Introduction—General Considerations—Enumeration of the Primary Causes of Slips and Subsidences in Cuttings and embankments, and Earthworks Constructed to Contain or Exclude Water—Some Dominant Principles to be Remembered in Determining the Location of Earthworks.

Earthslips and subsidences may be caused by the terrible power of an earthquake or other dreaded subterranean destroying force, upheaving, cracking, and shattering the earth’s crust and dealing death and havoc in its awe-inspiring course. They may also originate from the untiring efforts of the meanest rodents or the most minute crustaceous animals burrowing passages for aqueous action, the chief agent of the instability of the surface soils of the earth.

The ceaseless mutability of the created elements has been thus magnificently described:—

It is the constant absence of peace and rest in the earth that produces instability, however great, however small, for the disastrous landslips that have occurred in Switzerland and other parts of Europe, in India, and recently at Quebec, and in all regions of the world were caused by the same disintegrating operations as those which generate an earth-slip of comparative insignificance. In proceeding to a practical consideration of earthslips and subsidences, it may be well to call attention to the complexity of the subject, the character and conditions of earth and the impairing elements being so very variable and numerous that it is impossible to determine any rules even for a particular soil; and, moreover, it is necessary to separately consider slips in cuttings and those in embankments, as movement is somewhat differently created, and it does not necessarily follow because earth stands well in a cutting that it will do so in an embankment, or vice versÂ.

It may be said every kind of earth will slip or weather under certain conditions, even the hardest rock if superimposed upon an unstable stratum; therefore, some of the main questions to be considered are:—

I. The Probability of the Occurrence of a Slip.

II. The Effect of a Slip.

III. Should every Precaution be taken to prevent a Slip when a Cutting is being Excavated or an Embankment being Deposited; or is it better to Repair a Slip as it happens?

It is obvious in railway cuttings and embankments a mere crumbling of the surface may be disregarded, but in dock, canal, or any works containing or expelling water, the smallest movement, crack, or aperture must immediately receive due attention.

In order to effectually remedy a disease it is necessary to ascertain its character. Many of the primary causes of slips in cuttings and embankments are, therefore, here enumerated; but, of course, they are not named in their order of importance, which cannot be established.

1. The want of uniformity of the earth, particularly as regards percolation, cohesive power, and resistance to change by the action of water or meteorological influences.

2. The temporary or permanent exposure of the earth to the effects of the atmosphere, rain, frost, and snow.

3. The opening to the air and weather, &c., of thin seams of an unstable character, which, when unsupported, gradually crumble away and cease to support superimposed strata.

4. The tapping of springs.

5. The lower portion of a slope being impaired or undermined through an infiltration and flow of water.

6. The erosion of the slope.

7. The earth having intermediate unstable seams.

8. The unprotected surface of a cutting in light soil being loosened and blown away by a storm of wind, especially when it is accompanied by rain.

9. The slopes being honey-combed and disturbed by rodents, particularly in clay soils, clay marls, and clay loams; and upon submerged earthwork, and in certain districts in the tropics by a mollusk which will penetrate and even destroy rocks.

10. From one portion of a cutting being more exposed than another to disintegrating meteorological influences.

11. By the discharge of water from land drains following the old drainage course, and by the localisation of the surface or land water flow.

12. The improper or imperfect drainage of land outside a railway fence, causing land water to accumulate in and discharge itself through the slope, thus disturbing the established equilibrium.

13. By an interference with the natural flow of any underground waters.

14. By allowing water to accumulate in the gullet, or upon the formation during the process of the excavation of a cutting.

15. The local percolation of water through the slopes of a cutting from the defective construction, wrong location, or permeability of the surface of a drain upon the cess.

16. An accumulation of water caused by the unevenness of the slopes.

17. The acceleration and inducement of a flow through the slope of any water contained in land outside a railway fence, and consequent incitement to the land-water to exude.

18. Vibration.

19. Insufficient flatness of a slope either at the time of excavation or after exposure to meteorological influences.

20. The strain upon the face from lumps of earth being allowed to remain upon a slope during the construction of the works, or the gullet being excavated for a considerable distance in advance; the cohesion of the soil being thereby unduly and unequally strained.

21. By overweighting.

22. By unequal loading.

23. The establishment of spoil banks upon the cess, or the additional loading of the ground near and outside a railway fence, in soft soils having practically no cohesion or tenacity.

24. The excavation removed destroying the continuity of support, especially in soils partaking of a semi-fluid character.

25. The want of uniformity of the covering of a slope causing unequal percolation or exudation of water.

26. The neglect to fill up, or otherwise remedy, cracks or fissures in the slopes or cess.

27. By artificial or irregular consolidation either of the formation or slopes superinducing movement and weathering in any portion not so compacted.

28. From an accumulation of water behind a retaining wall at the foot of a slope, resulting in the stability of the wall being overcome by pressure.

29. By unequal pressure upon the foundations of a retaining wall at the foot of a slope caused by lateral over-pressure tilting it, or by its unequal settlement.

30. In sidelong ground, by the removal of support against the action of sliding, which, without artificial aid, may not be arrested until the slope of a cutting on the higher side approaches the steepest inclination of the face of the hill.

31. By blasting laminated rock dipping at a considerable angle towards a cutting in the side of a hill; the result sometimes being that a cavity is made depriving the upper beds of support and causing them to overhang, and a mass extending to the top surface of the hill to slip along the unsupported stratum.

The first ten “heads of the chief causes of slips in cuttings” might be classed as NATURAL, i.e. produced or effected by nature and, therefore, beyond the power of man to entirely prevent; the remaining heads as ARTIFICIAL, and therefore, in some degree to be prevented, unless obviously the result of the unavoidable exigencies of construction.

Heads of the Chief Causes of Slips and Subsidences in Embankments.

1. The percolation of surface water into the toe and under an embankment upon the original surface of the ground, and also downwards through the formation.

2. Unequal percolation of water through the formation or the slopes.

3. The surface of the ground upon which an embankment is tipped inclining in one direction, or falling on each side from the centre.

4. The effects of rain, frost, snow, and the atmosphere on the deposited earth.

5. By a crumbling of the lower portion of a slope.

6. By a hurricane or extreme wind force, especially when accompanied by rain and the location is that of a narrow, steep valley, blowing away or dissipating the top and the surface of an uncovered embankment.

7. By a slope becoming honey-combed by rodents, and in some few countries by an embankment in a river or the sea being bored and disturbed by crustacea.

8. Insufficient slope for permanent stability at the time of or after deposition.

9. The want of adhesion between the surface of the ground upon which an embankment is tipped and the deposited material.

10. By the accumulation at the foot of an embankment of boulders or lumps having no cohesion, and no adhesion to the surface of the ground upon which they are deposited.

11. The weight being too great upon the ground upon which an embankment rests.

12. By unequal loading.

13. An accumulation of water, caused by the unevenness of the surface of an embankment, or by the ground not being prepared so as to prevent a lodgment of water.

14. By obstructing the established discharge of land-water or by attempting to divert the natural flow of underground waters.

15. The localisation of the surface drainage.

16. Water percolating into a benching trench, made to receive the toe of a slope, thereby impairing the cohesion of the soil and reducing its weight-carrying capacity and stability.

17. Vibration.

18. The different nature and state of the earth tipped into an embankment and the consequent localisation of water in the more pervious soil, causing unequal settlement, subsidence and movement.

19. An embankment being tipped of material in a different state of dryness, moistness, hardness, softness, or in a frozen condition.

20. The size and character of the earth, as excavated in the cuttings; for instance, whether picked and shovelled soil, or lumps of excavation simply barred away and deposited in small masses with earth approaching a state of dirt or mud.

21. Overpressure upon the material forming an embankment.

22. The different conditions of the weather, when an embankment is tipped, causing portions to become dry, wet, or frozen.

23. The lead or distance from a cutting to the place of deposition being of considerable length.

24. The earth being loosened from vibration and concussion during transit in the wagons, and in the process of deposition, thus causing it to be non-homogeneous.

25. By the earth being tipped with greater impetus at one part than at another place.

26. An embankment not being tipped to its full width as it progresses, whether in one or more wagon roads.

27. First tipping the central portion of an embankment, and completing the width and slope by side deposition after some time has elapsed.

28. Tipping the contents of earth wagons from a considerable height, thereby loosening and separating the soil, causing the larger and heavier material to be near the foot of a slope and in lumps, and an embankment to have interstices and be temporarily or permanently unstable.

29. Irregular consolidation, artificial or otherwise.

30. Unequal exposure, particularly in embankments upon sidelong ground.

31. Insufficient width of the formation, especially in high and exposed embankments.

32. The junction of two embankments tipped from cuttings in different kinds of earth.

33. No time being allowed for subsidence or consolidation before the deposited earth is subject to varying loads and vibration.

34. By allowing water to collect upon the formation and to form channels down the slopes.

35. The neglect to fill, or otherwise remedy, cracks or fissures.

36. The want of uniformity of the covering of the slopes or top of an embankment causing unequal percolation of water.

37. By a retaining wall at the toe of a slope preventing the discharge of water that has percolated through the formation and the slope.

38. By an abnormal increase of the load upon the foundation of a wall caused by lateral thrust and tilting forward, or fracture of the footings or the concrete bed.

The first seven heads of “the chief causes of slips in embankments” might be classed as NATURAL, i.e., produced by nature, and, therefore, beyond the power of man to entirely prevent; the remainder as ARTIFICIAL, and, therefore, more or less to be prevented.

In the case of earthworks made to contain or exclude water for the purposes of docks, canals, waterworks, reclamation of land, irrigation or drainage, &c., may be added, without reference to their construction:—

1. Leakage along a discharge culvert, sluice, or tunnel in an embankment, or through the earthwork.

2. Erosion of the land slope or backing of a retaining wall by waves, or spray falling thereon and necessarily passing over the top of an embankment.

3. From abrasion and damage, caused by vessels or barges rubbing or colliding against a slope.

4. From erosion caused by wave action produced by the passage of boats or ships propelled by machinery; or by wind waves.

5. Variation in the water level, causing unequal pressure.

6. In the case of a reclamation embankment, it being closed from the ends, and not by raising in layers, from the ground level.

7. Variation of the submerged area, and consequent change in the degree of exposure to deteriorating influences.

To sum up, the principal causes of slips in earthwork may be stated to be air, water, frost and thaw, over-pressure, and vibration; the chief agent both in cuttings and embankments being water, which forces forward the surface of the slopes and destroys the cohesion of the soil, and impairs its frictional resistance until the earth is unable to sustain the weight upon it; vibration aiding and completing the movement, as it not only tends to loosen the soil, but may disturb the equilibrium of earthwork which is almost moving, and only requires a slight shock to set it in motion: in fact, vibration is frequently the complementary agency that causes a slip, and is obviously felt most in loose soils; but if there should be fissures in earth of a tenacious character, or boulders in clay to disconnect it, the effect of vibration will be more serious in the latter case, as whole masses of earth may become detached, instead of an equal settlement proceeding as with soils, such as sand and gravel, which may become consolidated by shaking, owing to wedging of the particles, should the slopes be sufficiently flat to prevent lateral movement; but it may detach portions of the slopes in soil having little or no cohesion, and thus initiate a slip.

In the following chapters the chief causes of slips in cuttings and embankments are considered, together with others bearing upon a solution of the subject, which is so interwoven that it is impracticable to preserve a successive order, but an endeavour has been made to separately indicate the cause and some remedies that may be adopted: but before proceeding to particularise, it may be well to name a few dominant principles of the alignment of public works, which if duly regarded may tend to prevent slips of serious importance.

Consequent upon financial and other causes, an engineer is usually required to so quickly prepare the necessary parliamentary plans and sections of public works, more particularly for railways, that it is beyond the power of the most experienced to set out in a few hours the best line of railway, &c., across a country, giving due consideration to the parliamentary, constructional, economical working, district and through traffic, and financial requirements of the undertaking. There are, however, a few points which he may be able to regard respecting the stability of earthworks, some of which are now enumerated.

1. Avoid cuttings or embankments in drift soil in or upon the side of a hill.

2. Avoid all damming back or flow of the natural drainage waters, or heaping of snow by the erection of an embankment, especially in mountainous, hilly, or sidelong ground, and in an undrained district.

3. When any excavation is in the side of a hill, observe the natural configuration of the ground in the wettest part, and remember that the slope of a cutting may not stand unless at the same inclination, or the toe of the slope is supported by a massive retaining wall and extensive draining, and that any disturbance may cause it to require a flatter inclination.

4. Avoid river or stream diversions in earth of a very porous character; and should an embankment have to be erected near to a deep river having a steep bank, locate the line a sufficient distance from the edge that the slope of the river bank may be flattened when required, as an extraordinary flood may cause it to be unstable and to fall in, and in order to restore it to a condition of stability, it may be necessary to widen the river and reduce the angle of inclination of its banks.

5. In treacherous earth do not locate a railway close to a road, except at a station, as if the line be placed above the road, a slip upon the railway may result in a slip upon the road; and should the line be below a road, the extra weight or vibration may cause the road to follow the railway and act, as it were, in unison with it.

6. In treacherous soil, where practicable, have stations nearly upon the surface of the natural ground.

7. Remember that upon one side of even the narrowest gorge or valley the earth may be much more solid than upon the other.

8. In exposed situations in a hilly country ascertain upon which side snow remains the longer, note which receives the greater amount of sunshine, and is the wetter and more covered with trees and vegetation. In a hill or mountain, the side to leeward of the prevailing winds almost invariably receives the greater rainfall.

9. In mountainous or hilly districts it may be advantageous to place a railway or road at a high level upon the sunny side of a hill or valley, as obviously it dries quicker; snow does not accumulate with the same facility, unless it happens to be exposed to the direction of the prevailing storms; the sunny side may be practically clear of snow, the shady almost impassable; but there is one drawback to the sunny side, namely—the more frequent occurrence of snow-slips, which may or may not be serious in extent. On the whole, experience seems to show that the wooded side of a valley is the best to select—sometimes one side is bare and the other wooded—unless there are special reasons to the contrary. In the winter season in certain districts abroad, for instance, in parts of Afghanistan and the adjacent mountain passes, the days are sometimes as hot as the summer of European countries, but at night the thermometer may fall below freezing-point, hence the value of tree-protection. It is obvious that in such climates the soil is peculiarly liable to disintegrating forces, and likely to slip, unless of a solid character, and that upon one side earthworks may be stable, and upon the other treacherous.

10. In many countries it may be advisable to adopt a valley in preference to a hill-side line, especially if the district is free from floods, or if the waters flowing down the side of the hills are considerable and suddenly appear, and there is a river and ample means available of controlling the hill-side torrents and conducting them to the river. If not, and the line must be on the mountain side, it may be advisable, in exposed places where water rapidly accumulates and becomes a torrent, to adopt the system of short tunnels round the hill spurs, in preference to deep cuttings, drainage, and slope protection works, and to place the railway or road at the highest level, so as to be free from the influence of floods.

11. Bear in mind that in high mountainous districts the drift deposit is generally torrential alluvium.

12. Avoid as much as possible high embanked approaches to a river bridge, especially when a deep river, which frequently changes its course, is in a flat country.

13. Consider if the simple erection of an embankment may in time cause its destruction, by the arrest or attempted diversion of the usual flow of the land waters.

14. In treacherous soils, on the side of a cliff facing the sea, determine whether it is preferable to erect timber trestles at its base instead of a solid embankment, or to place the line in a tunnel. The trestles can either be erected upon sills, resting on the ground and on short piles well secured from movement, or on piles driven some distance into the ground. The system may also be adopted if the ground be of a yielding character.

15. It has been noticed in some mountainous districts that the clouds break against the highest main ranges, discharge themselves on the smaller ranges, and generally do not reach the inner ranges which rise on the high table-lands; therefore, consider whether by locating a railway or road near the latter, there is much less probability of a slip in the earthworks and less provision required for surface and flood waters.

16. The configuration of a district through which a railway or road must be aligned, may be such that its location becomes one more based upon placing it on soil which is less bad or treacherous, than upon firm or stable ground. This is especially the case in hilly countries contiguous to the sea or large rivers. It may be optional to construct the works upon low or valley ground upon the side of a mountain or hill, or close to the sea shore or a river bank, which may require continuous defence works to protect it from waves and erosion; or on table-land which, however, if impervious and retentive of moisture may act as a catchment reservoir between hills, and cause the ground to be always in a damp state.

The character of the soil, the magnitude, and especially the average height of the embankments, or the depth of the cuttings, the easy drainage and discharge of the rainfall, and an economically constructed, maintained, and worked line are the chief conditions to inseparably bear in mind in determining the location.