A Treatise On The Principles And Practice Of Dock Engineering

DAMS. I03 considered desirable, whereas the action of a single chain grab is instan- taneous. A grab dredger with a bucket capacity of 1 ton may be assumed capable, from actual trial, of raising from 50 to 60 tons of mud per hour, when working in from 15 to 20 feet of water. Of clay, very little more than one-half this amount can be reckoned upon. The grab is an excellent tool and invaluable in confined situations, but it is scarcely suitable for general adoption in works on a large scale. It is not an economical instrument for the removal of stiff clay ; its hest performances are in regard to mud and soft earth. It cannot be counted upon to work with the same regularity and evenness as the ladder dredger; in fact, its tendency is to pit the surface of the ground with a series of hollows and depressions. But, in spite of these drawbacks, it has demonstrated its utility to such an extent that it is looked upon as an essential accompani- ment of most dock and harbour undertakings. Cost of Dredging.—The conditions prevailing in regard to dredging are of so variable a nature that no information respecting the cost, at any locality, is of much use elsewhere. Further than this, the available statistics are far from uniform, and there is considerable diversity of extent in the operations included. It can only be said broadly that, witliin ordinary limits, dredging is effected at some price between a penny and half a crown a cubic yard, distributed, roughly, somewhat as follows :— Suction dredging, Id. to 6d. Grab dredging, 3d. to 8d. Bucket dredging, 6d. to 2s. 6d. These figures do not apply to rock-dredging, the cost of which exceeds the higher limit, often very considerably. AUXILIARY APPLIANCES. Dams. _____In dock construction, a dam is a temporary contrivance for the exclusion of water from a site during the progress of the undertaking. It is accordingly composed of material susceptible of easy removal, eithei in bulk or in parts. Timber aud clay form two of the most prominent sub- stances for the purpose. Stone and concrète are occasionally used, under restrictions to be noted later. Iron is rarely employed, and then only with a view to its ultimate incorporation in the permanent work. In spite of its temporary character, a dam should be substantially made. The damage and delay, to say nothing of possible loss of life, resulting from the failure of any part of it, during a critical period, would far more than counterbalance any economy in construction. Too much stress cannot be laid upon this point. It is infinitely better to err on the side of excessive strength than to run the risk of disaster through an insufficient margin of stability.