A Treatise On The Principles And Practice Of Dock Engineering

CONSTRUCTION IN PNEUMATIC CHAMBERS. 201 clear of all mud and silt before berthing the dam. A good supply of clay will be necessary to replace wastage in the puddle. The length of the dam in question was 246 feet, divided into 16 bays of 15 feet each, with an overlap at one end. On the conclusion of the work the piles were drawn, and the sides of the dam removed separately. In a similar manner the concrete walls of a tidal basin at Ardrossan were constructed.* Fig 134 is a section of the wall and of the box dam within which it was built. Construction in Pneumatic Chambers.—This system, in one or other of its forms, represents a very considerable proportion of Continental practice, but it does not seem to have been adopted in any noteworthy instance in English ports, if, as is intended, we restrict the use of the diving bell to the actual construction of the wall. The system dates back some considér- able time, and walls have been con- structed on its principles, notably at Antwerp, Marseiiles, Genoa, and else- where. The following account of its appli- cation to the recently constructed quay walls of the Bassin de la Pinède, at Marseilles, is extracted and condensed from an article by M. Batard-Razeliére, Engineer - in - Chief of the harbour ■works there :—t “ The foundation of the quay walls is laid on stiff ground (ballast, grit, or hard clay), when that ground is met with above a level of 40 feet below Fitting 2'5" Mud Firm 'tiroujid Fig. 135. —Dock Wall at Marseilles— Section A. - - 42' 0 Batter 1 in. 20 the datum of ordinary low-water level. The profile of the wall is then represented by fig. 135. The masonry is bedded into the ground for a width of about 10 feet at its base. When stiff ground is only to be found below the above-named level, the site is dredged to that depth, the material con- sisting mainly of mud, sand, and decayed seaweed. A bank of rubble stone is then formed and brought up to a level of 30 feet below datum, having at this level a width of 41 feet, and the normal section of the wall is founded upon this base, as in fig. 136. To within 5 feet of low-water level the work is executed, by means of compressed air, in the interior of large metallic chambers (caissons J), acting like diving bells. From 5 feet below to 18 inches above datum it is * Robertson on “Ardrossan Harbour Extensions,” Min. Pruc. Inst. C.E., vol. cxx. + Bulletin de la Société Scientifique Industrielle de Marseille, 2me Trimestre, 1900. t The word “ caisson” in this connection has not quite the signification which it has when applied to the apparatus for closing a dock entranee.