STONE COLUMNS

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GROUND IMPROVEMENT USING SPECIAL TECHNIQUE STONE COLUMNS

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GROUND IMPROVEMENT USING SPECIAL TECHNIQUE

STONE COLUMNS

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INTRODUCTION To Ground Improvement

Techniques.

• Ground improvement techniques are commonly used at sites where the existing soil conditions are expected to lead to a unsatisfactory performance.

• Soil improvement techniques are used to increase the strength and stiffness of soil deposits.

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Methods for Soil Improvement

Ground Improvement

Ground Treatment

•Stone Columns•Soil Nails•Deep Soil Nailing•Micropiles (Mini-piles)•Jet Grouting•Ground Anchors•Geosynthetics•Fiber Reinforcement•Lime Columns•Vibro-Concrete Column•Mechanically Stabilized Earth•Biotechnical

•Deep Dynamic Compaction•Drainage/Surcharge•Electro-osmosis•Compaction grouting•Blasting•Surface Compaction

•Soil Cement•Lime Admixtures•Fly ash•Dewatering•Heating/Freezing•Vitrification

Ground Reinforcement

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INTRODUCTION To Stone Columns.

• Stone columns is used for both cohesive soils and silty sands to increase strength and decrease compressibility.

• Involves discrete inclusions that reinforce the soil.

• Suited for wide spread loads.

• Soils having low to medium SBC these are found economical and faster in construction.

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Cont…

• Stone columns in saturated cohesive soils

work as drainage system and decrease

consolidation time.

• Lateral expansion of the column due to

ramming will induce excess pore pressure

in clayey soils

• Effectively used for large area

stabilization.

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What are stone columns?

A ground improvement technique

where vertical columns of compacted

aggregates are formed through the

soil to be improved.

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Mechanisms For Improving Liquefiable Soil Deposits.

• Due to their high density, strength and

stiffness they reinforce the soil.

• Closely spaced drainage boundaries inhibits

development of excess pore water pressure.

• Installation densifies the surrounding soils.

• Lateral stresses are increased in

surrounding soils.

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• Diameters of 0.3m to 0.5m.

• Granular material ranges from 75 mm to 2mm.

• Pattern Spacing

– Triangular. 0.866S²

– Square. 1.0S²

Treatment depth in India 15m to 20m

DIMENSIONS

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Triangular Arrangement Of Stone Column.

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Square Arrangement Of Stone Column.

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Load Carrying Capacity

• Resistance due to lateral deformation by surrounding soil(Q1).

• Increase in resistance offered by surrounding soil due to surcharge(Q2).

• Bearing support provided by the intervening soil between the column(Q3).

• Overall safe load =Q1+Q2+Q3.

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Settlement analysis

• Settlement of treated ground is estimated using reduced stress method based on:– Stress concentration factor.– Replacement ratio.

• A unit cell comprises of stone column and surrounding soil.

-applied stress is shared between column and surrounding soft soil.

- consolidation settlement of treated ground.

- consolidation settlement of untreated ground.

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Settlement Reduction Ratio (B)

B = settlement of treated ground

settlement of untreated ground

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Field load test

• For evaluation of load settlement behavior. - Number of columns is 7 for a single column test. - Number of columns is 12 for a three column group test.

• Testing equipments-Steel plate or precast circular concrete footing. -Footing should cover the equivalent circular effective

area. -Sand blankets of size not less than 300mm. -Ground water table to be maintained at base of footing by

dewatering.

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Test Procedure.

• Load should be applied to footing by a kentledge to avoid impact,fluctuations or eccentricity.

• Load settlement observations is taken to 1.5 times the design load.

• Load increment is 1/5th design load.

• Settlements is recorded using 4 dial gauges (L.C=0.02mm).

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» contd…

• Applied test load to be maintained for a minimum of 12 hours.

• Settlements to be observed at time intervals of 1min,2min,4min,8min,16min,30min,1hr,1.5hr, 2hr,3hr,4hr….

• Load settlement and time settlement relationship to be plotted.

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Single Column Test

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Three Column Group Test

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Loading Arrangement For a Single Column Test.

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cont…• Acceptable Settlement Criteria:

Test Settlement

Single column

10mm to 12mm

Group of three column

25mm to 30mm

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Failure mechanisms

• Depends on the length .

• Length of column >4 times the dia failure is due to bulging.

• Length of column <4 times the dia failure is in general shear.

• Column experiences less bulging when loaded over an area greater than its own.

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• When interlayering of sand (thick) and clay compaction achieved during installation provides rigidity to effectively disperse the applied stress.

• In mixed soils failure should be checked for both sandy and clayey soils.

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Different Type Of Loadings Applied To Stone Columns.

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Failure in a homogeneous soft layer.

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Failure in a non homogeneous cohesive soil.

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Installation techniques.

• Non displacement method.

• Displacement method.

• Vibro-replacement.

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Non- displacementMethod.

Bailer and casing method

Direct mud circulation method

Rotary drill method

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Bailer And Casing Method.

-Borehole is advanced using a bailer and sides retained by a casing.

-To avoid loss of ground, water level is maintained 2m above GWT.

-Bailer dia to be less by 75mm to 100mm than the internal dia of casing.

-When the desired depth is reached well graded crushed rock of 75mm to 2mm is placed.

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Contd..

• And aggregates are filled to a depth of 1m to1.5m.

• Compaction is achieved by a rammer of suitable weight and fall to produce a ramming energy of 20knm per blow.

• Extent of ramming is measured by penetration of rammer into the backfilled material.

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Direct mud circulation method.

• Borehole walls are stabilized with bentonite mud.

• Prior to putting in the stone charge a casing is lowered to the bottom of borehole bentonite mud is completely removed and is replaced by water.

• Backfilling of the hole and compaction is done same as previous method.

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Rotary drill method

• Boring is done using augers or buckets.

• Sides of borehole is stabilized using a casing or bentonite mud.

• Pouring of stone and its compaction is achieved same as the earlier case.

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VIBROFLOT.

Practiced in several for

• vibro–compaction

• stone columns

• vibro-replacement

Vibroflot :Length = 2 – 3 mDiameter = 0.3 – 0.5 mMass = 2 tonnes

Suitable for granular soils

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Stone Columns

• 1-vibrator makes a hole in weak soil.

• 2,3-hole backfilled and compacted.

• 4-densely compacted stone column.

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VIBROFLOTATION

• Vibroflotation is a technique for in situ densification of thick layers of loose granular soil deposits. It was developed in Germany in the 1930s

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Vibroflotation-Procedures

• Stage1: The jet at the bottom of the Vibroflot is turned on and lowered into the ground

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

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

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

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Simple auger boring method.

• Size:• stone aggregate 20mm to

30mm. Sand(20%-25%)

of Cu=2.• Hammer :Weight=125kg.Falling

height=750mm

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Conclusions:

•Resistance to liquefaction and lateral load. •Reduction of foundation settlements and increases load carrying capacity.•Free drainage accelerates consolidation settlements thus minimizes post compaction settlements.•Construction is simple and cost effective.•Improves slopes of embankments and natural slopes.

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Summary

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Method Stone column,vibro replacement, sand piles

Principle Hole jetted into soft fine grained soil and back filled with densely compacted gravel or sand

Most suitable soil conditions or type

Soft clay and alluvial deposit

Maximum effective treatment depth

20m

Material required Gravel or crushed rock

Special equipment required Vibro float

Properties of treated material Increased bearing capacity and reduced settlements

Special advantages and limitations

Faster than pre compression avoids dewatering required for removal and replacement, limited bearing capacity

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References

• Soil engineering in theory and practice volume-3 by Alam Singh.

• Foundation analysis and design by Joseph E bowles.• Foundation design by Nainan p kurian Codes: IS15284 design and construction for

ground improvement-(part 1) stone columns.

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Thank - you