A Treatise On The Principles And Practice Of Dock Engineering

RESTRAINING FORCES. 171 TABLE XVI.—Approximate Weight per Cubic Foot of Various Kinds of Earth. Lbs. Fine dry sand, loose, ... 90 ,, ,, well shaken, .... ... 98 Coarse pit sand, 100 Damp river sand, ... 118 Quartz sand, 170 Gravel, . 90 to 95 Loose, dry shingle, 106 Mud, 102 Dry, common earth, loose, .... ... 95 Common earth, slightly moistened, 106 Densest and most compact earth, . 125 Loam, 125 Marl, ........ 100 to 120 Clay, 120 to 135 Chalk 117 to 174 Shale, ........ ... 162 Rubble filling (with interstices), 100 Surcharge.—The amount of surcharge upon a quay or dock wall can be determined by reference to the weights of cargo to be deposited there and of any superstructure upon the quay. A definite limit, however, is generally fixed in the former case, beyond which wharfingers and others should not be permitted to load quay spaces or shed floors, and an allowance of about 3 tons per superficial yard will generally be found adequate to cover all reasonable contingencies of sur- charge. The effect of the surcharge should be considered as extending from the vertical back (actual or virtual) of the wall to the intersection of the line of rupture with the quay surface, and its line of action taken as passing downwards through the centre of this distance. Fig. 94 shows the method of combining the effective pressures due to the earth wedge and the surcharge. The distance, F G, between their respective centres of gravity is divided inversely in the ratio of their weights, and the sum of the latter is taken as acting through the point, K, thus found. It will be noticed that, in this way, the- effect of the surcharge is not merely to increase the direct horizontal thrust against the back of the wall, but, at the same time, to raise its point of application and thus still further increase the overturning moment. Having dealt with those forces which tend to disturb equilibrium, we- now turn our attention to those which tend to maintain it. Restraining Forces.. — The magnitude and line of action of the restraining forces are open to less controversy and difference of opinion than is the case- with the overturning forces. If the wall have a vertical back the dead.