Concrete Sewers

188. Construction in Open Cut.—In the construction of sewer pipe of cement and concrete one of two methods may be employed; 1st, to manufacture the pipe in a plant at some distance from the place of final use, or 2nd, to manufacture the pipe in place. The methods of the manufacture of cement and concrete pipe which are to be transported to the place of use are treated in Chapter VIII. The process of constructing the pipes in place is ordinarily used for pipes 48 inches or more in diameter. For smaller sizes, brick, vitrified clay, and precast cement pipes are usually more economical.

The preparation of the foundation of a concrete sewer is similar to that for a brick sewer. If the ground is suitable the trench is shaped to the outside form of the sewer and the concrete poured directly on it. In soft material which would give poor support to a sewer with a rounded exterior, the bottom of the trench is cut horizontal and a concrete cradle of poorer quality than that in the finished sewer is poured on the soft ground, on a board platform, on piles, or on cribbing supported on piles.

If the invert of the sewer is so flat that the concrete will stand without an inside form the shape of the invert is obtained by a screed or straight-edge which is passed over the surface of the concrete and guided on two centers, or on one center and the face of the finished work. The construction of a flat invert sewer at Baltimore is shown in Fig. 1. The center for the concrete is shown in the foreground. When the concrete for the next section is poured it will be smoothed to shape by a screed or straight-edge resting on the face of the finished concrete and the center. The center is shaped to conform to that of the finished concrete. It is firmly staked in position and acts as a bulkhead for the concrete as it is poured, as well as a guide for the screed.

TABLE 68
Rate of Progress on Brick Sewer Construction
(Based on 8–hour day)
Diameter of SewerShapeNumber Rings, BrickNumber MasonsBricks per Mason per DayNumber LaborersFeet Progress per DayLocationAuthorityRemarks
7′ 0″
8′ 11″		Circular and Oval2½647103960GaryGillette9–hour day
4′ 0″Circular232500 36 Metcalf and EddyGeneral average
6′ 8″Circular3 arch
1 invert		18 62 DenverGilletteConcrete invert
2′ 9″Egg1 arch
2 invert		2 3 Springfield, Mass.Eng. Con., Jan. 16, 1907
5′ 6″Circular26457035110GaryGillette
6′ 6″Circular 44800 GilletteExceptional speed
2′ 9″Circular222080513.9SyracuseGilletteTunnel 12–hour day
16′ 0″Circular585 cu. yd. 22ChicagoGilletteFirst year
16′ 0″Circular512 70–7535ChicagoGilletteSecond year
3′ 6″Egg 2300 St. LouisGillette
9′ 6″Circular 3000 12.5ChicagoH. R. Abbott
3′ 6″Circularblocks2 1330 Lock joint and tile. 10–hour day

If inside forms are to be used they are made as units in lengths of 12 or 16 feet for wooden forms, and 5 feet for steel forms. The inside form is supported by precast concrete blocks placed under it and which are concreted into the sewer. It is held in position by cleats nailed to the outside form, to the sheeting, or wedged against the outside of the trench. In some cases, particularly where steel forms are used, the inside form is hung by chains from braces across the trench as is shown in Fig. 129. The form is easily brought to proper grade by adjustment of the turnbuckles and is then wedged into position to prevent movement either sideways or upwards during the pouring of the concrete. It may be necessary to weight the forms down to prevent flotation. Cross bracing in the trench which interferes with the placing of the form is removed and the braces are placed against the form until the concrete is poured. They are removed immediately in advance of the rising concrete.

Fig. 129.—Blaw Standard Half Round Sewer Form, Suspended from Overhead Support.
Courtesy, Blaw Steel Form Co.

The sewer section may be built as a monolith, in two parts, or in three parts. In casting the sewer as a monolith the complete full round inside form is fixed in place by concrete blocks and wires. The full round outside form is completed as far as possible without interfering too much with the placing and tamping of the concrete. The concrete is poured from the top, being kept at the same height on each side of the form, and tamped while being poured. The remaining panels of the outside form are placed in position as the concrete rises to them. An opening is left at the top of the outside arch forms which is of such a width that the concrete will stand without support. The casting of sewers as a monolith is difficult and is usually undesirable because of the uncertainty of the quality of the work. It has the advantage, however, of eliminating longitudinal working joints in the sewers which may allow the entrance of water or act as a line of weakness.

Fig. 130.—Construction Joints for Concrete Sewers.

If the sewer is to be cast in two sections the invert is poured to the springing line or higher. A triangular or rectangular timber is set in the top of the wet concrete as shown in Fig. 130. When the concrete has set the timber is removed and the groove thus left forms a working joint with the arch. After the invert concrete has set, the arch centering is placed and the arch is completed. This is the most common method for the construction of medium-sized circular sewers.

Large sewers with relatively flat bottoms are poured in two or three sections. First the invert is poured without forms and is shaped with a screed. About 6 inches of vertical wall is poured at the same time. This acts as a support for the side-wall forms. The side walls reach to the springing line of the arch and are poured after the invert has set. At the third pouring the arch is completed. The sewer shown in Fig. 1 is being poured in two steps, as the side walls are so low that they are poured at the same time as the invert. A transverse working joint similar to one of the types used in Fig. 130 is set between each day’s work.

The length of the form used and the capacity of the plant should be adjusted so that one complete unit of invert, side wall, or arch can be poured in one operation. The forms are left in place until the concrete has set. Invert and side-wall forms are generally left in position for at least two days, and in cold weather longer. The arch forms are left in place for double this time. For example if 20 feet of invert and arch can be poured in a day, 60 feet of invert form and 100 feet of arch form will be required. As the forms are released they must be moved forward through those in place. For this reason collapsible or demountable forms are necessary and steel forms are advantageous. Wooden arch forms are sometimes dismantled and carried forward in sections, but are preferably designed to collapse as shown in Fig. 131, so that they can be pulled through on rollers or a carriage.

189. Construction in Tunnels.—In tunnels the invert and side walls are constructed in the same manner as for open cut work. The tunneling, which acts as the outside form, is concreted permanently in place. The concreting of a tunnel by hand is shown in Fig. 132. If the work is to be done by hand the concrete is thrown in between the ribs of the arch centering and behind the plates or lagging, which are set in advance of the rising concrete. The lagging plates are 5 feet long which makes it possible to throw the concrete in place at the arch, and to tamp it in place from the end. A bulkhead and a well-greased joint timber are placed in position as the concrete rises.

Fig. 131.—Section through a Collapsible Wood Form.

Pneumatic transmission of concrete is also used for filling the arch forms as well as the side walls and invert forms. In using this method the mixer may be placed at the surface or at the bottom of the shaft or other convenient permanent location which may be some distance from the form. The mixture is discharged into a pipe line through which it is blown by air to the forms. The starting pressure of about 80 pounds per square inch can be reduced after flow has commenced. In constructing the St. Louis Water Works tunnel the compressor equipment for moving the concrete had a capacity of 1,600 cubic feet per minute at a pressure of 110 pounds. The tunnel is horseshoe shaped, 8 feet in height and with walls varying from 9 to 20 inches in thickness. The extreme travel of the concrete was 1,100 feet in an 8 inch pipe. The amount of air consumed at 110 pounds varied from 1.2 to 1.7 cubic feet of free air per linear foot of pipe. By the time the batch had been discharged the pressure had reduced to 25 to 40 pounds, depending on the length of the pipe. It is reported that a 6–inch pipe line would probably have given better results.

Fig. 132.—Ogier’s Run Intercepting Storm-Water Drain, Baltimore, Maryland.
Placing concrete in Arch. The steel lagging of the forms is carried up in sections as the concrete is deposited. The drain is horseshoe shaped, and is 12 feet 3 inches high and 12 feet 3 inches wide.

The end of the concrete conveying pipe is provided with a flexible joint the simplest form of which can be made by slipping a section of pipe of larger diameter over the end of the transmission line. The concrete is deposited directly on the invert or into the side-wall forms and can be blown into the arch forms for 20 to 25 feet.

190. Materials for Forms.—The materials used in forms for concrete sewers are: wood, wood with steel lining, and steel alone. The first cost of wood forms is lower than that of steel but their life is relatively short. If the forms are to be used a number of times steel is more economical. With proper care and repairs steel forms will outlast any other material. Because of the increasing price of lumber and improvements in steel forms, wood forms are not frequently used. A common type of specification under which forms are used is:

The material of the forms shall be of sufficient thickness and the frames holding the forms shall be of sufficient strength so that the forms shall be unyielding during the process of filling. The face of the form next to the concrete shall be smooth. If wooden forms are used the planking forming the lining shall invariably be fastened to the studding in horizontal lines, the ends of these planks shall be neatly butted against each other, and the inner surface of the form shall be as nearly as possible perfectly smooth, without crevices or offsets between the ends of adjacent planks. Where forms are used a second time, they shall be freshly jointed so as to make a perfectly smooth finish to the concrete. All forms shall be water-tight and shall be wetted before using.

Any material in contact with wet concrete should be oiled or greased beforehand in order to prevent adherence to the concrete.

191. Design of Forms.—The design of forms for reinforced concrete work requires some knowledge of the strength of materials and the theories of beams, columns, and arches. Forms can be constructed without such knowledge but that they will be both economical and adequate is an improbability. The ordinary beam and column formulas are applicable to the design of forms. The maximum bending moment for sheeting and ribs is taken as wl2
8, where w is the load per unit length, and l is the length between supports. Sanford Thompson recommends that the deflection be calculated as wl3
128EI, in which E is the modulus of elasticity of the material, and I is the moment of inertia of the cross-section referred to the neutral axis. The horizontal pressure of the concrete against the forms has been expressed empirically by E. B. Smith,[[101]] as

P = H0.2R0.3 + 120C − 0.3S

in which P = lateral pressure in pounds per square inch; R = rate of filling forms in feet per hour; H = head of fill. Ordinarily taken as ½R, but in cold weather or when continuously agitated it may be as high as ¾R; C = ratio, by volume, of cement to aggregate; S = consistency in inches of slump.

Earlier investigators have usually concluded that the pressures were equal to those caused by a liquid weighing 144 pounds per cubic foot, but the tests of the United States Bureau of Public Roads, from which the above formula was devised, show the pressures to be decidedly below this amount under certain conditions.

Fig. 133.—Centering for Large Forms.

With these units and formulas the design of the lagging becomes a matter of substitution in, and the solution of, the equations produced.[[102]] The forces acting on the ribs are indeterminate. No more satisfactory design can be made for the ribs than to follow successful practice, or what is seldom done, to determine the stresses in the forms by the application of one of the theories for the solution of arch stresses. The sizes of the lumber used in the ribs varies from 1½ × 6 inches to 2 × 10 inches, depending on the size of the sewer. If vertical posts are used at the ends to support the arch forms they are computed as columns taking the full weight of the arch. If the span is so wide that radial supports are used as shown in Fig. 133 the load at the center is assumed as one-fourth of the weight of the arch.

192. Wooden Forms.—Norway and Southern pine, spruce, and fir are satisfactory for form construction. White pine is satisfactory but is generally too expensive. The hard woods are too difficult to work. The lumber should be only partly dried as kiln-dried lumber swells too much when it is moistened, warping the forms out of shape or crushing the lagging at the joints. Green lumber must be kept moist constantly to prevent warping before use and when it is used it does not swell enough to close the cracks. The lumber should be dressed on the face next to the concrete and at the ends. Either beveled or matched lumber may be used for lagging. The joint made by beveled lumber shown in Fig. 134 is cheaper but less satisfactory than a tongued and grooved joint.

Fig. 134.—Beveled Joint for Wood Fords.Fig. 135.—Collapsible Wooden Invert Form for Concrete Sewers.Fig. 136.—Support for Arch Centering.

Fig. 137.—Wooden Forms Used in Tunnel, North Shore Sewer, Sanitary District of Chicago.
Journal Western Society of Engineers, Vol. 22, p. 385.

Types of wooden forms are shown in Figs. 135 and 136 for use in sewers to be built as monoliths or in two portions. Fig. 137 shows the details of a built-up wooden form used in tunnel work for a 42½ inch egg-shaped sewer.

193. Steel-lined Wooden Forms.—Sheet metal linings are sometimes used on wooden forms. They permit the use of cheaper undressed lumber, demand less care in the joining of the lagging, and when in good condition give a smooth surface to the finished concrete. Their use has frequently been found unsatisfactory and more expensive than well-constructed wooden forms because of the difficulty of preventing warping and crinkling of the metal lining and in keeping the ends fastened down so that they will not curl. Sheet steel or iron of No. 18 or 20 gage (0.05 to 0.0375 of an inch) weighing 2 to 1½ pounds per square foot is ordinarily used for the lining.

Fig. 138.—Blaw Standard Full Round Telescopic Sewer Forms, Showing Knocked-Down Sections Loaded on a Truck.
Courtesy, Blaw Steel Form Co.

194. Steel Forms.—These are simple, light, durable, and easy to handle. The engineer is seldom called upon to design these forms as the types most frequently used are manufactured by the patentees and are furnished to the contractor at a fixed rental per foot of form, exclusive of freight and hauling from the point of manufacture. The forms can be made in any shape desired, the ordinary stock shapes such as the circular forms being the least expensive. The smaller circular forms are adjustable within about 3 inches to different diameters so that the same form can be used for two sizes of sewers. The same form can be used for arch and invert in circular sewers. Fig. 138 shows the collapsible circular forms and the manner in which they are pulled through those still in position. Fig. 129 shows a half round steel form swung in position by chains and turnbuckles from the trench bracing, and Fig. 139 shows the free unobstructed working space in the interior of some large steel forms.

Fig. 139.—Interior of Steel Forms for Calumet Sewer, Chicago.
Sewer is 16 feet wide. Note absence of obstructions. Courtesy, Hydraulic Steelcraft Co.

195. Reinforcement.—It is essential that the reinforcement be held firmly in place during the pouring of the concrete. A section of reinforcement misplaced during construction may serve no useful purpose and result in the collapse of the sewer. In sewer construction a few longitudinal bars may be laid in order that the transverse bars may be wired to them and held in position by notches in the centering and in fastenings to bars protruding from the finished work. This construction is shown in Fig. 1. The network of reinforcement is held up from the bottom of the trench by notched boards which are removed as the concrete reaches them, or better by stones or concrete blocks which are concreted in. Sometimes the reinforcement is laid on top of the freshly poured portion of the concrete the surface of which is at the proper distance from the finished face of the work. This method has the advantage of not requiring any special support for the reinforcement, but it is undesirable because of the resulting irregularity in the reinforcement spacing and position.

In the side walls the position of the reinforcement is fixed by wires or metal strips which are fastened to the outside forms or to stakes driven into the ground. Wires are then fastened to the reinforcement bars and are drawn through holes in the forms and twisted tight. When the forms are removed the wires or strips are cut leaving a short portion protruding from the face of the wall. The reinforcing steel from the invert should protrude into the arch or the side walls for a distance of about 40 diameters in order to provide good bond between the sections. The protruding ends are used as fastenings for the new reinforcement. The arch steel may be supported above the forms by specially designed metal supports, by small stones or concrete blocks which are concreted into the finished work; or by notched strips of wood which are removed as the concrete approaches them. Strips of wood are not satisfactory because they are sometimes carelessly left in place in the concrete resulting in a line of weakness in the structure. Metal chairs are the most secure supports. They are fastened to the forms and the bars are wired to the chairs. In some instances the entire reinforcement has been formed of one or two bars which are fastened into position as a complete ring. This results in a better bond in the reinforcement, requires less fastening and trouble in handling, but is in the way during the pouring of the concrete and interferes with the handling of the forms.

196. Costs of Concrete Sewers.—Under present day conditions a general statement of the costs of an engineering structure can not be given with accuracy. Only the items of labor, materials, and transportation that go to make up the cost can be estimated quantitively, and the total cost computed by multiplying the amount of each item by its proper unit cost obtained from the market quotations.

A summary of some of the items that go to make up the cost of a concrete sewer and the relative amount of these items on different jobs is given in Tables 69 and 70.

TABLE 69
Division of Labor Costs For the Construction of 96–inch Circular Concrete Sewer
Classification of LaborClassification of Work
Task or TitleNumber of menTotal dollars per dayType of WorkDollars per foot
Superintendent16.00Excavation1.80
Engineman13.50Sheeting and bracing0.58
Hoister (engineman)12.00Bottom plank0.17
Tag-men23.30Pulling sheeting0.45
Earth diggers1016.50Backfilling0.33
On dump cars23.30Making and placing invert1.17
Carpenter on bracing23.00Making and placing arch1.54
Carpenters’ helpers23.30Laying brick in invert0.29
Laying bottom23.30
Moving pumps, etc.23.30Bending and placing steel in arch0.20
Pulling sheeting35.25
Mixing and placing concrete1626.40Bending and placing steel in invert0.09
On steel forms35.25Moving forms and centers0.62
Water boy11.00Watchmen, water boy, etc.0.62
Coal and oil 5.00
Total 90.40Total7.86
Notes.—Trench was 12½ feet wide and of various depths. At depth of 12 feet the cost of excavation was $1.61 per foot. From Engineering and Contracting, Vol. 47, p. 157.