VELOCITY AND DISCHARGE FOR EGG-SHAPED SEWERS (NEW FORM) FLOWING FULL (n= .015; Q in cubic feet per second; v in feet per second) 48 X 72 60 × 90 6.90 123.0 4.87 86.8 3.43 61.2 2.41 42.9 1.85 33.0 1.03 18.3 5.28 119.0 3.72 84.0 2.61 59.0 2.01 45.3 1.12 25.3 5.67 158.0 4.00 111.0 2.81 84.3 78.3 2.16 60.2 1.21 33.7 77.9 1.30 43.8 98.4 1.38 Egg-shaped sewers have a larger hydraulic radius than circular sewers when the flow is shallow; consequently, they reduce the likelihood of deposits. The general form of the cross-section of an egg-shaped sewer is shown in the accompanying illustration. The part above the line OO' is a semicircle, the part below the line 00' is formed by the three arcs DE, EG and GA, the arcs DE and GA having equal radii. It will be noticed that three different radii are used in constructing the figure; namely, CA = CB =CD=r for the upper semicircle, OD OE = O'G = O'A =ri for the two side arcs DE and GA, and C'E=C'F=C'G =ro for the lower arc EFG, commonly called the invert. The proportions of the different dimensions, as well as other useful data, are given in the accompanying table, and the discharge and velocities for various grades in the preceding table. ELEMENTS OF CROSS-SECTION OF EGG-SHAPED DIMENSIONS OF SEWER PIPES The standard lengths of sewer pipes are 2, 2, and 3 ft. The latter is the most desirable, because it reduces the number of joints in the pipe line. In diameter, they are made 4, 5, 6, 8, 9, 10, 12, 18, 21, and 24 in. Special sizes, such as 20, 24, 27, 30, and 36 in., are also carried by some factories. Thickness and Strength.-The practice of factories is to make pipe of two thicknesses, one known as standard pipe and the other known as double-strength pipe. The accompanying table shows the thickness that well-made pipe should have by the custom of the best factories. DEPTHS OF SOCKETS FOR STANDARD AND FOR DEEP-AND-WIDE SOCKET Kind of Socket Standard. Diameter, in Inches 6 8 9 10 12 15 18 21 24 30 36 1 2 Deep-and-wide... 2222 333 32 33 4 4 5 Tests indicate that standard pipe as made can carry a uniform load of about 2,000 lb. per lin. ft. of pipe, and doublestrength pipe, about 4,000 lb. The load that sewer pipes must carry is the weight of the earth in the trench above them, with the additional weight of a wagon wheel or a steam-roller wheel, either of which may add 1 T. loading to the pipe. A 12-in. pipe in an 8-ft. trench will have a mass of 1X8X1-8 cu. ft. of earth, or about 1,000 lb. with 2,000 lb. pressure on the top resting on it. Only a fraction of this loading, however, is transmitted to the pipe, the rest being supported by the sides of the trench. A factor of safety of 3 should be employed. It is safe practice to use double-strength pipe when the pipe is in a trench less than 6 ft. deep, and heavy surface loads may be expected. Under other conditions, standard pipe may be used, though double-strength pipe is always safer. Depth of Socket.-There are two types of socket, the standard and the deep-and-wide, or deep, socket. The depths of socket, in inches, are shown in the preceding table. The advantage of the deep-and-wide socket lies in the fact that the jointing material can be rammed into the sockets to a greater depth, and there is therefore less leakage through the joints. BRICK AND CONCRETE SEWERS Brick Sewers.-Sewers of a larger diameter than 24 in. are generally built of brick or concrete and can be made in any desired form. For ordinary conditions, the following empirical formula will generally be found satisfactory for indicating the number of rings required: in which R is the number of 4-in. rings or courses; D, the internal diameter of a circular sewer, or horizontal diameter of an egg-shaped sewer; and H, the total depth of the trench-all in feet. Any fraction greater than .25 in the value of R should be considered as 1. Concrete Sewers.-The concrete used for sewers should be of first-class quality, carefully proportioned to have as small a percentage of voids as possible. The concrete must be strong, to take up the tensile stresses in the arch; and impervious, to keep ground water out of the sewers. A mixture of 1-2-4 may be used for the arch, and a mixture of 1-24-5 for the bottom. The mixing must be very thorough, and the tamping into place carefully done. For sewer work, the mixture should be so wet that a spade can be readily thrust down into the mass to work the mixture into homogeneity. The thickness of circular concrete sewers built in firm and stable ground and at a depth not exceeding 12 ft. may be taken to be approximately as follows: This thickness must be varied, however, with the character of the soil and the depth of cutting. In wet, running soils, the lower half of the sewer may be from two to four times these thicknesses, with extra thickness at the sides. In trenches 30 ft. deep, the thickness of the arch may be twice the thickness given. ROADS AND PAVEMENTS HIGHWAYS GRADES, CROSS-SECTION, AND CURVES In order that a road may be satisfactory for travel, it must be dry and solid, and have easy grades, easy curves, and a smooth surface. These conditions refer to the use of the road, but there are other conditions that are essential to economic construction and maintenance; namely, (1) that the length of the road shall be a minimum; (2) that its surface shall be so placed with reference to the natural surface of the ground that the amount of excavation and embankment shall be a minimum; and (3) that it shall be so located as to be free from landslides, washouts, and snowdrifts. These different conditions often conflict with one another, and there is generally a great deal of difficulty in reconciling them. The question of cost |