=47. Surface Profile.=—A profile of the surface of the ground along the proposed lines of the sewers should be drawn after the completion of the computations for quantity. An example of a profile is shown in Fig. 26 for the line between manholes No. 3.5 and No. 147. The vertical scale should be at least 10 times the horizontal. A horizontal scale of 1 inch to 200 feet can be used where not much detail is to be shown, but a scale of one 1 to 100 feet is more common and more satisfactory and even one inch to 10 feet has been used. The information to be given and the method of showing it are illustrated on Fig. 26. The profile should show the character of the material to be passed through and the location of underground obstacles which may be encountered. The method of obtaining this information is taken up in Chapter II. The collection of the information should be completed as far as possible previous to design, and borings and other investigations made as soon as the tentative routes for the sewers have been selected.
=48. Slope and Diameter of Sewers.=—After the quantity of sewage to be carried has been determined, and the profile of the ground surface has been drawn, it is possible to determine the slope and diameter of the sewer. A table such as No. 20 is made up somewhat similar to No. 19, or which may be an extension of Table 19 since the first 6 columns in both tables are the same. The elevation of the surface at the upper and lower manholes is read from the profile.
The depth of the sewer below the ground surface is first determined. Sewers should be sufficiently deep to drain cellars of ordinary depth. In residential districts cellars are seldom more than 5 feet below the ground surface. To this depth must be added the drop necessary for the grade of the house sewer. Six-inch pipe laid on a minimum grade of 1.67 per cent is a common size and slope restriction for house drains or sewers. An additional 12 inches should be allowed for the bends in the pipe and the depth of the pipe under the cellar floor. Where the elevation of the street and lots is about the same, and the street is not over 80 feet in width between property lines, a minimum depth of 8 feet to the invert of sewers, 24 inches or less in diameter is satisfactory. This is on the assumption that the axes of the house drain and the sewer intersect. For larger pipes the depth should be increased so that when the street sewer is flowing full, sewage will not back up into the cellars or for any great distance into the tributary pipes.