Earthwork Slips and Subsidences upon Public Works / Their Causes, Prevention, and Reparation

by John Newman

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,

Overhanging, vertical, to ⅛ to 1.

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,

Perpendicular, to ¼ to 1.

But, if non-weather-resisting,

½ to 1 TO 1 to 1,

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

Friable rock, consisting of hard particles,

½ to 1 TO ¾ to 1.

Loose rock,

¼ to 1 TO 1 to 1.

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

1½ to 1 TO 2 to 1.

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.