A Treatise On The Principles And Practice Of Dock Engineering

io6 DOCK ENGINEERING. failure from which the former are exempt. The continuai subsidence of the clav hearting involves more than the mere replacement of the disappearing material, since the latter in sinking exerts powerful pressure of a hydro- static character against the sides of the dam, producing a strong tendency to rupture, which has indeed taken place in at least one instance to the author’s knowledge. Again, the presence of horizontal walings in the interior of the dam for the guidance of the piles in their descent, and of transverse ties, is a source of much troublesome attention, because the °lay> in settling, leaves cavities and interstices immediately underneath these parts, which serve as channels and duets for leakages. The evil may be minimised by the withdrawal of the inner waling, after the driving of the piles and before the insertion of the clay, also by the substitution of timber diaphragms, extending from top to bottom, for transverse tie-rods. Where rods or bars are used, several flat washers or plates of large area with per- forations near the upper edge, for the insertion of the through bolts, will sometimes serve to check the passage of water in case of a slight sinkage of the clay. 6 From these considerations it is clear that no useful object per se is served by any great thickness of clay puddle ; the disruptive force is only increased thereby, and sources of leakage are more difficult to locate. A minimum width of 5 feet in the interior of a cofferdam will generally prove an adequate allowance for impermeability, but, on the other hand, as regards the stability under external pressure, the height of the dam will exercise most influence in determining its width, though this factor can be discounted to some extent by the use of auxiliary shoring. The only external force at work upon a cofferdam is the hydrostatic pressure against its outer face. If we call this P, the height of the water hr and the weight of a cubic foot w, then the pressure per foot run (as explained in detail in Chapter viii.), is p _ wh2 and the overturning moment about the base, the centre of pressure beingat one-third of the height from the ground, Now, the dam derives its stability in varying proportions from three sources. These are—(1) its dead weight or inertia, treated as a heavy, detached mass ; (2) its resistance to transverse stress, treated as a cantilever firmly fixed in the ground; and (3) the support afforded by the external strutting, if any. (1) The moment of resistance due to the intrinsic weight of the struc- ture is Mi=wx g, .... (6)