Bridge Engineering (4) Foundations – Caisson Foundations 1. *Caisson foundations 1.1 IntroductionIntroduction A caisson is basically a structure with a hollow portion, which may be circular or box shaped in plan, having one or more chambers/cells Cutting edge: the tapered bottom edge of a caisson formed with steel plates and angles Well-body: the main structure of a caisson which support the pier(s) and the superstructure and transmits the loads onto the subsoil Diaphragm: the member (wall) in a caisson whose purpose is to divide the well into individual cells and to strength the structural rigidity and thus reduce the buckling of the caisson wall 1 Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015

Bridge Engineering (4) Foundations – Caisson Foundations 1. Caisson foundations 1.2 Types of Caisson Foundations According to material: concrete, RC, steel, masonry, timber, According to cross-sectional shape: circular (having no directional property), rectangular, round-ended; According to the number of cells/chambers: single- celled, double-celled, and multiple-celled, According to installation: * Open caisson (well foundation): a caisson that has no top or bottom cover during its sinking. * Closed caisson (pneumatic caisson): a caisson with a permanent or temporary roof near the bottom so arranged that men can work in the compression chamber under it. 2 Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015

Bridge Engineering (4) Foundations – Caisson Foundations 1. Caisson foundations 1.3 Open Caisson 1.4 Pneumatic Caisson Major components: caisson, air brake, air compressor, hydraulic excavating devices, and communication and lighting system Principle of pneumatic caisson: Bend or the caisson disease. 1.5 Installation Prefabrication: * On land or in shallow water: the cutting edge and the part above of caisson is assembled in site; *In deeper water: the part-caisson is assembled either on shore or on temporary purpose-made island in shallow water 3 Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015

Bridge Engineering (4) Foundations – Caisson Foundations Prefabrication: built on ways on the shore; on pontoons anchored near the shore or over the site where it is to be sunk; on a temporary platform supported by piles Floating of caisson: 'floated' into position by tug barges when the tide and tide wind are just right, controlling the movement, the line, the location, and the grounding of the caisson by theodolite.Floating of caissontug barges Sinking of caisson: the prevention of tilting or overturning, esp. rectangular caisson; the application of synthetic slurry Splicing of caisson: 4 Saturday, August 08, 2015

Bridge Engineering (4) Foundations – Caisson Foundations Excavating in caisson: by grab bucket dredger for open caisson, chamber for a pneumatic caisson Concrete sealing: by tremie method or pumping Top slab and preembedded steel stub. 2. *The consideration in foundation design: 2.1 the maximum likely scour depth, 2.2 the minimum grip length required, 2.3 the soil pressures at the base, and 2.4 the stresses in the structure constituting the foundation. 2.5 Scour at abutment and piers: the pattern depending on discharge, bed slope, bed material, direction of flow, alignment, shape, and size of piers. 5 Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015

Bridge Engineering (4) Foundations – Caisson Foundations The maximum scour in the case of abutment occurs at upstream corner, and at the downstream end in the case of a pier; and the scour will be further aggravated if the pier is not aligned in the direction of flow. 2.6 Grip Length: The purpose is to ensure stability under heavy flood conditions and to facilitate mobilization of passive pressure against horizontal forces adequate grip length below the maximum scour level should be provided unless the foundation rests on rock The minimum required grip length is specified as one-third of the maximum scour depth for road bridges and one-half for railway bridges. 6 Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015Saturday, August 08, 2015