A treatise on highway construction . Fig. 79.—Pole Drain. Fig. 80.—Stone Fig. 83.—Silt-basin. Fig. 82.—Outlet. Fig. 84.— pipe; the removal of sand or dirt from the fine gravel sur-rounding the pipe removes the possibility of its entering the pipe; DRAINAGE—CULVERTS. 539 and the coarse stone in the upper part of the trench interceptsthe ground-water in its flow toward the subgrade. These drains areplaced on either or both sides, as the contoiir of the ground requires. 691. Fall of Drains.—It is a mistake to give too much fall tosmall drains, the only effect of which is to
A treatise on highway construction . Fig. 79.—Pole Drain. Fig. 80.—Stone Fig. 83.—Silt-basin. Fig. 82.—Outlet. Fig. 84.— pipe; the removal of sand or dirt from the fine gravel sur-rounding the pipe removes the possibility of its entering the pipe; DRAINAGE—CULVERTS. 539 and the coarse stone in the upper part of the trench interceptsthe ground-water in its flow toward the subgrade. These drains areplaced on either or both sides, as the contoiir of the ground requires. 691. Fall of Drains.—It is a mistake to give too much fall tosmall drains, the only effect of which is to produce such a currentthrough thpm as will wash away or undermine the ground aroundthem, and ultimately cause their own destruction. When a drainis once closed by any obstruction no amount of fall which could begiven it will again clear the passage. A drain with a considerablecurrent through it is much more likely to be stopped from foreignmatter carried into it, which a less rapid stream could not havetransported. A fall of 1 inch in 5 feet will generally be suffic
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