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Bridge Engineering (4)

Foundations Caisson Foundations

1. *Caisson foundations1.1 Introduction

1.1.1 A caisson is basically a structure with ahollow portion, which may be circular or boxshaped in plan, having one or more chambers/cells.

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

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

1.1.4 Diaphragm: the member (wall) in acaisson whose purpose is to divide the well intoindividual cells and to strength the structuralrigidity and thus reduce the buckling of the caissonwall

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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): acaisson with a permanent or temporary roof nearthe bottom so arranged that men can work in thecompression chamber under it.


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1. Caisson foundations

1.3 Open Caisson

1.4 Pneumatic Caisson

1.4.1 Major components: caisson, air brake, aircompressor, 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 Installation

1.5.1 Prefabrication:

* On land or in shallow water: the cuttingedge and the part above of caisson is assembled insite;

*In deeper water: the part-caisson isassembled either on shore or on temporary

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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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1.5.5 Excavating in caisson: by grab bucket dredger foropen 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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2.5.2 The maximum scour in the case of abutment occursat 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 scourlevel 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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