20 BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
erosive action, the extent of which can not be foreseen with any cer- 
tainty. • Location of substructure should allow sufficient unobstructed 
waterway at time of greatest flow and abutments should be so lo- 
cated as to prevent water seeping behind them after possible erosion 
of adjacent banks. Foundations should extend below the anticipated 
erosive effect of the current and never less than 3 feet below the 
bed of the ditch in material other than rock, hardpan, or clay. 
Piers or bents should not be placed in the watercourse. 
Since they perform the same service, these bridges should afford 
the same security as those elsewhere on the railroad or highway and 
should be designed and constructed according to the same principles 
of permanence as well as economy. 
The permanent crossing types are reinforced-concrete box and arch 
culverts, slab and T-beam spans, steel girders, low trusses, and 
treated-timber trestles. Where the passage of floating equipment is 
necessary, spans which can be lifted to one side should be provided. 
Most open-drain crossings, however, are for secondary or farm 
roads where permanent structures are not justified. Extreme flood 
conditions are seldom provided for in designing bridges of this type. 
They are generally simple beam spans or timber trestles. 
The former are satisfactory for narrow ditches, banks of which 
suffer little erosion and give firm support to the mud sills or blocks. 
Short, longitudinal mud blocks give greater and more uniform bear- 
ing area than sills placed directly on the ground. The life of sills 
and blocks may be increased by embedding them in gravel or 
broken stone, to drain off the water. 
Decay of the stringers at the ends of trestles may result from their 
contact with the ground. A clearance of at least 12 inches should be 
provided to prevent this. 
Timber trestles (fig. 14) should consist of a channel span over 
the center of the ditch and approach spans long enough to place the 
ends of the trestle out of range of ordinary erosion. (See pi. 2, C 
and pi. 2, I). This arrangement is better than two-span trestles 
with a bent at or near the center of the ditch, because of the ob- 
struction to flow offered by the central bent. Single-span trestles 
with timber back walls higher than 2 feet are not desirable because 
the timber in the back walls is subject to rapid decay, and earth 
pressure against a back wall on a timber bent tends to push the 
bent out of place. Longitudinal bracing to prevent this obstructs 
the waterway. 
A timber trestle may be supported on either pile or frame bents. 
Pile bents should consist of three or more piles and be braced with 
double diagonal sway bracing. The cap should be drift bolted to 
each pile and the bracing should be bolted or spiked to the cap and 
each pile. Under some conditions pile bents are not economical for 
small jobs, because they can not be driven to good bearing by hand 
and the cost of pile-driving equipment is not justified for the few 
piles needed. 
Frame bents consist of three or more posts and a cap and sill, 
which may be placed directly on the ground on mud blocks or on 
concrete piers or pedestals. They should have double diagonal brac- 
ing bolted at the ends to the cap and sill and at intermediate points 
to the posts. The caps and sills should be dapped to receive the ends 
