. Railroad construction. Theory and practice . and connected at the center, will answer the purpose. 142. Abutments. When suitable stone for masonry is athand and a suitable subsoil for a foundation is obtainable withouttoo much excavation, a masonry abutment will be the an abutment would probably be used when masonry foot-ings for trestle bents were employed (§ 139, c). Another method is to construct a crib of 10Xl2 timber^ § 143. TRESTLES. 163 laid horizontally, drift-bolted together, securely braced andembedded into the ground. Except for temporary constructionsuch a method is gen
. Railroad construction. Theory and practice . and connected at the center, will answer the purpose. 142. Abutments. When suitable stone for masonry is athand and a suitable subsoil for a foundation is obtainable withouttoo much excavation, a masonry abutment will be the an abutment would probably be used when masonry foot-ings for trestle bents were employed (§ 139, c). Another method is to construct a crib of 10Xl2 timber^ § 143. TRESTLES. 163 laid horizontally, drift-bolted together, securely braced andembedded into the ground. Except for temporary constructionsuch a method is generallyobjectionable on account ofrapid decay. Another method, used mostcommonly for pile trestles, andfor framed trestles having pilefoundations (§ 139, a), is to usea pile bent at such a place thatthe .natural surface on the up-hill side is not far below thecap, and the thrust of the material, filled in to bring the surfaceto grade, is insignificant. 3Xl2 planks are placed behindthe piles, cap, and stringers to retain the filled Fig. 79. FLOOR SYSTEMS. 143. Stringers. The general practice is to use two, three,and even four stringers under each rail. Sometimes a stringeris placed under each guard-rail. Generally the stringers aremade of two panel lengths and laid so that the joints few roads use stringers of only one panel length, but this prac-tice is strongly condemned by many engineers. The stringersshould be separated to allow a circulation of air around themand prevent the decay which would occur if they were placedclose together. This is sometimes done by means of 2 planks,6 to 8 long, which are placed over each trestle bent. Severalbolts, passing through all the stringers forming a group andthrough the separators, bind them all into one solid construc-tion. Cast-iron ^^spools or washers, varying from 4 to fin length (or thickness), are sometimes strung on each bolt soas to separate the stringers. Sometimes washers are usedbetween the separating
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