A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 252 as gate platforms. The objection is the great tendency for any depres- sion in the floor to form a mud trap, but this may be partially obviated by arranging the culvert inlets so as to exercise their influence at such parts. The sectional profile of a sill is often curvilinear, but the outlines of modern naval architecture render it desirable that the sill should be as flat as possible. The height of the sill depends upon the amount of cover required to form a watertight joint with the gate or caisson, and the clearance necessary for truck-wheels, rollers, or slides, as the case may be. Six or eight inches will generally be sufficient in the first case, and the total depth usually varies from 18 inches to 3 feet. The vertical abutment face of the sill may be formed by stone, wood, or iron, assuming that there is always a wooden member of the gate or caisson to corne into contact with it. The dressing of this timber face necessitates great care and good workmanship, for upon a close-fitting joint depends the absence of leakage. On account of their proximity to the unprotected earthen floor of a dock, the sills of passages and the inner sills of locks are at times subject to very great hydrostatic pressure, if the underlying stratum be in any degree porous. Instances have even occurred in which, with a rock foundation, water has percolated into the bed joint between the sill and the rock, causing the former to uplift. To minimise the danger arising from this cause it will be advisable to pierce the sills with a series of vent holes, lightly covered with pieces of flagstone. If the bed joint remain intact these vents will not be called into action, but if through any mischance water should penetrate beneath the sill at a time when there is little or no hydrostatic counteraction, it is infinitely preferable that there should be a means of escape for the water rather than that the full effect of the fluid pressure should be exerted against the underside of the sill to its detriment and possible disruption. From the foregoing considérations it is obvious that weight and homogeneity are distinct advantages to a sill. To prevent undermining by the wash of the tide or the scour of a current, the outer sills of entrances should be provided with a masonry or concrete apron extending some distance in front of the sills. 2. Platforms. —-These form the floor over which gates and caissons are moved in and out of position. If for gates fitted with truck wheels or caissons with rollers or slides, they will be provided with granite, or iron, or steel tracks, the last two firmly bolted down to the masonry or concrete. Metal roller paths for gates form segments of circles in plan, and their upper surfaces are bevelled to the inclination of the truck wheels, which are truncated cones, on account of the greater amount of travel to be performed by the outer edge. The axis of the cone will intersect the axis of the pivot. Caisson tracks are either flat metal surfaces or rails. Occasionally, the wheels are attached to the floor, and the track or sliding surface to the underside of the caisson. A platform should be sufficiently strong