RAILROAD CONSTRUCTION 313 
TIMBER WORK 
The construction of trestles, culverts, cribbing, and other 
timber work is done just previous to track laying. 
Trestles are used in crossing streams where some form of bridge 
is required and to span depressions when it is necessary to elevate 
the roadbed above the ground level in order to maintain a given 
grade. They usually are cheaper than a fill when the grade line 
is 4 feet or more above the ground level, and although less per- 
manent, the life of the wooden structure is generally ample to 
meet the logger's needs. Trestle timbers also may be salvaged 
when the road is abandoned. 
They are built in two types known as pile trestles and framed 
trestles, and are made in sections, called bents, which are spaced 
12 or 14 feet apart. 
Pile Trestles. — These are used in stream beds and swampy 
spots where suitable foundations for framed trestles cannot be 
secured, and also for structures 75 feet or more in height when the 
cost of constructing framed trestles is high.^ 
Pile trestles are cheaper than framed trestles when the rail- 
road grade makes an oblique angle with the contours, because of 
the saving in excavation for mud sills which would have to be 
cribbed up on one side of the bent and sunk into the earth at the 
opposite side. 
Low pile trestle bents often have three round piles from 12 
to 15 inches in diameter, driven in a row across the roadbed. 
On a standard-gauge road one pile is placed in the center of 
the roadbed and the outer piles are placed from 24 to 28 inches 
on either side of it. On medium-height trestles for standard- 
gauge track four piles are used, the two inner ones being spaced 
3 feet apart, center to center, and the outer piles 26 inches, 
center to center, on either side of the middle ones. When the 
height exceeds 100 feet, five or sLx piles may be used. They are 
driven with a pile driver to bed rock, or solid bottom, and are 
sawed off at the required height above ground. A 10- by 10- 
inch, a 12- by 12-inch, or a 15- by 15-inch timber, called a "cap," 
is drift bolted on top of them with drift bolts. 
1 Pile trestles 120 feet in height and nearlj' 400 feet long have been erected 
in the Northwest, to span canyons, at a cost far below that for any other 
form of suitable structure. 
