A Treatise On The Principles And Practice Of Dock Engineering

FAILURES. 215 ing. As long as the water from the hills can percolate freely through the sand and escape there is no danger, but when the outlet is blocked the sand becomes sodden, and the clay acquires a slippery surface conducive to landslip. The quay wall consists of a solid face of three thicknesses, 4|, 6, and 9 feet respectively, formed by offsets at 5^ and 9J feet above the base. It is backed by a series of counterforts, arranged at intervals of about 30 feet, and well bonded into the wall. The spaces between the counterforts are spanned by two tiers of arches, the lower of which sustains the sand filling behind the wall, and the upper forms a foundation for the line of steam cranes which serve the quay front. The materials of which the wall is constructed are hard bricks and cement mortar, the latter in the propor- tion of 1 of cement to 4 of sand. The wall rests upon a level base, formed by a strong horizontal mortised framing of longitudinal and transverse timbers, covered with planking and supported by vertical and oblique bearing piles. A row of tongued and grooved sheet piling is driven to retain the bank of earth below the platform level. A corrugated iron shed, founded upon a distinct system of piling, stands a little distance back from the face of the quay. In August, 1890, shortly after the completion of the work, cracks were observed in the wall and in the brick gables of the shed, and it was found that both the quay and the shed had perceptibly shifted their positions. The backing at the rear of the wall was removed forthwith, in order to lighten the pressure. Very shortly afterwards the movement of the shed was found to have been arrested, evidently by the resistance of its founda- tion piles on the landward side, which had been driven well into the lower clay. Meanwhile, the quay wall continued a slow but uniform movement outwards ; so gradual and minute, however, as to permit a series of observations to be taken systematically, from which the source of the mischief was accurately traced, and the means devised for remedying it. It was found that the whole stratum of earth above the clay, extending as far as the hill top, was sliding bodily forward towards the Elbe, and, as the lower ends of the quay piles were bedded in the clay, the upper masonry was turning about the leet of the piles as about a pivot. In order to check this movement, and restore the stability of the wall, a series of 29 iron stays, placed about 15 feet apart, were secured to the upper face of the quay piles, and led to anchorages, some 164 feet back, sunk well below the surface. These stays consisted of links about 16 feet long and 10 square inches sectional area, alternately of round and bar iron, the latter being double and attached to the rods by bolts passing through their ends. At the quay face the extremity of the stay was made fast to a heavy iron plate, bearing against horizontal beams below the surface of the water, which transmitted the pressure evenly to the foundation piles. The anchorage on the landward side (figs. 155 and 156)