274. Small culverts are made by covering two side walls with large flat stones; the bottom being paved with stone at least nine inches deep, laid dry. The general dimensions of such structures depend somewhat upon the class of masonry, but as this is generally the third or fourth, will not vary much.
| Opening. | Side walls. | Cover. | Heads. |
|---|---|---|---|
| 2 × 2 | 3 × 2 | 12 | 2 × 10 |
| 2 × 3 | 3 × 3 | 12 | 3 × 10 |
| 3 × 3 | 3 × 3 | 12 | 3 × 11 |
| 3 × 4 | 3½ × 4 | 15 | 4 × 12 |
| 4 × 4 | 3½ × 4 | 15 | 4 × 13 |
| 4 × 5 | 3½ × 5 | 18 | 5 × 15 |
| 5 × 5 | 4 × 5 | 18 | 5 × 16 |
| 5 × 6 | 4 × 6 | 18 | 6 × 18 |
Figs. 130, 131, and 132, show plans for culverts of from 5 to 25 feet span.
Fig. 130.
Fig. 131. Fig. 132.
RETAINING WALLS.
275. A wall made to sustain a mass of earth or water, to resist overthrow, requires a certain thickness. A body of earth assumes what is termed the natural slope, the inclination of which depends upon the adhesion of the soil, but may be taken as one and one half horizontal, to one vertical, (1½ to 1), as an average.
The problem is, knowing the height of the wall and the form of the mass of earth to be supported, to find the thickness of the wall.