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 If sheeting is to be withdrawn from the trench it should be withdrawn immediately ahead of the backfilling, and in trenches subject to caving it may be pulled as the backfilling rises.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.