally be packed in an excavation than was taken out of it, but sand and gravel occupy more space than originally even when carefully tamped.
Tamping machines have not come into general use. One type of machine sometimes used consists of a gasoline engine which raises and drops a weighted rod. The rod can be swung back and forth across the trench while the apparatus is being pushed along. It is claimed that two men operating the machine can do the work of six to ten men tamping by hand. The machine delivers 50 to 60 blows per minute, with a 2 foot drop of the 80 to 90 pound tamping head.
Backfilling in tunnels is usually difficult because of the small space available in which to work. Ordinarily the timbering is left in place and concrete is thrown in from the end of the pipe between the outside of the pipe and the tunnel walls and roof. If vitrified pipe is used in the tunnel, the backfilling is done with selected clayey material which is packed into place around the pipe by workmen with long tamping tools. The backfilling should be done with care under the supervision of a vigilant inspector in order that subsequent settlement of the surface may be prevented.