Fig. 292.—Cheaper Stone Culvert.
Fig. 293.—Arched Concrete Culvert.
Where flat stones enough cannot be easily procured, culverts may be built of concrete. The abutments are first made, as in other cases; then empty barrels or sugar hogsheads, according to the capacity of the opening, are fitted in, or better still, a temporary arch is made of rough, narrow boards. The concrete of cement, sand and gravel, is then prepared and poured in, temporary supports of lumber having been fixed across each end of the culvert to keep the concrete in place until it hardens. Small stones may be mixed with the concrete as it is poured into place, and the whole topped off with a row of them. This protection of stones on the top is valuable, in case the covering of earth is worn or wasted away at any time while it is in use. For a longer culvert a flattened arch is made of concrete, as shown in [figure 293]. Light timbers are laid across, the ends resting lightly on the abutments. Across the middle of these a round log is placed to support the crown of the arch. Elastic split poles are sprung over all, and upon these are nailed thin narrow boards, extending lengthwise of the culvert. The ends being temporarily protected, the concrete is mixed and poured on, as before. As soon as the concrete has become thoroughly well “set,” the light cross-sticks are cut in two and the temporary work removed.
Fig. 294.—Angular Concrete Culvert.
A cross-section, showing another form of concrete culvert, and the method of construction, are shown in [figure 294]. Such a culvert is more easily built than the last, but is not as strong. The best and most durable culvert is of stone, with a regular half-round arch. Such work can only be done properly by a regular mason, but in the end it is cheaper, where the stone can be obtained, than any kind of make-shift.
