Engineering Wonders of the World
Volume I

184 ENGINEERING WONDERS OF THE WORLD. SECTION OF TYPICAL DOCK WALL AND FILLING BEHIND. a powerful floating derrick. On these blocks a masonry quay wall was built. Of course many other methods of making foundations have been tried, with, varying success. The problem is, and always must be, how to get th© best foundation for particular cases at the lowest possible cost. The majority of quays in wet docks, tidal docks, and rivers are of masonry or concrete. Obviously the quays of wet docks need not be so stoutly built as quays exposed to the variations of pressure caused by the rise and fall of tides. Unless the back- ing of the latter kind be carefully calculated and filled in, the quay will incline to slip for- ward as the tide ebbs and withdraws the sup- porting pressure of the water. In a wet dock there is no variation of pressure to speak of; but the engineering problem is just as delicate, though perhaps not so intricate. At their bases both, kinds of wall are spread out to pre- vent their slipping. The part of the base which spreads outward is called the “toe,” and largely augments the resistance of the wall to the pressure of the backing. In some river walls on the Clyde the blocks forming the faces are, with the same object, tied back to anchorages by means of steel rods. Where wall bases should spread out, sheet piles are driven into the ground below. Formerly, the resistance of back pressure was further promoted by inclining the wall slightly backward; but nowadays, with vessels of practically square sections amidships, the faces have to be vertical. Providing their design is sound, the walls themselves are not difficult to build, the practice varying accord- ing to the material used and corresponding generally with that which is followed in land operations. Locks and dry docks are much more diffi- cult to make than the other kinds of docks described. The foundations demand special care, as the floors and sides are practically one structure, and C°nstruction ++i 4- t 4. 4- of Locks* settlement of any part affects the whole. Particular care has to be taken that water does not get under the sills. Occa- sionally, with this object in view, the whole site is enclosed by sheet piling ; generally sheet piles in some places and bearing piles gener- ously used in others suffice. Where it has been difficult to get solid foundations for the sill, coffer-dams have been employed to exclude water until good ground is reached ; then the excavated soil is replaced by concrete right up to the level of the lock floor. » Sometimes, how- ever, it is impossible to obtain a foundation in the ordinary way. At Havre large steel caissons were sunk by means of compressed air, and filled with concrete ; on these the side walls, sills, gate floors, and aprons were founded. What has been said regarding the sill foundation holds true with regard to the side walls containing the sluice ways. The foundation of these must be unimpeachable. The lock-chamber walls are subject to much the same variations of pressure as are the walls of tidal docks, and have therefore to be built with equal care. They have this advantage, however, over ordinary dock walls, that the invert of the lock floor is a much more effect- ive support. The thickness of the invert