A Treatise On The Principles And Practice Of Dock Engineering

156 CHAPTER V. DOCK AND QUAY WALLS. Definition—Functions under Various Conditions—Stresses in Retaining Walls —OvBRTURNING FORCES—ANGLES OF REPOSE—ÏHEORY OF CONJUGATE PRESSURES— Coulomb’s Theorem—Chaudy’s Theorem—Weight of Earthwork—Surcharge —Restraining Forces—Counterforts—Tie Bars—Weight of Masonry —Em- pirical Formulæ—Conditions of Stability—Centres of Gravity—Typical Example—Practical Points—Natural Foundations—Stratified Sites—Arti- ficial Foundations—Piling—Wells and Cylinders—General Methods of Construction, with Examples or Quay Walls at Newcastle, Cork, Glasgow, Liverpool, Belfast, Ardrossan, Marseilles, Antwerp, Rotterdam, Dublin, Kurrachee, Suez, Bougie, and Sfax—Considération of Instances of Failure at Altona, London, Southampton, Calcutta, and Liverpool—Underpinning— Miscellaneous Types of Wall at Hull, Greenock, London, Liverpool, and Manchester. Definition.—A dock wall may be said to be a special case of a dass of walls termed Retaining or Revetment walls. Under normal conditions it derives a certain, albeit varying, amount of support from the hydrostatic pressure on its face, which more or less neutralises the earth pressure from the rear. Should, however, the dock at any time be allowed to run dry, the identity of its functions with those of an ordinary retaining wall would be complète. This is a possibility which may have to be faced, voluntarily, on account of repairs and alterations, or involuntarily, for other reasons, such as an accident to the entrance gates. Accordingly, it is advisable to neglect any frontal sustaining force and to treat a dock wall as if it were a retaining wall, pure and simple. But, even in so doing, it must be admitted that the range of contingen- cies to which a dock wall is liable far exceed those affecting an ordinary retaining wall. “Hydrostatic pressure alone may more than double or halve the factor of safety in a given wall. Thus, with a well puddled dock bottom, the subsoil water in the ground at the back of the wall will frequently stand far below the level of the water in the dock, and the hydrostatic pressure may thus wholly neutralise the lateral thrust of the earth, or even reverse it. On the other hand, with a porous subsoil at a lock entrance, the back of the wall may be subjected, on a receding tide, to the full hydrostatic pressure due to the range of that tide plus the lateral pressure of the filling. Again, the water may stand at the same level on both sides of the wall, but may or may not get underneath it. If the wall is founded on rock or good clay, there is no more reason why the water