Presentation on theme: "Construction Below the Water Table"— Presentation transcript:
1Construction Below the Water Table Exclude the waterLower the water tableSolidify the groundIgnore the water
2Exclude the WaterCaissons – 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.
3Box 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.
4Open CaissonsOpen 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.
5Pneumatic CaissonPneumatic Caissons can be sunk with the aid of compressed air.Provides a dry working chamber.Regulations applyVolume air supplyCaisson sicknessThe bendsStructural integrityMan management
6Simple cofferdam Cut off walls sunk into low permeability material Sheet pilesUsually steel interlockingContiguous bored pilesProblems with seals at jointsVibrated beam wallVibrate “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.
7Cofferdam 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.
8Sealed 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.
9Lower the Water Table Effectively confined to land sites with low permeability soilsto lower water table slightly over large areaSink a series of wellsgenerally on a grid pattern.Pump water from wellsGround water will flow towards excavationConsider environmental effect of pumped water.
10Solidify Ground - then dig it out (Not common – not easy to control) Freeze the water.Requires a lot of energy.Soil mass expandscan cause damagechanges properties of soil massCement groutingCement reacts with waterPermanently changes properties of soil massGenerally used as ground strengtheningOther chemical reactants
11Ignore the Water For processes that can be carried out underwater. WeldingConcretingAssembly workInspectionsDiversRemote controlled equipmentRemote handling
12Concrete 1Cement, 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.
13Typical concrete mixes Traditional 1:2:4batched by weight or volume1 part cement2 parts fine aggregate (sand)4 parts coarse aggregate (gravel)Often 2 parts 10mm approximate sizePlus 2 parts 20mm approximate sizew/c ratio of (say) 0.5 means 0.5 parts waterQuantity of water should allow for any wet aggregate
16Water/cement ratio is critical Want all water to be used in chemical reaction (w/c=0.25 is optimum for this)Too littleStiff paste that is difficult to place – can use plasticiserUn-reacted cement which can react later if it gets wetAggregate not properly bondedToo muchVoids when water evaporatesDrying shrinkage greaterLower density, lower strength reduced durabilityFor good workability usually need w/c>=0.3
17Aggregate 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.
18ReinforcementPlain concrete is strong in compression and weak in tension.Steel (mild or high yield) reinforcement usedTo carry tensile stresses.Links used to carry shear stresses.To increase compressive strength.Steel rusts in presence of water & oxygenRust has greater volume than steelExpansive forces damage concreteCorrosion worse if salt present
19Casting 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.
20Techniques for concreting under water Use pre-cast concrete units and lower into placeLight enough to placeHeavy enough to stay in place – or anchorPlace wet concrete inside sacrificial bagUse a hopper with a bottom gate & skirtUse tremie pipe or flexible hose
21Hopper & skirt Fresh concrete placed in skip Skip lowered to sea bed Gate openedSkip raised slowlyConcrete protected by skirt as flows onto sea bedOK for mass fillFresh concretehopperskirtSea bedHinged or sliding gate
22Tremie Pipe – (not to scale) for small quantities only Crumpled paper used to block tube initiallyFresh concrete placed within existing massFormwork required – can be pre-cast unitsScour may be a problemCofferdams can provide protectionCan use flexible hose & pumped concreteFresh concrete in hopperWater levelSea bed level
23Things to Remember about Concrete Designs based on 28 day strengthNo load until 7 days (approx)Hardens quickly but strength remains lowIs subject to sulphate attackSulphates found in some clay soilsHealth & safety issues to be consideredAllergy common & can be developedDemolition must be considered