Notes upon the Failure of Dock and other Walls from a Forward Movement of the Earth Filling or Backing.—Consideration of the Causes of such Action and some Preventive and Remedial Measures.

The most serious consequences often result from slips or “boils” of sand or loose soil during the construction of docks, and from the sliding forward or subsidence of quay walls, as in addition to the great expense of restoring the damaged wall must be considered the loss of receipts caused either by delay in opening the docks or from interference with the traffic. In this chapter no attempt is made to lay down rules and to declare that if they are followed failures from pressure, subsidence, or a slip of earth, can be obviated by careful attention to them. Many of the most renowned engineers and contractors throughout the world, whose scientific attainments and practical knowledge are universally and deferentially acknowledged by those competent to form a judgment, have had anything but pleasant experiences in the construction of docks and similar works.

The two banes of dockworks may be said to be:—

1. Defective foundations.

2. “Boils,” or the rushing up of loose soil and water in the foundations.

To say that a failure of a dock-wall cannot but be the result of defective engineering is a statement which it would be difficult to substantiate. To disturb the earth below the required depth of the foundations at intervals over the area of such works in treacherous soils in order to ascertain its character would simply be to offer a general invitation to it to commence movement, vide p. 208. It would be worthy to rank with the erudite report of a local committee formed to declare the best means of preventing floods in a district, which, after deep consideration reported: “That the committee were of opinion the floods would be permanently removed provided the engineer diverts all rivers, streams, watercourses, and ditches, to the next valley, and closes the up-stream hole in the hill, or causes the water to be discharged elsewhere, and reduces the rainfall within the limits required from time to time by the inhabitants of the district.”

Inasmuch as it is impracticable to ascertain the exact character and condition of any earth over a large area in such a location as that usually occupied by a dock, and to be certain that no unstable seams exist, and as the forces of nature are infinitely more powerful than those of man, it cannot be said that a failure is absolutely impossible. All that can be done is to render improbable any slip or subsidence by the employment of every known means to promote stability justified by the importance of the work and with a due regard to economy.

Dock-works are peculiarly liable to slips and subsidences because of their position necessarily being near to the sea, an estuary, river, or lake, and therefore in usually treacherous superficial soil of an insecure, water-charged character. The foundations are also at but comparatively little depth below the lowest water-level, and although they are not subject to the frequent vibration railway cuttings and embankments have to withstand, they are jarred by blows from vessels and falls of heavy goods, vibration from cranes and machinery, and also are liable to be suddenly and unevenly surcharged. The earth backing may also become quickly water-charged, and a force caused by the tide varying in height has to be considered in earth embankments for docks or reclamation purposes, particularly in sandy or loose soil, namely, that of the foundations being “blown” up by the upward pressure of the tidal or permanent head of water above the level of the bottom of the foundation; thus inside an enclosure embankment, or cofferdam, a depth of water of 30 feet would give an upward pressure of, say, 0·85 ton per square foot, and when the excavation has extended to a depth of 30 feet an insistent natural weight of a column of sand 1 foot square upon the foundation has been removed by the excavation equal to 1·65 ton, vide Chapter V. Thus the upward and relieved pressure 0·85 + 1·65 equals 2·50 tons per square foot, 0·85 ton of which constantly varies. It is obvious that in very permeable soils, such as sand, the hydrostatic pressure will be most felt, and should the range of tide be considerable the strain upon the earth will change and an active disturbing force be constantly present. This varying disturbing agent must be seriously considered in all dockwork or excavation near the sea or a tidal river, and also approximate to water, for there will always be danger of the water percolating, and when the foundations are not sufficiently deep of their being undermined. The weight and depth must therefore be sufficient to balance or exceed any upward pressure of the water in the soil due to its head level. The bottom of the foundation will almost invariably be below the depth at which water is usually found in the locality, and it should not be forgotten that sooner or later the earth upon which dock-walls rest is nearly certain to become wet and perhaps saturated, and that when saturated any additional water must flow away. In sandy or loose soils this may become dangerous. Care must also be taken in loose soil that it is not loaded by any temporary works so as to force the earth in any direction, or a slip will occur and any trenches may become filled, and every effort should be made to cause equal loading and pressure and to prevent unequal settlement.

The selection of the site of a dock is generally confined within narrow limits such as railway connection, roads, location of the want of dock accommodation, currents, the proper position of the entrances, &c., &c.; nevertheless the nature of the soil should be considered, for upon a tidal river a few hundred feet one way or other may cause the works to be upon various kinds of earth each more or less treacherous, and when intermixed perhaps more unstable. Even upon one side of a comparatively narrow river the earth may be in a much more stable condition than upon the other; and when an old watercourse or a dry ditch passes across the site of a dock it must not be disregarded, as failures and damage have occurred because the same section of a wall was continued throughout. It is then necessary to adopt a system of construction suitable for softer or less stable ground, and especially so if the wall has to be built upon a firm stratum overlying sand, as a diminution in the thickness of the bed may cause a settlement and the wall to be pushed forward; for there is always danger in sandy soils, unless the superimposed layer is of considerable thickness, that the latter may be pierced and the whole site be in jeopardy from a flow of the sand.

It may be that a dock must be built at a particular place, although the ground may be full of springs, the beds inclined, or a level stratum overlies an inclined bed of rock or softer earth incapable of standing at any but a flat slope, the whole being an unstable mass, for the deposits may extend to such considerable depths that to go below them is impracticable; however, uniform weighting and consolidation by the deposition of firm soils having insoluble particles will generally make it firmer, and by compression and admixture will ultimately cause the ground to be more solid. It should be loaded as long as possible, and the material be deposited beyond the site so as to prevent the outside and less consolidated earth slipping upon the works, and as it will be disturbed by the loaded portion having subsided it may require slight support upon the lower portion of the slope. When the settlement over the base and the slopes to the original bed is equal and regular and care is taken that the soft soil cannot run, by a due protection of its surface it can be built upon. The rise of the mud in front and for some distance from the wall is generally from about one-third to one-half of its subsidence when loaded. The rate of sinking and uplifting of the soil should be noticed, and whether it is equable and the depth to which settlement extends is the same over a considerable area and practically ceases at a certain depth.

Where a dry dock had to be made upon soft fine sand, Portland cement concrete has been successfully used to cover the whole site. In such soils, to avoid the danger of the bottom being blown up when a dock is empty, and has not the counterbalancing weight of a ship, arrangements should be made so that sufficient water is let in to counteract the uplifting force. If it can be avoided it is advisable not to erect a graving dock upon very porous earth, as the expense of an impervious lining, whether of Portland cement concrete or clay puddle, and of keeping it dry will be considerable; and when unequal settlement takes place a run of the soil may occur and a slip of earth cause it to become fissured and separated, especially in an estuary or bay with headlands, as natural springs may make the sand a quicksand. Much depends upon the head, if the water comes from the adjoining hills it may be so great as to prevent any but a strongly constructed dock, acting as a watertight box, being successful. The bearing power of the soil will also be practically the same whether the foundations are a few feet or at a considerable depth, as the head will be sufficient to make the springs issue, and therefore disturb the earth.

The drainage of the site requires to be carefully deliberated, and no rules can be laid down, but the flow of all surface water and springs should be controlled. The chief aim should be rather to prevent water reaching the site than to allow it to be present and then pump it away, hence it is well to have the deepest sump and all water-raising apparatus, if possible, outside the area of the works of art and in the direction of the land or river flow. The first operation in draining the site is to construct the necessary surface drains around it and also the sinking of a sump, cylinder, or wells where required, which to be effectual must be some few feet deeper than the lowest level of the works of construction; but the depth should be no more than is absolutely requisite, as the greater the lift of a pump the less the efficiency; care being taken that the bottom of the sump is sufficiently low that water gravitates to it, and that the pumping apparatus is at such a height that it cannot be “drowned.” When the soil is of so loose a character that pumping will cause it to “run,” the bottom can be closed and holes made in the ring and water pumped in through them, and in addition coarse gravel or broken stone can be inserted to act as a filter. The act of drainage may cause soil that has been full of springs to be entirely free from them, and cannot but always tend to consolidation, if only because of settlement and the interstices or fissures being less occupied by water or being reduced in size. In loose soils, especially at the commencement of pumping operations, it is advisable that it be not very rapid, as a quick change of condition may cause the ground to be disturbed too much, whereas slow pumping may clear the water as effectually and enable the soil to settle to its new state. The location of the sump should be at or about the lowest level of the ground well away from any wall or the dock area, and on the line of the greatest flow of the land waters or natural watercourse, thus, dealing with them before they reach the site of the works.

In loose and almost all earth the foundations should be covered up quickly so as to preserve the natural condition of the soil. It is also important to offer every possible support to the earth to prevent initial movement, and slight assistance may afford the required strength.

Unless there are cogent reasons to the contrary, the excavation for dock-walls is better executed in trenches, the frames of which being struck as the work progresses can be used again; the wall can thus be supported by the earth on either side, and no lateral thrust is brought upon the wall until it has set, and therefore the danger of bringing, perhaps, the greatest strain upon it when in an imperfectly solid condition is obviated, the back and top of the wall being protected from the weather, and any movement or slip of the earth prevented; hence a wall should act as quickly as practicable as a monolithic mass, and the material forming it be thoroughly bonded and set in Portland cement or the best hydraulic lime mortar, or be wholly of strong Portland cement concrete.

The trench system of constructing dock-walls, covered ways, and all work below ground is the safest to adopt, and in loose soils or clay that expands and is quickly affected by atmospheric influences, and where there are buildings or the ground is weighted, the erection of side walls in trenches, and then the completion of the invert or arch is often the only way in which the work can be executed. Even should the formation be rocky, and the beds be inclined towards the site, the short 15 to 20 feet in length trench system is the best to adopt; and it may be advisable or necessary to work day and night in order that the ground may be left exposed for the least possible time and as machine mixed Portland cement concrete walls can be erected much quicker than masonry or brickwork, in such situations it is the best material to use. The excavation for the walls should be confined within the smallest limits, but no under-cutting should be allowed in the excavation trenches, or the portion undercut may strain the upper part and cause it to slip and subside.

In Chapter VIII. soft soil or weak ground is referred to, and as in dockworks this so frequently occurs, the necessity of making every reasonable provision against undue settlement is apparent. In such foundations the weight upon them should be as equable as possible, for any excess of load will cause subsidence at that place, and a large bearing area is required which may not be easy to obtain, except by increasing the area at the back, as the face must be nearly straight, or only slightly curved, to enable vessels to get alongside, but the wall should not be the heavier in front as it will aid an overturning movement. Any dredging operations should be effected at such a distance that the stability of a wall cannot be impaired, or a slip of earth may occur, as the bared soil may commence to “blow” or run consequent upon the dredging having relieved the ground of weight and, perhaps, removed an impervious covering. A row of sheet piles inserted before dredging is commenced, if well supported in front, may be sufficient to prevent movement.

In dockwork, the margin allowed to resist lateral strain being considerable, walls seldom fail by being fractured from excess of lateral pressure; but either by vertical or lateral pressure causing subsidence or the whole of a wall to slide forward upon the foundations; and not from defective form, workmanship, or materials; but from a compressive strain upon the face of a wall caused by a force tending to overturn it; and, as a rule, when a wall inclines towards the face very little more pressure will upset it. Generally the foundations are sufficiently firm to support the insistent load, and the chief cause of failure or undue movement is principally the result of slipping forward and the want of adhesion of the soil and frictional resistance of the wall upon it. How can this forward movement of the earth be prevented? Before proceeding to name some means that can be employed it is advisable to remember the cardinal principles of the construction of dock-walls to resist lateral thrust, because a structure may be so designed as to induce a movement or a slip of earth. They may be briefly stated to be as follows:—

1. That the maximum weight is not required in the front of a dock-wall but at the back, in order that the centre of gravity may be as far distant from the face as the exigencies of sound construction will allow.

2. That the centre of gravity should be as low as possible consistent with the due strength of the upper portions of a wall so as to increase the resistance to an overturning movement.

3. To have an equal load upon the ground, and to guard against any extra strain upon the foundations near the face of a wall consequent upon the lateral thrust, which has a tendency to cause a wall to overturn.

4. Particularly to provide against any forward movement either by having deep foundations, protective works at the base of a wall, or by other means.

Note.—It is obvious, in order to counteract a sliding movement, that the foundations should be at right angles to the face batter at the base, and not be horizontal. These inclining inward foundations, however, can only conveniently extend for a portion of the width in very thick walls, but throughout in those of less thickness. When no support is possible in the front of a wall, such as an extra depth of foundations, an invert, or a firm connection between the lower portion of opposite dock-walls, &c., it is advisable to counterfort the wall.

5. That the wall should be as symmetrical in form as possible, particularly upon a soft foundation, in order to ensure equal loading and settlement, and that it subsides without a forward movement or tilting.

6. That it should be homogeneous, especially when the strata dip toward it, sufficient provision being made to prevent any lodgment of water at the back.

7. That it must have the usual margin of stability above the sufficient weight and mass to resist the calculated lateral thrust of the earth and that caused by it being surcharged by buildings, goods, cranes, or machinery, which may cause unequal and sudden loading and strain; and to withstand in sidelong ground the additional lateral thrust caused by the inclination of the strata.

8. That the face be sufficiently perpendicular to allow ships to lay close alongside, and that it be of the required hardness to resist the rubbing of vessels, and especially of barges.

9. That, when a wall is constructed of different materials, a complete union is effected of all the parts, and anything tending to make it of varying strength, unless exceptionally required, should be avoided; such as the junction of lime concrete with Portland cement concrete, or the employment of the hardest and strongest bricks or stones and their connection by means of a weak mortar.

10. That the resultant of the vertical and lateral pressure of the wall, whether surcharged or not, falls well within the middle third of the wall at its base. If not, the foundations will be unequally strained.

11. That any mooring posts or bollards, gate or sluice chamber machinery be so placed and secured that they cause no serious additional strain upon a dock-wall.

12. In soils that expand, such as the clays, dry backing should be provided of absorbent and even material capable of compression sufficiently to relieve a wall of severe pressure from the expansion of the earth.

13. That selected material be used for the backing, and that it be raised in layers inclining in a direction opposite to that of the wall, care being taken that it cannot become water-charged.

Having briefly referred to a few of the chief principles of construction of dock-walls, the connection between them and slips and subsidence in earthwork has to be considered.

It is seldom a dock-wall fails from insufficient mass or incorrect design. The chief element of danger being imperfect foundations causing a forward movement of the wall, which may either result in a bulge or a complete advance of the whole section for a considerable distance, and usually when this forward motion commences it extends for a long length, and has an apex, and is not only of a disastrous character, but also difficult to quickly and permanently restore to a state of stability. The wall may be most carefully designed, and be amply sufficient to resist lateral thrust due to the pressure of the earth and any load upon the quay, and nevertheless, although as a wall it is perfect and unimpaired, the whole mass from want of sufficient depth of the foundations or hold in the ground frequently slides forward: and there are few extensive docks that have been constructed in any but the firmest earth that have been free from such a mishap. In order to limit the extent of a sliding movement, it might be said, reduce the lengths of the wall and make the unsupported from the front length as short as possible, and in doubtful earth it may be advisable to abandon all long straight walls and adopt the system of short jetties, with ample width at their heads for unloading and loading operations, for a vessel to lay alongside, for store-house room, and a double line of rails and the necessary free action of machinery; so as to avoid the expense of slips pushing forward the wall: but to do this may almost ruin the commercial prospects of a dock, for naturally owners and captains of ships will send their vessels where the greatest facilities are offered and the impediments to movement the least. Local considerations in each case alone can determine the shortest required clear length. However, in designing jetties to dock-walls it is advisable to so construct them that they may act also as face counterforts to a wall, and be located at any apparently weak place so as to give support where it is needed.

The chief danger is from variation of the earth in the foundations and the existence of thin seams of an unstable character, or fissures in rocky soil allowing percolation of water, most difficult, if not bordering upon the impossible, to discover unless by trial pits and borings over the whole area of the site of a dock, which would be a most unwise proceeding, as has been previously named in this chapter. The marvellous labours of geologists and others have enabled an accurate opinion to be formed of the locality and the depth at which water and certain minerals are to be found and also to indicate the earths in which fissures and seams are frequent; but it must ever be impossible for man to absolutely state that no unstable seams or veins exist over a considerable area of ground, more especially in the superficial beds upon which a dock-wall has to be constructed, although the probability of their presence may be determined with some degree of precision. The importance of a complete examination of the ground of the site and neighbourhood is imperative, and particularly of any cuttings and embankments in the locality.

In Chapter II. many conditions of earth are referred to in which slips and movement are probable, but that now under consideration is usually caused by a wall resting upon seams or weak veins of unstable soil; therefore, there is danger when a comparatively thin seam of soft slimy earth interposes between the hard bed upon which the foundations may rest and one below the vein; for, but a little additional moisture may sufficiently lubricate the surface to enable the firm bed to slide upon it, the frictional resistance being thereby so reduced that movement results. Similarly, in the case of a seam of gravel overlying clay, the firm gravel bed may slide upon the clay: the foundations should then be carried down to the clay. Piles at the base driven into the gravel will be of little use; in fact, for permanent work, they are now almost abandoned except for jetties, cills, aprons, dolphins, &c., for uniformity of support is very difficult to obtain in pilework and cannot be proved to exist; but piles may be useful to help to sustain a structure until it has taken its permanent bearing and is in possession of its full strength. For purposes of permanent support they are somewhat unreliable, for failures have happened because of their weakness, and walls have consequently been overturned; their chief use is as auxiliary temporary aids to lateral stability, such as confining concrete or lessening the percolation of water. However, the earth may be in such a condition that it cannot retain them in position when they are strained, then they are useless for lateral support. Their resistance to horizontal strain varies much with the nature of the soil and not necessarily according to its cohesive power; for instance, experiments have shown sand to afford the greatest horizontal resistance; clay, less; and in loose ashes the power is further reduced.

When a clay stratum is thin and overlies gravel or sand, provided the necessary precautions are taken that the sand does not “boil,” it is better to have the foundations upon the underlying gravel or sand, as the layer of clay may bulge and slide upon the gravel and thrust out the wall. Should a stratum of clay be superimposed upon another of similar character, it will always be in a damp state conducive to lateral instability, as there will be two sliding surfaces, and the upper may be forced forward and carry the wall with it, and should a seam of sand be interposed it may become “quick” and flow away. In such cases the foundations should extend to a safe depth in the lower clay stratum or be below the unstable seam, and when the depth of silt or unstable soil overlying a firm bed, as rock, is considerable, the well system can be adopted, provided the bottom is levelled. As seams, weak veins, and fissures are so frequently met with the examples might be continued almost ad infinitum. To ascertain whether they exist and their location is one of the primary precautions to be observed, as dock-walls usually fail from bulging or slipping forward, causing fracture or overturning.

Additional weight or increase of the thickness of a wall may not suffice to arrest movement of the base or forward motion of the earth. To place the foundations at a greater depth may be impracticable, although it may be the best method of restoration; it then becomes necessary to insert an invert or strut between the walls in a narrow dock, or in a large dock to attempt to remove the unstable seam in front of the wall and prevent movement of the vein by a curtain-wall, additional weight, or other means of consolidation; or by removing the solid backing, draining the back, and the erection of a timber platform instead of the earth, so that it reposes at its natural slope without creating a thrust upon the wall; or by having packed rubble filling instead of ordinary solid backing so as to remove or lighten the lateral pressure upon it, by draining the ground at the back; or by counterforts from the foundation to the ground level, at the front of the wall, especial care being taken to prevent their parting from the main wall.

The arched wall system is sometimes adopted for docks, but as it requires a longer time to construct, and is obviously more liable to be damaged from settlement and by pressure of the earth and the failure of joints and weak places, solid walls of Portland cement concrete with a hard face are more to be desired; as in such situations a monolithic and equally resisting mass is required, weight and mass being of importance. The relative bulk of materials for a certain expenditure should be considered, as it may happen that greatly increased weight and mass may be obtained for no extra expense by the employment of a certain substance. Combinations of brickwork and masonry and concrete are being abandoned in favour of one homogeneous material throughout, and no yet known aggregate fulfils this condition for such work as Portland cement concrete, owing to the difficulty of making secure and perfect joints, although hard brick facing may be necessary to protect the face from wear by the rubbing of vessels. By a judicious adoption of material for a dock-wall in order to give it weight and mass, and sufficiently deep foundations, neither counterforts nor other special means of protection against failure may be required, and slips and movements of the earth may be prevented.

In the case of a stratum of soft soil of considerable depth overlying a firm foundation, dock and quay-walls upon arches, not exceeding about 30 feet span, have been successfully erected upon wells sunk in or to the solid ground, when by reason of the cost it was impracticable to carry a solid wall to the firm ground; and no slipping, sliding forward, or subsidence has occurred.

To prevent movement of the earth at the back of a wall pushing or fracturing it and being the cause of a slip, it may be well to briefly state a few points to which attention should be especially directed.

In adopting two systems of design of a wall, it is advisable that the change of form be gradual and not made abruptly, or cracks may occur from unequal settlement or load upon the foundations, and also when a concrete bed is used under a wall a lime concrete layer cannot be firmly joined to one composed of more solid and unyielding material, such as Portland cement concrete. Similar walls erected upon different soils are not equally strained, as one earth may scarcely change from the effects of air and water, and another may vary daily according to the weather.

To counteract any overturning tendency, a wall should have a large bearing area under the face portion, all projections and heavy copings should be avoided, or anything that increases the load near the face at the top. Weight and width of base and considerable depth of foundations are most important, for well designed walls when placed upon stable ground have been pushed forward simply from an insufficiently deep hold in the ground; and the load upon the earth must be within its safe bearing power. A wall should be designed so that its centre of gravity is as far from the face as practicable, and when it is constructed of stone or brick it is necessary that the joints be as strong as possible, and be capable of resisting varying strains. Should lime mortar be used it is probable it will be partly washed out, and Portland cement mortar should always be preferred upon the face, or water may percolate into the wall and even pass through it to the earth and cause slips and subsidences.

Counterforts erected upon the outer face of retaining walls are much to be preferred to any placed at the back, for support at the face is what is required: in the latter case there is always a chance of their becoming separated from the main wall, although when so located at the back they are considered to lessen the lateral pressure of the earth by dividing it, but any reduction caused by friction of the soil against the sides may vary so greatly, according to the condition and subsidence of the earth, that it is prudent to disregard it, and it is well to remember that the pressure may become intensified or uneven upon the main wall by deflection and concentration caused by the counterforts, the result being the creation of weak and unduly strained places; it may therefore be better to uniformly increase the thickness of the wall. Counterforts of triangular shape on plan have been adopted in order to lessen the lateral thrust by directing it to the sides of the triangles, their bases being against the wall. It is doubtful whether the thrust of the earth so acts under the usual varying conditions of work. The necessity of mooring vessels alongside a dock or quay-wall prevents the adoption of face counterforts except to a very limited extent; however, although the form of a ship permits of only a slight batter for the upper half, the lower portion may be inclined for a few feet from the bottom. As the adoption of a batter on the face increases the area of the base of a wall and its frictional surface upon the ground, and tends to lessen overturning movement and undue strain upon the face, a dock, quay, or retaining wall having a batter is to be preferred to one with a vertical face; in fact, it acts as a triangular front counterfort, with the great advantage of being an inherent part of a wall.

In foundations of a sandy or silty character, a dock or quay-wall having wells in the cross section to within a few feet of its base filled with rammed light dry material, the width of the bottom nearly approaching that of the depth of water, has been adopted in preference to a solid wall of less thickness, as affording a wider base, greater resistance to overturning, and a reduction of the weight upon the foundations.

A slip of the earth backing is frequently caused from insufficient drainage and a consequent accumulation of water behind a wall, producing such pressure that the friction of the weight of the wall upon the foundations is impaired and the support in front insufficient to resist a forward movement, therefore the back drainage should receive due attention. When a dock-wall has been thrust forward by the earth it may be a serious movement, likely to increase and culminate in the destruction of the wall, or be merely a slight lateral settlement. Such slips are usually of considerable length, and have an apex where the greatest forward motion has occurred. The chief remedies consist in securing the foundations, supporting the face, lightening or removing the pressure of the backing, draining the backwaters and preventing their reaching the wall. Provided a wall is intact and not damaged, but solidly and horizontally pushed forward, it need not be taken down, but if vertical subsidence has taken place, or a settlement, at the face only, it is generally of serious importance, and it may be necessary to remove and replace it.

When a wall bulges slightly from the pressure of the earth at the back and then ceases to move, the ground behind may have become in a state of permanent equilibrium, and the lateral thrust, which before movement was too great, may be so reduced that in the altered position it may remain stable, provided the void is filled with light porous backing, and the drainage of the earth receives due attention. Counterforts of piles reaching to the level of the ground in front of the face have been adopted to arrest and prevent a forward movement, but when the ground is much disturbed and in a loose state, it is unadvisable to drive piles because of vibration which may induce further motion. Instead of timber pile counterforts, trenches might be cut at intervals along the toe and be filled with quick-setting cement concrete, the filling to follow the excavation as closely as possible, and no unsupported earth to be allowed to remain.

As improper backing often induces a slip and subsidence of the earth and the failure of a wall, it is of importance that it should be carefully executed, for many well-designed walls have failed or bulged from being badly and hurriedly backed with unsuitable and soft retentive material in a deteriorated condition, the result being movement of the earth. Care should be taken that the material of which the backing is composed will always maintain the slope of repose upon which the calculations have been based that determined the dimensions of the wall; therefore, it should always have an angle of repose which is not much affected by moisture. It is important to remember that according to the nature of the backing so will be the pressure upon a wall, depending principally upon the coefficient of friction of the earth upon similar earth, its cohesion, its friction upon the surface of the back of a wall, the inclination of the layers, the general character of the soil and the effect of moisture upon it; and no vegetable, decayed, or “made” earth should be used for such a purpose, nor material which gives a varying thrust according to the state of dampness or dryness, and the lightest dry, firm, and stable earth should be preferred. When rubble backing is adopted and is carefully packed, the lateral thrust may be greatly reduced, as in some degree, instead of backing a dry wall may be considered to have been built at the back of the face wall. Clean ashes are an excellent material for filling damp places in a wall, because they not only absorb moisture, but are light, and stand permanently at a slope of about 1 to 1 TO 1¼ to 1, but they can seldom be obtained in sufficient quantity to be used in considerable masses; also broken bricks and burnt ballast, although heavier, make good backing.

Earth that is much affected by air or water, or that expands, contracts, or fissures, such as clay, is not good for filling, and dry masonry retaining walls should not be hacked with earth but with rubble, or in countries with a heavy rainfall the wall will most probably be forced forward.

Care should be taken to have plenty of weep-holes in a wall so as to obviate any accumulation of water, and it is well if the back has a rough face in order to increase friction and prevent cavities in the backing down which water may percolate. During construction a few weep-holes in a dock or quay-wall should always be provided, until the water is to be let in to a dock, when it is best to fill them with strong Portland cement concrete as water would proceed along them from the face and accumulate at the back of a wall; the permanent drainage being affected by other means.

In pulling down walls that have been built with offsets at the back it will usually be noticed that the filling does not rest upon them, but that hollows occur as the earth subsides; for this reason, a straight batter at the back is to be preferred. No support from the weight of the earth, which is supposed to be upon them, should be relied upon, although it may temporarily exist.

The filling should be commenced at the wall, and the layers be so deposited that their slope approximates to that of being at right angles to the surface of the slope of the ground rather than parallel to it. When thought desirable and the natural ground is solid, it can be benched, and the backing be damped and rammed.

The backing of a wall may consist not only of an inverted triangular piece of earth, but extend for some distance to the rear, and may have a top surface at a considerable height above it; the wall will then have to sustain a severe thrust. The angle of repose of the earth should be ascertained and the filling be thoroughly drained, or it may be disintegrated by moisture, the whole mass gradually become unstable, and finally push the wall forward with great force.