Bridge Engineering (4) Foundations – Caisson Foundations 1. *Caisson foundations 1.1 Introduction 1.1.1 A caisson is basically a structure with a ho

llow portion, which may be circular or box shaped in plan, having one or more chambers/cells.

1.1.2 Cutting edge: the tapered bottom edge of a caisson formed with steel plates and angles.

1.1.3 Well-body: the main structure of a caisson which support the pier(s) and the superstructure and transmits the loads onto the subsoil

1.1.4 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

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Bridge Engineering (4) Foundations – Caisson Foundations 1. Caisson foundations1.2 Types of Caisson Foundations 1.2.1 According to material: concrete, RC, steel,

masonry, timber,1.2.2 According to cross-sectional shape: circular (having

no directional property), rectangular, round-ended; 1.2.3 According to the number of cells/chambers: single-

celled, double-celled, and multiple-celled,1.2.4 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.

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Bridge Engineering (4) Foundations – Caisson Foundations 1. Caisson foundations1.3 Open Caisson1.4 Pneumatic Caisson1.4.1 Major components: caisson, air brake, air

compressor, hydraulic excavating devices, and communication and lighting system.

1.4.2 Principle of pneumatic caisson: 1.4.3 Bend or the caisson disease.1.5 Installation1.5.1 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

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Bridge Engineering (4) Foundations – Caisson Foundations

1.5.1 Prefabrication:

1.5.1.1 built on ways on the shore;

1.5.1.2 on pontoons anchored near the shore or over the site where it is to be sunk;

1.5.1.3 on a temporary platform supported by piles.

1.5.2 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.

1.5.3 Sinking of caisson: the prevention of tilting or overturning, esp. rectangular caisson; the application of synthetic slurry.

1.5.4 Splicing of caisson:

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Bridge Engineering (4) Foundations – Caisson Foundations

1.5.5 Excavating in caisson: by grab bucket dredger for open caisson, chamber for a pneumatic caisson

1.5.6 Concrete sealing: by tremie method or pumping

1.5.7 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:

2.5.1 the pattern depending on discharge, bed slope, bed material, direction of flow, alignment, shape, and size of piers.

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Bridge Engineering (4) Foundations – Caisson Foundations

2.5.2 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:

2.6.1 The purpose is to ensure stability under heavy flood conditions and to facilitate mobilization of passive pressure against horizontal forces.

2.6.2 adequate grip length below the maximum scour level should be provided unless the foundation rests on rock.

2.6.3 The minimum required grip length is specified as one-third of the maximum scour depth for road bridges and one-half for railway bridges.

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