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CASE STUDY:

STONE COLUMN

Stone column technique for ground improvement is being extensively used to undertake

constructions in weak soils. The stone columns essentially increase the bearing capacity

of loose cohesionless soils. In cohesive soils, along with the increase of bearing capacity,

the consolidation settlement of the ground under loading is also considerably reduced. In

addition, in cohesive soils stone columns act as drainage paths to accelerate the rate of

consolidation of the residual settlement. Even though stone columns are very useful for

these purposes, designs made without proper concept of the behavior of the stone column

and execution of work not understanding the stone column and behavior pattern of the

non-treated ground leads to complications. Failures have occurred where stone columns

have been used for ground improvement. This paper attempts to highlight these factors. A

case study where the foundation failure of a structure constructed on a soil improved by

stone columns highlights the various aspects discussed.

SHAPE OF STONE COLUMN AFTER LOADING

SUITABILITY OF THE GROUND FOR STONE COLUMNS

Two common methods of stone column constructions are:

· Vibro-floatation

· Bored rammed system

In the floatation technique, a vibrating needle working with the water jet reduces the

friction of the surrounding cohesionless soil, filling the voids and thus achieving

compaction. This creates a cavity into which borrowed granular material is filled and

compacted. Thus, in general, the density of the ground increases with the increase in

friction angle. As a result, bearing capacity increases. This technique is therefore possible

only in cohesionless soils.

The bored rammed stone columns are used in cohesive soils. In this technique, a casing

pipe is used to remove the cohesive soil protecting the sides of the bore, thus minimising

disturbance to the surrounding soil. The stones are laid into the bore and rammed to a

larger diameter as the casing pipe is withdrawn. These columns achieve their strength by

the lateral restraint offered by the surrounding soil. It is therefore very essential that the

shear strength of the surrounding soil not be reduced by the construction of the stone

column. Hence, the stone column technique could be adopted in clays of low sensitivity.

These columns also act as drainage paths to accelerate settlements under loading.

BORED RAMMMED STONE COLUMN VIBRO-COMPACTION

TERMINOLOGY:

Displacement/non-displacement type of installation process-If the soil is laterally

displaced while making the whole due to driving of a tube or a casing ,it is the

displacement type of boring .when the soil is taken out during boring ,it is non-

displacement type of installation.

Ground improvement-To improve the load bearing capacity of the loose or soft soil to

the required depth by some practical methods.

Sensitivity of clay-The ratio of the unconfined compressive strength of clay at its natural

state to the remoulded condition.

Working stone column-A stone column forming part of a foundation system of a

structure.

Initial test column-Columns,which are not working columns, but are installed for

assessing the load carrying capacity of stone column. Such columns may be tested either

to its ultimate load capacity or to at least 1.5 times the design load .The test is called

initial load test.

Routing test column- The column that is selected for load testing &is subsequently

loaded for the purpose. The test column may form working column itself, if subjected to

routine load test with loads up to 1.1 times the safe load. This test is called routine load

test.

Ultimate load –The maximum load which a column can carry before the failure of

ground or column material, whichever is lower.

Safe load-Load derived by applying factor of safety on the ultimate load capacity of the

column or as determine by column load test.

Factor of safety-Ratio of ultimate load capacity of column to the safe load capacity of

stone column.

Allowable load-The load which may be applied to a stone column after taking into

account its ultimate load capacity, column spacing, allowable settlement , etc.

1. SOME IMPORTANT FEATURES OF STONE COLUMN TREATMENT:

1.1 Influence of soil type

Subsurface soil whose un-drained shear strength range from 7 to 50kpa or loose sandy

soils including silty or clayey sands represents a potential class of soils requiring

improvement by stone columns. Subsurface conditions for which stone columns are in

general not suited include sensitive clays and silts (sensitivity greater than 4) which loose

strength when vibrated and also where suitable bearing strata for resting the toe of the

column is not available under weak strata.

1.2 Influence of construction methodology-

The disturbance caused to the soil mass due to the particular method of constructing the

stone columns significantly affects the overall effects of the composite ground. The

availability of equipment, speed of construction and the depth of treatment would

normally influence the choice of construction technique.

1.3 Treatment depth

The treatment depth with stone column for a given soil profike should be so determined

that the most significant compressible strata that contribute to the settlement of the

foundation.

1.4 Area of treatment

Stone columns work most effectively when used for large area stabilization of the soil

mass. Their application in small groups beneath building foundation is limited and it is

not being used. Thus, large loaded areas which apply uniform loading a foundation soils,

such as beneath embankments, tank farms and fills represent a major area of application.

1.5 Termination

End bearing is not a specific requirement for stone columns. However, they should

extent through soft compressible strata. The soil near the ground surface has a dominating

influence on the settlement and ultimate bearing capacity of stone columns.

2. BASIC DESIGN PARAMETERS

2.1 Stone column diameter, D

2.1.1 Installation of stone in soft cohesive soils is basically a self compensating process

that is softer the soil, bigger is the diameter of the stone column formed. Due to the

lateral displacement of stones during the vibrations/ramming, the completed diameter of

the hole is always greater than the initial diameter or the casing depend upon the soil

type, its undrained shear strength, stone size, characteristics of the vibrating probe used

and the construction method.

2.1.2 Approximate diameter of the stone column in the field may be determined from the

known compacted volume of material required to fill the hole of known length and

maximum and minimum densities of the stones.

2.2 Pattern

Stone columns should be installed preferably in an equilateral triangular pattern which

gives the most dense packing although a square pattern may also be used.

2.3 Spacing

2.3.1 The design of stone column should be sight specific and no precise guidelines can

be given on the maximum and minimum column spacing. However, the column spacing

may broadly range from 2 to 3 depending upon the site conditions, loading pattern,

column factors, the installation technique, settlement tolerances, etc.

2.3.2 For large projects, it is desirable to carry out field trials to determine the most

optimum spacing of stone columns taking into consideration the required bearing

capacity of the soil and permissible settlement of the foundations.

2.4. Equivalent diameter

2.4.1 The tributary area of the soil surrounding each stone column forms regular hexagon

around the column. It may be closely approximated by an equivalent circular area having

the same total area.

2.4.2 The equivalent circular has an effective diameter (De) which is given by following

equation:

De = 1.05 S for an equilateral triangular pattern, and

= 1.13 S for a square pattern

where

S = spacing of the stone columns

The resulting equivalent cylinder of composite ground with diameter D inclosing the

tributary soil and one stone column is known as unit cell.

2.5 Replacement ratio (as)

2.5.1 For purpose of settlement and stability analysis, the composite ground representing

an infinitely wide loaded area may be modeled as a unit cell comprising the stone column

and the surrounding tributary soil. To quantify the amount of soil replaced by the stone,

the term replacement ratio, as, is used. Replacement ratio (as) is given by:

as = As/A= As/(As + Ag)

Where

AS = Area of stone column,

Ag = Area of ground surrounding the column,

A = Total area within the unit cell.

2.5.2 The area replacement ratio may also be expressed as follows:

as = 0.907 (D/S)2

Where the constant 0.907 is a function of pattern used which, in this case, is commonly

employed equilateral triangular pattern.

2.6 Stress Concentration Factor (n)

2.6.1 Stress concentration occurs on the stone column because it is considerably stiffer

than the surrounding soil. From equilibrium consideration the stress in the stiffer stone

columns should be greater than the stress in the surrounding soil. The stress concentration

factor, (n) due to externally applied load is defined as the ratio of average stress in the

stone column to the stress in the soil within the unit cell.

n =

The value of (n) generally lies between 2.5 and 5 at the ground surface. The stress

concentration factor (n) increases with time of consolidation and decreases along the

length of stone column. Higher (n) values at the ground surface may result if load is

applied to the composite ground through a rigid foundation as compared to flexible

foundation. The stress concentration factor (n) may be predicted using elastic theory as

function of the modular ratio of the stone and clay assuming equal vertical displacements.

However, as the modular ratio can vary within wide limits.

3. FAILURE MECHANISMS:

Failure mechanisms of single stone column loaded over its area significantly depends

upon of length of column. For column having length greater than its critical length (that is

about four times the column diameter) and irrespective whether its end bearing or

floating, its fails by bulging. However, column shorter than the critical length are likely

to fail in general shear if it is end bearing on a rigid base and end bearing. In practice,

however, a stone column is usually loaded over an area greater than its own in which case

it experiences significantly less bulging leading to greater ultimate load capacity and

reduced settlements since the load is carried by both the stone column and the

surrounding soil.

Wherever inter-layering of sand and clay occurs, and if the sand layer is thick enough as

compared to the size of the loaded area, the general compaction achieved by the action of

the installation of the stone column may provide adequate rigidity to effectively disperse

the applied stresses thereby controlling the settlement of the weak layer. However,

effective reduction in settlement may be brought about by carrying out the treatment of

stone columns through the compressible layer.

When clay is present in the form of lenses and if the ratio of the thickness of the lense to

the stone column diameter is less than or equal to 1, the settlement due to presence of

lenses may be insignificant.

In mixed soils, the failure of stone columns should be checked both for the predominantly

sandy soils as well as the clayey soil, the governing value being lower of the two

calculated values.

4. DESIGN CONSIDERATIONS

4.1General

By assuming a tri-axial state of stress in the stone column and both the column and the

surrounding soil at failure, the ultimate vertical stresses a1, which the stone column can

take, may be determined from the following equation:

σ1 = 1 + sinϕ1 σ2 1 + sinϕ2

Where

= lateral confining stress mobilized by the surrounding soil to resist the bulging of

the stone column;

Øs = angle of internal friction of stone column;

= coefficient of passive earth pressure kp of the stone column.

This approach assumes a plane strain loading condition and hence does not realistically

consider three dimensional geometry of single stone column.

4.1.1The bearing capacity of an isolated stone column or that located within a group may

be computed using the other established theories also .Besides the passive resistance

mobilized by the soil, the increase in capacity of column due to surcharge should be taken

into account.

4.1.2 Particular attention should be paid to the presence of very weak organic clay layers

of limited thickness where local bulging failure may takes place .therefore capacity of

column in such weak clays should also be checked even if they are below the critical

depth.

4.2 Adjacent Structures

4.2.1when working near existing structure, care should be taken to avoid the damage to

such strictures by suitable measures.

4.2.2 In case of deep excavation adjacent to stone columns, prior shoring or other suitable

arrangement should be done to ground against lateral movement of soil or loss of

confining soil pressure.

4.3 Ultimate load capacity and Settlement

4.3.1 The ultimate load carrying capacity of stone column may be estimated

approximately on the basis of soil investigation data or by test loading. However, it

should be preferably determined by an initial load test on a test column specifically for

the purpose and tested on its ultimate load particularly in a locality where no such

previous experience exists.

4.3.2 Procedure for estimating the load capacity and settlement of a single column is

given in Annex A and Annex B, respectively. Any other alternate formulae with

substantially proven reliability depending upon the sub-soil characteristics and the

method of installation may be used.

4.4 Environment Factors

Design consideration should take into account the environment factors, such as presence

of aggressive chemicals in the subsoil and ground water an artesian conditions etc.

4.5 Load Test Result

The ultimate load capacity of single column may be determined from load tests with

reasonable accuracy. The settlement of a stone column obtained at safe /working load

form load test result on a single column should not be directly used in forecasting the

settlement of the structure unless experience from similar foundation in similar soil

conditions on its settlement behavior is available. The average settlement may be

assessed on the basis of sub soil data and loading details of the structures as whole using

the principles of soil mechanics.

4.6 Factor of Safety

4.6.1 The following factors should be considered for selecting a suitable factor of safety:

a) Reliability of the value of ultimate load carrying of the column,

b) The type of superstructure and the type of loading,

c) Allowable total and differential settlement of the structure, and

d) The manner of load transfer from stone column to the soil.

4.6.2 It is desirable that the ultimate capacity of column is obtained from an initial load

test. The minimum factor of safety for such a load test should be 2.5

4.6.3 When ultimate capacity is derived from soil mechanics consideration, the minimum

factor of safety recommended in each formula should be applicable.

STONE COLUMN IN CLAY SPACING

5 GRANULAR BLANKETS:

5.1 Irrespective of the method used to construct the stone columns, the blanket laid over

the top of the stone columns should consist of clean medium to coarse sand compacted in

layers to a relative density of 75 to 80 percent.

5.2 Minimum thickness of the compacted sand blanket should be 0.5 m. This blanket

should be exposed to atmosphere at its periphery for pore water pressure dissipation.

5.3 After ensuring complete removal of slush deposited during boring operations, a

minimum depth of 0.5m, preferably 0.75 m below the granular blanket should be

compacted by other suitable means, such as rolling /tamping to the specified densification

criteria.

TRIANGULAR ARRANGEMENT OF STONE COLUMN

6 FIELD CONTROLS:

6.1 In the methods involving boring the set criteria and the consumption of granular fill

form the main quality control measures for the columns constructed by the non-

displacement technique. For ascertaining the consumptions of fill, the diameter of the

column as formed during field trials should be measured in its uppermost part for a depth

of four diameters and average of these observations taken as the column diameter.

6.2 In the case of vibrofloats, the following minimum field controls should be observed:

a) Vibroflot penetration depth including the depth of embedment in firm strata,

b) Monitoring of volume of backfill added to obtain an indication of the densities

achieved, and

c) Monitoring of ammeter or hydraulic pressure gauge readings to verify that the

maximum possible density has been achieved in case of vibrofloted columns.

7. FIELD LOADING TESTS:

7.1 Irrespective of the method used to construct the stone columns ,the initial load test

should be performed at a trial test site to evaluate the load settlement behavior of the soil

stone column system. The test should be conducuted on a single and also on a group of

minimum three columns.

7.2 For the initial load test, in order to simulate the equivalent steel plate of adequate

thickness an rigidity may be based on the effective tributary soil area of stone column for

a single column test and three times the effective area of single column for a three

column group test in each case, the footing may cover the equivalent circular effective

area centrally.

7.4 The initial and final soil conditions at trial site should be investigated by drilling at

least one borehole and one static cone test/pressure meter test/dynamic cone test prior and

subsequent to the installation of column. All the tests including the standard penetration

test, field vane shear test and collection of undisturbed/disturbed samples and laboratory

testing on the samples should be as per relevant Indian standards.

7.5 A granular blanket of medium to coarse having thickness not less than 300 mm

should be laid over the test column. Over the blanket, a properly designed footing should

be laid. The footing may be cast away from the test site and transported to the test

location son as to fixed it properly over the sand blanket.

7.6 In case of high water conditions exist at site, the water level during the test should be

maintained at the footing base level by the watering.

7.7 Following procedure should be followed for application of load:

a) The load should be applied to the footing by a suitable kentledge, taking care to avoid

impact or fluctuations.

b) The kentledge will be minimum 1.3times the maximum test load.

c) Load settlement observations should be taken to 1.5 times the design load for a single

column and three column group test respectively.

d) The settlement should be recorded by four dial gauges (sensitivity ) fixed at

diametrically opposite ends of the footing.

e) Each stage of loading should be near about 1/5 of the design load and should be

maintained till the rate of settlement is less than 0.05mm/h at which instant the next stage

of loading should be applied.

f) The design as well as the maximum test load should be maintained for a minimum

period of 12 h after stabilization of settlement to the rate.

g) Load settlement and time settlement relationships should be plotted from the

settlement observed from each increment of load at intervals of

1min,2min,4min,8min,16min,1/2h,1h,1.5h,2h,3h,4h…… till the desired rate of the

settlement has been achieved the time intervals may be suitably modified if so desired.

h) The test load should be unloaded in 5 stages. At each stage enough time should be

allowed for settlement to stabilize.

j) The load test should be considered. Acceptable if its meet the following settlement

criteria:

i) 10 -12 mm settlement at design load for a single column test.

ii) 25-30 mm settlement at design load for a three column group test.

k) For routine load test few of columns may be tested up to 1.1 times the design load

intensity with minimum kentledge of 1.3 times the design load.

Single column test arrangement: a) column area loading; b) entire area loading

8. ELIMATION OF LOAD CAPACITY OF A COLUMN:

8.1. STONE COLUMNS IN COHESIVE SOILS

Load capacity of the treated ground may be obtained by summing up the contribution of

each of the following components for the wide spread loads, such as tankages and

embankments:

1. Capacity of the stone column resulting from the resistance offered by the surrounding

soil against its lateral deformation under axial load.

2. Capacity of the stone column resulting soil due to surcharge over it,

3. Bearing support provided by the intervening soil between the columns.

8.2. Capacity based on bulging of column

Considering that the foundation soil is at failure when stressed horizontally due to

bulging of stone column:

a) Initially, the surcharge load is supported entirely by the rigid column. As the column

dilates some load is shared by the intervening soil depending upon the relative rigidity of

the column and soil. Consolidation of soil under this load results in an increase in its

strength which provides additional lateral resistance against bulging.

b) The surcharge load may consist of sand blanket and sand pad. If thicknesses of these

elements are not known, the limiting thickness of surcharge loading as represented by the

safe bearing capacity of the soil may be considered.

c) The increase in capacity of the column due to surcharge may be computed in terms of

increase in mean radial stress of the soil as follows:

ro = qsafe/3 (1+2ko)

Where ro is the increase in mean radial stress due to surcharge, and qsafe is the safe

bearing pressure of soil with the factor of safety of 2.5

qsafe=Cu.NC/2.5

Increase in ultimate cavity expansion stress = roFq'

Where

Fq = vesic’s dimension less cylindrical cavity expansion factor

Fq’=1 for Øg =0

Increase in yield stress of the column =kpcol ro

d) Allowing a factor safety of 2, increase in safe load of column, Q2 is given by the

following formula:

Q2 = kpcol roAS/2

The surcharge effect is minimum at edges and it should be compensated by installing

additional columns in the peripheral region of the facility.

8.3 Bearing support provided by the intervening soil:

This component consists of the intrinsic capacity of the virgin soil to support a vertical

load which may be computed as follows:

Effective area of stone column including the intervening soil for triangular pattern

=0.866 s2

Area of intervening soil for each column, Ag is given by the following formula:

Ag =0.866s2 –πD2/4

Safe load taken by the intervening soil,

Q3 =qsafe Ag

Overall safe load on each column and its tributary soil

Q= Q1 +Q2 +Q3

Mechanisms of load transfer for (a) a rigid pile and (b) a stone column.