The Probability of a Slip—Time of the most frequent Occurrence—Some Conditions under which Slips and Subsidences in Cuttings and Embankments may be expected in different Earths, such as Rock, Chalk, Sand, Gravel, Clay, &c., &c.,—Notes on the Slopes of Repose.

It is of importance to know when serious slips are most likely to happen and under what conditions they are probable, for the process of disintegration may commence immediately the earth is excavated, and be very gradual, although the soil may remain stable for many months, or even a year or two, because the earth has not had time to be affected to the point of instability.

The history of recorded slips appears to indicate that the most serious movements of earth and those most difficult to remedy occur in the following soils.

Drift Earth upon rock in sidelong ground.

Chalk Soils, as witness the slips in the early part of 1877 in the cuttings near Folkestone, and the more recent on the Calais-Boulogne Railway, and that in an embankment of chalk at Binham’s Wood, near Balcombe, in October 1853, when in a length of about 200 yards some 70,000 cubic yards of earth slipped towards a valley. Probably this is one of the most extensive recorded slips of a railway embankment of chalk; however, in this case the traffic was not stopped, but only delayed.

Clay Soils, especially the yellow clay; illustrated by the notable slip at New Cross, near London, when some 90,000 cubic yards of yellow clay moved upon the smooth surface of a shaly clay bed and covered the formation: also the brown, and boulder clay, and the lias clays, as witness the well-known recorded slips in the Midland counties of England, in which either aluminous or calcareous material may preponderate.

There are few, if any, earths in which the cohesion, weight-sustaining power and ability to resist the action of water and meteorological influences are practically the same at all depths, the different conditions, arrangement, and character in which they are found being almost infinite, and there are earths which may become consolidated and watertight if in a constantly moist and protected state, that when dry and exposed will shrink, fissure, and soon become unstable.

Consequent upon cohesion, a cutting may stand for some time almost vertically; nevertheless stability cannot be considered as solely regulated by the cohesion of earth, for an embankment of gravel, sand, or broken rock with a proper slope and protected from erosion will usually safely bear more load than an embankment of clay, although the former material may be said to have no cohesion; but the lateral thrust of dry, firm sand is known to be small, provided the sand is not disturbed; also no earth can be said to be immovable under every condition, but consideration of the soils particularly liable to disturbance or mutation is, under ordinary circumstances, the main question to be determined.

With few exceptions the exterior or faces of cuttings and embankments will, at certain times, become impaired or soddened by the infiltration of water. In cuttings there is the additional danger, owing to the geological formation, of the excavation reaching the depth at which water is generally found in the locality, and it is therefore advisable to ascertain this level, and also to decide whether the ground must be excavated below it, as necessarily there will be a downward flow, and the slopes and formation will consequently have to sustain a pressure due to the difference between the normal level of the water-bearing stratum in the neighbourhood and that of any depth beneath it. In such a case, in addition to the usual softening and loosening aqueous action, there is the particular insecurity of the formation and slopes being undermined and eroded by springs: also in pervious soil in a drained district a cutting will be found to be comparatively dry to about the level of the bottom of the existing contiguous drains, but below that depth water will be present, probably in considerable quantity. Land drains also frequently cause slips, as they localise the flow of the surface or underground waters, and when in excavating a cutting they are intercepted, the discharge should be led away from the slopes; but difficulty may be encountered in effecting this, as water will usually follow its original course, and it may be impossible to entirely divert the direction of the flow, and the only thing to do may be to gently conduct the water down the slope by means of pipes, rubble, burnt brick, gravel surface drains, or timber ducts.

An important question to determine is, when are slips in earthwork most likely to occur.

In Europe they are most frequent in the autumn and winter months; but no rule can be established, nor is it reasonable to conclude, because any earthwork has remained stable during the usual period when slips may be expected, that, therefore, none will happen, for the heaviest rain may descend at an unusual season, and as moisture is the chief cause of the instability of all earths, it is rather to the quantity of the rainfall at any time than to the fixed seasons that attention should be directed.

Spring being the driest season in England and autumn the wettest, October and November being the months of heaviest rainfall, slips are more probable in the latter than the former season; but the first heavy and continuous rainfall after a period of estival drought is that particularly to be feared, or the first rainy weather after a dry period irrespective of the season of the year; but serious slips may not occur for many days or until the expiration of even a month or two after such rainfall, as the ground waters require time before they percolate to or reach the site of a cutting, and, therefore, all danger may be thought to be past when it is steadily approaching. The autumnal rains have to replace the moisture that has evaporated during the summer, and this may not, and usually does not if it slowly proceeds, produce instability in earthwork; but immediately the rainfall approaches or becomes in excess of the power of the natural absorption of the soil, the rain must flow away, for the earth being fully charged cannot contain it, the surface becomes wet or the mass soaked, according to the degree of the permeability of the soil, and the quantity of rain necessary to produce saturation; therefore, the state of the earth that induces a slip is that most desirable to know; this cannot be absolutely established in every case, for it depends upon so many influences, and obviously varies according to the character of the earth, the varieties and conditions of which are practically infinite.

In countries that have dry and wet seasons, which cause the earth to become parched and then to be rapidly saturated, mere surface waters to become streams, and rivers torrents, slips are probable soon after the commencement of the rainy season.

When frost follows rain or a fall of snow, and the latter has descended upon a frozen surface and a thaw sets in, particularly if it be accompanied by a warm wind causing it to be very rapid in action, the earth is severely tried, for the frozen water in the ground becomes suddenly liberated, while the surface is in a state of saturation. Probably the worst event that can occur for causing floods is when a sudden and rapid thaw follows a heavy snowfall upon frozen earth, as then the snow will melt, and water cannot gently percolate the earth, as the surface will be in a more or less frozen and impermeable condition, and the snow-water consequently must flow away.

Should any excessive or violent rainfall succeed a period during which the heat of the sun has caused fissures in the surface of the earth, allowing water to enter, the state of the soil is favourable to movement. Land has also become unstable in mountainous countries because a district has been deforested, or tree-protection much reduced, movement of the earth usually happening after the first heavy rains or thaw succeeding frost or snow.

It has also been noticed that when an earth has become completely saturated or water-charged, a sudden fall of the barometer to a low pressure will liberate the pent up water which the soil cannot contain and cause it to burst out, the equilibrium having been so delicate. Under such circumstances slips are nearly sure to ensue, and to be serious from their sudden action.

Extensive slips in earthwork seldom occur during the excavation, or a short time after the completion of a cutting; on the other hand, movement in an embankment frequently happens during deposition. In the case of an embankment, time may cause the earth to become consolidated, but in a cutting the disintegrating and disturbing forces, and the combined action of air and water percolating until they force forward the earth, are usually gradual in their operation, and often require a year or two to cause a state of instability; in fact, the history of slips, with a few exceptions, in soils whose condition is very readily changed by water, indicates that serious movement in cuttings does not generally occur until a cycle or two of the seasons has elapsed, during which period meteorological influences, aided by vibration and other deteriorating operations, are slowly and regularly proceeding, until at length such a change in the general condition is caused that a slip happens, apparently from some sudden agency, whereas the stability of the earth has been gradually and surely wasting away for a long time; hence the importance of continual careful observation in cuttings even of moderate depth in doubtful soil.

In canals and works of a similar character constructed to contain water, if any movement or slip of earthwork takes place, it usually occurs within a short time of the water being admitted, and generally within a few months and seldom after so long a period as a year, the ground in a short time becoming consolidated, being exempt from severe vibration and many of the disturbing agencies present in railway cuttings and embankments.

In endeavouring to ascertain the probability of a slip occurring, not only should the superficial strata be considered, but also the original formation of the country; for instance, drift-soil, which is generally met with upon the surface of sloping rocks, may consist of various earths intermixed in endless variety, and in every conceivable shape, and is not necessarily produced by a weathering of the rock upon which it lies, for it may have been brought from a distance. In any case, drift-soil is the result of decomposition and disintegration, and from its nature is unreliable and ever subject to change, to slip, and to subside, and so are most of the glacial deposits and moraine found in mountainous countries; and whenever the contour of a district is irregular and has numerous clefts, soft and marshy places, valleys and hills, earthworks will require to be protected against slips; also, should a cutting be at the base of a cliff or hill, it will probably have to be excavated in drift deposit and, perhaps, in silt if below the water-level of adjacent sea or river, and the ground dips towards the natural outfall of the land-waters. Such drift-soil may be alternately dry and charged with water from the rocks above, especially if they are much fissured and water-bearing and permit easy percolation of water, and must always be in a state of mutability.

Rock.

With regard to slips in rock, or earth generally classed as rock, the unstratified or igneous rocks, although they are sometimes traversed by mineral veins and dykes, are the less likely to slip; but rocks liable to surface decomposition and disintegration, such as some varieties of basalt and clay-slate, which latter by atmospheric and aqueous action will partly return to its original state of being fine mud, thrown down from the metamorphic rocks, may change their condition and are likely to slip; and also limestone rock, which however resists the eroding action of water better than sandstone, may become separated by frost although its surface soon dries.

Simply knowing the general character of a rock without ascertaining the proportion, state of the different particles of which it is composed, and whether any metamorphic action has taken place, is not necessarily a reliable guide to its stability. In districts situated at a high level, rocks are usually less permeable than in low-lying lands, and the surface discharge is greater and quicker because of the increased rainfall, and less absorption and retention of water.

It should be noticed whether there are dips in the surface of rock, as they often contain unreliable material, such as pockets and pot-holes of clay, sand, mud, silt and detritus; and movement may be expected if it be carelessly tipped with the rock into an embankment. Rocks which oppose vegetation are usually hard and weather-resisting, and the faults and fissures local; but it is not so much the equal weathering of the face of rocks that is to be feared, as the presence and interspersion of seams, breaks and fissures, and it should always be borne in mind that the condition of a rock varies considerably—it may be sound in one place, and be fissured, disintegrated, and quickly weather in others—and that all laminated and fissile earths are liable to slip because of the percolation of water down veins and crevices.

The durability of a rock may be approximately known by a careful examination, commencing at the surface of the ground and proceeding until it is reached, the thickness and character of the different top soils being noted, and particularly whether the degradation is uniform; but rock, such as some sandstone, which allows water to ooze, permeate, or force a passage, is of doubtful stability. Weathering may be possible only upon the surface or may gradually extend downwards, and as it can hardly be called a slip, the point to ascertain is not so much that it is sound and weather-resisting, as to know that there is no chance of any portion becoming detached or sliding, through the cohesion of the joints being impaired or destroyed by water, frost, or other agencies; for in the case of rocks which show irregularities of stratification, much cleavage, or are separated by upheaval, or have synclinal and anticlinal folds, masses become detached along the line of cleavage and independently of the normal stratification, therefore fissures or faults, weak veins between masses of rock and crevices, or inclined beds through which water may flow and always be present; the direction and inclination of the dip of the strata, effects of weather upon the veins, and to know the weight upon sloping ground which the rock will bear without sliding down a hill are the main considerations; for rocks may be distorted, upraised, contorted and tilted at every angle, and even horizontal beds may repose upon the upturned edges of other strata.

An inclined water-bearing stratum between rock loosely bedded and inclined towards a cutting, unless drained and supported, will probably cause a slip consequent upon the action of water or frost; on the other hand, veins may alone hold the masses together, and, therefore, when they are affected the cementing medium is destroyed; however, inclined water seams are a frequent cause of slips, for where any water-bearing earth meets a closer and consequently less pervious stratum, damp surfaces are produced, and an unstable condition; consequently, mixtures of rock, clay, and sand, are usually troublesome. Also in a cutting in sidelong ground if a stratified rock dips parallel, or nearly so, to the slope of a hill, slips are probable, as it may slide towards the cutting. Similarly, in a cutting in drift or alluvial soil, or any that will quickly weather, resting upon rock, especially should it have a smooth bed inclining towards the formation, the superimposed earth will usually be unstable, and even the act of penetrating the top stratum, or the erection of a retaining wall or the weight of a small embankment upon it, may cause it to move; and when motion has commenced it is difficult to arrest it; and should water trickle upon the surface of the rock, it may cause the upper stratum to slide; also when water flows or remains upon rock having a superimposed bed of shale or clay, the top stratum may not remain at rest even though the surface of the rock may be nearly level and practically waterproof, and where rock beds overlie shale which is liable to become softened by time and water and to perish, particularly when the beds are twisted or contorted; as, for instance, limestone or sandstone upon shale or marly-shale, the latter becoming softened by the action of the atmosphere, water, or frost will form a sliding medium upon which the rock may move, or should shale overlie rock, as it frequently does, it may slide upon the hard surface. All alternate beds of shale or any softer earth than the rock, particularly broken shales when found mixed with sand and clay and the lias shales, and rock should be regarded as treacherous and liable to slip. Also some of the slate rocks, as they frequently have veins of limestone, &c., and as the latter decomposes it mixes with the clay and becomes of a marly character. Dark blue shale or indurated slaty clay is sometimes difficult to excavate, but when exposed to atmospheric and aqueous action it breaks into pieces and becomes little better than a treacherous clay. The cohesion of shale becomes less as it approaches a greasy clayey condition, and, therefore, one readily affected by water or air, and it may then not stand at a steeper slope than 3 to 1. Rock and shale, which may stand at a steep inclination provided the beds are horizontal, it has been found, do not permanently repose when they dip towards a cutting until as flat a slope as 2 to 1 is given; and where clay and shale beds in cuttings are present, a slope of 1½ to 1 has been insufficient, and they have not been stable till an inclination of 2 to 1 has been adopted.

As cuttings in rock are frequently in the side of a hill, the dip of the strata should be ascertained, and in the case of an unstratified rock, it should be known whether it is fissured or lies upon a solid and firm bed considerably below the level of a cutting, so that it may be prevented from movement. It should also be ascertained if the top stratum is a mere crust, such as a capping of conglomerate resting upon clay-rock, through which water may burst and cause it to separate, and sometimes the rock may be more solid in the valleys than upon the hill-side because of greater diluvial action, and induration caused by exposure.

The crystalline rocks are the least easily destroyed and are generally rough and jagged. The science of geology shows that limestones, sandstones and clays, were originally heaps of mud deposited, removed or arranged by water; and that boulders are transplanted masses from the parent rock, and are worn and rounded by mechanical attrition. Consideration of the manner in which rocks have been formed affords a fair indication of their stability in earthwork; for instance, many clay rocks are reduced to a pasty condition by the action of water and air, but with different results according to their nature, some requiring blasting to excavate them. On the other hand, there are sandstones which although soft in the quarry become hardened when exposed to the atmosphere. A dock cut in red sandstone when exposed to the atmosphere may slip and fail, but if the rock be protected from the weather or constantly covered with water it may be reliable.

As cuttings are near the surface and seldom at greater depths than 100 feet, it is hardly possible to know the angle at which a rock dips or whether there are faults and fissures in it, unless an examination is made upon the site, and this notwithstanding the geological character may be thoroughly understood. Local conditions may cause peculiarities which no law can determine, and although at considerable depths, deeper than railway or any works with the exception of well-sinking and mining are likely to reach, the nature of the earth is accurately ascertained; the surface soil may be in almost every conceivable variety, and also so dislocated, denuded and rearranged, that usually horizontal strata may be nearly vertical. Its character may be accurately known, but the lie of the surface beds or dip of the upper strata, or the order of supraposition cannot be invariably absolutely established; for instance, when crevasses, fissures and veins are frequent in earth upheaved and disintegrated by volcanic action, earthquakes and other disturbing causes, as in parts of South America, Japan, and other eastern countries, Italy, the Tyrol, Spain, &c., &c., slips of earthwork are to be expected, and the soil is likely to be much inclined, full of faults and probably water-bearing seams.

Under the comprehensive name of rock is usually included any earth from the hardest mass to be found to that which will crumble in the hand, as soft sandstone rock. The chief absorbent rocks with which engineers have to do are the limestones, sandstones, chalk, and clay rocks. Rock may also be simple or present the appearance of being homogeneous, or it may be a mass of different substances, be flat bedded, have open or close joints, and be what is called—

Solid rock.

Hard rock.

Dense, or compact, rock.

Loose rock.

Rock in loose layers.

Loose rock with cavities, caverns, and pot-holes of various earths.

Fissured rock.

Friable rock.

Indurated earth liable to be disintegrated by atmospheric influences.

Decomposed rock.

Rotten decomposed rock.

Or any mass of earth cemented together by a substance, weather-resisting or not, requiring blasting or that can be excavated by means of bars and picks.

The slope of repose required may range from overhanging or vertical to that of the earth of which it consists when disintegrated and dissolved; therefore the angle of repose varies considerably, but the following cardinal principles may be followed without fear under ordinary circumstances and conditions.

Granite. Quartz, if not fissured, and when little mica is present in it, and most of the igneous rocks,

Also porphyry, gneiss, trap, but their stability varies considerably.

Compact hard sandstone and limestone and other solid sedimentary rocks producing stone sufficiently hard and weather-resisting that it can be used in construction,

But, if non-weather-resisting,

The slope becoming flatter as the rock becomes softer and more easily disintegrated.

Friable rock, consisting of hard particles,

Loose rock,

Soft shaly limestone and the argillaceous rocks may not be permanently stable until the slope is,

Schistose rock is troublesome in earthwork, being fissile in structure and deleteriously affected by rain and the atmosphere. On the Panama canal works in the Culebra cutting, maximum depth 333 feet 6 inches, numerous slips occurred, displaced the roads, and overturned the excavators.

Mica-schist is variable and frequently has numerous water-bearing fissures. Its hardness depends upon the quantity of quartz it contains; when the proportion of mica is greater than the quartz it is soft and very fissured and contains veins, sometimes of clay, often yielding a considerable flow of water. Should the percentage of quartz be large it becomes hard, holds little water, and is of a gneissose character.

As water is the chief disintegrating agent and cause of instability, it may be well to mention that Professor Prestwich has stated that “hard quartzites, slates and grits (Silurian), purple and grey shales, schists and fissile sandstones with hard compact limestones and dolomites (Devonian), rarely contain any levels of water, and that it is only encountered in fissures”; hence the importance of knowing the position of the fissures, and taking the necessary precautions to promote stability. Where rocks, especially if generally known as water-bearing, crop out at a high angle, and are in well-defined beds, water may be expected, as although it may not percolate vertically, it will along the inclined beds. Sandstone and limestone bands in rock usually cause small springs.

The stability of a sandstone for earthwork and purposes of construction is dependent upon the material which cements or holds it together, whether iron rust, lime, free silica, alumina, &c., &c., &c., its quantity and condition, and the degree of hardness imparted when it was formed, and the nature of the agglutinant; therefore the varieties and degrees of fineness and hardness are numerous. Sandstones are generally found to be laminated when hard; and bare of vegetation if pure and free from marls; they contain and part with water in different proportions, and sometimes have watertight bands crossing them, severing water communication, which may cause earthworks to be of unequal stability. The firmest and strongest are close-grained and fine in texture, the weaker are coarse and gritty, and have a sandy appearance. They may be white, yellow, green, black, red, grey, brown, or other colour, and although of the same hue, their character may not be identical; for instance, the red sandstone is hard and also very soft. Sandstone of a greenish hue is generally hard, much fissured and full of water. When firm, greensand may stand at a steep slope, the surface being protected; but it varies considerably, and may at one place be close and yet be gradually deteriorated until it is of the character of fine loose sand.

Should it be found upon excavating sandstone that it is not upon its natural bed, but distorted, upheaved, or vertical, it will probably split and become detached under the destructive action of air and water. In tropical climates it has been found that sandstones generally dissolve and become disintegrated when used in damp foundations. Limestone also varies much in character, and is treacherous, whether it is hard or soft, when pockets of clay or sand are present and beds of clay are contiguous. Percolated water having carbonic acid in it may also soften or dissolve it. The softer kinds if in fragments as ballast, or when deposited in an embankment, often become quickly disintegrated by frost and the weather, as do sandstones.

Should any rock strata be vertically inclined instead of horizontal, although it may be known in the latter case they are generally watertight, fissures in the upheaved beds may become channels for the passage of the subsiding or rising waters, and may cause saturation of the soil over a considerable area and induce a flow through the slopes or the seat of an embankment. Upon such a site no reservoir, dock, canal, or any earthen structure to hold water should be placed; but although the nature of the ground may be fatal to the stability of such an embankment, so long as the underground waters do not rise to the level of the seat of a railway embankment or flood a cutting or burst the slopes, they may not seriously affect the stability; for, unless the head level of supply is great, more danger maybe expected from downward percolation saturating the ground upon which the embankment is placed than from an upward flow. The insecurity of erecting a reservoir or similar work upon such a site, in which water is brought into a district in greater quantity than its natural flow, is obvious, as the earth may gradually become saturated from the constant leakage down the upheaved fissures, until it becomes in an unstable condition and finally slips and subsides.

Chalk.

As in most public works, with the except:on of tunnels, wells and mines, the chalk with which an engineer has to deal is surface chalk, or the top layers of that deposit known as the upper chalk, almost invariably containing much more water than the lower chalk, although it rises quicker in the lower beds, as it is under greater pressure, and which vary in hardness, purity, and solidity and may have frequent fissures and holes, with or without flints, and be anything from hard, compact chalk rock to mere marly calcareous earth; considerable judgment is required to successfully determine the slope of stability and the precautionary works that may be necessary to attain repose; for some of the upper beds soon weather, and being soft, friable, and fissured are permeable and liable to slip; in fact, the Oolitic series, as it consists of alternating bands of limestones and clays and occasionally sandstone, is frequently fissured and has loose joints and therefore requires to be carefully treated.

The range of the slope of permanent stability obviously depends upon the nature of the chalk, whether it is denuded or covered or mere loose-jointed strata, the effect ground and surface waters may have upon it, and also the position of the beds, and whether a cutting or embankment is on the side or the base of a hill, and consequently at the place where it is likely to be in a wet condition.

The Needle-rocks in the Isle of Wight and Beachy Head may be mentioned as familiar examples, showing that firm and comparatively pure chalk will stand practically perpendicular, even when much exposed, if pure and free from faults and homogeneous in texture; and in blocks with beds inclined away from a cutting it will permanently stand

a slow regular crumbling of the surface or falling down of small fragments, which seldom produce serious movement in such material, being the only deleterious effect of weather influences.

As the chalk becomes broken and less evenly bedded,

Loose, friable chalk in surface beds will often not permanently stand at a less slope than from

according to the depth and degree of exposure. The most usual slopes being

Impure wet chalk and marly chalk will require a slope not less than

Much depends upon its freedom from faults, crevices, and pot-holes, as they hold water, and the surrounding soil may fall away, for water quickly passes in quantity through the fissures and crevices which are generally numerous in the upper chalk, especially at the bottom of a bed of flints which in consequence of their impermeability lessen the upward flow; but flint beds in soft chalk are an advantage, as they act as drains.

The affinity chalk has for water, which has been considered a reason for the absence of important rivers in that formation, as water does not flow away freely upon it, causes it to be readily affected by rain and disintegrated from the effects of the expansive and contracting action of frost and thaw; hence draining and covering the surface may be important, but care must be taken not to interfere with natural springs. This property of chalk, viz., its affinity for water, although a disturbing cause in earthwork in that formation is of value for covering or filling in open trenches, counterforts, or drains in other soils, as the chalk attracts water, and therefore dries the surface of other earths.

Anything that localizes the percolation or flow of water, or helps to make water seams, veins, fissures, and hollows, which are sometimes filled with sandy gravel, loam, and detritus readily admitting water, will tend to break the chalk into separate masses and cause it to become loose and unstable by the action of rain, frost and thaw, and vibration. Should flint beds occur in chalk, and they frequently do in the upper beds if they are horizontal or nearly so, much more water may be expected to flow along their bed, as it forms a water-passage, than when they are in inclined or vertical seams. As chalk absorbs much water, but does not readily exude it, although it may soon become dry upon the surface after rain, it is advisable to lessen percolation in order to prevent slips. It is known that the angle of friction of water in chalk will affect the flow and that the discharge varies greatly according to the character, fracture, and other conditions of the soil; for instance, it has been proved that a hydrostatic pressure due to a gradient of about 1 in 132 is required to enable water to pass through the chalk as found at Dover, whereas in the Hertfordshire beds much less is required, namely, that equal to a gradient of about 1 in 350 to 1 in 420. This is named as showing, even when unfissured, the varying perviousness and character of chalk, and that it cannot be treated as a material of even approximate consistency of texture. The power of capillary attraction of chalk has been proved to be great and the evaporation from the surface practically unlimited. These properties and its known affinity for water render it liable to constant change; also the particles of calcareous soils being affected by moisture and to a certain extent soluble, water will take up lime in them, and therefore they are treacherous earths and liable to slip and subside.

Professor Ansted has shown that a cubic foot of the upper chalk when dried will absorb 2½ gallons or 40 per cent. of its bulk of water; the lower chalk 2 gallons, or 33 per cent. of its bulk; and that the pores of a cubic foot of chalk are equal to 40 per cent. of the bulk, and are therefore equivalent to the area of a pipe about 9 inches in diameter.

Ordinary drainage will not remove the water, hence chalk is a difficult soil to treat successfully, and slips and subsidences may result in such large areas as the surface of cuttings and embankments simply through the difficulty of preventing it becoming saturated.

Another characteristic of chalk, which requires careful observation to prevent slips, is that water does not generally issue through a mass, or equally over a consider able area, but is discharged through fissures, and crevices, and flint beds; hence one of the chief means of preventing slips cannot be adopted, namely, to disallow a localization of the flow of any water; therefore, the disturbing element of water seams is in greater or less degree present in all chalk-earth that is not solid and homogeneous in texture. The flow from such water-veins or seams should not be interfered with, as any obstruction, and possibly diversion, which is likely to fail, will only result in the spring saturating the adjacent soil, and in its bursting out at another place. There is no safe remedy but to gently lead away the water, for where springs occur, either in chalk or rock, they will find the line of least resistance; consequently the waters of percolation will tend to flow to one place, and cause a spring.

Chalk is found in regularly stratified and separated masses, sometimes caused by beds of flints, and although the position of the layers may indicate their successive ages, age can hardly be taken as an absolute indication of the increased stability of chalk in earthwork. When overlaid with clay it is usually harder than when bare, probably owing to pressure, non-exposure to atmospheric influences, and to the absorbed water being of a different character, which has been proved by analyses. It is especially advisable in chalk soils to know the head level of water in the district, and to note if the bottom of a cutting is below the usual water-bearing line in the open wells, which may not necessarily be at the same depth; their average level being ascertained, an idea can be formed of the probability of springs bursting out, and according as the rainfall is excessive or not, so usually will be the flow.

If chalk beds incline across a valley, and have an impervious stratum of clay upon them, it has been found that the most water issues at or about the point where the impervious seam first overlies the chalk, i.e., at the edge of the basin, and the greater its depth, the less the flow; therefore, should a cutting be located at a place where this stratum is thinnest, more water from springs may be expected than at any point where the impervious layer is thicker.

It is also well to remember that the line of water-flow is not necessarily a horizontal plane, for it frequently follows the contour of the chalk, and that the causes of surface irregularities of subterranean water are unknown; but rain-water accumulating in chalk principally rises and issues most rapidly along the bed lines; consequently the flow along these must be gently discharged, or slips will occur; but chalk uniform in character and of solid and close texture, without flints and fissures, usually is not water-bearing, and will stand almost vertically. As a rule the cohesion of the upper beds, if they are homogeneous, is greater than the lower beds, although the mass may be softer.

Should the drainage or natural outlet of the land waters of a chalk district be obstructed or dammed back, from the quantity of water being in excess of that the fissures or water seams in a chalk hill can discharge, and the pent up waters be unable to escape, hydrostatic pressure, in addition to a weakened condition of the chalk through excess of moisture, will be caused, and extensive slips may be expected along the escarpment, the displacement being gradual, the ground separating and fissuring until at length it is pushed out by hydrostatic pressure. Such a slip usually occurs in large masses, resembling a fallen cliff, for the disturbing agent is all-powerful, and the area affected very considerable, and particularly so if the chalk is superimposed upon different soil, or harder ground, as then the whole mass will probably move forward.

Although some approximate inclinations have been previously given, the varieties of chalk are so numerous that no absolute slopes of repose can be named, for chalk or calcareous earth may be:—

Marble or crystalline limestone.

Ordinary limestone rock.

Hard, compact chalk rock.

Lower white chalk.

Upper white chalk.

Hard grey chalk.

Ordinary grey chalk.

Pure white chalk.

Friable white chalk.

Yellow, light and dark blue, soft chalk becoming of marly character.

Hard chalk marl.

Grey marly chalk.

Grey clayey chalk.

Note.—The preceding chalk marls contain so large a proportion of argillaceous matter as to become almost clays so far as regards treatment in earthworks. Many serious slips have occurred in chalk soils, and their history indicates that the chief disturbing element was water, whether held back over a large surface until the hydrostatic pressure became too great for the slopes to withstand it, or from its bursting out in springs, and so separating and disintegrating masses of the earth.

Near the entrances to tunnels slips appear to be most frequent in cuttings in chalk. This would seem to lead to the belief that in places where it is known the chalk soil is likely to be troublesome from land-water and springs, it would be advantageous to prolong tunnels beyond the economic depth of a cutting, and even to continue them until such a depth as 40 feet is reached, to so arrange the gradients that they drain the interior, and to provide a complete system of pipes and drains, even if side galleries have to be driven to tap the water, before it reaches the lining, to relieve the sides, crown, and invert of a tunnel so that no water can pour down the roof or walls unless under control.

In a tunnel so situated and liable to water-pressure, the thrust of the soil will be very variable, and cannot be foreseen. At one place during construction, the walls and lining may be finished without movement of the earth, at another, the pressure maybe great and act unequally, either upon the side walls or the arch. As a rule, at the entrances to tunnels the pressure is greater upon the arch than the sides, for then the whole of the wedge-shaped mass within the boundary of the angle of repose of the soil is disturbed, and its cohesion impaired or destroyed, and therefore it presses upon the arch, this pressure tending to counteract the lateral pressure; but as the depth increases, the load from this wedged-shaped mass becomes less upon the arch, although generally greater than any lateral pressure, because the earth above is not disturbed or impaired, consequent upon the depth of the hill being greater and the cohesion and side-pressure of the earth tending to support it, but the lateral pressure is increased because the normal pressure of the soil due to the depth is augmented. It is this disturbance of such friable soil as chalk at tunnel entrances, causing the particles to be loose and separated, and in a state especially disposed to percolation of water, that probably causes the earth to be in a condition favourable to slips, and for them particularly to occur at or about the entrances to tunnels. When, therefore, the depth of open cutting at the entrances is reduced, any slip cannot be of the same magnitude as it would be if it happened at a greater depth. The circular or one closely approaching it would appear to be the best form for the lining, where variable or great pressure, vertical or lateral, is to be expected; for the pressure in a tunnel will always be unequal, and the surface of the earth must be supported.

In some experiments to join substances by pressure it was found that though great pressure forms chalk into hard blocks, the particles are not firmly united, and that they separate along the surfaces of contact of the original particles and not through them; these tests tend to show that masses of chalk are usually in a state not indisposed to separation. The same result occurred in similarly testing pulverised sandstone.

Sand and Gravel.

In fine sand-cuttings springs may be expected, and the earth become in a semi-fluid state if there is water at a higher level to filter through it; also in the case of all porous and open soils. Any drawing away of the sand must be prevented, as it will induce a slip, and cause the earth to become running sand, especially dangerous near buildings, for its egress must be prevented, or subsidence will ensue, and serious erosion. The excavation in such cases should be in as short lengths as practicable, so that the surfaces are not unsupported, and walls and structures should be quickly erected. The sands that are met with in estuaries are frequently in such a condition, that a slight obstruction to the tidal flow will cause movement, the equilibrium being easily destroyed. Should there be a break in the continuity of a clay stratum, overlying light loose soil, the latter will probably boil up, and in determining the depth of a cutting, care should be taken that this impervious stratum is not broken or injured.

Marl, clay and sand beds are likely to slip when they are superimposed, and there are some districts in which sandy soil is so charged with water that, unless the drainage of the slopes and formation suffices to drain for some distance the land outside a cutting, the sand will become overcharged with moisture and will act as a fluid and slip, the lateral support being removed by the act of excavation and its normal condition altered. Being so delicately balanced the least additional disturbing force, such as a spoil bank being tipped upon the surface, or the inducement or acceleration of a flow of water, will set it in motion and make it a quicksand. For instance, small sand islands have been removed by making cuts in them from 15 to 20 feet in width, and by men shaking bars, &c., inserted in the soil; the sand along the edge becomes loose, falls, and the current sweeps it away. As an example of the changeability of the condition of sand may be named that in sinking pits by congelation in loose sand it has been found that the grains during the freezing of the water, by means of tubes containing a freezing mixture, were additionally separated about 5 to 7 per centum.

As the sand met with in public works is seldom in very deep beds, it has not been subject to the steadying forces which many earths have undergone, and it may have been constantly moving until its final deposition, and therefore it is easily set in motion; and although sand will subside less from a load after it is saturated with moisture, the water in it trying to escape may cause it to slip upon an unsupported surface such as a slope.

Many experiments have shown that the power of absorption of sand decreases with the fineness of the grain, and that sand when thoroughly wet will contain water equal to about one-third to two-fifths of its bulk, and that almost all this can be drained; hence its varying condition and instability. If a well be sunk in sandstone and regularly pumped it will drain the rock around for some distance, the drainage space being conical, its vertex the bottom of the well, and its base the surface, varying in extent according to the nature of the soil and depth of well, showing the porous nature of sand.

The interstices of silicious sea-sand, when not compressed, have been ascertained by Mr. J. Watt Sandeman, M. Inst. C.E., to amount to about 40 per cent. of the volume of sand. For coarse or fine sand, or a mixture of the two, the interstices did not vary much. When it was compressed by a rammer in water, its bulk could be reduced to the extent of 12½ per cent. The interstices of broken red sandstone, varying in size to that which would pass through an 8-inch ring, were found to be 36 per cent. of the whole volume, but as the stones were in contact 10 per cent, must be added, and if under water 15 per cent.

The experiments clearly show the known great capability of subsidence in sandy and open sandy gravel soils, their clear water space, and how easily fine sand may, by a current of water, become running sand, and their adaptability for ramming and consolidation by moisture. Tipped sand when rammed will subside if saturated with water nearly as much as it can be beaten down, which shows how greatly its bulk is affected by water, and although its rapid consolidation is an advantage in embankments, it is of importance that percolation should be equal.

The chief conditions of a safe foundation upon pure sand, namely, that it cannot escape laterally or be undermined, are obviously not to be attained in either cuttings or embankments, as the lateral support is removed, and the slopes are liable to be undermined and unequally charged with water, and the influence of water on sandy soils is the principal cause of their instability, for in excavating cuttings the face will frequently stand at a very steep slope if dry, but upon its becoming saturated the sand may flow, and in the case of gravel and sand, although the stones forming gravel do not change, the whole subsides.

As gravel is found in various conditions, it may be well to classify it as it is herein regarded.

Clean gravel is considered as that which nearly approaches the condition of a pebbly beach. If an appreciable quantity of sand is present, it is sandy gravel. If loam, or marl, or clay, it is loamy, marly, or clayey gravel.

Gravel hills are large accumulations of water-worn rocks, and may have boulders in them intermixed with the freshwater deposits of sands and marls, and by means of natural cementing material between the particles seem to be firmly set and to be so conglomerated as to appear to be in a similar condition to weak concrete; but there is always a chance of the matrix becoming dissolved, therefore it is advisable to test a mass by the application of water and to expose it to the atmosphere before relying upon its permanent stability, and with the view to determine whether it is hard cemented gravel or not.

Gravel may be made more compact and will subside if water is pumped upon it and allowed to filter through, and in making an artificial foundation of gravel, it is not reliable without water percolation and consolidation by ramming.

All earth consisting of particles having rounded surfaces is liable to become loose, and upon weight being placed upon it the grains are inclined to roll and become detached, but if they are angular fragments, which seldom is the case, this tendency will be lessened, and the angle of repose will be steeper.

With regard to the slopes necessary in sand and gravel, the more angular, rough, hard, and clean the particles, the steeper the inclination.

Earth that can be properly called gravel seldom requires a flatter slope than 1½ to 1, and usually a less inclination is sufficient, but if loose it will not stand vertically even for a depth of a few feet.

Solid indurated masses of gravel will stand perpendicularly and as rock.

If the gravel consists of quartz or sandstone boulders, or is very coarse with stones of considerable size, or like a clean pebbly beach, 1 to 1 TO 1¼ to 1.

Ordinary clean gravel of uniform size at about 1 to 1.

Thoroughly compressed, hard, clean sand, about 1 to 1.

Looser sand and light gravel, 1¼ to 1 TO 1½ to 1.

Irregular beds of sand, gravel, clay, and fragments of rock, 1¼ to 1 TO 1½ to 1.

Sand mixed with vegetable matter, argillaceous or loamy sand, about 1½ to 1.

As the proportion of mould or clay in the sand becomes greater a flatter slope is necessary according to the nature of the earth with which it is incorporated, the degree of wetness, and also the exposure of the surface.

Clay loams require slopes from 1½ to 1 TO 3 to 1, and shifting sand when a current of water reaches it will become a quicksand, and not be stable even when horizontal, but if drained and the toe is secured it will usually stand at an inclination of from 3 to 1 TO 4 to 1. On the other hand, an embankment of hard, clean, angular sand, rammed but left bare, when exposed to tidal action with little wave disturbance, has reposed at 2 to 1 TO 2½ to 1 slopes.

Loamy soil and vegetable mould will, for any height not exceeding about 5 feet, stand nearly vertically for a reasonable time.

Clay.

With respect to cuttings and embankments in clay soils, perhaps no earth is more affected by water and air, or more difficult to treat, as it will expand if only exposed to the atmosphere and without contact with water, 6 inches being no unusual dimension to allow for expansion in tunnel-work. This property and its contraction upon drying alone make it an earth particularly liable to slip and induce fissures and cracks through which water can trickle, notwithstanding the surface of the clay may be almost impermeable. If clay could always be kept dry or in its natural condition it would be stable and free from slips; but this cannot be effected, for water is held in suspension in clay for a considerable period, its plastic nature preventing gravitation, and evaporation is known to be a very slow process; and as the same clay under different circumstances may stand nearly vertically or only at a very flat slope, its liability to constant change makes it very treacherous, and it should be classed as a most deceptive earth of a dangerously unstable and unsafe description, for it may be so hard as to nearly turn a pick, and yet water and air will rapidly cause its disintegration, but if weather influences can be prevented from reaching it when in such a hard compact state it will afford a firm foundation.

London clay in its natural condition usually contains about 10 per cent. of water. The more permeable the clay the more likely are slips to occur and the face to become soft, loose and disintegrated, slimy beds being thus produced which are difficult to prevent or remove; therefore a covering of close grass turf, or layer of burnt ballast, ashes, or chalk, upon any damp place or fissure after it has been filled is advantageous.

Solid blue clay, which generally requires the use of the pick, of the clays is, perhaps, the most stable, being almost impermeable if free from delaceration; but yellow and most other clays are unequal in texture, faults and breaks are frequently numerous, and water penetrating converts the surfaces and the mass into a muddy and semi-fluid condition resting only when horizontal, which has been painfully experienced in the crushing in, during construction, of some tunnels in the London district. The trickling of water down fissures forms a slimy and easy-sliding surface most difficult to treat or prevent, and so long as the natural contour of the ground does not offer resistance to movement, a slip may extend for a long distance, either in deep or shallow cuttings, and there may be considerable hydrostatic pressure.

The disruptions, variableness of character, and existence of fissures cause any but the most homogeneous clays to be treacherous and likely to slip, particularly the yellow and any laminated clays, as they allow water to enter by the veins which are usually frequent in the mass. Yellow clay has a greater tendency to crack upon drying than blue clay and does so much more quickly, hence its dangerous nature, and although a mass may be only damp, fissures will enable water to penetrate and reduce it to a state of instability; it is also not infrequently in a plastic state, having fissures and cavities full of water.

It should, however, not be forgotten that in the endeavour to prevent the deleterious effects of aqueous action upon clay soils that they may be over-drained, so that they become too dry, as then the clay will shrink, crack, and fissure; the chief aim should be to keep it always in a sufficiently moist state so as to obviate the formation of cracks and fissures, and at the same time cause it to be dry enough to be firm, and never in a pasty or pulpy condition; in other words, to maintain its natural state if one of stability, and prevent any excess of water penetrating it or reaching its surface.

The lias clays are treacherous chiefly owing to the presence of much calcareous matter, and therefore approach a marly state; heavy slips have occurred in the has formation in the midland counties of England, notwithstanding that a slope was adopted which experience had shown produced stability, namely 3 to 1. A slight variation in the composition of this soil or an unfavourable position will cause a slip in such treacherous earth.

Pure clay shrinks some 5 per cent. in drying, the contraction being less as sand is present in it, for when it is mixed with twice its weight of sand it is reduced to 3 per cent., and as impurities increase in clays the less impervious they become. Most clays have silicious earth in them, but if sand is present the clay is then more open, and water will permeate and drain more freely; but mixtures of clay and sand may assume a pulpy condition when impregnated with water, consequently it is always advisable to test such earth. The varieties of sandy clay are many, and all are usually more or less unstable. Among them may be named red clay with sand and mica, blue sandy clay, sandy green clay, stiff red sandy clay, the loamy clays of various hues, dark grey, red sandy, and black clayey loams.

If clay could be kept in a moist state fissures would seldom occur. The constant alternation of wetness and dryness creates the fissures, and water completes the disintegration. There are a few clays which are stable when kept in solid masses, as then only a small surface is affected by air and water, but if they are loosened and broken up, as in the process of excavation and deposition, they readily become in a muddy condition. Any mud or silt which may be soft and readily pressed when wet, but cakes and shrinks in to detached lumps when dry or upon being exposed to the atmosphere, is a treacherous soil, as it will return to its original state upon becoming wet. Clays or any soils that cake should always be regarded with suspicion, as although having the appearance of solidity and the possession of weather-resisting qualities in their natural position, when disturbed, quickly become worthless for earthwork purposes, and may stand in one situation almost as a soft clay, and when disturbed and wet assume a horizontal surface. Such ground may repose at a 4 to 1 TO 8 to 1 slope, because its crust has become caked or case-hardened, yet when it is broken it may become, upon being exposed, simply fluid mud. To prevent clay soil weathering upon the surface, a layer of gravel 1 foot or so in thickness has been placed upon it, the idea being that it is not only a protection, but the weight of the covering upon the clay will cause the water to be pressed out from the soil into the gravel through which it can percolate to the drains.

A crude test to indicate the probable character of a clay as regards its stability in earthwork is to burn a piece of it and notice the colour. If it becomes white or of a whitish tint, the clay is generally less likely to slip than when it is of a reddish or yellowish tinge. Another rough experiment can also be made. Get a piece of clay, place it in water, and note the time taken and the depth to which the surface has become saturated, and whether it is very slimy and will easily slide down a slightly inclined plane; its tenacity may then be approximately judged. Also by weighing, an idea of the amount of sand may be imagined; the more sand there is in clay the lighter it will be, all other conditions being identical. A comparison between two clays will enable some opinion to be formed of their relative stability in earthwork, though, of course, there are many other features to be considered. All impure clays, such as shaly clay, sandy clay, loamy clay, and marly clay require to be carefully treated, although they may be easier to manage than yellow or brown clay.

When two retentive clay beds overlap or overlie, and have no intermediate permeable stratum, they must be in a humid state, as is the case in the Fen country, unless they are constantly drained; but serious slips are not so likely to occur in them as when two masses of clay have an interposing seam of sand or silt liable to be eroded by water falling down fissures in the clay, which probably extend to considerable depths, with the result that two slimy surfaces are formed and the clay slides. Clay underlying gravel often contains numerous pockets and seams filled with running sand, and should there be a permanent head of water the discharge will be in large quantities and at a considerable velocity. A cutting in wet sandy clay is generally treacherous and difficult to manage.

As a clay bed near the surface of the ground is sometimes upheaved, if a permeable stratum such as gravel or sand overlies it, the drainage of water through or down the slopes will be arrested, and the earth at the back of the slope will be constantly wet and may ultimately become saturated through the damming back of the water; then a slip may be expected.

All upheaved, dislocated, and twisted superficial beds of clay, which will generally be of varying consistency and therefore settle unequally; over or underlying seams of sand or gravel, are likely to slip and subside, and their stability much depends upon whether or not the lie of the beds obstructs the permeation of water. If the dip of the clay-beds is towards the natural outfall, most probably an adjacent river, slips are probable because of the creation of sliding surfaces and the continuity of the beds being destroyed by a cutting and the consequent loss of support.

Should permeable soil lie between the top stratum and a bed of clay, water will accumulate upon the clay, make it slimy and cause a flow upon the bed; for example, when a thin bed of vegetable earth rests upon gravel, sand, or peat, and that upon clay, water will percolate, and perhaps air, through the top soils, and may cause them to slip upon the clay-bed. Also should a layer of clay overlie permeable strata, as clay upon sand, or clay upon gravel, unless it is sufficiently thick and solid to prevent infiltration, it may slide upon the permeable soil as its lower surface becomes wet. When clay-hills have veins, water may accumulate in them and flow, and if very dark yellow clay overlies light yellow calcareous clay, which may rest upon hard blue clay, it is obvious each stratum is somewhat differently affected by weather and air, and therefore movement is to be expected. The edges of clay-hills are always likely to slip, especially should they be in the form of spurs.

Boulder-clay is seldom reliable, because, although it may be hard and stand vertically in dry weather, in wet it swells, weathers quickly, becomes soft and cakes upon drying. The stability of such soil is governed not only by the nature of the clay, but by that of the boulders and their effect upon the earth in which they are embedded, and much depends upon the degree of changeableness upon exposure to air and moisture of all the particles of which they are composed; hence boulder-clay, although hard to excavate, may quickly dissolve. On the contrary, it may occasionally be so hard that it seems to be solid rock, and may even resist erosion and weathering as well as if it were rock; but care must be taken to prevent indurated mud being mistaken for solid clay-rock, and therefore it is advisable to test the soil with water.

Seams of silt, soft pasty soil, or soapy earth met with in clay, which have become decomposed by atmospheric and aqueous action, are to be feared, and the brown clay, especially when soft: red, or dark yellow clays that break into laminÆ and crumble upon exposure to the air, and although tenacious in the flakes and when fresh-cut are loosely held together in bulk, often have thin veins of sand in them; and when water percolates it remains, and is very difficult to drain. It has also been found that when minute non-adhesive particles of mica are present in clay that it will become disintegrated by water, although it may be hard to excavate. Some of the gault clays, although stable when dry, become soapy when wet and are not easily managed, but the bluish grey gault is usually tenacious and almost impermeable. The gault clays have little sand in them but much calcareous matter, and, as a rule, they do not swell and bulge like the London clays.

Some clays, when dry, appear to be stable and firm, though they are often drift or dried-up mud simply requiring the influence of water and the atmosphere to cause them to return to their original state; the brown and boulder-clays are of this nature and are distinctly perishing, and therefore treacherous clays.

As in clay and all retentive soils, water is always present, and as the impervious nature of clay prevents water gravitating and being drained, and as owing to wet seasons the permeable portion of the clay may gradually become overcharged and be reduced to a muddy condition, slips may occur years after a clay cutting has been excavated or an embankment deposited. In clay embankments the greater weight upon the centre may gradually press water towards the slopes and cause them to slip; hence the value of covering the formation in order to lessen percolation. Cracks and fissures may be so produced in clay embankments, and the danger is that they form sliding surfaces and cavities from the presence of water, and gradually soften the interior until it fails from the thrust of the earth and want of uniform support; therefore, clay earths are more difficult to treat than granular soils, as local weakness is the cause of failure, a mass may be sound, but a crack or fissure may soon become large and pass through an embankment to the seat or the slopes. Although the filtration of still water may not cause an embankment to slip as long as the water merely restores it to its normal absorbing power, when that is exceeded the percolating water will be dammed-up and cause hydrostatic pressure and a soddened surface, destroy cohesion, and disintegrate the particles.

When clay is contained within the walls of a building, injurious action and unequal settlement may arise, as the earth inside and outside may be in a different condition; consequently in soils that expand and contract there may be external or internal pressure according to the state of dampness or dryness, consequent upon the ground outside being subject to weather influences and the contained earth being comparatively free from such operation.

Shale, whether black or brown, may become decomposed by water and be softened by time, and is either loose or firm. If loose, it requires protection, and the surface should not be exposed to the weather. The presence of iron pyrites in shale has been found to be a cause of its becoming treacherous when water has access to it, as the mass becomes decomposed.

Marly soils are of various hues, red, blue, grey, and yellow, and consist principally of clay and lime, and are usually called clay-marls when clay preponderates, and marl-clays or chalk-marls when chalk is in excess. They are dry to the touch and will effervesce with acids, the presence of lime being thus made evident by the ebullition produced, and some idea of the relative quantity may be judged by comparing results with different lumps: they vary in character according as lime or clay predominates.

Chalk-marl may act as an almost impermeable stratum and arrest the percolation of water from more permeable soil, but it is liable to slip because of fissures being present in it, which are common to most calcareous earths; and when these crevices are bared in cuttings by the ground being excavated, the flow from the previously-confined or diverted springs is facilitated, and perhaps the harmless underground passage of water is prevented, and consequently the earth may become loosened and unstable owing to the changed condition.

Although marl may be so hard that it cannot be excavated by picks and bars, but requires blasting, some varieties crumble and become soft under weather influences, and the slopes need protection, or a constant trickling and wearing away of the surface will result. Grey marl generally weathers quickly, and if there is much clay in it, it often falls to pieces upon exposure to the air, and becomes broken and so split into disjointed pieces as to admit water, and as it sometimes contains a considerable quantity of sand it is soon reduced to an unstable state, for marl-clay and sand beds are always likely to slip. The variegated marls are treacherous, but red marl is usually stable, still it must be lined in tunnels; and so are those that are impermeable or closely approach that condition, but any marl in which the lime may separate from the mass is unstable. All clay or marl soils that soon work into “slurry” may be classed as treacherous, or any earth that quickly becomes in a liquid state; for instance, some of the hard “pan” soil met with in Canada melts away by the action of air, rain, snow and frost, and becomes unmanageable and like blue paint, and will quickly fill drains and run down the slopes and cover the formation of a cutting. Also the top black cotton soil found in India expands by aqueous action, and contracts in drying as clay, and is of doubtful stability.

As might be expected, the more argillaceous or clayey marls when exposed to weather influences expand, contract, and act almost as clays, becoming decomposed, disjointed and separated, and when superimposed upon rock, especially if it be inclined, are very likely to slip, and are treacherous and unstable soils, as they soon become in a muddy state and have slimy surfaces, which, when resting upon any dipping bed, cause them to be disposed to move upon the smallest disturbing element being present or becoming increased, such as water or vibration. When a comparatively permeable stratum overlies even hard marl, water will penetrate to the latter earth and the superimposed layer will rest upon an unstable bed, and therefore be likely to slip. Blue, or any marl when found mixed with layers of small gravel and sandy clay is very treacherous, as air and water cause it to swell and crumble, and it becomes in a soft state requiring a very flat slope. All clay marls that swell when wet, and crack and fissure when dry, are unstable, as water percolates to them, and should sand veins occur in marl, water may trickle to a considerable depth and cause the ground to be in a loose and doubtful condition below the formation or bottom of a cutting. Such soil when tipped into an embankment is worse to treat than in a cutting, as it will absorb and retain water so that it is practically impossible to drain or extract it.

Professor Ansted has classed clay soils as under:—

When combined with 30 or 40 per cent. of sand they are CLAY LOAMS.

When combined with 40 to 70 per cent. of sand they are TRUE LOAMS and LOAMY SOILS.

But not until they have 90 per cent. of sand in them are they SANDY SOILS.

When combined with from 5 to 20 per cent. of lime, the soils become MARLY.

But not until they have more than 20 per cent. of lime in them are they CALCAREOUS SOILS.

With respect to the slope of repose in clay earths, it may be anything from 1 to 1 TO 12 to 1, and when the clay is mere dried mud and becomes saturated it may not be at rest until it is horizontal.

In cuttings, strong compact clay, if not seriously impaired by air and moisture, will stand at from,

Ordinary clay, such as the blue clay, with protected slopes and proper drainage, and firm boulder clay,

Plastic clay from,

Lias clays of doubtful character,

Superficial beds of London clay,

and not less for any weak clay.

In embankments, much depends upon the height, mode of tipping, state of the soil when deposited, the protection afforded and uniform character of the mass, the larger it is the more difficult it will be to drain.

Although firm clay may for a short time stand at nearly a vertical slope for any height of face not exceeding about 10 to 12 feet, few clays will permanently stand at a less slope than,

although they may for a time do so at 1 to 1 in unsubmerged work, but if the slope is covered with water, as in a canal, and subject to wash or wave action, however shallow the depth, it is seldom they repose at a less inclination than 1½ to 1.

Should an embankment be more than about 30 feet in height

Firm clay from,

Plastic-clay, consequent upon the percolation and pressure of water in cuttings, may stand at a steeper slope in a surface protected embankment, and if carefully tipped, it will repose at from

but from hydrostatic pressure the same clay may require from 2 to 1 TO 3 to 1 slopes in cuttings, and even an inclination of 5 to 1 has been found to be necessary.

The varying slope-system is referred to in Chapter VI., as also the range of slopes.

It is always well to bear in mind that in the case of clay, loamy or marly soils, or any possessing soluble particles, the quantity of water in them governs the required flatness of the slope, and as it may be variable, a clay that will repose at a certain angle may slip upon receiving an additional amount of water, hence the importance of drainage and protection. The cohesive strength of clay also varies greatly; and as it is likely to be impaired, and, perhaps, destroyed by water and other causes, it should be considered as of fluctuating value, and therefore as generally unreliable.

The slopes of repose required in river-banks have a wide range according to the degree of exposure and opposition to the free flow of water, and the protection given to the surface. In canals, and drainage-channels, or ordinary rivers, they range from 1½ to 1, when protected, to 5 to 1; but there are numerous small, shallow, and sluggish streams with almost vertical banks. As the earth of river-banks is frequently of a clayey nature the slopes are referred to under the head of Clay; usually it is mixed in character, and, therefore, the slope of stability cannot be determined from that of any particular and unalloyed soil. Towards the mouth the land often consists of detritus liable to be washed away by a stronger flood than that by which it was deposited; it may also be eroded by constant or increased wave action. Ordinary clayey, loamy, tenacious, or not easily moved, silt and sand river-banks, in a current that will not erode them at moderate depths, if the face is protected, will stand at an inclination of about 1½ to 1, when the channel is freshly cut.

If not covered or protected at from 2 to 1 TO 3 to 1, but when in clay soil of a doubtful character and non-homogeneous, or boulder-clay, about 4 to 1.

2 to 1 TO 3 to 1 is the most general slope; but in marsh land, unless the surface is protected, they may not permanently stand at 5 to 1. Fascine-work may be the easiest and most convenient protection, or a gravel or turf-covering may suffice, and should new channels have to be created, the question to decide is, will it be cheaper to make the slopes sufficiently flat so that they can be left unprotected, or is it preferable to have a steep inclination, such as 1½ to 1, and to carefully cover and protect the surfaces, the probable cost of maintenance of the protecting cover being duly considered?

As a rule in homogeneous earth, such as clay, underground excavation is the most free from water, and therefore from slips; but the enormous force clay possesses upon swelling—and this disturbance of the soil may occur many times by the action of air and moisture—and its contraction upon drying, cause tunnels in clay to be risky undertakings. The chief precautions against failure are to allow plenty of space for the expansion of the clay, reduce the uncovered lengths to the least dimensions, have dry, yet hard and compressible, filling between the arch and the walls and the clay, use the strongest bricks, or material with great compressive strength for the lining, adopt a form giving as equal support as possible in every direction, i.e., one closely approaching the circular, leave plenty of weep-holes to prevent any accumulation of water, and thoroughly drain and gently conduct to the proper drainage channels all water as it exudes.