Notes upon the Location, Preservation, and Protection of Sea, Estuary, Reclamation, Canal, and Reservoir Embankments of Earth constructed to Contain or Expel Water.

In the first place, care should be taken in determining the site of an embankment across an estuary that there shall be no concentration or alteration of the general direction of the currents, or scouring action will be created; for the erosive and other deleterious effects of wave action upon a shore are influenced by the angle at which they are impelled against it. A prudent course to adopt is to carefully preserve the usual channels by means of bridges, particularly in ground of a loose character, such as is usually found in partly landlocked waters; for if the velocity is increased, the earth which has been deposited by the original current being reduced or impeded resulting in the suspended matter in the water falling to the bottom, will be subject to a force that will again disturb and cause it to return to its previous suspensory condition; and any disturbance of the normal currents may destroy the equilibrium of stability and alter the flow, and when they are affected it may be most difficult to restore them to their original state, for water will always endeavour to obtain the easiest channel.

It is essential to know the heaviest flood discharge of any river that may flow into an estuary, the greatest depth and velocity of the river, the normal and flood channels, their sectional area and direction, and the extent and shape of the catchment area, so as to establish the required opening to give the natural waterway; for it is important not to interfere with the tidal capacity of an estuary or the volume or flow of any upland waters into the sea, as a navigable channel may become filled with silt, especially when the shore is flat or sandbanks exist; and upon a sandy coast an embankment across an estuary with openings for a navigable channel will probably cause it to become difficult to navigate and, perhaps, impossible, without constant dredging and other works of maintenance, as an embankment may obstruct and deflect the currents and prevent them carrying away the suspended matter. An open viaduct instead of an embankment is almost always to be preferred, and may be necessary; for the power of a current to scour or move particles is greatly augmented by a small increase of the velocity, and the earth may be in such a delicately balanced condition that any increase of scouring action may destroy the seat of an embankment.

To prevent leakage and scour of the base of an embankment near a river resting upon loose soil, curtain walls are sometimes inserted upon both sides extending to a considerable depth, thereby affording security against an embankment merely resting upon a mound which may gradually erode, with the result that it must finally slip and be destroyed; and it may happen, unless the foundations are carried down into impervious soil, that water may escape underneath and undermine it; such action is obstructed by carrying the slopes a few feet below the ground so as to prevent through surface percolation.

As a rule, an embankment across an estuary with one or two openings in it is to be avoided, and it should be most carefully considered whether it will not be better to expend a larger sum and erect a pile viaduct which will not interfere with the currents or channels and only require ordinary precautions to be taken against erosion, instead of depositing an embankment with openings at the channels and the necessary protective works which may consist of pitching the slopes, covering them with fascines, mattress work, or sods, erecting short or long, low or high, groynes, as the former may be ineffectual in causing a deposit or a shoal in front of the toe of the slope and in preventing scour, for in loose silty and sandy soil of considerable depth and not sufficiently firm to resist erosion, they will probably fail by reason of the space between them being washed away; and a complete covering of the foreshore may be requisite, or a protecting apron of homogeneous impermeable soil, and continuous training walls to prevent the creation of shoals: all of which protective works will constantly need to be repaired, and the failure of any one may cause not only an embankment to give way but the remaining preservative works. Undoubtedly there are many estuaries upon which if an embankment even with many openings for channels had been deposited, it would have failed, the soil being in such a tender state that the least additional weight upon the surface would destroy the equilibrium. Also interference with the littoral currents is a risky operation in any but a hard rock bed as regards the foundations, quite apart from injury to navigable channels. When there are numerous ditches, creeks, or channels crossing the line of a proposed estuary embankment of variable stability, especially should hills be near and the earth be so porous as to be saturated every tide, without doubt the safer plan is to erect a low viaduct, as the old waterways will be an endless source of trouble should the ground be anything but firm clay, and interference with the currents will not be preventable, as either their velocity will be increased or decreased, the result being scour or fresh deposits which will affect the channels; and such alteration may change the direction of the motion of the waters, which must be prevented in front of an estuary embankment; or waves will be created by the water travelling and rushing over shoals, for the even configuration of the bottom is a wave lessener, and should there be a deep channel near an estuary embankment severe wave action may be created. The piers or piles of such a viaduct should be cylindrical, thus offering no flat surface for the waves to break against, and yet temporarily to divide them without serious shock to a structure. When of very considerable length, a projecting embankment deposited upon a flat shore of a tidal estuary has caused a heaping up of the water on one side at low tide, and therefore it is unequally strained. This was found to be the case with an estuary embankment on the Scheldt, 2½ miles in length.

The form of the slope has also to be considered, but all the principles that determine the best profile in each case of a pier or breakwater do not necessarily apply to an embankment in an estuary. The recoil of the waves washing away the ground in front and at the toe is particularly to be guarded against, and the action of spray or a broken mass of water falling upon the formation, as also the direct action of the waves. A concave form should not be adopted throughout, as waves roll up until they approach the top portion, when they turn over and fall upon the flatter portion and often breach it, but when the face is straight, excepting for a few feet at the toe, the waves are diffused in travelling up, although they proceed to a higher point upon a flat slope than a steep one, and the recoil is greatly diminished, but with the view of avoiding direct wave action, when the ground is inclined in front of a sea or estuary embankment, it is well to curve the lower part of the slope and to make it cycloidal to the surface of the ground and the slope for a little distance, so as to reduce obstruction to a minimum. The method of making a level terrace or stepping the slope instead of a curved face has the advantage of checking the rising of the sea up the face, altering its direction and acting as a wave-breaker, and also combines these effects with giving as large an area of the base as can be obtained by a curved face, and in bringing the centre of gravity of the cross section of the embankment nearer to the seat, but the face must be securely protected. The width of the cess should increase according to the degree of exposure. If an embankment consists of earth the slope of the cess should be from 6 to 10 to 1, or the system is better avoided and a continuous face adopted.

Short groynes will often protect the toe of an embankment and prevent any longitudinal current undermining it. As a rule, in a tidal estuary the cost of protecting the slopes is considerable, and more above low-water level than below it. In adopting groynes formed of single or double rows of piles, in order to prevent erosion of the toe and a slip and subsidence in an estuary or sea embankment, the littoral currents must be considered before determining their direction and position, the object of their erection being to prevent the waves, and especially the prevailing waves, from scouring the shore, and also to cause a general deposition of shingle, and, therefore, they are usually placed at an angle to the set of the waves, so as to cause the latter to be diffused. The angle will vary; the best guide is to examine the effects of any that may be erected in a similar position to that to be built. An angle of 50° to 70° with the foreshore, in a leeward direction is frequently adopted, and with respect to the distance apart, this depends principally upon the direction of the current and prevailing wind, contour of the shore, degree of exposure, and the length of the groynes. When the whole of an open coast has to be protected, no natural defence existing, the line of the prevailing set of the current and wind on a straight shore can be set off at the end of a groyne, and before the point at which it meets the foreshore another can be erected. They are generally successful when properly placed, and are most frequently straight; if not, they have a concave face to the direction of maximum eroding force; a convex must be avoided, as it will not permanently retain the deposits; their practical effect being that the breach is heaped up or retained upon the prevailing wind, set of current, or wind-wave side to a height of some feet above the leeward side, therefore, a groyne should be so constructed that planks can be added to it as required. They have been proved to prevent the formation of bars when judiciously located, but their success greatly depends upon their proper position and direction, or in loose soils such as sand they may cause a deposit upon the windward and prevailing current side, but an erosion and falling away upon the other; and when placed in front of a reclamation embankment upon shifting sand until the ground at the back of the embankment becomes dry by the tidal waters being excluded, or the surface is impermeably coated, they may be of comparatively little use to prevent a slip or movement of the shore; for as the tide recedes below the level of the toe of the slope, a seaward flow of the tidal and land waters will be created in very porous soil under the seat of an embankment, and may continue until the tide returns and rises to the level of the ground, thus causing the shore to be constantly changing place and permanently established accumulation impossible. Short transverse spurs have been adopted to lessen this action but their effect is hardly noticeable. As failure of a groyne will probably cause erosion of the foot of an embankment and a slip, a cheap and effective means, attested by the experience of nearly half a century, of preserving the timber from the attacks of most marine worms may here be named. It consists in scorching the piles, thus preventing fermentation of the sap, and immediately tarring them: also in placing the wood in the opposite direction to that in which it grew; the latter operation has been found to increase the durability 50 per cent., the reason, it is believed, being that the capillary tubes in the trees are so adjusted as to oppose the rising of moisture when the wood is inverted.

In excavating for an enclosure embankment, the earth should not be disturbed nearer than is economically necessary, and a cess should be left of about 40 to 50 feet in loose permeable soil, the width being governed by the character of the earth, the depth of the excavation, and the height of the embankment. Should dredging have to be executed near an embankment, a considerable distance should be left between the toe of the slope and the line of operations. Before commencing an estuary or enclosure embankment, it is advisable to notice whether a deposit is left upon the shore by the incoming tide, and to ascertain whether it forms in some degree a protective covering, for if this should be the case, any increase of the velocity of the flowing water which might be caused by dredging or a concentration of the littoral currents should be avoided, or the tidal matter in a state of suspension will not gradually sink to the ground. The tidal deposit, although the earth forming the embankment should not be tipped upon it, may also be of importance as tending to prevent or lessen any percolation of water through the foreshore and under the seat of an embankment; for instance, on the Nile, the deposited slime is found to make a practically watertight covering on the loose sand. Mr. Thomas Stevenson has also stated that, according to the depth below the surface of low water that mud reposes, may be approximately judged the force of wave disturbance and degree of exposure; the less the depth, the less the power possessed by the waves.

In the case of treacherous soil which circumstances compelled to be partly used in a reclamation embankment of moderate height, but which it was found would gradually become firm by compression, rough sheet piles with a plank at top have been inserted, giving the outline of the finished slope of the material to be afterwards tipped upon it, the piles and planking not being removed, and, therefore, affording the required temporary support before the earth became consolidated and stable by compression and time.

In Chapter IX. the deposition of embankments is referred to as it affects slips and subsidences in earthwork, but in an enclosure embankment an additional precaution is particularly necessary, namely, that immediately tipping from the ends increases the velocity of the flow of the outgoing or incoming tidal water through the opening, and consequently augments its scouring action; its deposition from a spurn head should be abandoned, and the embankment be uniformly raised from the base. The employment of cofferdams, piling and planking for effecting a closure is now generally discarded in favour of the horizontal system of equally raising the height of an embankment from its base; and is even to be preferred to fascines, unless the latter are merely used to distribute the weight over the base, to protect the surface of a slope, or form a shield against scour either temporarily or permanently.

In previous chapters the protection of a slope is examined; here some reference is made to the particular preservation of the slopes of an estuary or reclamation embankment.

When the protection afforded is not uniform care must be taken that although it makes one part secure it does not weaken another. In the case of river-banks in a soil that is in a delicate state of equilibrium, it may occur that soon after one portion has been protected, another is being scoured, whereas previously it was stable; therefore, to prevent localization of the erosive action, whether on the foreshore of an estuary, reclamation, or a river-bank, and consequent slips and subsidences, the covering should extend over a considerable length. In a sheltered position simply sodding the slopes may be effectual. Some other means of protection are a hard chalk or gravel counterfort founded a few feet below the ground at the toe of an embankment, and a covering of similar chalk or gravel upon the slope, should the soil be favourable. When an embankment of earth in an estuary or river is of firm soil and only requires to be made proof against wave action, stones may be simply deposited evenly upon a slope and so that they will not be washed out, and pitching be not required, as the rough face will tend to break up the waves; but where a simple covering is adopted, whether close or comparatively loose, the slope should be straight, as a concave form causing a recoil of the waves will in time damage or separate the face shield. A coating of clay about 2 feet in thickness, upon a slope with stakes driven into it, and large bushy boughs of trees fixed thereon with the tops downwards, is frequently used in India as a protective cover to a crumbling bank of a river, and to training spurs erected to prevent erosion and slips. Mattresses, fascine or wattled work, besides being expensive, will slide down a slope unless well secured to it, and therefore a constant strain is produced as in all stake-held coverings; it has also been observed by the experienced that although so largely and successfully used in Holland and on its coast, there is very little ground swell on the Dutch shores, and that in a very exposed situation, or where heavy ground swells exist, they may not answer, and may become disintegrated by the much greater weight and force of the sea; and this, notwithstanding the surface breakers of thin water and little mass broken up by the wind may produce more visible agitation.

To prevent a river-bank slipping, and also to maintain a channel in a muddy river, half-tide longitudinal training walls made of wattled work or fascines have been used, so as to cause the deposition of the suspended matter in the tidal water and to gradually restore the impaired slope and secure it from crumbling into the river. The stones brought down by heavy floods have also been used to maintain a river channel and protect its banks from slipping, the interstices gradually becoming filled with mud deposited by the water when the floods subside.

When sudden and unexpected scour of the slope or bed near an estuary embankment upon soft soil has to be immediately arrested to prevent a slip, gunny bags filled with sand afford a ready means of repairing any cavities, the interstices between the bags usually being filled rapidly. Material should be added as required and any concentration of the erosive currents should be avoided.

The required height has to be determined of an embankment in an estuary or the sea to prevent any flow over it or waves falling upon the inner slope; 4 to 5 feet above the highest water mark appears to be adopted in the lower reaches of the Thames and unexposed estuaries in England. In Holland 10 to 15 feet, depending upon the degree of exposure. It is of paramount importance to prevent any waves washing over the top, as damage and, perhaps, a breach may be caused thereby. The height of the highest known wave must therefore be ascertained.

Should the shore be sandy and loose, a characteristic of estuarine accumulations, although its usual bed may be preserved in any storm, when an embankment is erected the rapid and ceaseless process of wasting of the sand and loose soil by the recoil of the waves from the face may in time lay bare the toe of a steep slope and undermine it; for it has been found where the foundation was sand and a rubble mound, which should be so constructed that its interstices become filled in order to make it more solid and stable, was placed upon it, and the superstructure upon the mound, that the sea being resisted by a vertical wall recoiled and made the soil a quicksand, although the sand would be stable at its natural slope in still water. Obviously the less the action of the waves is impeded, the less the looseness of the sand. Experiments have shown that a slope of 1 to 1 will reflect waves, on a flatter slope they are broken. In such situations light structures, offering little resistance to the action of the waves and not causing an impediment to the current, should be adopted in preference to a massive or solid erection; but when an embankment is necessary, it should have a long sloping mound or foreshore upon which the waves will gradually become lessened and dispersed, the desired object being to prevent deep water close to the work. When a railway or road follows the shore, instead of erecting a retaining wall to protect an embankment, a preferable plan to adopt may be to have open trestle-work offering the least possible obstruction, and when the formation is at the base of a cliff of variable and doubtful soil it is the best construction, as the cliff is not touched, and slips and subsidences are avoided. Should the deposition of an artificial beach be considered necessary for the preservation of the foot of a cliff in addition to the trestle road, experience seems to indicate that the contour affording the most protection is one in which the slope has a flat terrace or cess at not above three-fourths of the vertical height, another short slope, and a nearly horizontal space some distance from the foot of the cliff; but a storm will straighten the face, and it may be impossible to economically maintain it; however, should such a slope be assumed it should not be disturbed. Vide Chapter VI. for information respecting slopes. When a sea or an estuary retaining wall is necessary in order to prevent the slipping of an embankment consisting of loose soil, an inner dwarf wall at the edge of the formation upon the land side should be erected so as to hold the embankment in a box, and not allow any spray or water passing over the retaining wall to erode the inner portion.

Deep water is generally required close to the work in railway piers or jetties to enable vessels to get alongside; a heavy and monolithic wall must consequently be erected; however, in the case of loose soil, the vertical system simply should not be used unless the foundation is thoroughly protected and below the reach or effect of wave action, and no re-entering or right angles should exist, as they increase the action of the waves. When a rubble mound is cast in and a vertical structure placed thereon, great care must be taken that there are no holes, except the natural interstices between the stones, and that they have a firm foundation and sink equally, or the random mound may give way and the superstructure will then necessarily follow.

As an illustration of the deleterious effects of the recoil of waves may be mentioned that a high vertical wall with a parapet has been found to endanger the toe, but when the parapet has been removed in order to allow the head of the waves to leap over the top of the work, the structure remained stable. To prevent the recoil of the sea and the scooping away of the base and the foreshore in much exposed situations, and consequent slips and subsidences, breakwaters which simply act as wave screens, and not as wind screens, are sometimes kept a little below high-water mark so that the heads of the waves may have free action, although their onward motion is prevented. On the contrary, light open work, such as a lattice screen, although it may somewhat lessen wave action, does not prevent it passing through.

The principal causes of the failure of vertical walls when placed upon an easily eroded foundation are by the scouring action of the recoiling waves, therefore their magnitude should be reduced to the lowest limit: by the impounded air driving out particles of the structure: by waves travelling upon the top, therefore their forward motion should be deflected and rendered vertical: by the hammering action of the mass caused by its being alternately quickly submerged and unsubmerged; the practical effect being that the foundation is intermittingly released of a portion of the load and then fully strained, therefore the height of the waves should be reduced as much as possible. Respecting the action of falling water, experiments were recently made in India, which proved that “the greatest intensity of pressure does not exceed that due to a column of water of a height equal to the fall;” the greatest intensity of pressure being always fractionally under the hydrostatic head.

As an illustration of the constant change of the load upon the foundations of an embankment in tidal waters, the following calculations have been made.

The weight of a cubic foot of sea water is taken as 0·028 ton.

The weight of a cubic foot of sand is here taken as 0·056 ton.

A. The weight of a lineal foot of the embankment when unsubmerged equals 201·60 tons, computed as follows:—

B. At high water the weight of the embankment is reduced by the weight of the water displaced, which equals 84 tons, calculated as under.

The submerged contents of the embankment are—

From this must be deducted the weight of the water resting upon the two slopes, which equals 33·60 tons—

C. Thus the insistent load at high water upon the whole area of the foundation is reduced by

D. At high water a vertical pressure is imposed upon the ground beyond the toe of the slope due to the 20 feet head of water—

This latter weight and element of stability tends to prevent movement of the ground, and also the toe of the slope, but is entirely removed at low water when the insistent pressure at the foot of the embankment is the greatest.

For the purposes of illustrating the varying load upon the surface of the ground caused by a rise and fall of a tide, it will be sufficient to take one slope of the embankment.

E. The weight of a lineal foot of one slope, if unsubmerged =

F. The weight of water resting upon the slope per lineal foot at high tide =

G. The weight of the water displaced by a lineal foot of the submerged portion of one slope of the embankment at high tide =

H. The area of the base of the slope per lineal foot =

I. Therefore the insistent pressure upon the surface of the ground at the base of one slope at low water, when the bed is presumed to be dry, is,

J. At high water it is

of a ton per square foot, or 22·60 per cent. less.

It has been shown that the vertical pressure of the water upon the ground beyond the slope is 0·56 ton per square foot at high tide, vide D., therefore the weight upon the seat of the slope is only in excess of the normal weight of the water upon the ground beyond the slope,

An inspection of the diagram shows that the flotation power of the whole of the shaded portion of the slope is balanced by the weight of water resting upon the whole of it, the areas of the triangles being similar; and that the portion W. of the slope is that which loses weight by immersion.

Calculating the pressures at low and high water upon the base of the portion W., the relative vertical pressures would be—

K. The area of the base = 30 ft. × 1 ft. = 30 square ft., consequently the pressure upon it =

L. The unsubmerged portion =

The submerged portion =

30 ft. × 20 ft. × 1 ft. = 600 × (0·056 – 0·028) = 16·80
25·20

a difference of 0·56 ton per square foot, or 40 per cent. less load.

M. And upon the base of the central portion—

N. The unsubmerged portion =

The submerged portion =

also a difference of 0·56 ton per square foot, or 33 per cent. less load.

The difference in weight, assuming a wave of 5 feet in height, measured downwards from high water level, to simultaneously roll against the embankment upon both slopes and completely recoil, would be equivalent to the displacement of 5 feet depth of water for a strip =

The cubic contents per lineal foot are—

O. The flotation power =

For the purposes of this calculation, this weight is taken as if it were spread over the whole area of the seat of the embankment at a depth of 15 feet from the top =

The vertical pressure per square foot therefore =

which is equivalent to a hammering action upon the foundations of 275
144 = say, 2 lbs. per square inch occurring each time the 5 feet waves recoil.

The weight, 1·05 ton upon each slope, of the water upon that portion of the slope which is alternately submerged and unsubmerged is not considered.

This wave action may, and generally will, happen upon one side only of an embankment owing to the direction of the wind, the current, and the “fetch” of the water. In that event the lateral pressure upon the embankment will also constantly change, and there will be a varying horizontal force from the 5 feet in height wave and its percussive action upon the slope tending to produce unequal strain and movement.

The object of the preceding calculations is to show the variation of pressures an embankment in an estuary or a tidal river has to sustain in addition to those of an ordinary embankment upon dry land, and its especial liability to slip and subside; and also to demonstrate that the vertical and necessarily the horizontal pressures may be in a perpetual state of mutation and vary considerably, and that the vertical pressure of the water outside an enclosure embankment may reach a point when the water may be forced upward upon the land side. Usually, subsidence is greatest in the wet seasons and at the time of the lowest tides.

In choosing between two materials for submerged work practically equal in other respects, the heavier should be preferred, as by reason of its own weight it has greater power to resist the action of the waves and scour, and the decrease of its specific gravity by the weight of the bulk of water displaced is relatively not so large.

With respect to the earthwork of canals and embankments constructed to hold water, each chapter of this book contains information relating to the promotion of the stability of the soil, and it is not here intended to refer to the most approved methods of construction, but only to name some points requiring attention.

A barge or ordinary ship canal is usually placed at a shallow depth in the ground and meanders through a district, avoiding deep cuttings and heavy embankments, such an undertaking as the Suez, or the Panama, ship canals being altogether exceptional undertakings; and also the Manchester Ship Canal. Some of the most treacherous soils are named in Chapter II., but probably the worst earth in which a canal can be made is peat-bog land; the method of procedure is then different to that required in making a railway or a road, and in such soil the construction of a canal should be avoided, as it necessitates most experienced and skilful treatment and extensive drainage, which causes subsidence; difficult maintenance to preserve the channel and retain the water of navigation, control the drainage waters, and keep unimpaired the towing-path, which must be firmly covered throughout. Fine sand is also an unfavourable soil as it so readily becomes a quicksand, but almost all other earths usually met with can be so protected that failure should be an improbable contingency, except when the cuttings are of extraordinary depth, as on the Panama Canal; the chief danger being when there is considerable diversity, irregularity, fissuring, looseness, and upheaval of the soil, for varieties of almost every surface earth may be encountered, each with its own characteristics and behaviour when dry or water-charged, and requiring great attention, especially at the joints, and not a few separate treatment, not only of the soil but of the same earth in a cutting and in an embankment.

It sometimes happens that there is not enough clayey earth to form a canal bank, but if the whole of the excavation, except the mere surface earth, is used there is sufficient, and that it must be employed for reasons of economical construction. In such a case the clayey earth should be deposited upon the water side, and the gravelly or sandy soil on the land side and as a towing-path covering, great care being exercised that no stratification of the earth takes place, or it may separate, and water seams be created. The bottom and slopes of a canal embankment must be covered with an impervious layer, puddle or concrete being most frequently used for the bottom, and puddle or a coating of well rammed vegetable soil sown closely with grass seeds for the slopes, or stone pitching in a wide canal liable to wash: also in loose or doubtful soil. In canal cuttings when the water is drawn off the counter-thrust against the slopes is removed, and unless this is maintained the earth, if in a delicate or loosened condition, may slip, as a flow may be caused of previously dammed up waters. Prior to any works being commenced upon a canal embankment it is advisable to strengthen it for a distance of not less than about 60 feet in length on each side of a proposed railway bridge or other structure, particularly when it will be subject to vibration.

Water will soon find a weak place in any earthwork and, certainly, when an engineer can maintain heavy earthworks in treacherous soils in canal construction, where part of one slope is submerged in an embankment and the other dry, and in a cutting part wet and part dry, he should be able to do so in any analogous situation, making due provision for vibration, the chief disturbing agency canals are not subject to, but which is so potent in its effect upon railway earthwork.

Canal and reservoir embankments are so similar as regards the stability of earthwork that they are here considered under one head. The object of a canal, reservoir, or river embankment is in a desired position to hold water without leakage, subsidence, or deterioration of the earth, or other works connected with the general construction. In dock, canal, or any earthworks made for the purpose of containing or expelling water, it is obvious the position is entirely dissimilar to that of railway works: for whereas a slip or a subsidence in an embankment upon railway work may be of slight moment and give comparatively little trouble or anxiety, any movement in earthwork constructed for the purposes named is of grave importance, as it may mean the destruction of the work, loss of life and property, not only upon the site but also in the surrounding district. No leakage or marked weeping of such an embankment should be disregarded, as unless arrested it will gradually deteriorate the earth until the equilibrium becomes so affected that the embankment fails. As a rule, upon railway works, serious slips, except in very unstable soil, such as drift earth, or those quickly decomposed by atmospheric influences, seldom occur, vide Chapter II., until a year or two has elapsed, the process of disintegration from water being necessarily slower in its action, the material having an opportunity to return to its normal condition, and the surface being more equally exposed. In earthwork for hydraulic purposes, although not subject to frequent and sudden vibration, the slopes being unequally exposed and part generally either permanently or temporarily submerged or alternately wet and dry, and the top and land side unsubmerged and fully open to the action of the weather, percolation cannot be equal and regular upon its surfaces, and slips and subsidences therefore usually occur within a short time, varying from a few weeks to a few months; and after about a year or a cycle of seasons they are not frequent, unless the inherent design and construction of an embankment is faulty, permitting leakage through the submerged slope or bottom, pools of water to collect upon the top, and the bank to become fissured and unequally wet or dry. Especial care should be exercised to ensure a complete and impermeable connection with the earth upon which it is placed.

In embankments to contain or expel water time should always be allowed for an embankment to consolidate before the admission of the water, but in the case of a cutting the presence of the navigation water as soon after construction as practicable may be an advantage, depending upon the nature of the earth in each case; as it may protect the submerged portions of the slopes from the deteriorating effects of being in a constantly changing degree of dryness and wetness, and shield them from the sun and drying winds. Except under peculiar circumstances the unstable places will be known in a few months, and provided a canal is properly constructed, it will not cause much trouble after six months or a year from earthwork movement if the embankments and cuttings receive ordinary attention, as the navigation water will quickly indicate the unconsolidated places. In clay, clay marls, clay loams, and where loamy soil is intermixed with permeable and water-bearing strata, movement of the earth in canals will speedily occur unless proper precautions have been taken in the construction, as they are seldom homogeneous.

Earthworks to contain or expel water should be made proof against even improbable deterioration and accident, and in proportion as the soil is less solid and firm it should be consolidated by ramming or other means, or be covered and protected to prevent it cracking, and only firm and binding material should be used in canal embankments or those holding or expelling water in order to prevent slips and subsidences. Chapter II. treats of some conditions under which slips and subsidences may be expected. No precaution should be omitted that will render the work solid and uniform, and nothing should be left to chance. The location is of great importance, for the earth may vary in stability within a very short distance, not only in character but as regards the superimposition of the strata. Upheaved and distorted beds should be avoided, and all loose and fissured earths, especially rock or other soils having more or less vertical seams: for all embankments constructed upon fissured soils are sure to cause anxiety and trouble, and will always be liable to become in a dangerous state from an excessive rainfall. Impermeable horizontally deposited earth of considerable thickness is that to be desired, or firm ground that cannot slide upon another stratum or be affected by any artificially brought addition to the percolating waters consequent upon the construction of waterworks. Having carefully selected a site, which for waterworks purposes must almost invariably be upon high ground and may have to be upon the top cap soil of a hill and be peculiarly exposed, the next step to guard against a slip or a subsidence is to prepare the foundations so that no leakage or trickling of water can undermine or gradually deteriorate the seat of the embankment; and, therefore, a thorough connection between the deposited embankment and the ground must be established and the whole be prevented from movement.

The selection of the best available earth is one requiring careful consideration, and it may be necessary to experiment to test the capability of the soil to be made watertight by compression or other comparatively inexpensive means. In some cases none may be obtainable; if so, the only course to pursue may be to consolidate the earth as much as possible, and protect it with an impermeable homogeneous and durable covering, and one that experience has shown can be trusted to equally resist percolation. The whole practice of stable earth dam construction is comprised in the employment of homogeneous, fine, and tenacious earth uniformly deposited in thin layers, gently watered sufficiently to aid ramming and consolidation, rolled, pressed, or trodden down by the passage of carts, men, or animals, and in its due surface protection. Heavy rolling is to be preferred, for it is more effective than ramming, as may be judged by the greater compression; the thickness of the layers and the weight being so regulated that it compresses the earth to its state of maximum solidity, and does not pulverize it, as the compression is more uniform, and irregularly compact masses are not created, which not only destroy uniformity of condition of the mass, but cause seams and veins and destroy homogeneity. After rolling, the compressed layer should be gently watered, as the weight of the roller will have made the top crust drier than the lower portion, unless it has been made into too moist a state. The thickness of the layers should not exceed, when the soil is to be heavy steam-rolled, about 6 inches in earth, and 4 inches in clay soils; they may be compressed to about four-fifths to two-thirds of their normal thickness, the degree of compression to ensure maximum solidity indicating the openness of the original earth. In sand, the layers when simply wetted and rammed with an ordinary rammer, should be about 4 to 5 inches in thickness, and ordinary earth about 2 inches. The volume of the sand will be reduced from 10 to 15 per cent. Simple ramming of sand will only reduce it about 6 to about 9 per cent., and water about 4 to 6 per cent., making the total compression as before stated, the quantity of water used being some 20 per cent. of the volume of the sand. Embankments of moderate height have been made of sand when no other earth was available, except clay loamy soil in comparatively small quantities, the surface of the embankment being worked by thin bars, and the loam being incorporated with the sand and made to fill its interstices, surface protection being thus afforded, and all the usual means to promote consolidation being adopted. Wet earth well mixed with a grout of quicklime has also been used to make a firm embankment of little height. Clay is not a good material to use unless it is incorporated with a considerable percentage of sand to prevent fissures and to lessen expansion. Many prefer the sand to be in the proportion of about two-thirds of the mass, only one-third being clay; the soil being then a loam, and cannot be classed as a clay.

There is much diversity of opinion as to the relative value of a central puddle wall or protection of the slope and toe. Such a wall is placed in the centre, not only to give support to an earth embankment and prevent any through leakage, but also to keep the puddle in a uniformly moist condition, and therefore to prevent it fissuring and cracking from exposure to the sun or air. In an embankment erected upon an earth foundation it may be advantageous and warrant the expense, but when the embankment of earth is placed upon rock it should not be erected, for it is impossible to make a watertight joint between rock and clay puddle, and should it be placed in a rock trench leakage will not be prevented by it, and water will accumulate when it does not at once pass through the seat of an embankment until the surface of its base becomes softened, and the whole mass almost worthless either as a means of support or of prevention of through percolation; and, moreover, there is the danger that the clay will draw away from the sides of the rock. What is wanted is, as it were, to insert as far as the top of an embankment an artificial rock having a thorough connection with the foundation, consequently a Portland cement concrete wall should be adopted in such a case and not one consisting of clay puddle. Two great advantages of a Portland cement concrete central support over that of a clay puddle wall are that it does not settle and the weight is evenly distributed, whereas a puddle wall in subsiding may draw away from the earth of the embankment and cause cavities and cracks and consequent leakage. The chief consideration is to produce an embankment having impermeability, solidity, and homogeneity, and a thorough connection at the foundations; and as the central puddle wall system rarely does so, and usually only under circumstances which would probably hardly warrant the expense of its adoption, it is in a state of obsolescence; and should central wall support be necessary, which may not infrequently be the case, Portland cement concrete is being adopted in its place, a solid and durable material being used instead of one varying in shape and size, according to its state of dryness or wetness, liable to fissure, and incapable of being permanently and firmly joined to any material. This system may be described as one of arresting the percolation of water in the interior of an embankment; but the great aim is to prevent any percolation into the mass, as when once a passage is made the water will escape along the line of least resistance, and a fissure become a cavity, a cavity a breach.

The protection of the slope and toe has for its object the prevention of any percolation into the mass, and it may effect all that is requisite, but its efficiency and completeness much depends upon the preparation and deposition of the layers during the construction of an embankment, and also the time that can be allowed for consolidation, so as to prevent any cracking or fissuring of the face covering from settlement of the embankment. It is obvious in the case of a reservoir embankment or a slope alternately submerged and unsubmerged, that an exposed clay puddle covering cannot be used, as heat or the sun’s rays will cause it to crack, although if constantly wet it would succeed. A puddled clay covering with a broken stone bed placed upon it to receive dry or mortar-set stone pitching has been frequently adopted; a simple cement concrete facing about 6 to 12 inches in thickness, depending upon the depth of the water and the nature of the soil, or in conjunction with an asphalt coating of ordinary thickness. In the two latter cases care must be taken that they do not separate from the embankment either from the force of pent-up water, frost, or shrinking of the earth embankment.

As any perforation of the surface must be prevented, in certain districts the covering should be capable of resisting the attacks of rodents and crustacea, and, therefore, a stone pitched or concrete covered slope is necessary, or the puddle towards the surface must be well incorporated with small stones or ashes or other tough material. In European countries the effects of the burrowing of rats may be insignificant, but in warmer climates, as, for instance, on the coast of Coromandel and in some parts of Bengal, where rats may measure as much as 2 feet in length, their attacks are not to be disregarded with impunity.

The causes of failure of water-containing embankments afford an indication of the direction in which especial care should be exercised in their construction. Assuming a reservoir embankment to be of the necessary form and bulk, and to be properly constructed, the principal causes of failure are as follows:—

1. Leakage along the line of a culvert or pipe passing through the lower portion of an embankment.

2. Leakage under the seat of an embankment.

3. Water overflowing the top and eroding the land slope and so destroying the equilibrium.

4. Bursting of springs over the site. Vide Chapter XII. on “boils” in loose soils.

With regard to the first, the most frequent cause of the failure of a water enclosure embankment; the culvert or outlet passages have been rendered unnecessary by conducting the waters in a tunnel passage under the seat of the embankment and without interfering with it; but this method is expensive. One of the causes of failure is the weight of the embankment owing to unequal settlement producing a breach in a culvert, the probable result of insecure foundations or want of a firm concrete base to the culvert to evenly distribute the weight; or it being placed upon clay puddle, which should never be done; or the embankment being constructed without due care. All culverts should be sufficiently large for a man to easily pass through and should be equally watertight within and without, for a leakage from the culvert to the bank is equally dangerous, and means should be provided so that it can be closed in short lengths. Reports on the temporary failures of reservoir embankments almost invariably state, failure occurred from water penetrating between the puddle and the culvert, from water percolating between the rock foundation and the central puddle wall, or the embankment gave way as water issued through interstices in it caused by settlement of the masonry outlet passage.

Careless construction has often been shown by inspection to be the reason of so many embankments yielding along the line of the culvert; but when the latter is properly designed and built it seldom causes a temporary failure of a well-made reservoir embankment. Respecting the second cause of failure, it generally proceeds from want of care in thoroughly binding an embankment to the solid ground and protecting the toe; by “boils” in the foundations; or by a porous earth seam, such as fine sand, existing under the stratum upon which the embankment is erected becoming a quicksand on flowing water reaching it.

In connection with the third, it may be said that it is not a frequent cause of failure, as provision is almost always made to prevent it, but it may occur in a reservoir embankment from extraordinary circumstances, or in a much less degree from sufficiently high waves being generated upon a lake or impounding reservoir that the top of the bank may be loosened, and the water dash over it and erode the land slope, and convert it into a kind of tail-race; tarpaulins have been temporarily laid upon the surface on an emergency to protect a soft place, also sand bags and planking to prevent an overflow. In an embankment erected for such a purpose the top width is more severely strained, consequent upon the greater exposure, than in a reservoir embankment, and the height must be sufficient to prevent water passing over it.

With reference to the fourth cause of failure, “boils” in loose soil and the bursting of springs are referred to in Chapter XII.

When a reservoir is emptied the weight of the water on its bottom is removed, but the load from the embankment is the same, and should the ground be soft the embankment may subside towards the reservoir and the bed be uplifted; hence it may be advisable not to draw off the water unless the bottom is weighted or movement prevented.

Due regard of the different causes of failure herein enumerated and care in construction, will reduce to a minimum the probability of a slip or a subsidence in an earth embankment erected to contain or expel water.

The temporary or permanent diversion of rivers or streams being so often necessary in public works, a few paragraphs are here devoted to it so far as regards earthslips and subsidences. If it be possible when the soil is very porous and incapable of retaining water, it is advisable not to divert a river.

Some of the most vulnerable places in a newly-formed river-bank are:

The ends that join it to the old bank or to the land, to which it should be thoroughly connected.

The toe of the slope and seat, which should be tied into the bed of the river or be well protected by making the slope flat towards the base.

Any abrupt bends or angles should always be avoided as they increase erosive action.

The wind and water line which requires especial protection.

Provided these points are remembered and the usual precautions taken in forming a river-bank to make it thoroughly sound and homogeneous, a slip or subsidence of serious moment is improbable.

In order to protect the sandy bed of a river and to prevent the banks slipping and subsiding, it may be necessary to guard against scour of the bed and consequently of the toe of the banks. Stone thrown in will settle and compress the bed by weighting and consolidation. By periodical depositions the sand becomes more protected and the quantity of stone required is reduced, but especial care should be taken to preserve the normal bed, to offer no obstruction, and not to cause whirlpools or to interfere with the current except to direct and train it, or the erosive action so created will cause movement. Stones simply cast in and allowed to sink and find a permanent bed until the regular surface of the bottom of a river is so reached, have been proved in many instances to be a sure protection in sandy soils provided eddies do not exist. The preservation of the slopes is particularly referred to in Chapter VII.