. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . r quite IJ : diffi-culty of incorporating the added material with the oldembankment and preventing its sliding off frequently makesthese repairs disproportionately costly. 6i. Compound When the cut consists partly ofearth and partly of rock, a compound cross-section must bemade. If borings have been made so that the contour of therock surface is accurately known, then the true cross-section maybe determined. The rock and earth should be calculated sepa-rately,
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . r quite IJ : diffi-culty of incorporating the added material with the oldembankment and preventing its sliding off frequently makesthese repairs disproportionately costly. 6i. Compound When the cut consists partly ofearth and partly of rock, a compound cross-section must bemade. If borings have been made so that the contour of therock surface is accurately known, then the true cross-section maybe determined. The rock and earth should be calculated sepa-rately, and this will require an accurate knowledge of where therock runs out—a difficult matter when it must be deter- 68 RAILROAD CONSTRUCTION. § 62. mined by boring. During construction the center part of theearth cut would be taken out first and the cut widened until asufficient width of rock surface had been exposed so that therock cut would have its proper width and side slopes. Then theearth slopes could be cut down at the proper angle. A^bermof about three feet is usually left on the edges of the rock cut as. Fig. 41. a margin of safety against a possible sliding of the earth the work is done, the amount of excavation that has beenmade is readily computable, but accurate preliminary estimatesare difficult. The area of the cross-section of earth in the figUriemust be determined by a method similar to that developed for^borrow-pits (see § 89). 62. Width Off foadbed. Owing to the large and oftert dis-proportionate addition to volume of cut or fill caused by theaddition of even one foot to the width of foadbed, there is anatural tendency to deduce the width until embankments bec^omeUnsafe and c^uts afe too narrow foi* proper drainage. The cdstof maintenance of roadbed is so largely dependent bn thd drain-age of the roadbed that there is tni6 economy in making anample allowance for it. The practice of some of the leadingrailroads of the country in this respect is gi
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