Granite is principally used in situations where great strength is required, such as for copings and facings to dock walls, quoins and sills to entrances and locks, column and pivot bases, girder beds, paving setts, and road metal.

The stone is procured in various parts of the United Kingdom, chiefly in Aberdeenshire, Kirkcudbrightshire, Cornwall, Devonshire, Leicestershire, Wicklow, Wexford, and the Channel Isles. Cornish granites have generally a very coarse grain.

Sandstone has a crystalline structure composed of grains of quartz cemented together by various substances, such as carbonate of lime, carbonate of magnesia, &c., upon the weathering qualities of which the durability of the stone depends. A good sandstone should possess a uniform, compact, bright, well-cemented grain. A dull appearance is not a good sign. Some sandstones are very friable, others are but moderately durable, but a few of the harder varieties are very serviceable for dock work, such as those from the reputed quarry of Bramley Fall,* near Leeds, from the Forest of Dean, in Gloucestershire, and elsewhere.

Limestone is a somewhat vague term for a stone, the principal constituent of which is carbonate of lime; and a class which includes chalk, Portland stone, Kentish rag and marble, has a very wide range of characteristics indeed. The most durable specimens, as a rule, are heavy, dense, and homogeneous, with a fine, crystalline grain. Portland and Purbeck limestones, perhaps the best known varieties in general use, differ slightly from this criterion; the first has a fairly large grain, and the second is conchoidal and non-crystalline. Both these stones, and, indeed, limestones generally, and in a lesser degree sandstones, are vulnerable under the attacks of the Pholas, and this acts as a deterrent to their extensive use in marine situations. The limestone blocks at Plymouth

*The original quarry of Bramley Fall is reported to be practically worked out, but much of the stone from neighbouring quarries goes by the same name.

+ For a very valuable and complete series of experimental results, dealing with the crushing strength of stone, the reader is referred to a paper on "The Building Stones of Great Britain," by Professor T. Hudson Beare.-Vide Min. Proc. Inst. C. E., vol. cvii.

DESTRUCTION OF STONE.

155

break water had to be replaced by granite blocks owing to the ravages of the mollusc. Apart from this, the growing popularity and the ready adaptability of concrete have caused it to largely supersede natural rock for dock construction and harbour works.

Destruction of Stone. The softer kinds of stone will frequently wear away under continued attrition and the chemical action of an unsuitable atmospheric environment, but the destructive agencies most in evidence, in regard to the more adamantine varieties used in dock work, are living organisms.

The Pholas dactylus is a mollusc, living in sea water, which bores into limestone, shale, sandstone, and timber, but does not attack granite. It is a small animal, with a maximum length of about 5 inches, but one which is quite capable of doing extensive mischief by boring its holes in close proximity to each other, causing the ultimate collapse of the masonry.

The Saxicava is another mollusc known to bore into limestone to a depth of 6 inches. It has manifested its presence at Plymouth, Folkestone, and elsewhere.

There is apparently no remedy for the ravages of these marine borers, except the substitution of some other kind of material for the stone attacked.

156

CHAPTER V.

DOCK AND QUAY WALLS.

DEFINITION-FUNCTIONS UNDER VARIOUS CONDITIONS-STRESSES IN RETAINING WALLS -OVERTURNING FORCES-ANGLES OF REPOSE-THEORY OF CONJUGATE PRESSURESCOULOMB'S THEOREM-CHAUDY'S THEOREM-Weight OF EARTHWORK-SURCHARGE -RESTRAINING FORCES-COUNTERFORTS-TIE BARS-WEIGHT OF MASONRY-EMPIRICAL FORMULA-CONDITIONS OF STABILITY-CENTRES OF GRAVITY-TYPICAL EXAMPLE-PRACTICAL POINTS-NATURAL FOUNDATIONS-STRATIFIED SITES-ARTIFICIAL FOUNDATIONS-PILING-WELLS AND CYLINDERS-GENERAL METHODS OF CONSTRUCTION, WITH EXAMPLES OF QUAY WALLS AT NEWCASTLE, CORK, GLASGOW, LIVERPOOL, BELFAST, ARDROSSAN, MARSEILLES, ANTWERP, ROTTERDAM, DUBLIN, KURRACHEE, Suez, Bougie, aND SFAX-CONSIDERATION OF INSTANCES OF FAILURE AT ALTONA, LONDON, SOUTHAMPTON, CALCUTTA, AND LIVERPOOL-UNDERPINNING— MISCELLANEOUS TYPES OF WALL AT HULL, GREENOCK, LONDON, LIVERPOOL, AND MANCHESTER.

Definition. A dock wall may be said to be a special case of a class of walls termed Retaining or Revetment walls. Under normal conditions it derives a certain, albeit varying, amount of support from the hydrostatic pressure on its face, which more or less neutralises the earth pressure from the rear. Should, however, the dock at any time be allowed to run dry, the identity of its functions with those of an ordinary retaining wall would be complete. This is a possibility which may have to be faced, voluntarily, on account of repairs and alterations, or involuntarily, for other reasons, such as an accident to the entrance gates. Accordingly, it is advisable to neglect any frontal sustaining force and to treat a dock wall as if it were a retaining wall, pure and simple.

66

But, even in so doing, it must be admitted that the range of contingencies to which a dock wall is liable far exceed those affecting an ordinary retaining wall. Hydrostatic pressure alone may more than double or halve the factor of safety in a given wall. Thus, with a well puddled dock bottom, the subsoil water in the ground at the back of the wall will frequently stand far below the level of the water in the dock, and the hydrostatic pressure may thus wholly neutralise the lateral thrust of the earth, or even reverse it. On the other hand, with a porous subsoil at a lock entrance, the back of the wall may be subjected, on a receding tide, to the full hydrostatic pressure due to the range of that tide plus the lateral pressure of the filling. Again, the water may stand at the same level on both sides of the wall, but may or may not get underneath it. If the wall is founded on rock or good clay, there is no more reason why the water

OVERTURNING FORCES.

157

should get under the wall than that it should creep under any stratum of a well-constructed masonry or puddle dam, and under those circumstances the presence of the water will increase the stability by diminishing the lateral thrust of the filling. If, however, as is perhaps more frequently the case, the wall is founded on a porous stratum, the full hydrostatic pressure will act on the base of the wall, and reduce its stability in practical cases by about one-half." These mutable conditions can manifestly only be met by providing a considerable margin of strength.

*

Stresses in Retaining Walls.-The forces at work in the case of an ordinary retaining wall are three in number:

(1) There is the overturning influence of a wedge-shaped mass of earth, DCE (fig. 77), behind the wall, which tends to slide down some plane of rupture, C E, in the absence of proper support.

(2) To this must be added the effect of any surcharge upon the surface of the ground constituting the wedge.

(3) And, lastly, there is the weight of the wall acting vertically downward, and consequently offering resistance to the overturning tendency. If the back of the wall be not vertical, as in fig. 78, it is obvious that the perpendicular line, CD, must still be considered the virtual boundary of the opposing influences and that the weight of the earthwork, F C D, must be included in the weight of the wall.

It will be well to consider these forces a little more in detail.

Fig. 79.

Overturning Forces.-The actual extent of the wedge and its effective pressure can only be matters of conjecture. It is common experience that unsupported earthwork stands at widely differing slopes, according to the nature and condition of the particles of which it is composed. To a limited degree, experiments have determined some of these slopes and fixed what is termed an Angle of Repose (p, fig. 79) for the more prominent kinds of

* Baker on "Lateral Pressure of Earthwork," Min. Proc. Inst. C. E., vol. lxv.,

p. 180.

earth. But the values attached to these angles can only be regarded as of an approximate nature, as will be evident from a glance at the following table comprising maximum and minimum results obtained by different experimentalists :—

Ranges so extensive render it an exceedingly difficult matter to assign any angle to a variety of soil, however specific, especially in view of a further modification due to its degree of humidity. The amount of moisture present in the sample under consideration very materially influences the experimental result obtained for its angle of repose. A slight quantity, just sufficient to occupy the interstices between the grains of solid matter, has been found to increase the frictional resistance to movement, and, accordingly, to produce a correspondingly greater angle of repose. Any excess of moisture, however, over and above this trifling amount, results in a diminution of the frictional resistance; and if the humidity be indefinitely increased, the material eventually acquires a muddy consistency to which there is no angle of repose worth noting. Ordinary clay, for instance, in the dry condition crumbles at 40°; moderately moist, its inclination may be increased to as much as 50°; allowed to become saturated, it subsides at an angle of 10°.

Argillaceous earths are most susceptible to the deteriorating influences of moisture, and any admixture of sand with the clay only produces an accentuation of the evil, because the impermeability of the clay offers an obstacle to the escape of water which has entered through the pores of the sand. A striking instance of this is afforded in a notable landslip behind wall at Altona, to be dealt with at a later stage.

a quay

The foregoing considerations distinctly emphasise the necessity for the prompt and adequate drainage of earthwork, and particularly so in the case of dock and river walls, where the earth backing is generally in a state of intermittent immersion. Under the head of a rising tide, water penetrates to an equal height behind the wall, and, unless there be adequate means for its withdrawal with the ebb, the volume of water thus confined will prove a serious augmentation of the overturning forces.