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. Identical structures on land are not usually called cofferdams and the name seems to be falling out of use.

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 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

Simple cofferdam

• Cut off walls sunk into low permeability material– Sheet piles

• Usually steel interlocking• http://www.land-water.co.uk/U

serFiles/images/Case%20studies/03110BeverleyBeckv2.pdf

– 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.

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.

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.– Welding– Concreting– Assembly work– Inspections

• Divers• Remote controlled equipment• Remote handling

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.

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

Specifying Concrete – BS8500

Exposure Class continued

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

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.

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

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.

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 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

hopperFresh concrete

skirt

Sea bed

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

Water level

Sea bed level

Fresh concrete in hopper

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

Useful Links

• http://www.cementindustry.co.uk/

• http://www.ictech.org/index.htm

• http://www.concreterepair.org.uk/

• http://www.cba-blocks.org.uk/index.html

Any Questions?