Slope failure

“ We have already seen that erosion is favored by declivity. Where the declivity is great the agents of erosion are powerful; where it is small they are weak; where there is no declivity they are powerless. Moreover it has been shown that their power increases with the declivity in more than simple ratio.
It is evident that if steep slopes are worn more rapidly than gentle, the tendency is to abolish all differences of slope and produce uniformity. The law of uniform slope thus opposes diversity of topography, and if not complemented by other laws, would reduce all drainage basins to plains. ”

“ But in the case of a cutting in drift or permeable soil upon rock or an impermeable surface liable to become wet, which not infrequently occurs, the conditions of equilibrium may easily be destroyed, and the slope upon the higher side may not stand at a less inclination than that of the hill. ”

“ In the worst places the guides point out to the traveller where to plant his feet and hold him up if he begins to slip. Shrubs, tough grass, and here and there a stunted pine-tree give a welcome hold, but on the steepest slopes they are wanting. The last slope up to the foot of the cliff—a very long and steep declivity of loose gravel which gives way at every step—is most fatiguing. As I was struggling slowly up it with the guides, we heard the furious barking of dogs away up the mountains on the opposite side of the glen. ”

“ When cuttings are upon the escarpment of a clay hill the slope upon the valley side need not be so flat as that against the hill. An inclination of 3 or 4 to 1 may be necessary upon the hill-side, whereas a 11⁄2 to 1 slope may be sufficient upon the other. When a slip may involve the destruction or injury of any adjacent buildings, the best plan is to erect a retaining wall at the foot of a slope, not less than about one-fourth of the height of an embankment or the depth of a cutting, ample provision being made for drainage and the expansion of the earth. ”

“ The bushes that littered the upper slope were more infrequent here. There was indication of a recent landslide, which might continue down to the sea-level or might end abruptly and disastrously over the edge of some steep cliff. Slipping, sometimes on her back, sometimes sideways, sometimes on her face, she felt her momentum increase with every yard she covered. The ends of the ski-sticks were frayed to feathery splinters, and already the desired plateau was above her. ”

“ In places we scrambled for hours along precipitous slopes, cumbered with fallen rocks, and with swirling rapids below us; a mile or so of easier going where the country was more open, and again a mountain spur would shoulder the river aside. This time the abrupt slope would be dense with high and tangled thorn-bush through which we had to cut a way, whilst here and there a huge fallen rock or a whole landslide from the cliffs above would bar further progress, till we had crawled round, over, or under them, at imminent risk of breaking our ponies’ necks or legs, as well as our own. ”

“ But if we examine the final result of this double waste, we shall see that when this ultimate ratio of equality is established throughout the upper and lower talus and the natural slope of the hill, the road must disappear altogether, instead of maintaining, as it often does, a breadth of 70 feet upon an uniform slope of the face of the hill. There is another circumstance in their construction equally repugnant to this hypothesis. In no one instance is the surface level, or even nearly so, as the profiles will show. ”

“ The air was heavy with the noxious vapor as he went rapidly toward the slope up which crowds of miners were passing, and as some of the men loitered behind the others it became necessary he should hide in the drift to escape detection.
"Why don't the fools move faster," he said, in a hoarse whisper. ”

“ The height or depth at which a slip in an embankment or 51cutting becomes disastrous cannot be ascertained by any rule, but knowledge of the flattest slope at which any particular earth has remained stable in the same state as the embankment or cutting under deliberation, will enable a close approximation to be determined of the point to which a slip in a cutting is likely to reach, and that to which an embankment will subside. ”

“ 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. ”

“ These blocks lie on one another, often on a steep slope, owing to the roughness of their surfaces, which prevents their sliding. They are then more dangerous even than slaty rocks, since this very roughness beguiles a climber into feeling that the footing is safe at a steeper angle than on the smoother surfaces, while the rocks are merely in unstable equilibrium. ”

“ The protection of the toe of a slope is of importance, as it is usually the most vulnerable and the weakest part. When the earth is of the same character, the quantity of water is usually greater at the lowest level than above it, and the stability of the soil in its vicinity is therefore lessened. In clay soils this softening action at the base may cause a slip and probably can only be prevented by reducing the percolation by surface drainage, which has been referred to in Chapter IV. ”

“ A coating of clay about 2 feet in thickness, upon a slope with stakes driven into it, and large bushy boughs of trees fixed thereon with the tops downwards, is frequently used in India as a protective cover to a crumbling bank of a river, and to training spurs erected to prevent erosion and slips. Mattresses, fascine or wattled work, besides being expensive, will slide down a slope unless well secured to it, and therefore a constant strain is produced as in all stake-held coverings ”

“ George.
"Yes, sir," said Philippe; "it is right up a steep slope of rocks."
"Is there good footing," asked Mr. George, "or are the rocks loose, or slippery?"
"It is very good footing," said Philippe. "In one sense the rocks are loose, for the whole side of the mountain where we go up is formed of slag and scoriæ. But then the pieces are wedged together, so as not to move much, and the foot clings to them, so that you don't slip. On the whole, it is good footing. The only difficulty is, it is so steep. It is a thousand feet up rough rocks, as steep as you can go." ”

“ Should it be determined to pitch the lower part only of a slope, in order to prevent slips, the pitching must continue to such a height that a flow of water down the slope is impossible, or it may become detached. If the pitching is not rough squared on the joints, but of various forms, it is preferable that the face having the largest area be laid downwards, smaller stones being carefully wedged in between the interstices. Any defective execution is usually followed by a falling of the stones, and should this happen the surface will be broken and erosion and slips will ensue. ”

“ The great slope on which we spent so much time was, from being denuded by the avalanche of which I have spoken, deprived of its greatest danger. Had it had the snow still resting upon it, and had we persevered with the expedition, we should almost without doubt have ended with calamity instead of success. The ice of that slope is always below, its angle is severe, and the rocks do not project sufficiently to afford the support that snow requires, to be stable, when at a great angle. ”

“ In such a case the only means of preventing a slip is by erecting a wall at the toe of the slope, and as it is necessary that the foundations be in the rock beneath the surface earth, the simple operation of excavating may impel the drift down the hill. Also should an embankment be required upon drift soil its weight may be sufficient to cause the mass to slide and a diversion of a railway may be imperative, for the cost of a retaining wall, because of its height and the great pressure it would have to sustain, or be built to sustain, would prohibit the adoption of such a remedy. ”

“ If there be cliffs at the top of the slope, as is often the case, very extensive falls of rock may take [Pg 174] place from it, the masses descending with such speed that they directly attain the stream. If the steeps be low and the rock divided into vertical joints, especially where there is a soft layer at the base of the steep, detached masses from the precipice may move slowly and steadfastly down the slope, so little disturbed in their journey that trees growing upon their summits may continue to develop for the thousands of years before the mass enters the stream bed. ”

“ Rate of Erosion and Declivity. In general erosion is most rapid where the slope is steepest; but weathering, transportation, and corrasion are affected in different ways and in different degrees.
With increase of slope goes increase in the velocity of running water, and with that goes increase in its power to transport undissolved detritus.
The ability of a stream to corrade by solution is not notably enhanced by great velocity; but its ability to corrade by mechanical wear keeps pace with its ability to transport, or may even increase more rapidly. ”

“ If we have any doubt as to the security of a slope, which we shall have when the angle of the slope and quality of the snow are varying and the resultant stability is indeterminable, we must ascend or descend it vertically, if possible, or on very steep zigzags. ”

“ I speak of some rocky slopes as lines of rest, because, whenever a mountain side is composed of soft stone which splits and decomposes fast, it has a tendency to choke itself up with the ruins, and gradually to get abraded or ground down towards the débris slope; so that vast masses of the sides of Alpine valleys are formed by ascents of nearly uniform inclination, partly loose, partly of jagged rocks, which break, but do not materially alter the general line of ground. ”

“ Experiments have shown that a slope of 1 to 1 will reflect waves, on a flatter slope they are broken. In such situations light structures, offering little resistance to the action of the waves and not causing an impediment to the current, should be adopted in preference to a massive or solid erection; but when an embankment is necessary, it should have a long sloping mound or foreshore upon which the waves will gradually become lessened and dispersed, the desired object being to prevent deep water close to the work. ”

“ A simpler remedy is the insertion of a dry rubble lining on the surface of the original ground upon which a slip has occurred, and the restoration of the slope by means of firm earth or rammed and dried slipped soil; but the system of rammed earth counterforts, although it has succeeded when effected during the dry season, the earth being wetted sufficiently to make it cohesive by ramming, will not succeed in wet or winter weather, as the soil cannot then be consolidated by ramming operations, for it will be overcharged with water from the effects of rain, frost, or snow. ”

“ In all earth such as shingle, gravel, or sand, consisting of pieces of rock or rubble, or having round particles, it is important to remember that any surface disturbance may cause serious movement, particularly should it commence at the bottom of a slope, as then the revolving action may not cease until a flatter inclination is 108produced by material rolling from the top and not reposing until it nearly reaches the base. The quantity of moisture is the chief governing condition of the slope of repose in clay soils, and it should not be forgotten that this may vary considerably. ”

“ They are, of course, much more apt to side-slip when moving across a slope (especially a steep one) than when running straight down it; indeed when traversing a steep slope they may side-slip even in soft snow if that is shallow and rests on a slippery crust. But though they will generally run straight downhill with absolute steadiness on snow that makes them side-slip badly when traversing, the surface may be so extremely slippery that they will side-slip even in a direct descent owing to small lateral undulations of the ground. ”

“ Again, if we are forced to cross danger-areas of stones or snow, we have the better chance in the case of stones. Each falling stone only occupies in its fall a fraction of the area of danger, and may be eluded; but every inch of snow in an area of avalanche risk is alike pregnant with danger. It is no longer a question of artful dodging individually, but of an escape in time by the whole party, possibly over a considerable extent of slope, all of it equally treacherous. ”

“ As long as it remains incoherent, every removal of substance at the bottom of the heap, or addition of it at the top, occasions a sliding disturbance of the whole slope, which smooths it into rectitude of line; and there is hardly any great mountain mass among the Alps which does not show towards its foundation perfectly regular descents of this nature, often two or three miles long without a break. ”

“ In all regions where earthquake movements are frequent, and the shock of considerable violence, the trained observer notes that the surfaces of bare rock are singularly extensive, the fact being that many of these areas, where the slope lies at angles of from ten to thirty degrees, which in an unshaken region would be thickly soil-covered, are deprived of the coating by the downward movement of the waste which the disturbances bring about. ”

“ The fells drop down fairly sharply on each side, and in the failing light there seems no object in following the stream any further, when quite suddenly the green slope on the right stands out from a scarred wall of rock beyond, and when we are abreast of the opening we find ourselves before a vast fissure that leads right into the heart of the fell. The great split is S-shaped in plan, so that when we advance into its yawning mouth we are surrounded by limestone cliffs more than 300 feet high. ”