Underwater Construction

UNDERWATER CONSTRUCTION Construction Below the Water Table

Exclude the water Lower the water table Solidify the ground Ignore the water

Exclude the Water Caissons – usually refers to structures which are constructed offsite and then brought to site in one piece or in a series of independent modules. Cofferdams – usually refers to structures in water that are constructed on site, often from standard parts. Design to keep water and/or soil out of the excavation.

Box Caissons

 (a) Box caisson floated into place with ballast as required.  (b) Caisson filled with appropriate material – water may be

pumped out first.  Hollow caissons can be used to house equipment – filled they can be used as foundations.

 http://www.skye-bridge.co.uk/caisson.htm

Open Caissons  Open caissons permit

excavation or other work to be carried out inside the caisson.  The caisson will sink down into the soil as excavation proceeds.  Sections can be added on top to increase height.  Water can be pumped out to permit dry work.

Pneumatic /Compressed Air Caisson  Pneumatic Caissons can be

sunk with the aid of compressed air.  Provides a dry working chamber.  Regulations apply  Volume air supply  Caisson sickness  Structural integrity  Man management

Simple cofferdam  Cut off walls sunk into low

permeability material  Sheet piles  

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Contiguous bored piles 

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Usually steel interlocking http://www.landwater.co.uk/UserFiles/ima ges/Case%20studies/031 10BeverleyBeckv2.pdf Problems with seals at joints

Vibrated beam wall 

Vibrate “H” pile into ground and inject grout as pile removed – usually permanent. System can be used for construction

 Pump water from sump.

below water table on land or in rivers etc.

Cofferdam with de-watering wells  Can lower water table

by sinking wells and pumping water (at a rate faster than the reentry rate) to a suitable location. Must consider silt content etc. of pumped water and effect on ground water flow.

Sealed Cofferdam  Completely sealed

system.  Must cater for upthrust.  Only direct rainfall needs to be pumped out.  Horizontal barrier can be concrete, clay, ground freezing etc.

Lower the Water Table  Effectively confined to land sites  with low permeability soils  to lower water table slightly over large area  Sink a series of wells  generally on a grid pattern.  Pump water from wells  Ground water will flow towards excavation  Consider environmental effect of pumped water.

Solidify Ground - then dig it out (Not common – not easy to control)  Freeze the water.  Requires a lot of energy.  Soil mass expands  can cause damage  changes properties of soil mass

 Cement grouting  Cement reacts with water  Permanently changes properties of soil mass  Generally used as ground strengthening  Other chemical reactants

Ignore the Water  For processes that can be carried out

underwater.  

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Welding Concreting Assembly work Inspections

 Divers

 Remote controlled equipment  Remote handling

Casting Concrete Under Water  Concrete will set under water.

 Need to protect wet paste from strong

currents.  Concrete at surface contaminated by sea or river water & cement leaches out.  Need to keep mass of concrete intact & minimise new surface area as it is placed.

Techniques for concreting under water  Use pre-cast concrete units and lower into

place  

Light enough to place Heavy enough to stay in place – or anchor

 Place wet concrete inside sacrificial bag  Use a hopper with a bottom gate & skirt  Use tremie pipe or flexible hose

Hopper & skirt  Fresh concrete placed    

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in skip Skip lowered to sea bed Gate opened Skip raised slowly Concrete protected by skirt as flows onto sea bed Sea bed OK for mass fill

Fresh concrete

hopper skirt

Hinged or sliding gate

Tremie Pipe – (not to scale) for small quantities only  Crumpled paper used to     

block tube initially Fresh concrete placed within existing mass Formwork required – can be pre-cast units Scour may be a problem Cofferdams can provide protection Can use flexible hose & pumped concrete

Fresh concrete in hopper

Water level

Sea bed level

Things to Remember about Concrete  Designs based on 28 day strength

 No load until 7 days (approx)  Hardens quickly but strength remains low  Is subject to sulphate attack 

Sulphates found in some clay soils

 Health & safety issues to be considered 

Allergy common & can be developed

 Demolition must be considered