A Treatise On The Principles And Practice Of Dock Engineering

WEIGHT OF WALLS. 175 however minute, is sufficient to nullify the advantages of counterforts, and even to invest them with dangerous potentialities, for, in falling back, they add some portion of their own weight to the earth pressure against the wall. Provided, however, the counterforts be adequately bonded into the body of the wall (and this may be effected very satisfactorily in the case of walls con- structed of Portland cement concrete), there can be no doubt as to the advantage to be derived from their aid. The thickness of the wall may then, for theoretical investigation, be regarded as increased to the extent of the thickness of the counterforts, divided by the distance apart at which they are set; in other words, the wall may be taken at its mean thickness. At the same time it is a matter of opinion as to whether the material may not be more economically distributed uniformly. In instances where it is rendered necessary, additional security may be afforded by the use of tie-rods or tie-bars firmly connected to the wall near the top and carried to a secure anchorage in the ground some distance away. The very great leverage (measured from the base) at which such a tensile force would act, renders a comparatively slight rod capable of counteraeting a considerable degree of earth thrust. The expedient has often been adopted for the purpose of strengthening walls which have showed signs of yielding. Means should be provided for properly tightening up the bars or rods by means of gibs and cotters, screw shackles, or other contriv- ances. A rough and ready way is to heat the whole length of the bar before completing the attachment ; the contraction in cooling will generally be found sufficient to bring the bar into stress. Weight of Walls.—The weight in air of the various kinds of material of which a dock wall may conceivably be composed is stated below :— TABLE XVII.—Approximate Weight per Cubic Eoot of Mineral Substances. Lbs. Lbs. Basalt, . .. 187 Masonry, . . . 116 to 144 Brick, . . 115 to 135 Mortar, . . . . 109 Brickwork in mortar, 112 Quartz, . . . . 165 Felspar,.. 162 Sandstone— Flint, 164 Gatton (Surrey), 103 Granite— Calverley (Kent), . 118 Cornish, 164 Whitby (Yorks.), . 126 Aberdeen, 166 Red (Cheshire), 133 Dublin,. 170 Craigleith (Edinburgh), . 141 Guernsey, 187 Darley Dale (Derby), 148 Limestone— Talacre (Flint), 150 Bath, 120 York, . . . . 157 Portland, 130 Auchray (Dundee), 159 Chalk, 145 Abercarne (Monmouth), 168 Purbeck, 150 Slate— Chilmark, 155 Cornwall, 157 Kentish rag, . 166 Westmoreland, 173 Marble, 170 Welsh,. 180 Magnesian, . '. 175 Trap, . . . . . 170