The permissible intensities of pressure on foundations in various classes of material allowed by the building codes in different cities are given in Table 25. These figures are based on the assumption that the material is restrained laterally, which is generally the condition in sewer construction. In the softer materials it becomes necessary to spread the foundations not only to reduce the intensity of pressure, but also to care for the thrust of the sewer arch. The arch thrust may be found by one of the methods of arch analysis, and the haunches spread to care for this, or the sewer invert may be transversally reinforced to assist in caring for this action. Some sewer sections in hard and soft material are shown in Fig. 22 and 23. TABLE 25 ALLOWABLE BEARING VALUE ON SOILS IN VARIOUS CITIES From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906 ─────────────────────────┬───────────────────────────────────────────── Quicksand and alluvial │½ to 1 ton per sq. ft. for Providence, R. I., soil │ ½ ton per sq. ft. for 6 cities ─────────────────────────┼───────────────────────────────────────────── Soft clay │1 ton per sq. ft. for 27 cities, ¾ ton per │ sq. ft. for New Orleans, 2 to 3 tons for │ Providence, R. I. ─────────────────────────┼───────────────────────────────────────────── Moderately dry clay and │2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½ fine sand, clean and │ tons for Louisville, 2 to 4 tons for dry │ Providence, 3 tons for Grand Rapids and Los │ Angeles ─────────────────────────┼───────────────────────────────────────────── Clay and sand in │2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3 alternate layers │ to 5 tons for Providence ─────────────────────────┼───────────────────────────────────────────── Firm and dry loam or │3 tons for 24 cities, 2½ tons for 2 cities, 2 clay, or hard dry clay │ to 3 tons for Atlanta, 3½ tons for or fine sand │ Philadelphia, 4 tons for 6 cities ─────────────────────────┼───────────────────────────────────────────── Compact coarse sand, │4 tons for 25 cities, 3½ tons for Buffalo, 3 stiff gravel or natural│ to 4 tons for Atlanta, 4 to 5 tons for earth │ Cincinnati, 5 tons for Denver, 4 to 6 tons │ for 3 cities, 6 tons for Rochester, N. Y. ─────────────────────────┼───────────────────────────────────────────── Coarse gravel, stratified│6 tons for 3 cities, 5 tons for 2 cities, 8 stone and clay, or rock│ tons for 1 city inferior to best brick │ masonry │ ─────────────────────────┼───────────────────────────────────────────── Gravel and sand well │8 tons for 5 cities, 6 tons for 2 cities, 8 cemented │ to 10 tons for 1 city