act ground water

7/29/2019 Act Ground Water

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Dewatering/Ground Water Control

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DefinitionGround water can be defined as water which is heldtemporarily in the soil above the level of water table

Dewatering is a process to lower down the water table togive reasonably dry working conditions especially for

excavations activities.

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Control of Ground WaterCan be divided into two groups:

Permanent exclusion of ground water (cut-off wall tothe flow of ground water)

Temporary exclusion of ground water by lowering downthe water table

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Control of Ground Water

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SUMP PUMPING Pumping from sumps is the most widely used since it can

be applied to all types of ground conditions and iseconomical to install and maintain.

The only problem is the settlement: the ground is likely tomove as the water flows towards the sump area. There is also a risk of instability at the formation level in

supported excavations, owing to the upward movement ofwater.

These problems can be partially overcome by positioningthe sump at a corner of the excavation at a level below theformation level

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OPEN PUMPING / SUMP PUMPING

pumping from sumps and ditches

least expensive method: from the standpoint of direct dewatering cost

if conditions are wrong, it can result in delays, cost

overrun or catastrophic failures

must identify those conditions that are not favorable toopen pumping before deciding to proceed with it:

impairing the foundation of proposed structure or of

existing structure nearby

delaying the project or escalating cost of excavation

endangering workers

SUMP PUMPING

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SUMP PUMPING

sump below formation level in corner of

excavation

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SUMP PUMPING

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Jetted Sump Detail

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WELLPOINT SYSTEM

has been used> 50 years, most versatile method and

effective in all type of soil

may not be the most economical method for a given

job due to advances in other pre drainage tool

most suitable for shallow aquifer (water level need

to be lowered < 6 m)

multi stages will be used for lowering > 6 m

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well points13


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DEEP BORED WELLS

Each with individual pump (involve a high unit cost) Advances in well design (aquifer analysis) and

construction techniques, and pump technology have

made it practical to utilize deep well

Best suited to homogeneous aquifers that extendedwell below the excavation level

Can be installed to greater depth (up to 12 m)

Volume pumped by each well is high thus wider

spacing is practical

Careful exploration, including a pump test, must be

carried out before undertaking this method

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DEEP BORED WELL

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Horizontal Ground Water Control

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Horizontal Ground Water Control Installing into the ground 100mm diameter PVC

perforated suction pipe covered with a nylon filtersleeve to prevent the infiltration of fine particles

Using special machine which excavate a narrowtrench, lays the pipe and backfill the excavation

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Horizontal Ground Water Control

Procedure:

To sink a lined shaft by boring methods into claylayers.

Shaft is large enough to allow men to go down & installhorizontal wells, which are jacked into ground.

Wells consist of outer and inner casing.

Wells are provided with valves, where they pass

through the wall of shaft. It the method of permanent drainage measure.

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Electro-osmosis

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Electro-osmosis Uncommon method

Costly

Cohesive soils- clay and silts On the principles that soils carry a negative charge.

Insert two electrodes and passing electric chargebetween them, anode and cathode (well point)

Electric current is passed between the anode andcathode causes the positively charged water moleculeto flow the well point.

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GROUTING METHOD CEMENT GROUT

- form a curtain. Mixture of neat cement and water

CLAY/CEMENTS GROUT

- suitable for sand and gravels where soil particles are too small for cement grout.

Bentonite with additive (Portland cement/soluble silicates) CHEMICAL GROUTING

- for medium to coarse sands and gravels. The chemical form a permanent gel

RESIN GROUTING

- for silty fine sand, similar application of chemical grouts

BITUMINOUS GROUT

- For fine sand to decrease permeability not increase the strength thus unsuitable forunderpinning works

GROUT INJECTION

- Grouts of all kinds are usually injected into the subsoil by pumping in the mixture athigh pressure through tubes.

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GROUTING

Grouting is a displacement process in which a fluid, overlying the voids inGroundmass (soil or rocks) is placed by another fluid which is more suitablefor improving the properties of the groundmass.

This fluid will solidify overtime by physio-chemical action and interaction with

pores. This grouting increasing the strength and/or reducing the permeability of

groundmass. Thus improving the characteristics of ground mass.

The success of grouting is dependent in selection and type of grout materialsand suitable grouting technique.

There are mainly two types of grouts which commonly used, cement grout andchemical grout/Chemical consolidation.

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TYPICAL SKETCH OF GROUTING EQUIPMENT


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Characteristics of Grout

Stability of grout

Particle size

Viscosity Strength

Permanence

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Types of Grout use Cement-based:

* OPC/SRC.

* Cement/ Bentonite.

* Cement/Sand.

* Cement/ Fly ash

* Micro fine Cement

Chemical based:* Sodium Silicates.

* Polyurethanes.

* Resins.

* Asphalt emulsion

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Procedure for cement grouting A small (100-200mm dia.) hole is drilled to the requireddepth.

Injection pipes are installed into soil at design locations todesired depths.

After placing pipe to the desired depth, the grout- a rock lessconcrete, is forced through the pipes.

Injection continues until a pre-determined refusal pressureis reached or uplift occurs at the surface.

The injection pipes are then raised to a shallower depth andthe process is repeated.

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Installation Process of Grid Pattern The installation of grouting for large area treatments follows the

phase system as applied for Impact Compaction.

The effective radius of the grout hole varies with the type of soilbeing treated.

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The neat cement grout isinjected through a smallnozzle at high pressure andmixes with the in-situ soil.

This method produces themost homogeneous soil-cement element with thehighest strength and the

least amount of grout spoilreturn.


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Clay Grouting Injection of bitumen emulsion or slurries of clay or

bentonite.

Grouting with chemically treated bentonite clay can

be used for impermeable cut offs in alluvial strata. Method is applicable by using bentonite clay in

combination with Portland cement , soluble silicateand other agent in different proportion.

Larger voids are filled with clay cement grout followedby clay- chemical grout with finer material.

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Chemical Consolidation Chemical injection is applicable for sandy gravels and

sand of finest grading.

Chemical Used is sodium silicate in conjunction with

other chemical which forms silica get. Pipes are driven into the ground about 0.5m apart &

Calcium chloride is injected down followed by sodiumsilicate ,which is known as two shot method.

Complete cut off of water can be obtained byrepetition of injection.

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Freezing Suitable method for all types of subsoils with a

moisture content in excess of 8% of the voids

Insert freezing tubes into ground and circulate afreezing solution around the tubes to form ice in thevoids

Thus creating a wall of ice to act as impermeable

barrier Circulating solution magnesium chloride/calcium

chloride at -15 and -25 c

Liquid Nitrogen is more suitable then chilled brine.

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Freezing

highly effective cutoff and ground support 37


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Vibro Floatation

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Vibro flotation is a techniquefor in situ densification of thicklayers of loose granular soildeposits. It was developed inGermany in the 1930s. Vibrators are inserted intoloose granular soils & withdrawnby leaving a column ofcompacted soils.

Vibro flotation Procedures


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Vibro flotation-Procedures

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Stage1: The jet at the bottom of the Vibro float is turned on and lowered into theground.

Stage2: The water jet creates a quick condition in the soil. It allows the vibratingunit to sink into the ground

Stage 3: Granular material is poured from the top of the hole. The water from thelower jet is transferred to the jet at the top of the vibrating unit. This water carriesthe granular material down the hole

Stage 4: The vibrating unit is gradually raised in about 0.3-m lifts and held vibratingfor about 30 seconds at each lift. This process compacts the soil to the desired unit

weight.


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VibroflotationGround Type Relative Effectiveness

Sands Excellent

Silty Sands Marginal to Good

Silts Poor

Clays Not applicable

Mine Spoils Good (if granular)

Dumped Fill Depends upon nature offill

Garbage Not Applicable


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Appropriate dewatering MethodsEffective Grain Size(D10)

Example Dewatering Method

Larger than 0.1mm

0.1 004 mm

0.004 0.0017 mm

Gravel, rock, boulders

Sand

Silt, clay

Sumps, ordinary wellpoints

Vacuum wells or wellpoints

Electro osmosis

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