EFFECT OF GEOSYNTHETIC ENCASEMENT ON SAND COLUMN · PDF fileEFFECT OF GEOSYNTHETIC ENCASEMENT...

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Int. J. Struct. & Civil Engg. Res. 2013 Uttam Kumar et al., 2013

EFFECT OF GEOSYNTHETIC ENCASEMENT ONSAND COLUMN IN SOFT SOIL

Uttam Kumar1*, Tandel Y K2 and Solanki C H2

The soil improvement method mostly used in the current state of the practice with densification,consolidation, reinforcement, chemical treatment and stabilization of soil. Soil reinforcementcan be an ideal solution for improvement of clay. Out of other conventional methods, stonecolumns (or sand columns) are effectively being used for ground improvement, particularly forflexible structures such as road embankments, oil storage tanks, etc. The load capacity of thesand columns mainly depends on the shear strength of the surrounding soft soil. The sandcolumn is found useful in improving load capacity and reducing the settlement of clay deposit. Inaddition to this, the encasement of geosynthetic all-round the sand columns is suggested forenhancing the load carrying capacity of the sand column in treated ground which also ensuresthe easy formation of columns in weak strata. The present study investigates the effect ofdiameters of geosynthetic encased sand columns in soft soil deposit during loading. The loadresponses of sand columns are also investigated with the variation of encasement length of thecolumn.

Keywords: Geosynthetic encased sand column, Ordinary sand column, Geosynthetic, Bulkunit weight, Relative density, Reinforcement, Soft clay

1 Assistant professor, Civil Engineering Department, CGPIT Bardoli, Surat-394350.2 Research Scholar, Applied Mechanics Department, SVNIT, Surat-395007.

*Corresponding Author: Uttam Kumar,[email protected]

ISSN 2319 – 6009 www.ijscer.comVol. 2, No. 3, August 2013

© 2013 IJSCER. All Rights Reserved

Int. J. Struct. & Civil Engg. Res. 2013

Research Paper

INTRODUCTIONThe techniques for soil improvement have beenchanging during the last three decades. A lotof soil improvement methods have been usedto deal with soft soi l problems. Thereinforcement of ground by tension resistantelements can be applied for improvement inweak strata. This reinforcement can be

provided with stone columns or sand columns.Stone columns, also known as granular pileshave been used to a large extend for severalapplications. Stone columns essentiallyincrease the bearing capacity of soft soils.Therefore, ground reinforcement by stonecolumns solves the problems of the soft soilby providing advantage of reduced settlementand accelerated consolidation process.

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In case of group of sand columns Bulgingis found the primary mode of failure. Thisdrawback can be overcome by wrapping theindividual sand columns with a suitablegeosynthetic. The geosynthetic encasementhelps in easy formation of the sand column andimproves the strength and stiffness of thecolumns. By reinforcing sand columns byGeosynthetic, the ultimate bearing capacity ofthat column can be increased to considerableamount. Thus the geosynthetic encased sandcolumn is the technique for reinforcement toimprove the loading capacity of the ground.

Van Impe and Silence (1986) was probablythe first to recognize that columns could beencased by geotextile. The loading capacityimprovement and reduction in settlement ispossible with a high-modulus geosyntheticencasement of sand columns of gravel columnsto avoid bulging (Raithel et al., 2000, Alexiewet al., 2005, di Prisco et al., 2006, Murugesanand Rajagopal, 2006, 2007, 2010, Gniel andBouazza, 2009, 2010). The results of thoseresearch works have been implemented forinstallation of geosynthetic encased stonecolumns in various projects.

Malarvizhi and Ilamparuthi (2004) reportedthe improved performance of geosynthetic-encased stone columns based on small-scalelaboratory tests on end bearing as well asfloating columns. Murugesan and Rajagopal(2006) reported that the bulging of stonecolumn upon loading will be predominant upto a depth of 1.5-2 times the diameter of stonecolumn from the ground surface. Hence, onlythe top portion of the stone column needs morelateral confinement in order to improve itsperformance. The performance of encasedstone columns of smaller diameters is superior

to that of larger diameter stone columnsbecause of mobilization of higher confining

stresses in larger stone columns (Murugesan

and Rajagopal, 2006, Kameshwar et al., 2011,Tandel et al., 2012).

Hence this paper investigates the

improvement in loading capacity of sand

columns in a square pattern after all-roundencasement by different types of geosynthetic.

This paper represents the load response ofdifferent diameters of the encased sand

columns in group load test. The effect of

encasement length of the sand column is alsoinvestigated. The results can be useful to save

the cost, effort and time for installation of stonecolumns.

MATERIALSSoil

The soil, taken from Vesu in Surat was sievedthrough 2 mm sieve to remove the coarserfraction. To find the undrained cohesion of thesoil sample, laboratory vane shear tests werecarried out at 38% and 43% water content(Table 1).

Table 1: Properties of Soil

Property Value

Liquid limit (%) 48

Plastic limit (%) 18

Plasticity Index (%) 30

Specific gravity 2.50

Indian Standard soil classification CI

Bulk unit weight at 43% watercontent (kN/m3) 17.25

Undrained cohesion at 43%water content (kN/m2) 9

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Sand

The clean river sand aggregates of a size lessthan 4.75 mm was taken to form sand columns.The sand compacted to a density of 1.62 g/cm3 and it was maintained constant throughoutall the tests (Table 2).

parameter, the ini tial modulus of thegeosynthetic was varied by using threedifferent types of geosynthetic as wovengeotextile, non-woven geotextile and geogridfor the encasement. The sand columns wereinstalled in the soil bed with a typical squarepattern.

Preparation of Soil Bed

The laboratory vane shear test resulted to theundrained cohesion of soil 9 kN/m2 at 43%water content. The proper mixed soil mass withcorresponding water content was placed ateach 5 cm up to the full height of 40 cm in thetank of size 50 cm × 50 cm × 45 cm.The surfaceof each 5 cm layer was provided with uniformcompaction up to the full depth of soil layer inthe tank.

Installation of Sand Columns

The sand columns of four in numbers were

installed up to full depth of soil layer in a squarepattern of spacing 25 cm center to centre of

each sand column in the tank.

The sand columns in the experimental work

were installed by displacement method usinga casing pipe having an outer diameter equal

to the diameter of the sand column. Theencased sand columns were installed by

wrapping the geosynthetic around the casing

pipe. The casing pipe along with a base platewas pushed into clay bed vertically at the

specified location in the clay surface till itreaches the bottom of the tank. The base plate

is to prevent the surrounding clay from entering

into the pipe during the lowering of casing pipe.The displaced clay was taken out and the

surface of the soil was trimmed to its originallevel.

Table2: Properties of Sand

Property Value

Specific gravity 2.74

Maximum unit weight (kN/m3) 18.0

Minimum unit weight (kN/m3) 15.0

Compacted unit weight (kN/m3) 16.20

Relative density (%) 45

Uniformity coefficient 3.50

Coefficient of curvature 0.73

Geosynthetic

Three types of geosynthetic material have beenused for the experimental program. Thematerials have been sewn and glued to providea circular shape for encasement purpose. Theresults of the initial tensile modulus of geotextileand geogrid was taken by wide-width tensiletest (ASTM D4595) (Table 3).

Table 3: Properties of Geosynthetic

Types of Geosynthetic Initialtensile

Modulus (kN/m)

Soft grid 7.50

Non-woven geotextile 11.50

Woven geotextile 43.70

METHODSThe laboratory tests were conducted on twodifferent diameters 50 mm and 75 mm of sandcolumns in order to predict the influence ofsand columns during group load test. The other

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The quantity of the sand aggregate requiredto form the stone column was pre measuredand charged into the casing pipe in layers of 5cm thickness up to the full height of sandcolumn. The relative density of sand wasmaintained at 45% for the installation of eachsand column.

Load Test on Sand Column Group

After installation of sand columns the entire tankset up is placed in the loading frame and theloading is applied through strain controlleddisplacement of loading plate at a constantstrain rate of 1.2 mm/min. The settlement inthe sand column group was measured with thehelp of LVDT (Figure 1).

75 mm diameter Whereas in case of RSC itis opposite 75 mm diameter have moresettlement (Figure 2).

Hence, in case of OSC, the 75 mmdiameter column has 6% less reduction insettlement than 50 mm diameter of sandcolumn. In case of RSC (Soft grid), the 50 mmdiameter column has 10% less reduction insettlement than 75 mm diameter of sandcolumn for a given stress of 100 kPa.

Figure 1: The Loading Frame withLVDT in Laboratory

RESULTS AND DISCUSSIONEffect of Diameter of Sand Column

For a given Stress of 100 kPa and ColumnEncased With Soft Grid, OSC Having 50 mmdiameter, has Settlement 34 mm and for OSCHaving 75 mm diameter, has Settlement 32mm. For RSC Having 50 mm diameter, hassettlement 25 mm and for RSC having 75 mmdiameter, has settlement 28 mm. WhichMeans in case of OSC settlement is less in

Figure 2: Stress-settlement Response OfSand Columns Reinforced With Soft Grid

For 100% Reinforcement

For a given Stress of 100 kPa and columnencased with Non-Woven Geotextile, OSChaving 50 mm diameter, has settlement 34mm and for OSC having 75 mm diameter, has

Figure 3: Stress-Settlement ResponseOf Sand Columns Reinforced With

Non-woven Geotextile For100% Reinforcement

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settlement 32 mm. For RSC having 50 mmdiameter, has settlement 20 mm and for RSCHaving 75 mm diameter, has settlement 23mm. Which means in case of OSC settlementis less in 75 mm diameter, whereas in case ofRSC it is opposite 75 mm diameter have moresettlement (Figure 3).

Again, in case of RSC (Non-woven), for agiven stress of 100 kPa, the 50 mm diametercolumn has 13% less reduction in settlementthan 75 mm diameter of sand column.

For a given Stress of 100 kPa and ColumnEncased with Woven Geotextile, OSC having50 mm diameter, has settlement 34 mm andfor OSC having 75 mm diameter, hassettlement 32 mm. For RSC having 50 mmdiameter, has settlement 12.5 mm and for RSChaving 75 mm diameter, has settlement 17.5mm. Which means in case of OSC settlementis less in 75 mm diameter whereas in case ofRSC it is opposite 75 mm diameter have moresettlement (Figure 4).

In case of RSC (Woven), for a given stressof 100 kPa, the 50 mm diameter column has28% less reduction in settlement than 75 mmdiameter of sand column.

There is significant reduction of settlementwith increase in initial tensile modulus ofgeosynthetic material. So the increase in initialmodulus of geosynthetic for encasement canimprove the performance of sand column.

The settlement response shows that forOSC as the diameter increases, the Stress isalso increases, whereas in case of RSC (SoftGrid), RSC (Non-Woven), and RSC (Woven)geotextile the pattern is opposite, that is withincrease in diameter stress is decreasing, the

only thing is that in case of RSC (Woven)geotextile the diameter is more dominant.

Effect of Encasement Length ofSand Column

The effect of reinforcement length on the loadcarrying capacity of the reinforced sandcolumn for 50 mm and 75 mm diameter is sandcolumn is discussed in this section. Thevariation of stress corresponding to 20 mmsettlement for different types of geosyntheticand different types of reinforcement length isdrawn (Figure 5). It can be conclude that asthe reinforcement length decrease loadcarrying capacity of reinforced sand column

Figure 4: Stress-Settlement Response OfSand Columns Reinforced With WovenGeotextile For 100% Reinforcement

Figure 5: Variation of StressCorresponding to 20mm SettlementFor 50mm Diameter Sand Column

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Figure 6: Variation of StressCorresponding to 20 mm Settlement

for 75 mm Diameter Sand Column

get decreases for different types ofreinforcement. But the decrease in the loadcarrying capacity from 100% reinforcement to50% reinforcement length is not significant.This suggests that, 50% reinforcement (4 timesthe diameter of column) may be adequate asper the result form the present study.

CONCLUSIONThe performance of smaller diameter sandcolumn is superior to that of bigger diametersand column. The reason for this is thedevelopment of larger additional confiningstresses in smaller diameter reinforcedcolumns. Increasing the initial modulus ofgeosynthetic for encasement of sand column,there is more improvement in performance ofthat column. The encasement up to the 4 timesthe diameter of sand column can be adequateto increase the performance of the sandcolumns.

REFERENCES1. Murugesan S and Rajagopal K (2007),

“Model tests on geosynthetic-encasedstone columns”, GeosyntheticsInternational, Vol. 14, No. 6, pp. 346-354.

2. Gniel J and Bouazza A (2010),“Construction of geogrid encased stonecolumns: A new proposal based onlaboratory testing”, Geotextiles andGeomembranes, Vol. 28, pp. 108-118.

3. Hamed Niroumand, Khairul AnuarKassim and Chong Siaw Yah (2011),“Soil Improvement by Reinforced StoneColumns Based on Experimental Work”,EJGE, Vol. 16, Bund. L.

4. IS 15284: Part 1 (2003), “Design andConstruction for Ground Improvement -Guidelines - Part 1: Stone Columns”,Bureau of Indian Standard, New Delhi.

5. Joel Gniel and Abdelmalek Bouazza(2009). “Improvement of soft soils usinggeogrid encased stone columns”,Geotextile and Geomembranes, Vol. 27,pp 167–175

6. Malarvizhi S N and Ilamparuthi K (2004),“Load versus settlement of claybedstabilized with stone and reinforced stonecolumn”, 3rd Asian Reg. Conf. onGeosynt., pp. 322-329.

7. Murugesan S and Rajagopal K (2010),“Studies on the behaviour of single andgroup of geosynthetic encased stonecolumns”, Journal of Geotechnical andGeo-environmental Engineering, Vol.136, No. 1, pp. 129-139.

8. Tandel Y K, Solanki C H and Desai A K(2012), “Reinforced stone column:remedial of ordinary stone column”,International Journal of Advances inEngineering & Technology, Vol. 3, No.2, pp. 340-348.

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