A treatise on architecture and building construction . oting bed cut level over its entire surface, nor evencut into a series of horizontal surfaces resembling steps, asis frequently done in softer soils, which method costs a greatdeal of time and money; but it is necessaiy that the surfaceof the rock shall be so roughened that the possibility ofthe footing slipping on its foundation will be this is done, concrete may be put in to bring thefoundation to its proper level. When the structure is threeor four stories in height, stone or brick may be used in placeof concrete, but a
A treatise on architecture and building construction . oting bed cut level over its entire surface, nor evencut into a series of horizontal surfaces resembling steps, asis frequently done in softer soils, which method costs a greatdeal of time and money; but it is necessaiy that the surfaceof the rock shall be so roughened that the possibility ofthe footing slipping on its foundation will be this is done, concrete may be put in to bring thefoundation to its proper level. When the structure is threeor four stories in height, stone or brick may be used in placeof concrete, but a concrete base is usually consideredpreferable. §7 MASONRY. 27 FOOTINGS OX SLOPING GROUND. 55. Footing- courses built on slopes—especially of clay—?are always likely to slide; this may be avoided by cuttinghorizontal steps in the slope as shown in Fig. 17, where theslope ^^y is stepped off, as shown at a, in order that the foot-ings b inay have a horizontal bearing. These footings rhaybe of either stone or concrete, but when the former material. Fig. 17. is used, great care miist be exercised to secure a perfectbond at the stepping places, and the foundations should belaid in as long sections as possible. SPREAD FOOTINGS. 5G. It is often found that compressible soils, even allu-vium and soft clay, will bear from 1 to 2 tons per squarefoot with but little settlement, and under a steady load, thissettlement is in most cases uniform. It is very oftencheaper, therefore, to spread the foundation over a largearea, than it is to drive piles. la Chicago, for example, the subsoil is of blue clay, found from 5 to 8 feet below the street grade. This clay bed, when below the level of ground or drainage water, becomes quicksand or blue mud, and has a bearing capacity of only a little over 1 ton per square foot, so the heavy weights of the high buildings are carried on spread foundations. This2-3 28 MASONRY. plan is usually adopted in the Chicago soil, because thequicksand, or muck subst
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