A Treatise On The Principles And Practice Of Harbour Engineering

iç6 HARBOUR ENGINEERING. suffered more than a very slight displacement. It was therefore decided to surround the pierhead with a ring of nine concrete blocks of extremely large size. These were formed on the buoyant caisson principle. Wooden boxes of grooved and caulked planking, 7 inches thick, were formed, at a timber-yard 14 miles distant, to a trapezoidal shape with widths of 23 feet and 15 feet, a length of 23 feet, and a height of 13 feet, so as to inclose a volume of 5650 cubic feet. Stiffened by transverse and longitudinal ties, and filled to a depth of 3 feet with granite rubble concrete, they were floated to the site of the head and there sunk close alongside one another, in about 11 feet of water, a level base having been prepared for them by blasting away the protubérances of the mound, and filling the hollows with stones, débris, and sacks of concrete. The boxes were then filled up with concrete in a couple of days, in this way attaining a weight of 400 tons. Of the projected nine boxes, six were delivered in the year 1901, and, in addition, four blocks of 370 cubic feet were set. The filling of the inclosed space was then put in hand. But in September of the same year a storm produced a fresh cross fracture in the pierhead, and a further sinking of its front face by 8 inches. The con- crete boxes were slewed about 18 inches, but without damage. In the succeeding December a further and more pronounced depression of the head took place, the settlement amounting in all to 6 feet 6 inches. By this time, however, the superstructure had found a firmer bearing on the foundation stonework. Such small cavities as remained were closed with concrete. During the following year three more concrete boxes were deposited, and the space between the boxes and the head densely packed with large stones. Owing to these improvements, the pierhead suffered but little during the gales of December 1902 and February 1903, which almost destroyed the north breakwater. The boxes, indeed, were considerably shifted, but they did not suffer much damage. The pierhead was again built up to the height of the original summit, and the breaches which had been made between the boxes were closed in 1903 by four additional boxes, and the remaining gaps were made good in 1904. At this point the interesting but calamitous narrative ceases. It would be too sanguine a view to conclude that perfect stability had at last been attained. But, as a record of disasters and of remedial expedients, it is most instructive, and the lesson it enforces is complète. Quays. The inner sides of breakwaters and the shore frontages of harbours are commonly bordered with quays for the reception of merchandise and passengers. A quay is, properly speaking, a paved space or area devoted to the purposes of loading and unloading craft, and it is usually bounded at the water’s edge by a wall or wharf founded at a sufficient depth to permit of vessels lying alongside. Vessels do not always remain afloat; in many cases with the recession of the tide, they take the ground. A quay and a quay wall