A Treatise On The Principles And Practice Of Harbour Engineering

!6° HARBOUR ENGINEERING. water, but Works on which they are adopted are liable to stoppages and delays arising from tempestuous weather. In certain instances, as at Dublin, solid blocks of masonry bave been built on an adjacent quay and transported by a floating crane, in an almost wholly immersed condition, whereby their effective weight has been very materially reduced. Apart from and independent of any particular system of excavation, there are general features of breakwater construction which call for careful considération. The first and most important of these is the foundation. Foundations. It would be impossible almost to devote to this subject more attention than it merits. Very great and serious harm may accrue to a breakwater founded upon a base insufficiently firm and secure. Even if the damage be remediable, there is the expense of repairs, which will probably become a matter of periodic récurrence. These repairs will naturally be of a more pronounced character in the case of regularly bonded structures, such as walls of ashlar work, which, when disturbed or deranged in any way, involve the provision of special appliances and skilled labour to reinstate them. Accordingly, it will be well to consider the characteristics and qualifica tions of a good foundation. These may be classed under two heads: Incom- pressibility and Permanence. Incompressibility. —A theoretically ideal foundation is incompressible : it does not yield in any way to the load imposed upon it. Such a foundation, however, except in the harder varieties of rock, is almost impossible of realisa- tion.1 The greater part of the material constituting the sea bottom is more oi less of a compressible nature, though in some cases the compression may be but slight. Thus, in addition to the softer kinds of rock, sand and gravel and some varieties of marl are very little, if at all, affected by heavy loads, provided precautions be taken to prevent lateral escape. All other materials are compressible to a marked degree : mud, silt, the softer kinds of marl, clay (particularly when moist and plastic), peat, etc. While an incompressible foundation is undoubtedly desirable, some slight yielding is no insuperable objection, provided the settlement be uniform. It is of no great moment if the whole superstructure sink a little; but if a portion only gives way, fracture between the stationary and yielding parts is bound to occur. Hence, a foundation should be as far as possible homogeneous. A building is safer on an all-clay foundation than on one of rock and clay. Where the foundation is varied in character, therefore, special precautions are necessary to ensure equal bearing power. The pressure on the weaker material should be distributed over a larger area ; the dividing line between the two strata should be distinguished by augmented bond, such as is afforded by 1 When obtained, it is not an unmixed Messing, as the levelling of an indurated surface is troublesome.