Different Techniques for under water construction
Exclude the water
Lower the water table
Solidify the ground
Ignore the water
1) 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. Identical structures on land are not usually called cofferdams and the name seems to be falling out of use.
1) 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.
2) 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.
3) Pneumatic Caisson
- Pneumatic Caissons can be sunk with the aid of compressed air.
- Provides a dry working chamber.
- Regulations apply
– Volume air supply
– Caisson sickness
– The bends
– Structural integrity
– Man management
1) Simple cofferdam
System can be used for construction below water table on land or in rivers etc.
Cut off walls sunk into low permeability material
– Sheet piles
- Usually steel interlocking
– Contiguous bored piles
- Problems with seals at joints
– Vibrated beam wall
- Vibrate “H” pile into ground and inject grout as pile removed – usually permanent.
Pump water from sump.
2) Cofferdam with de-watering wells
- Can lower water table by sinking wells and pumping water (at a rate faster than the re-entry rate) to a suitable location. Must consider silt content etc. of pumped water and effect on ground water flow.
3) 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.
2) 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.
3) 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
4) Ignore the Water
- For processes that can be carried out underwater.
– Assembly work
- Remote controlled equipment
- Remote handling
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
1) Hopper & skirt
- Fresh concrete placed in skip
- Skip lowered to sea bed
- Gate opened
- Skip raised slowly
- Concrete protected by skirt as flows onto sea bed OK for mass fill
2) 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
- 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