Note.—Clean fragmentary rock of uniform size, carefully tipped, would stand permanently at any height within reasonable approach to the safe compressive load; but in deposited earth, as in cuttings, in addition to the contingency that the natural ground at the base may not be able to bear the strain, the effect of water pressure must be considered as it may become of sufficient force to cause the face of the slopes to become loose and finally separate and slip. At a height of only 50 feet clear head, the pressure would be about 22 lbs. per square inch; therefore, it is not so much the weight the deposited earth will bear as the effects of water upon the earth, and the water pressure that have to be considered.
| Other Earths. | |
|---|---|
| Description of Earth Embankment. | Approximate safe permanent maximum height in feet, of an embankment in earth, conditions as described. Feet. |
| Peat moss, marsh earth, consolidated mud, silt, hard peat turf | 0 to 7 |
| Alluvial soil obtained from a river bed | 5 to 8 |
| Note.—When the river bed is rocky and the deposit firm, the height may be increased to 15 feet. | |
| Soft wet pasty clay, and marsh clay, moist and difficult to drain | 5 to 6 |
| Diluvial clay soil of river beds, according to its uniform character, degree of firmness and hardness | 6 to 20 |
| Alluvial soil. Loam and loamy earth. (Clay and 40 to 70 per cent. of sand.) Clay loams (clay and about 30 per cent. of sand) | 15 to 30 |
| Damp clay soil. Equably damp, that can be drained and will partly drain itself | 25 to 30 |
Note.—The earths named will generally have their bearing power increased by careful deposition in an embankment, for the act of equal separation of the mass will cause a decrease in the quantity of water contained in them: and they will be relieved of the water-pressure to which they were liable in their natural position. On the contrary, when carelessly tipped they may be deteriorated. Vide Chapter IX.
In the case of all earths that are readily impaired by water, it is the degree of permanent uniform dryness or wetness that governs the safe height of an embankment; for even the surface of the most impervious clay will be liable to become in a muddy state, although in the mass it may be dry or in a condition conducive to stability. Almost all aluminous or calcareous earths hold or retain water in varying quantity, and as in embankments this may constantly change, no safe permanent height of an embankment in such soils can be established. The most solid impermeable clay when separated into small pieces and impregnated with water to any quantity above that it normally contains, will have its bearing power and general stability reduced, until, when saturated, it becomes as mud. Impure and argillaceous chalk, marly clay, marl, and shale, are all more or less weakened by aqueous action, and their safe permanent height may be anything not exceeding the crushing strength of the earth, with a due allowance for the deteriorating influences of weather, vibration, and internal water-pressure.
In this chapter reference has been made to certain limits of the safe height of embankments; but were these to be adopted as the maximum safe height in each earth it would be an indefensible edict, for by the adoption of the required precautionary measures named in the several chapters of this book the height can be considerably increased, and what is more, has been much exceeded with impunity in numerous instances. The inclination that any earth assumes, after being exposed to the weather a sufficient time for it to be affected by it, will afford a good indication of the safe load it will support. As the slope of repose becomes flatter, the bearing power of the soil in an embankment will be reduced, all the deteriorating influences being identical.
For the best information “On the actual lateral pressure of earthwork,” vide the paper by Sir B. Baker, M. Council Inst. C.E., Minutes of Proceedings of the Institution of Civil Engineers, London, vol. lxv., part iii.
CHAPTER VI.
Slopes, General Considerations.—Table Showing the Usual Range of Slopes.—Table of Coefficients of Friction.—Notes on the Cohesion of Earth.—Form of a Slope.—Some Conditions Governing the Necessary Inclination.—Widening Earthworks within the Original Fences.
With regard to the inclination and the form of a slope, and the prevention of slips in earthwork, in any slip the surface must be affected; therefore, to determine the slope of permanent stability of any earth, whether in a cutting or an embankment, is of great importance. An unnecessarily flat slope is not only a monetary waste but may be a cause of instability, for it exposes a larger surface to deterioration by the weather. By what means is a correct decision to be attained? Experience has shown that certain earths under known conditions will repose at particular inclinations; however, to empirically assume that any earth will always stand under any circumstances is clearly imprudent and untenable. As a guide, a table of slopes for earths is valuable, but consideration of the consequences of movement, and the distinctive features of each case should govern a decision.
Should the earth be treacherous and require a flat slope, it may be advisable to reduce the quantity of excavation by the erection of a retaining wall, not only to effect a saving in expenditure, but also to permanently support the soil, and prevent instability.