A Treatise On The Principles And Practice Of Dock Engineering

STRATIFIED SITES. '85 wall may be comparatively economically constructed in the form of a thin veneer of masonry or conerete, securely attached by dovetailing, at intervals, to the vertical face of the rock. Such was the niethod adopted at the Herculaneum Dock, Liverpool, where the rock cutting was faced by 2 to 4 feet of masonry, with vertical dovetails 5 feet wide and 4 feet deep, at 20-feet intervals, as shown in figs. 110 and 111. If, however, the rock be very hard and durable, the necessity for veneering is obviated, as at Ardrossan (fig. 112). Clay is a very uncertain material. It varies in volume, texture, and consistency. When thoroughly dry, it is hard and friable ; when saturated, it becomes soft and viscous. Mixed with lime, it forms a brittle compound, known as marl. When the adulterant is sand, the more tenacious product is called loam. Clays possess so many purely local attributes that little can be said of their efficiency, as a class, for foundation purposes, beyond that they are usually satisfactory, if properly protected. One variety of clay—• the blue clay—however, possesses striking and dangerous characteristics, which call for especial précautions. Several instances of failures in dock walls have occurred by reason of its treacherous nature. Apparently firm in itself, it often conceals planes of non-adhesion—surfaces in such a state of greasiness that they slide over one another with the greatest facility. These planes may be some distance below the foundation level, and involve the upper stratum of clay in the forward movement of the wall, as actually took place at the S.AV. India Dock.* A blue clay foundation has been responsible for the sliding of dock walls at Southampton, Calcutta, Avon- mouth, and elsewhere. Nominally and generally bluish in colour, the upper layers of this clay are sometimes yellow, due to the change of a protoxide of iron into a peroxide, by the action of air and moisture. Sand and Gravel are usually firm and durable foundations, practically incompressible, but they must be confined laterally. They need protection from the action of currents. Very often beds or pockets of these substances are met with in the boulder, drift, or glacial clay. If too deep for excava- tion, they may be rendered very serviceable by the expedient of mixing some neat cement with the topmost layer. Stratified Sites—The question of the depth at which it is desirable to found a quay wall depends not only on the projected level of the dock bottom, but, to a far greater degree, upon the nature and disposition of the strata met with. Having reached a depth adequate from the point of view of design, a problem presents itself which may be resolved into four heads, the first and simplest of which has just been dealt with. 1. A sufficiently firm foundation of indefinite extent. The wall may be erected thereon, with such precautions as the nature of the case requires. 2. A hard stratum overlying a soft one. Here it is essential to preserve the hard covering intact. For example, if a bed of clay overlie a quicksand * Min. Proc. Inst. C.E., vol. exxi., p. 120.