| Name of Earth. | Weight. | |
|---|---|---|
| Decimals of a Ton. Cubic Foot. | Tons. Cubic Yard. | |
| Basalt, solid | 0·083 | 2·25 |
| Bath stone, solid | 0·052 | 1·40 |
| Chalk, damp to wet, loose to close | 0·056 to 0·074 | 1·50 to 2·00 |
| Clay | 0·054 to 0·059 | 1·45 to 1·60 |
| Flint, solid | 0·074 | 2·00 |
| Granite | 0·078 | 2·10 |
| Gravel and shingle | 0·046 to 0·055 | 1·25 to 1·50 |
| Limestone. Lias to compact mountain | 0·067 to 0·078 | 1·81 to 2·10 |
| Marl | 0·044 to 0·052 | 1·20 to 1·40 |
| Mud, at surface | 0·044 | 1·20 |
| Mud, at about 15 feet in depth | 0·048 | 1·30 |
| Peat, hard, and top mould | 0·036 | 0·98 |
| Portland stone, solid | 0·065 | 1·75 |
| Quartz, solid | 0·076 | 2·05 |
| Sand, dry river | 0·041 | 1·10 |
| Sand, damp and shaken | 0·055 | 1·50 |
| Sandstone, solid | 0·063 to 0·072 | 1·70 to 1·95 |
| Shale | 0·074 | 2·00 |
| Slate, solid | 0·080 | 2·15 |
| Trap, solid | 0·078 | 2·10 |
In determining the safe load upon deposited earth it is well to remember that:
1. Excavated earth cannot be restored in bulk to its original condition.
2. When the earth is simply deposited from a tip head, it cannot be immediately consolidated by the act of deposition.
3. Until subsidence may, for all practical purposes, be considered at an end, no deposited earth can be regarded as stable; but a mere uniform weathering of the surface, which cannot be prevented when the exterior is uncovered, will generally not cause instability of the mass.
4. Upon most public works the earth in an embankment is exposed in thin layers.
5. The earth is loaded, in the great majority of cases, soon after it is deposited and before it has settled or become consolidated, and is also subject to vibration from earth waggons, locomotives, &c.
6. The comparative dry or wet state of the deposited earth and its power of resistance to meteorological deterioration. Vide also Chapter IX.
Taking into consideration these and other deteriorating influences, the problem to be solved is, what deduction must be made from the safe load upon unexcavated earth in foundations, in order to know the safe sustaining power of the same earth when deposited in an embankment? Much depends upon the liability of the soil to become saturated or in a damper state than that in which it is known to be stable. For this reason the height of embankments in clay soils, which are so deleteriously affected by the weather, is generally made as little as possible. Of course, temporarily an embankment will stand with a greater pressure, i.e., at a greater height than the safe height, and, provided no lateral movement took place, an embankment would be stable until the earth was nearly crushed; but permanent stability and freedom from slips and subsidence is the object to be attained, and everything must be subject to local conditions, such as the amount of rainfall, the situation, the care exercised during deposition, the protection given to the surface, and the general drainage, and these must always govern the application of any general rule. It is, therefore, necessary to divide earths into two kinds, namely:—granular and non-granular; the former is assumed to have particles, for purposes of earthwork, insoluble in water, the latter to be liable to be dissolved by aqueous action, or to be so affected by it as to lessen the stability. It is impracticable to determine by a formula the permanent safe maximum height of an embankment; the values named are, however, an approximately reliable indication of the height, and are based upon the assumption that the slopes are sufficiently flat to be stable, and that the embankments are deposited in the ordinary way, the width of the formation or top being not less than about 15 feet. It is seldom economical to make an embankment more than 70 to 90 feet in height, except for short lengths, and where deep cuttings cannot be avoided, and it becomes a question of tipping spoil banks or main embankments, or the foundations for a viaduct are known to be of a treacherous and doubtful character. The heights named have been calculated under the worst conditions, i.e., that any weight upon the formation will be directly communicated to the base as in the case of a column, and not through the cross-section of an embankment, and over the whole area of the base; however, there is always the danger that the central portion may subside, as the weight upon it is the greatest, and that the slopes may be disturbed and pressed out, especially in soft or soluble earth likely to be quickly affected by moisture. The soil is considered to have no reliable cohesion.
In this country the limits of general practice for the height of embankments when unaided by retaining walls or other support, but with the slopes soiled, covered with grass, and only externally drained, has been as follows:—