Page 226 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.28 THE WORK

These structures may be distinguished from each other on a basis of width of opening. The criti-
cal width, or span, at which a bridge becomes a culvert varies from 5 to 20 feet in different localities.

Log. Figure 5.19 shows a type of log bridge suitable for carrying a pioneer or haul road, or a
driveway, across a small stream.
Sill logs are set into the bank parallel with the stream, far enough back from it to be secure
against being undermined. They may be braced by bolting to stumps or driven piles, or by cables
stretched to anchors behind them. This anchoring is very important, as streams often flood suffi-
ciently to float wood bridges.
Logs strong enough to carry expected loads are then placed close together across the stream,
resting on the sills and preferably being fastened to them and to each other by lag bolts in drilled
holes. Butts and tops should be alternated, so that log taper will not make one side wider and higher
than the other. These logs are called stringers.
Next, straight-grained logs of smaller size should be split in quarters or other wedge-shaped
fractions and placed, split sides down, between the tops of the big logs. They may be cut into as
short sections as necessary to fit snugly and to fill the cracks between the logs. Sections of round
poles may be used in the same way.
The split pieces should each be fastened to one log with spikes or lag bolts. Fastening can be
on either side, but must not be on both, as that would permit movements of the logs to split them
or pull the fastenings loose.
Quarter logs or poles are then fastened to the outside logs, to serve as curbings to prevent vehicles
from running off, and/or to retain surfacing. The wood surface may be left exposed for light or
occasional use, or covered with gravel or dirt to make it smoother and reduce danger of tire damage
if nails work up.
Decks may also be made of planks or split logs nailed at right angles to the stringers.
If the span is long, or the loads are heavy, a stone-filled log or timber crib may be used as a
center support. Cribbing may also be used at one or both banks if they are too low or soft to give
proper support.
The strength of wood of various species, and in different conditions, varies so widely that indi-
vidual judgment must be exercised in selecting the logs. If the bridge is to be used over a period
of years, resistance to rot may be more important than initial strength.
Greenwood is strong but lacks rigidity, and tends to give too much bounce to a long bridge. It
will usually bend and splinter before breaking.

FIGURE 5.19 Log bridge.