1. Construction Below the Water Table
Exclude the water
Lower the water table
Solidify the ground
Ignore the water

2. 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.

3. Box Caissons http://www.skye-bridge.co.uk/caisson.htm
(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.

4. 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.

5. 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

6. Simple cofferdam 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.
System can be used for construction below water table on land or in rivers etc.

7. 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.

8. 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.

9. 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.

10. 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

11. Ignore the Water For processes that can be carried out underwater.
Welding
Concreting
Assembly work
Inspections
Divers
Remote controlled equipment
Remote handling

12. Concrete 1
Cement, aggregate (sand & gravel) & water mixed together in appropriate proportions form concrete.
Cement powder reacts with water in mix to form a new compound.
Forms a hardened cement matrix with aggregate particles bonded to (and locked within) the matrix.

13. Typical concrete mixes
Traditional 1:2:4
batched by weight or volume
1 part cement
2 parts fine aggregate (sand)
4 parts coarse aggregate (gravel)
Often 2 parts 10mm approximate size
Plus 2 parts 20mm approximate size
w/c ratio of (say) 0.5 means 0.5 parts water
Quantity of water should allow for any wet aggregate

14. Specifying Concrete – BS8500

15. Exposure Class continued

16. Water/cement ratio is critical
Want all water to be used in chemical reaction (w/c=0.25 is optimum for this)
Too little
Stiff paste that is difficult to place – can use plasticiser
Un-reacted cement which can react later if it gets wet
Aggregate not properly bonded
Too much
Voids when water evaporates
Drying shrinkage greater
Lower density, lower strength reduced durability
For good workability usually need w/c>=0.3

17. Aggregate Purpose is dimensional stability
Volume of cement when set is less than that of paste – shrinkage inevitable.
Low coefficient of thermal expansion.
Must not absorb moisture.
Must be chemically inert.
Appropriate strength, size, shape & grading.
Form good bond with hardened cement.

18. Reinforcement
Plain concrete is strong in compression and weak in tension.
Steel (mild or high yield) reinforcement used
To carry tensile stresses.
Links used to carry shear stresses.
To increase compressive strength.
Steel rusts in presence of water & oxygen
Rust has greater volume than steel
Expansive forces damage concrete
Corrosion worse if salt present

19. 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.
Large dimensional tolerance required.

20. 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

21. 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
Fresh concrete
hopper
skirt
Sea bed
Hinged or sliding gate

22. 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

23. 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

24. Useful Links http://www.cementindustry.co.uk/

25. Any Questions?