A Treatise On The Principles And Practice Of Dock Engineering

172 DOCK ENGINEERING. weight of its structure constitutes the one and only element of stability, and its line of action is obviously vertical through the centre of gravity. If, however, the back of the wall be inclined to the vertical at an angle, 'LO h^ ß, as in fig. 81, the nett weight of the wall must be increased by —tan ß, the weight of the earth directly supported by the wall and manifestly assisting to maintain equilibrium. The combined weights must be taken as acting through a common centre of gravity. Such, at any rate, is the legitimate course to adopt from a purely theoretical point of view. At the same time it must be admitted, on unimpeachable testimony, that the assumption is not borne out by actual experiment. Sir Benjamin Baker states that “he has invariably observed that when a retaining wall moves by settlement or otherwise, it drops away from the filling and eavities are formed. A settlement of but ^ of an inch, after the backing had become thoroughly consolidated, would suffice to relieve the offsets of all vertical pressure from the superimposed earth, and the latter cannot therefore be properly considered as contributing to the moment of stability.”* Considering, however, that the purely theoretical aspect of the problem involves equal, if not greater, discrepancies on the other side, in unduly augmenting the effective overturning thrust, it is no inéquit- able arrangement to regard the advantages accruing to the weight of the super- imposed earth as compensating for the neglect of the cohesive power of the backing. Where the offsets at the back of the wall are continued to some depth, it may reasonably be urged that any indisposition of the earthwork to follow settlement in the wall argues a correspondingly high degree of cohesion between the particles and a considerable modification of the calculated thrust. Another point which calls for attention is the extreme likelihood of water finding a passage beneath the wall, especially in porous foundations, for, in this way, the effective weight of the wall is decreased by the weight of a volume of water équivalent to the immersed section. This may amount to as much as 45 or 50 per cent.; a reduction of great importance. The effect, however, is only felt when the dock is full of water, and then the support derived from the hydrostatic pressure on the face of the wall is sufficient to compensate for the diminution in weight, unless the water in the dock be lowered rapidly while the earth backing is imperfectly drained. The liquid head due to the water imprisoned behind the wall, combined with percolation through the foundation, is sufficient to produce a dangerous complication, resulting in more than one instance, from actual experience, in movement and disruption. Counterforts, or narrow pilasters, are often built at regular intervals behind a retaining wall with the view of adding to its stability. Their value in this respect is entirely a question of adhesion. In the case of masonry walls it has frequently been found that a separation has taken place between the counterfort and the body of the wall. Such a separation, * Min. Proe. Inst. C.E., vol. lxv., p. 181.