Mechanism of Geogrid Encased Stone Column

Malarvizhi, S.N. Ilamparuthi, K.1

Scientist Professor

e-mail: malar_sn@yahoo.com e-mail: kanniilam@gmail.com

Structural Analysis Division, Indian Space Research Organisation, Bangalore1Department of Civil Engineering, Anna University, Chennai

ABSTRACT

Model tests conducted in the laboratory on stone columns and encased stone columns are simulated numerically

using PLAXIS FE code and results are compared with experimental results. The results of numerical analysis are

analysed to establish the mechanism by which stone column and encased stone column derive their resistance.

The stone column derives its resistance by its bulging and the column material offers passive resistance against

bulging. This passive resistance coefficient was found to be close to the passive resistance of column material.

But, the magnitude of bulging of encased stone column for a given settlement is far less than the stone column due

to the stiffness of the encasement. The higher the stiffness of encasement the lesser is the radial strain. Despite

less dilation, the stones of the encased column also offer passive resistance and the passive resistance coefficient

was found to be close to the Kp value as observed in the case of stone column.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

Stone columns are increasingly being used for ground

improvement, particularly for stabilizing road

embankments, foundation for oil storage tanks, etc. Stone

column derives its axial capacity from the passive resistance

developed against the bulging of the column and increased

resistance to lateral deformation. If the surrounding soil

has the adequate strength, the stone columns formed are of

uniform diameter throughout consuming the estimated

amount of stone material. But in the event of inadequate

confinement from the surrounding soil, the performance

of stone column becomes poor.

To overcome this, rammed stone column with a geogrid

encasement in the form of tube is tried as an alternative.

The encasement, besides increasing the strength and

stiffness of the stone column, prevents the lateral squeezing

of stones when the column is installed even in very soft

soils, thus enabling quicker installation and preserves

drainage quality of the column and frictional properties of

stones used in the column. Though encased stone columns

possess many advantages, this technique is not practiced

widely as that of stone columns because of limited

understanding on its response as well as lack of simpler

installation techniques. In this study, emphasis is attached

to delineate interactive mechanism of encased stone

column.

2. LITERATURE

Bauer & Nabil (1996) carried out laboratory and analytical

investigation of sleeve reinforced stone columns through

the triaxial compression tests and reported that though the

friction angles remained the same, the strength increase

was due to the “apparent cohesion” developed. From their

triaxial tests, Sivakumar et al (2004) reported that columns

longer than approximately five times the diameter did not

show further increase in load-carrying capacity. The hoop

strains in the geosynthetic encasement are maximum near

the top surface, and decrease with depth (Murugesan &

Rajagopal, 2009). Alexiew et al (2005) provided a design

method for geosynthetic encased stone columns, wherein a

confining force in the ring direction of the encasement is

based on the complete stress-strain behavior of the encasing

material. The radial widening is calculated and

consequently the vertical settlement of the column, thereby

the average settlement of the stabilized ground.

3. OBJECTIVE

The focus of this paper is to bring out the mechanism with

which the stone column behaviour installed in soft clay

responds to the load and also to find the reason for the

improvement in the behaviour of stone column because of

encasement. In other words, the interaction mechanism

between the stone, the geogrid encasement and the

supporting clay medium is presented.

950 S.N. Malarvizhi and K. Ilamparuthi

4. MODELLING DETAILS

Laboratory model is a cylinder of clay bed with a single

stone column or an encased stone column at the centre of

the bed which is loaded over an area using a rigid circular

plate covering some part of clay around the column. The

ideal finite element model to simulate the laboratory model

would be an axisymmetric continuum, which composes of

the clay bed, the stone column and the geogrid encasement.

The geometric details of a typical laboratory model and

the finite element idealisation are shown in Fig.1. Proper

boundary conditions are simulated, mesh is refined at the

interface between the stone column, the geogrid and the

clay and the part of clay bed near the loading plate. The

material properties of clay, stone and geogrid determined

experimentally are used as input parameters in the analysis.

Initial conditions for pore pressure and K0 conditions are

calculated.

Fig. 1: Typical Axisymmetric Model of clay and Columns

5. MECHANISM OF STONE COLUMN

Fig. 2 shows a typical deformed mesh of stone column bed

for a specified settlement. The stone column of diameter

‘d’ bulged laterally over a length of ‘4d’ of the column.

The magnitude of bulging of column both in the radial and

along the length of the column increased with settlement.

Fig. 2: Deformed Mesh of Stone Column Stabilised Bed

Fig.3 shows the bulging response of an end-bearing

stone column along its length for different settlements. The

radial bulging of column (radius = r), ‘x’ (expansion of

outer face of the column in the radial direction due to load

on the column), increased gradually with the load as shown

and the bulging was maximum at the depth varying between

1.5d to 2d from the top. The bulging of stone column is

found to extend from the column head to a depth varying

between 4d and 6d for the various settlement levels. The

bulging predicted by the finite element analysis agrees

reasonably well with the observations made in the

experimental study as well as with the results of earlier

researchers (Greenwood, 1970, Hughes & Withers, 1974,

Rao et al, 1997). Similarly, the bulging of stone columns

of three diameters is compared in Fig.4, which shows that

the bulging increases with L/d ratio, where L is the length

of the column. Similarly, the comparison of bulging between

floating and end-bearing columns plotted in Fig. 5 shows

that the bulging increases with length and the bulging was

more in end-bearing column.

It is evident from the observations made that the stone

column derives its resistance basically from lateral

resistance offered by the surrounding clay to the expansion

caused by bulging of stone column under load.

0

2

4

6

8

10

0.0 0.2 0.4 0.6 0.8 1.0

x/r

L/d

fro

m t

op

5 mm

10 mm

15 mm

20 mm

Settlement

d = 30mm

D = 60mm

H = 300mm

L = 300mm

Ic = 0.25

Fig. 3: Bulging of Stone Column at Different Settlements

Fig. 4: Bulging of Stone Columns with Different Diameters

Mechanism of Geogrid Encased Stone Column 951

0

2

4

6

8

10

0.0 0.1 0.2 0.3 0.4 0.5

x/r

H/d

fro

m t

op

L/d = 5 (f)

L/d = 7.5 (f)

L/d = 10 (Endbearing)

d = 30mm

D = 60mm

H = 300mm

Ic = 0.25

Fig. 5: Bulging of Floating and End-Bearing Stone Columns

Further analysis was carried out by examining the

radial and vertical stresses mobilised at the point of

maximum bulging for various settlements. The values thus

obtained are plotted as ratio between vertical and radial

stress and settlement is presented in Fig.6 for columns of

three different diameters. From the figure, it can be seen

that the ratio is around 4 for 1mm settlement of column

and it increases with settlement. At settlement around

0.15times the diameter of the column, the ratio between

the stresses is maximum and is equal to 7.3 irrespective of

the column diameter. Further, with increase in settlement

there is a marginal decrease in value. However the value

remains constant for all the three diameters equal to 7 which

is close to the Kp value of the column material used in the

study. The observation made from the numerical studies

compares well with the mechanism of stone column

reported by earlier researchers (Hughes et al 1975, Rao et

al 1997, Wood et al 2000).

0

5

10

15

20

0 1 2 3 4 5 6 7 8

σσσσv/σ/σ/σ/σr

Set

tlem

ent

(mm

)

30

40

60

D = 2d

L = 300mm

H = 300mm

Ic = 0.25

d

Fig. 6: Variation of sv/s

r with Settlement for Stone Columns

with Different Diameters

0

5

10

15

20

0 1 2 3 4 5 6 7 8

σσσσv/σ/σ/σ/σr

Set

tlem

ent (m

m)

L/d = 5

L/d = 7.5

L/d = 10

d = 30mm

D = 60mm

H = 300mm

Ic = 0.25

Fig. 7: Variation of sv/s

r with Settlement for the Stone Column

with Different L/H Ratio

The ratio between vertical and radial stress and

settlement is presented in Fig.7 for floating columns and

the end-bearing column. From the figure, it can be seen

that the ratio is around 3.5 initially and it increases with

settlement. The settlement at which the ratio between the

stresses is maximum shifts downwards in the floating

columns and the value also decreases to 6.1 against 7.3.

Though the value of the ratio of stress is the same for both

the floating columns, the depth at which the maximum

occurs is at 2d and 2.5d for the columns with L/d ratio 7.5

and 5 respectively. However the value remains constant

for the higher settlements.

6. MECHANISM OF ENCASED STONE COLUMN

The deformed mesh of the encased stone column clay bed

is shown in the Fig. 8. From the figure, it can be seen that

the lateral deformation of encased column is not able to be

recognised visually from the deformed mesh since its

magnitude is rather small.

Fig. 8: Deformed Mesh of Encased Stone Column Bed

In Fig. 9, the radial deformations obtained at different

depths for the columns encased with three nets, net1, net2

and net3 (stiffness = 15, 40, 60kN/m respectively) are

presented. The columns exhibited lateral deformation over

entire length of column irrespective of the type of encasing

material. The net1, which is having least stiffness among

the materials used exhibited higher deformation than other

two materials upto the mid depth of the column. Below the

mid depth, its deformation is less than the other two

materials. The performance of column encased with net2

and net3 is identical but the magnitude of deformation is

marginally less for net3 than net2. Further in net3 column,

the radial deformation is almost uniform over the entire

length of the column. In general, the magnitude of

deformation is maximum at the depth close to 1d from the

column top for a given settlement irrespective of the

stiffness of encasing material.

From these results, it is inferred that the magnitude of

lateral deformation is controlled by the stiffness. The hoop

952 S.N. Malarvizhi and K. Ilamparuthi

force mobilised in the column material is shown in Fig. 10

for the encased columns. The hoop force is maximum at

depth around 1d and decreased with depth. This response

is seen in all the three columns irrespective of the stiffness

of the encasing material with higher hoop force in column

encased with material of higher stiffness. Maximum hoop

force is developed at the depth around 0.5d of the

encasement. For higher pressures, hoop forces are mobilised

over the entire length of the encasement.

0

2

4

6

8

10

0.00 0.02 0.04 0.06 0.08 0.10

x/r

L/d

fro

m top

net1

net2

net3

d = 30mm

D = 60mm

L = 300mm

H = 300mm

Ic = 0.25

Fig. 9: Radial Deformation of Encased Stone Columns

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0

Hoop force (kN/m)

L/d

fro

m t

op

net1

net2

net3

d = 30mm

D = 60mm

L = 300mm

H = 300mm

Ic = 0.25

Fig. 10: Hoop Force Variation in Encased Stone Columns

The (σv/σ

r) variation with settlement is presented in

Fig. 11 for the column diameters encased with three nets.

The ratio (σv/σ

r) is around 6 for all the column diameters

and the three reinforcements. Thus this ratio is close to the

Kp [=tan2(45+φ

c/2), where φ

c is internal friction of

compacted stone material, which is equal to 46°] value of

stone material used in this study

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10

σσσσv/σσσσr

Sett

lem

en

t (m

m)

net1

net2

net2

d = 60mm

D = 120mm

L = 300mm

H = 300mm

Ic = 0.25

Fig. 11: Variation of sv/s

r with Settlement of

Encased Stone Columns

7. CONCLUSIONS

The results of numerical analysis establish the mechanism

by which stone column and encased stone column derive

their resistance. The stone column derives its resistance by

its bulging over a length of 4d to 6d under the load with

maximum bulging at the depth around 2d. The column

material offers passive resistance against bulging. This

passive resistance coefficient was found to be close to the

passive resistance of column material. This response is seen

irrespective of the end condition of the column. The

magnitude of bulging of encased stone column for a given

settlement is far less than the stone column despite all the

conditions being the same for both except encasement. The

significant reduction in the radial bulging is due to the

stiffness of the encasement. The higher the stiffness of

encasement the lesser is the radial strain for a given

settlement of the stabilised bed. This shows that the

dilatancy of column material is reduced effectively by the

reinforcement, which is in agreement with the well

understood concept of lesser dilation at higher confinement.

Despite less dilation, the stone material of the encased

column offer passive resistance and the passive resistance

coefficient was found to be close to the Kp value as observed

in the case of stone column.

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