A Treatise On The Principles And Practice Of Harbour Engineering

i88 HARBOUR ENGINEERING. a certain individuality of treatment. They serve to mark the entrances to ports and harbours, and should therefore be clearly and readily recognisable. It is to be noted, in this connection, that they also run the risk of collision and impact with passing vessels. At night-time, and in misty weather, an entrance, especially if at all narrow, should be efficiently lighted, and, to this end, pierheads are often furnished with a lantern, fixed either to a mast or mounted on a platform, or set in a lighthouse. Pierheads, therefore, demand consideration from two different aspects : (1) as outlying works in particularly exposed situations, requiring special pré- cautions in regard to design and construction ; and (2) as a means of guidance and direction to vessels entering a port, more especially in times of stress and heavy weather. Form of Pierheads.—With respect to the former point of view, there are, in the first place, strong reasons for conferring upon a pierhead a shape which, in plan, is symmetrical about a point or axis. Accordingly, the ends of many moles and breakwaters are expanded as already stated, and given some geometrical form—circular, square, octagonal, rectangular, hammerhead, etc., as the case may be. Of these, the circular may be claimed as the most con- venient shape, from the point of view of manæuvring vessels, it being easier, in case of necessity, to warp a ship round a continuons curve rather than to swivel it through an entire quadrant. The widening of the pierhead, how- ever, somewhat interferes with the working of vessels alongside an inner quay, though, on the other hand, it serves to cut off any strong flow of current which might endanger the vessels moored there. (1) As regards intrinsie stability, a pierhead should be, as far as possible, homogeneous throughout, without joint or intersection. This is not always a Îeasible arrangement, nor in any case easy to achieve. There can be no doubt, however, that an ideal pierhead would be one of the nature of an enormous monolith. For the purpose of constructing such a monolith, a buoyant caisson chamber of the type which has already been described (p. 175) is often constructed, floated into position, sunk, and filled with concrete. Another method is to form an outer ring or boundary, of large blocks securely anchored together, and to deposit mass concrete in the interior. In this instance, it has been found essential to introduce a number of cross ties passing right through the ring, to prevent the blocks from being disturbed under the temporary fluid pressure of the concrete. Yet another method is the driving of an outer ring of sheeting piles, either of timber or steel, to form the necessary inclosure. But this operation is attended by some difficulties. The satisfactory alignment and driving of piles is not always accomplished with facility, even when the work is straight- forward and the conditions favourable. With both these factors acting in an adverse sense, the likelihood of a successful issue is more remote. Curvilinear work in piling is a particularly awkward undertaking. One word of caution is very necessary on the undesirability of forming anything of the nature of a terrace, or level platform, at the foot of the