A Treatise On The Principles And Practice Of Dock Engineering

MIMI 260 DOCK ENGINEERING. 6 inches, running alternately and intermittently, were found adéquate to- deal with all infiltrations of water. The North Loek at Dunkirk.* Prior to 1896 the port of Dunkirk was served by three entrance locks, the largest of which, the west look, had a serviceable length of only 384 feet and a width of 69 feet. As far back as 1883 this accommodation was found to be insufficient, and in 1887 the project of a large new lock (fig. 185) was- approved, at an estimated costof 9| million francs. The dimensions decided upon were: a width of 82 feet and lengths of 687 feet over all, 580 feet between outer sills and 558 feet available for actual use. The level of the sills was fixed at 16 feet 6 inches below the local datum (zero of marine charts), so that there is an available draught of 30 feet at lowest neap tides, 32 feet 6 inches at mean neaps, and 35 feet 9 inches at mean springs. The works were completed and the lock opened for traffic in 1896. The loek is provided with three pairs of metal ebb gates, by means of which it can be divided into two chambers, with lengths of 351 and 229 feet respectively, for the purpose of reducing the period of locking for vessels of moderate or short length. The outer gates are furnished with strut frames as a support against rough seas. The filling and emptying of the lock are achieved by means of two longitudinal colverts of 11 feet 6 inches by 5 feet 9 inches sectional opening running, one on each side, from one end of the loek to the other. These culverts are closed at the extremities and near the middle by swing gates, of the type called fan gates (portes en éventail), and they are in permanent connection with the loek chamber by means of 16 transverse openings, each 6 feet 6 inches wide. The dimensions given to the culverts are such that the lock can be filled in six minutes under a head of 10 feet. Ship caissons can be berthed at both ends of the loek in case of accidents and repairs. The opposite quays are in communication by means of a centrally situated two-leaved swing-bridge, with a single cart track, including a line of rails. A metallic culvert of circular section, 6 feet 9 inches in diameter, forms a syphon under the floor for the transmission of water, gas, electric, and hydraulic supply mains. The area of 10 acres which formed the site of the loek between the outer channel and the inner docks was enclosed by means of two cofferdams, one at each end. The outer dam (fig. 186) was based on the sill of an old sluicing lock after the removal of the masonry, closing the opening between the side walls. It consisted of a bank of sand having its outer slope covered by a thick layer of stiff earth (épaisse couche de terre forte), with stone pitching superadded as a protection against wave action. The inner dam formed a semicircle in plan, projecting into the adjoining basin. As in the previous * Vide L’Ecluse Nord et ses Abords, Dunkirk, 1896.