. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . ope is nearly or quite 1^ : difficulty of incorporating the added material with the oldembankment and preventing its sliding off frequently makesthese repairs disproportionately costly. 91. Compound sections. 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-sect
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . ope is nearly or quite 1^ : difficulty of incorporating the added material with the oldembankment and preventing its sliding off frequently makesthese repairs disproportionately costly. 91. Compound sections. 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- § 92. EARTHWORK. 107 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. Aberniof about three feet should be left on the edges of the rock cut as. Fig. 44. 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 figuremust be determined by a method similar to that developed forborrow-pits (see § 120). 02. Width of- roadbed. Owng to the large and often dis-proportionate addition to volume of cut or fill caused by theaddition of even one foot to the width of roadbed, there is anatural tendency to reduce the width until embankments becomeunsafe and cuts are too narrow for proper drainage. The costof maintenance of roadbed is so largely dependent on the drain-age of the roadbed that there is true economy in making anample allowance for it. The practice of some of
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