. Economic Papers. t of concrete about 6 inches thick. In small concrete•culverts the footing is frequently made solid across the entire bottom ofthe culvert, as in Plate X, the bottom serving both to distribute theload and prevent scour. In larger structures, a cut-off wall, as shown inPlate XIY, is often used. When no provision is made to prevent scourthe side walls should be not less than 18 inches below the bed of thestream. (2) Frost, while a serious matter farther north, seldom penetratesdeeper than 6 inches in this State. Any footing carried below the sur-face of the ground 12 inches is
. Economic Papers. t of concrete about 6 inches thick. In small concrete•culverts the footing is frequently made solid across the entire bottom ofthe culvert, as in Plate X, the bottom serving both to distribute theload and prevent scour. In larger structures, a cut-off wall, as shown inPlate XIY, is often used. When no provision is made to prevent scourthe side walls should be not less than 18 inches below the bed of thestream. (2) Frost, while a serious matter farther north, seldom penetratesdeeper than 6 inches in this State. Any footing carried below the sur-face of the ground 12 inches is safe from frost. (3) Undermining at the outlet is generally caused by water droppingsome distance at the end, or the increased velocity of the water due tocontraction in passing through the culvert. This may be prevented bypaving the outlet with stone or concrete, or by the use of a baffle wall, asillustrated in Plate X. As a last expedient, if considerable wash has *From Handbook of Cost Data by H. P. 20 CULVERTS AND SMALL BRIDGES FOR already occurred, rock mixed with earth may be dumped in to preventfurther scour. Where the water must fall a considerable height the use of an outletdrop as illustrated in Fig. 5, p. 19, is advisable. COFFERDAMS. When foundations must be prepared under water, a cofferdam is gen-erally necessary to keep out the water while the footing is being the water is deep or the current swift these cofferdams are veryexpensive and the construction of each one is a separate problem whichshould not be attempted except under the direction of a competent engi-neer. However, in the construction of small culverts and bridges, thewater seldom exceeds 6 feet in depth, and where there is a mud bottom,free from boulders, little difficulty should be experienced in constructingcofferdams, Fig. 6, p. 21, illustrates a cofferdam which, under favorable conditions,will cost about $200 for a pier 25 feet long. The 6x 6 uprights shouldbe driven firs
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