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.