ART. 41. SEWERS BUILT IN PLACE Sewers more than 5 feet in diameter are generally built in the trench; concrete, brick, stone or special forms of clay-block being the materials commonly used; although pipes (made outside the trench) have been used of diameters up to 7 feet, and sewers have been built in place that were smaller than 5 feet. (Sewers as small as 10 or 12 inches have been built in place by means of a movable inner and outer form, but this system has never come into commercial use.) During the past twenty years concrete has been used more than any other material for construction in place, replacing brick, which had been used generally before that time. Concrete possesses the advantages over brick that it does not require so much skilled labor, and is generally cheaper. Also, it can be made to any desired form. When well made it is equal if not superior to the best hard-burned sewer brick in resistance to abrasion in inverts. Concrete inverts twenty years old and more can be cited that show no appreciable wear under conditions that made it necessary to renew brick inverts in six or seven years. Carelessness in constructing concrete, however, can easily produce inferior work, causing infiltration of ground water and occasionally collapse. Concrete should consist of such proportions of aggregate as to secure maximum density and strength, the materials being thoroughly mixed and placed in the forms with the thickest consistency that will permit the concrete to fill them completely. The invert, especially, should be well mixed and troweled down smooth. In perhaps the majority of cases, concrete sewers are strengthened by the use of steel reinforcement. Large sewers have been made of almost every conceivable shape. A few of the shapes that have been used are adapted to peculiar circumstances only, and some have been freaks of invention adapted to no circumstance. For a given perimeter of cross-section, the circle gives a larger area than any other form; consequently if the thickness of shell be fixed, a sewer of a given capacity requires for construction the least material if circular. Brick sewers were gen erally made circular, or sometimes egg-shaped or oval. Some conditions or features of construction, however, may make it more economical to employ a different shape, the saving in labor, in foundation material, etc., more than offsetting the additional amount of material in the sewer barrel. For example, in some cases limited head-room necessitates a flatter shape than a circle. In designing a section, full consideration should be given to the requirements of minimum flow (dry-weather or house-sewage flow, if a combined sewer), and the invert so made as to insure ample velocity. Where the soil is not stable, the circular shape is generally not practicable, since neither the horizontal nor the vertical thrust of the arch can be sufficiently resisted by the ground. In such a case, the bottom may be made flat or approximately so, and side-walls vertical on the inside and battering on the outside may be used to take the thrust of the arch; or vertical side-walls may be covered with a flat roof of reinforced concrete slab construction, or of steel-beam and arch construction. Where the head-room is limited, the sewer may be divided into two or three circular sewers, or may change its section to a wide and shallow sewer with a flat roof. In some cases, one or two longitudinal walls in a wide sewer practically divide it into two or three sewers having a continuous flat roof. Such construction may be used under railroad tracks or other places where considerable weight is applied to a sewer, heavy sidewalls and a heavy roof securing an adequate strength of construction at less cost than would be practicable using a circular sewer. Where the sewer is built in tunnel, economy is generally effected by making the shape of the sewer as nearly as practicable that of the tunnel. This may give a section approximately rectangular in form where the sewer is through ground requiring the use of poling-boards or other tunneling methods giving a flat roof. Where the sewer must be supported on piles and a timber platform, owing to the poor support offered by the soil, the most economical construction would be one with a flat invert. Those at the left are for poor foundations and heavy loads. The left-hand sides of the right-hand sections are for pile foundations; the right-hand sides for firm soil. |