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