. Railroad construction. Theory and practice . anthat through the mortise, so that the cap will be drawn downtight when the pin is driven. Occasionally an iron dowel (aniron pin about Ih in diameter and about 6 long) is insertedpartly in the cap and partty in the pile. The use of drift-bolts,shown in Fig. 66 (6), is cheaper in first cost, but renders repairsand renewals very troublesome and expensive. ^Split caps,*shown in Fig. 66 (c), are formed by bolting two half-size stripson each side of a tenon on top of the pile. Repairs are veryeasily and cheaply made without interference with the traf
. Railroad construction. Theory and practice . anthat through the mortise, so that the cap will be drawn downtight when the pin is driven. Occasionally an iron dowel (aniron pin about Ih in diameter and about 6 long) is insertedpartly in the cap and partty in the pile. The use of drift-bolts,shown in Fig. 66 (6), is cheaper in first cost, but renders repairsand renewals very troublesome and expensive. ^Split caps,*shown in Fig. 66 (c), are formed by bolting two half-size stripson each side of a tenon on top of the pile. Repairs are veryeasily and cheaply made without interference with the trafficand without injuring other pieces of the bent. The smallerpieces are more easily obtainable in a sound condition; the 152 RAILROAD CONSTRUCTION. §129. decay of one does not affect the other, and the first cost is butlittle if any greater than the method of using a single piece. Forfurther discussion, see § 136. For very hght traffic and for a height of about 5 feet threevertical piles will suffice; as shown in Fig. 66 (a)o Up to a height. Fig. 66. of 8 or 10 feet four piles may be used without sway-bracing, asin Fig 66 (b), if the piles have a good bearing. For heightsgreater than 10 feet sway-bracing is generally necessary. Theoutside piles are frequently driven with a batter varying from1 : 12 to 1 ! 4. Piles are made, if possible, from timber obtained in thevicinity of the work. Durability is the great requisite ratherthan strength, for almost any timber is strong enough (exceptas noted below) and will be suitable if it will resist rapid following list is quoted as being in the order of preferenceon account of durability Red cedarRed cypressPitch-pineYellow pine White pineRedwoodElmSpruce 9. White oak 10. Post-oak 11. Red oak 12. Black oak 13. Hemlock 14. Tamarac Red-cedar piles are said to have an average life of 27 yearswith a possible maximum of 50 years, but the timber is rather § 130. TRESTLES. 153 weak, and if exposed in a river to flowing ice or driftw
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