Approximate Safe Maximum Load upon Different Earths.—Normal Pressure of the Earth.—The Safe Maximum Load upon Deposited Earth.—Approximate Safe Maximum Height of an Embankment.

There is no limit to the depth of a cutting except a due regard to economical construction, provided the slopes are sufficiently flat, and the lateral and upward fluid pressure in the slope, and formation and quantity of water not too great for the stability of the earth; but, in the case of embankments, the load upon the ground and the deposited material in great measure restricts the height, or necessitates an embankment being gradually spread out, so as to enlarge the bearing area as the weight is increased.

The following values of the safe maximum compressive load have been compiled from actual practice, but are, of course, only intended as a guide to the safe load for foundations excavated in ground not artificially deposited. The condition of the earth in each case should be considered, and in works of magnitude it is advisable to make experiments extending as long as practicable, and for at least a month, and it is false economy not to carefully ascertain the character, condition and other circumstances of a foundation destined to support any part of a structure, a failure of which may result in serious consequences; and it should be borne in mind that a continuous surface possesses greater sustaining power than the same area in detached portions, as the adhesion of the sides is not destroyed; similarly, the load that a tenacious earth will support upon a small area is somewhat greater than over a large area, because the lateral surfaces are relatively larger in proportion to the area, and, therefore, the effect of cohesion is proportionately greater; but in loose soils it is not so, for cohesion exists but in name, and the ground around would be upheaved upon an excessive load being superimposed. In testing the weight any earth will support, it is not so much the first settlement, provided it is not excessive, that it is desirous to know, but whether after the first settlement it ceases or the earth, as it were, reacts and rebounds, which it may do in firm ground to the extent of one-eighth to half an inch. If so, the ground is not overloaded.

After ascertaining by experiment the pressure any earth will bear over a given area, the object should be to make the soil neither drier nor wetter than that of its natural state when experimenting, and it should be maintained in that condition. In testing the weight which a soft earth will support, some days should be allowed for the sinking of the test platform, and such subsidence should be ascertained periodically by careful levels. A month is not too long for a reliable and complete test, as many soft soils continue to yield. In soft clay soils considerable depression often proceeds for weeks after a load has been applied, but except in peculiar earths such settlement will ultimately be imperceptible, and will practically cease. A considerable margin of stability should in all cases be allowed. Although it may not be absolutely necessary to experiment when the nature of the ground is well known, wherever stability is of great importance, the cost of a practical experiment being so small, there is no sufficient reason why an actual test of the sustaining power of the soil should not be made in the majority of instances, for there are many earths whose friction and cohesiveness can alone be depended upon for resistance to displacement. In such cases the initial pressure upon the earth should not be much exceeded.

In loose non-cohesive earths the load may be increased, when the depth is considerable, as the soil has been subject to a greater normal pressure due to the weight of the soil upon it at any depth; but it is not advisable to consider such increase of bearing power of the soil, unless at any depth it is found that the normal pressure augments the bearing power and makes the earth more dense, which may be approximately ascertained by experiment. In such event the load upon the base can be increased by the weight of the normal pressure removed. Supposing 5 tons per square foot was known to be the safe load upon the surface of the ground, and at any depth it was found that the normal pressure of the soil was 2 tons; 5 + 2 = 7 tons placed at that depth would equal 5 tons at the surface. In the worst case, when the loose earth is of great depth, and it is certain that it cannot be tapped or disturbed at the depth at which it is decided to place the foundations of a structure, provided the load is not more than the normal pressure, it is not probable that it will subside or slip, as no additional weight is imposed.

In foundation and general work, rocks are usually not loaded with a greater weight than from 8 to 18 tons per square foot, according to the character of the rock. As the crushing strength has been principally ascertained from cubes, and not from prisms, rectangular blocks, or irregularly shaped pieces, and as the resistance of rocks to transverse strain or breaking across is considerably less than the compressive strength, and varies greatly and not always according to the crushing resistance of the material, from 8 to 20 tons per square foot is a prudent limit for the safe load, and should not be exceeded unless under exceptional circumstances; as unequal bearing may greatly intensify the strain, and irregularity in the texture may reduce the resisting powers to that of the weakest part. Sandstone rock that can be crumbled in the hand should not be loaded with more than 1½ to 1¾ ton per square foot, or it will probably begin to flake and disintegrate. The strength of sandstone varies very greatly, and in experiments it has been found that when fine close grained, it supported before being crushed five times the weight that very coarse gritty sandstone, having a sandy appearance, would sustain; the respective crushing pressures per square foot being 362 and 67 tons.

Reference to authorities on the resistance of stones to crushing, tension and transverse strain, will give the approximate safe load per square foot; but in foundations, i.e., upon the rock in its natural location, it should not exceed one-tenth of the ultimate resistance, and the compressive strength should not alone be taken as a guide to the safe load, but the resistance of the rock to tensional and transverse strain should be considered. The value given for crumbling sandstone is for the softest material that can be called rock, and is merely stated to show that although some earths may be generally classed as rocks their bearing power may be limited. The safe load upon an artificial rubble mound foundation depends upon its character, and firmness and solidity when deposited, and upon that of the ground on which it is placed. No general values can be named, although it may be classed as clean or compact gravel.

In a cutting, by excavation, the normal pressure of the earth, which varies with the depth and weight of the soil, is removed.

Let

D = the depth in feet of a cutting from the original surface of the ground,

W = the weight of a cubic foot of earth in decimals of a ton,

P = the normal pressure in tons per square foot at any given depth,

On the other hand, in the case of an embankment the normal load upon the earth is not affected, but an additional weight is superimposed; consequently, to prevent slips or subsidence from overweighting, there is a limit to the height of an embankment, apart from economical considerations of its deposition. The problem to determine is, therefore, the limit of the height to which an embankment may be deposited without exceeding the safe load that the natural ground will permanently sustain. The safe load upon the earth of which an embankment is composed is referred to in due course.

Let

S = the safe load in tons per square foot upon the original ground or earth not artificially deposited.

H = the theoretical limiting height in feet of an embankment.

W = the weight in tons of a cubic foot of the deposited earth.

Note.—When the earth is of a mixed character, the safe load should be that of the weakest soil.

The condition of equilibrium is that the height is not greater than the safe load divided by the weight of the ground, and is consequently given by the expression—

In order to prevent a reference to other books, a table is appended of the approximate weights of different earths in their ordinary condition, compiled from the best authorities.

The weights are those of solid rock, therefore, when it is deposited they will be lighter according to the volume of the interstices.

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:—

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.

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.

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.