Surface soils, about 20 to 30 feet.
Boulder clay from 25 to 30 feet.
Yellow, or ordinary clay, 30 to 40 feet.
Ditto, ditto, in cuttings, 60 to 65 feet.
Chalk, 30 to 40 feet.
Gravel and sand, about 60 feet.
Ditto, in cuttings, 80 feet.
These have, however, been exceeded in many instances, especially upon the lower side of an embankment upon sidelong ground. Probably the highest artificial embankments unsupported by walls or any protective works, are quarry and mining-tips, and ballast-heaps. These have been cast out in all weathers, and allowed to assume any slope or shape, and to a height as great as 100 to 120 feet, and have remained stable, although usually in an exposed situation, but almost free from vibration.
In Chapter VI. a railway embankment 90 feet in height, in treacherous clay-soil is instanced, and there are many embankments of various earths from 70 to 90 feet in height on the centre line, but a railway embankment exceeding 100 feet in height on the centre line is rare, although the height from the toe of the slope to the formation level in sidelong ground may be frequently exceeded. A short and very high embankment may stand where one of considerable length would slip, consequent upon variations in the character and condition of the earth, the bearing capacity of the ground, the state of the weather, the flow of the surface water, general homogeneousness, the configuration of the ground, and other deteriorating influences; but draining, careful deposition, and a judicious adoption of the varying slope system, and other precautionary works, will conduce to stability. Unless in exceptional circumstances, it is advisable to avoid very high embankments, as a slip of earthwork seldom gives much prevenient notice, and the works of reparation can scarcely be effected by any means than manual labour. The quantity of earth to be treated may be so large that before the remedial works can be completed a wet season may arrest operations and cause additional movement and subsidence of the earth.
| Granular Earths. | |
|---|---|
| Description of Earth Embankment. | Approximate safe permanent maximum height in feet, of an embankment in earth, conditions as described. Feet. |
| Indurated compact gravel, cementing material imperishable. Clean ballast | 120 |
| Clean gravel, unwashed | 110 |
| Sharp compact clean sand | 100 |
| Firm, clean, angular, large-grained sand | 90 |
| Ordinary nodular sand, slightly loamy | 60 to 70 |
| Loamy sand | 50 to 55 |