. Railroad construction. Theory and practice . me uneven-ness of grade is inequitable through uneven settlement or im-perfect construction, a grade of 1 in 20 should preferably berequired, although much less is often used. The length of apipe culvert is approximately determined as follows: Length = 2s (depth of emhankment to top of pipe) + (width of roadbed), in which s is the slope ratio (horizontal to vertical) of the practice an even number of lengths will be used which willmost nearly agree with this formula. 186. Iron-pipe culverts. Simple cast-iron pipes are used insizes from 12
. Railroad construction. Theory and practice . me uneven-ness of grade is inequitable through uneven settlement or im-perfect construction, a grade of 1 in 20 should preferably berequired, although much less is often used. The length of apipe culvert is approximately determined as follows: Length = 2s (depth of emhankment to top of pipe) + (width of roadbed), in which s is the slope ratio (horizontal to vertical) of the practice an even number of lengths will be used which willmost nearly agree with this formula. 186. Iron-pipe culverts. Simple cast-iron pipes are used insizes from 12 to 48 diameter. These are usually made inlengths of 12 feet with a few lengths of 6 feet, so that am^ requiredlength may be more nearly obtained. The lightest pipes madeare sufficiently strong for the purpose, and even those whichwould l)e rejected because of incapacity to withstand pressuremay be utilized for this work. In Fig. 97 are shown the stand-ard plans used on the C. C. C. & St. L. Ry., which may be con-sidered as typical \ ^ \ \ ^o \ <M \ CM \ /^.^-^ r7 t \ / vf / cvr / / / k-T^/i -a-HO/ Fig. 97.—Standard Cast-ironPipe Culvert. C. C. C. &St. L. Ry. (May 1893.) § 187. CULVERTS AND MINOR BRIDGES. 205 Pipes formed of cast-iron segments have been used up to 12feet diameter. The shell is then made comparatively thin, butis stiffened by ribs and flanges on the outside. The segmentsbreak joints and are bolted together thiough the flanges. Thejoints are made tight by the use of a tarred rope, together withneat cement. 187. Tile-pipe culverts. The pipes used for this purposevary from 12 to 24 in diameter. When a larger capacity isrequired two or more pipes may be laid side by side, but insuch a case another design might be preferable. It is frequentlyspecified that ^double-strength or extra-heavy pipe shallbe used, evidently with the idea that the stresses on a culvert-pipe are greater than on a sewer-pipe. But it has b
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