A Treatise On The Principles And Practice Of Harbour Engineering

CHAPTER VIII. PIERHEADS, QUAYS, AND LANDING-STAGES. Importance of Pierheads—Forms adopted—Main Features—Lighthouses and Harbour Lights—Examples of Pierheads at Toulon, Sunderland, and Pillau -Quays —Landing Slipways—Stairways and Ladders—Spending Beaches—Entrance Booms—Landing- stages: Fixed and Floating—Pontoons—Conditions of Stability—Centre of Buoyancy and Metacentre—Case of Semi-immersed Pontoon—General Case—Case of the Ballasted Pontoon—Internal Stresses in Pontoons—Liverpool Landing-stage. Importance of the Pierhead.—Whatever the relative exposure of other parts of a mole or breakwater, there can be little doubt that its termination —the pierhead as it is called—is subjected to experienoes considerably more trying than any which fall to the lot of maritime structures in general. To begin with, it is unfavourably situated; and, in the second place, it is still more unfavourably adapted to its position. Exposed on three sides out of four, it is called upon to withstand and repel the most powerful subversive agencies without that uninterrupted lateral support which constitutes so important and valuable a mainstay of the breakwater proper. The absence of this support renders it a matter of the utmost moment to make a pierhead thoroughly self- sustained and independent—to treat it, in fact, as a perfectly detached and isolated structure, capable of resisting, unaided, all external influences ; for in its downfall is involved the destruction of more than is contained within its own limits. The pierhead removed, the section of the breakwater immedi- ately adjoining it has its security materially impaired, and becomes practically defenceless. It enters into that precarious condition which is inséparable from a “scar-end,” and the area of damage may be almost indefinitely extended. The pierhead. therefore, should be looked upon as the keystone of a breakwater’s stability. In itself, the pierhead may possibly not exhibit to the eye any specially marked features of height or width—many minor structures do not; but whether these features be in evidence or otherwise, the necessity for greatly increased strength and powers of resistance cannot be gainsaid. As a matter of fact, most moles and breakwaters of any importance are equipped with prominent pierheads of striking outline and substantial construction. There are several reasons why this should be the case. Not only do pierheads, as a rule, project into deep water, demanding a broad and extended base to ensure correspond- ing stability, but they have also special functions to discharge, which require 187