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INTRODUCTION

INTRODUCTION TO GROUND IMPROVEMENT

Ground Improvement refers to a technique that improves the

engineering properties of the soil mass treated. Usually, the properties

that are modified are shear strength, stiffness and permeability.

Ground improvement has developed into a sophisticated tool to

support foundations for a wide variety of structures. Properly applied,

i.e. after giving due consideration to the nature of the ground being

improved and the type and sensitivity of the structures being built,

ground improvement often reduces direct costs and saves time.

Need for Ground Improvement

• Mechanical properties of soil are not adequate. • Swelling and shrinkage • Collapsible soils • Soft soils • Organic soils and peaty soil • Sands and gravelly deposits, karst deposits with sinkhole formations • Foundations on dumps and sanitary landfills • Handling hazardous materials in contact with soil. • Use of old mine pits

Here I discuss Ground Improvement by ‘STONE COLUMN’ and ‘PREFABRICATED VERTICAL DRAIN’ techniqes.

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literature review on stone column and pvd

RA Barron (1944) presented the first comprehensive theoretical solution to the vertical

drain assisted consolidation of soft soil, based on existing solutions for one dimensional

vertical consolidation[ Terzaghi (1925)] . His analysis was based on the following

assumptions:

i) Darcy’s flow law is valid. ii) Soil is saturated and homogeneous. iii) Consolidation displacement is in vertical direction. iv) Excess pore water pressure at the drain well surface is zero. v) Soil beyond cylindrical boundary is impervious. vi) Excess pore water pressure at top boundary of the soil-mass is zero. vii) No vertical flow at the center cross-section of the soil mass.

Barron considered a single drain surrounded by single soil cylinder in his analysis. It

was assumed that the installation process of vertical drain does not affect the soil properties

adjacent to the drain, and permeability of the drain well was high enough for well resistance

to be neglected.

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S Hansbo (1981) proposed an approximate solution for vertical drain based on equal

strain hypothesis taking both smear and well resistance into account considering that rate of

flow of internal pore water can be measured by applying Darcy’s law. Hansbo modified the

equation developed by Barron by simplifying assumptions due to the physical dimensions

characteristics of the prefabricated drain and the effect of installations.

The modified expression of average degree of consolidation is given as

Uh=1- exp [−8𝑇ℎ

𝐹(𝑛)]

And F= F(n)+Fs+Fr.

Where F is the factor which express the additive effect due to the spacing of the drains

f(n).F(s) factor for Smear Effect .F(r) factor for well resistance. For the typical values of the

spacing ratio, of 20 or more, the spacing factor becomes

F (n) = ln [𝐷𝑒

𝑑𝑤]-

3

4

To account for the disturbance during the installation a zone of reduced permeability was

assumed. The smear effect factor is given as:

𝐹𝑠 = [(𝑘ℎ

𝑘𝑠) − 1] ln(

𝑑𝑠

𝑑𝑤)

Where ds is the diameter of the disturbed Zone and kh is the co efficient of permeability in

the horizontal direction in the disturbed zone .

Since the PVD s have limited discharge capacity Hansbo introduced a drain resistance factor

Fr assuming that the Darcy’s law can be applied for flow along the vertical axis of the drain.

Well Resistance factor is given as: F r = π z(L –z) Kh/ qw.

Where, z is the distance from the drainage end of the drain. L is the drainage length when

drainage occurs from at both ends. K h= is the coefficient of permeability in the horizontal

directions qw is the discharge capacity of the drain at hydraulic gradient of 1.

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Hansbo also showed that the process of consolidation for a circular drain and the band

drain is almost the same, when the circular drain is assumed do have a circumference equal

to that of the band drain is d w= 2(b+t)/л.

The discharge capacity of the PVD is a function of its filter permeability, cross sectional

area, lateral confining pressure & drain stiffness controlling its deformation.

Budhina Indraratna, A.S. Balasubramanium and P. Ratnayake (1994)

The Performance of an embankment constructed on a soft foundation stabilized with

vertical band drains was analyzed. The effectiveness of the vertical drains could be

evaluated by considering the excess pore water pressure, vertical and lateral displacement

& the surface settlement profile in relation to the consolidation behaviour of the soft clay.

For efficient vertical drains, the case study reveals, that the effect of the smear and well

resistance is negligible. In the short term the rate of dissipation of pore pressure at the

drain soil boundaries controls the drain efficiency, hence the consequent settlement. In the

long term (mere than 400 days) the concept of perfect drain seems realistic.

The short term, the assumption of zero excess pore pressure at the drain boundaries, over

estimates the vertical settlements but underestimates the lateral yield. For perfect drains,

although the prediction of initial or short term vertical settlement is conservative,

significant under predication of lateral movements is of greater concern. This is because the

propagation of failure surface is mainly a function of the incremental lateral displacement

within the upper soft clay.

Therefore to evaluate the stability of rapidly built embankment on soft clay, the excess-

pore-pressure condition in the drain boundaries must be correctly incorporated in the finite

element analysis at any given time.

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The study has shown that unless the un-dissipated excess-pore-pressure along the drain

boundaries are correctly accounted for, the vertical settlements and lateral displacement

cannot be predicted to an acceptable degree of accuracy. In this respect, additional

modification in the pore pressure ‘shape function’ of the elements along the soil drain

boundaries is desirable.

The accurate prediction of lateral displacement requires careful assessment of soil

parameters corresponding to the actual stress path response in the field, during

embankment loading. The installation and vertical drains curtails lateral displacement

substantially, thereby reducing the risk of shear failure. The existence of rigid crust just

beneath the embankment can also resist lateral displacement, facilitating construction of

higher embankments.

The use of plain strain finite element analysis (CRISP) cannot be regarded superior to a

more powerful three dimensional analysis of flow into vertical drains.

Nevertheless, considering the relative simplicity of the modified cam clay theories and the

user friendly CRISP. The current analysis is justified in terms of both computational effort

and acceptable quality and prediction.

Jitendrapal Sharma & Daping Xiao (2000)

The authors using a large scale laboratory model examined the extent of smear zone. They

conducted a test experiment simulating smear and another with no smear. Excess pore

water pressure and moisture content was measured at seven different radial locations

within the test set up. Several samples for recording moisture content, and void ratio were

collected. They observed that the distribution of excess pore pressure due to drain

installation gave a clear indication of the extent of smear zone. The effect of

reconsolidation of the clay was found to be greater than of remoulding of the clay.

The extent of smear zone was also confirmed from the change in the void ratio of the clay

layer in the smear zone from the moisture content measurement.

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It was found that the radius of the smear zone is about Four times the radius of the

mandrel.

The alteration of in the properties of the clay layer due to mandrel insertion is caused by

reconsolidation due to dissipation of pore pressure and remoulding of the clay layer due to

shear applied by outer surface of the mandrel.

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They had proposed that the smear zone to be further divided in two zones –

A remolded zone of limited extent close to the drain and

A reconsolidated zone of wider extent situated between remolded and

intact clay.

Cholachat Rujikiathkamjom & Buddima Indraratna (2009)

A study was presented by the authors on procedures of design of vertical drain. In this they

have significantly extended the earlier methods to include-

a. Linear reduction of lateral permeability in the smear Zone.

b. The effect of overlapping smear zones in a closely spaced drain net work.

c. The gain in untrained shear strength due to consolidation.

D.Basu, M.Prezzi .M.R.Madhav (2009)

The authors studied the 2D finite Element analysis of the effect of soil disturbance caused by

the installation of a vertical drain on the rate of consolidation. A rectangular pattern of PVD

installation was considered. Smear zone a zone of maximum disturbance was assumed to

exists on the immediate vicinity of the surrounding the PVD. A transition zone surrounding

the Smear zone ,where the effect of disturbance reduce gradually as the distance from the

PVD increases. The Hydraulic Conductivity in the transition zone is assumed to increase

linearly from a low value in the smear zone to the original in situ value in the un disturbed

zone.

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The extent of the soil disturbance ,size of the smear zone as well as that of the transition

zone, spacing of the PVD ,shape and size of mandrel was also investigated. Result of this

analysis was compared with that of an experimental study. It was observed that both the

results compare reasonably well.

Authors opined that, transition zone has a definite role in slowing the rate of consolidation.

Neglecting its, effect can lead to faulty estimation of consolidation and settlement.

Error in estimating the degree of consolidation caused by converting the rectangular unit cell

to equivalent circles will be less than 5%, .Accounting for the soil disturbance ,due to the

process of installation, reduces the error.

The factors relating to drain installation that affects the consolidation most are – soil

disturbance, extent of the disturbed zone, shape and size of the mandrel.

For a particular degree of consolidation the time factor is related to the ratio of conductivity

of the Smear and undisturbed zone.

The transition Zone can be replaced by a equivalent smear zone with increased dimentions.

The extent of extra length of the smear zone depends on the degree of disturbance caused

by the installation.

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Giridhar Rajesh B; Radhakrishnan, R. and Dey A ; (2014)

The authors in their article provides a detail report about the plain strain Finite Element

modeling of an embankment pre-load resting on a soft soil site treated with PVD.

The preloading has been modeled as a stage construction sequence. The pressure of the

vertical drains has been modeled by considering the change of permeability of the soil

surrounding the drain. The efficiency of the PVD has been illustrated with the aid of

comparative Settlement at the centre of the embankment in the presence and absence of

PVDs. Application of PVDs aided in the reduction of the excess pore pressure generated at

the end of the loading by a significant amount, as well as resulted in the accelerated

dissipation of the accumulated pore pressure. The effect of smear has studied with the aid

of varying horizontal to vertical permeability ratio of the soil, which revealed that the in-situ

measurement most possibly corresponded to a kh/ks magnitude in the range of 3 to 4.

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STUDY ON GROUND IMPROVEMENT STONE COLUMN

TECHNIQUE

1. ABSTRACT

Ground improvement is an important requirement in today’s construction industry as

land reclamation is becoming increasingly popular. The stone column technique is a

very efficient method of improving the strength parameters of soil like bearing

capacity and reducing consolidation settlement. It offers a much economical and

sustainable alternative to piling and deep foundation solutions. Ground improvement

when implemented through stone column technique aids in a much stable solution

to construction in weak cohesive soils. This is an attempt to discuss in detail about

this technique to improve soil stability, including it’s salient features, design

parameters, major functions and drawbacks.

2.1 INTRODUCTION

India has large coastline exceeding 6000kms. In view of the developments on coastal

areas in the recent past, large number of ports and industries are being built. In

addition, the availability of land for the development of commercial, housing,

industrial and transportation, infrastructure etc. are scarce particularly in urban

areas. This necessitated the use of land, which has weak strata, wherein the

geotechnical engineers are challenged by presence of different problematic soils with

varied engineering characteristics. Many of these areas are covered with thick soft

marine clay deposit, with very low shear strength and high compressibility. Out of

several techniques available for improving the weak strata, stone columns have been

used to a large extend for several applications. The design of stone column is still

empirical, based on past experience and needs field trials before execution .

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3. INFLUENCING FACTORS

3.1 Soil

Subsurface soils whose undrained shear strength range from 7 to 50 kPa or loose

sandy soils including silty or clayey sands represent 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 > 4) which lose

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

the column is not available under the weak strata

3.2 Treatment depth

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

determined that the stone columns extend through the most significant compressible

strata that contribute to the settlement of the foundation. Average depth of stone

column accomplished in India may be around 15m, although with equipment

modification, higher depths beyond 20 m are now becoming widespread.

3.3 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 foundations is limited and is

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

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

application.

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4. PRINCIPLE

The stabilization of soils by displacing the soil radially, with the help of a deep

vibrator, refilling the resulting space with granular material and compacting the

same with the vibrator is called vibrostone columns or simply stone columns. In

other words, stone columns are constructed where in the soft soil is strengthened by

replacing a certain percentage of soil with aggregate. The aggregate column will act

as a drainage channel to release the excess pore water present in the subsoil. The

degree of improvement of soft soils by stone columns is because of the densification

of the surrounding soft soil during the installation of stone column itself and the

subsequent consolidation process occurring in soft soil before the final loading of

improved soil.

5. INSTALLATION TECHNIQUES

A. Non Displacement Method

The process of installation where soil is taken out during boring is called non displacement

type of installation.

Bored Rammed System:

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

minimizing 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

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adopted in clays of low sensitivity. These columns also act as drainage paths to

accelerate settlements under loading.

B. Displacement Method

If the soil is laterally displaced while making the hole due to driving of a tube or

casing, it is the displacement type of boring.

.

Vibro Replacement Method:

In this method, creation of hole in the ground and compaction of granular fill

backfilled in the hole is done mechanically using a mechanical unit called vibrofloat.

Stone columns may be constructed using vibrofloat either by Wet process, which is

suitable for soft to firm soil with high water table condition where borehole stability

is questionable, or by dry process which is suitable for soils of relatively high initial

strength with low water table, where the hole can stand of its own upon extraction

of the probe, such as unsaturated fills.

Cased rammed stone column Vibro replacement method.

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6. INSTALLATION PROCEDURE

To form a vibro-replacement point the vibrator with its follower tubes is placed over the

selected point by means of a suitable supporting rig (crane). After starting the motor the

vibrator is lowered into the ground. It simultaneously releases water from the lower jets

which remove the soft soil directly under the vibrofloat nose forming a hole. This operation

allows practically an unimpeded penetration of the vibrofloat into the soil under its own

weight. No increase in density of the soil is achieved during this operation of the probe

penetration. When the vibrofloat has reached the desired depth, the water supply to the

lower jet is reduced suitably and the top jets are put on. Wash water from these upper jets

returns to the ground surface through the annulus between the outside of the follow on

tubes and the crater sides. This upward flow maintains an open channel along the sides of

the vibrofloat permitting backfill material shoved from the surface to reach the bottom and

it also prevents the probe from sticking. The annular wash water flow is established by

raising (surging) the vibrofloat twice or thrice to clean the loose soft soils from the hole.

When the water flow continuously returns to the surface, the probe is raised by suitable lift,

say 1.5 m and the backfill is poured into the annular space between the poker and the side

walls of the hole. The vibrator is then lowered back into the hole between 0.70 to 0.80 m,

thereby creating a 0.7m length of stone column. The horizontal vibrations generated by the

poker drive the stones laterally into the soil to form a column of an enlarged diameter.

Combination of bottom and top water jets may also be used depending upon the soil.

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. 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. Over the granular blanket, a geotextile mat is laid and then it’s again

covered with a granular blanket over which the foundation would rest. This is to uniformly

distribute the load coming on the the stone column and there by create a region of uniform

loading.

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A. Top Feed System

In the Top Feed System, the poker is completely withdrawn after initial penetration

to the design depth. Stone (12-75mm in size) is then tipped into the hole in

controlled volumes from the ground surface allowing fall under gravity to the bottom

side of hole.

The column is compacted in layers (the stone is forced downwards and outwards)

through continued penetration and withdrawal of the poker. The Top Feed System is

suitable if the hole formed by the poker will remain open during construction of the

column

B. Bottom Feed System

The gravel may be fed from a rig-mounted hopper through a permanent delivery

tube along the side of the poker, which bends inwards and allows the stone to exit at

the poker tip. This Bottom Feed process requires a smaller grade of stone (2-45mm).

By remaining in the ground during column construction, the poker cases its own hole

and hence is suited to ground with a high water table or running sand conditions.

Wet top feed process is called vibro-replacement and dry top/bottom feed process is

called vibro displacement.

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7. DESIGN

While stone columns will transmit some load to the soil by shear stresses (along the

column-soil interface) and end bearing (at the column base), the predominant load-transfer

mechanism (unless the column is very short) is lateral bulging into the surrounding soil. The

relevant column stresses are depicted in Fig. 3. The passive resistance of the surrounding

soil dictates the column performance under load. Generally the column bulging will be

greatest close to the top of the column where the overburden pressures are lowest.

A. Design Parameters

1) Stone Column Diameter: Installation of stone columns 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 lateral displacement of stones during vibrations/ramming, the

completed diameter of the hole is always greater than the initial diameter of the probe or

the casing depending upon the soil type, its undrained shear strength, stone size,

characteristics of the vibrating probe/rammer used and the construction method. Diameter

usually varies from 800 to 1500 mm

2) Pattern of arrangement: Stone columns should be installed preferably in an equilateral

triangular pattern which gives the densest packing although a square pattern may also be

used. A typical layout of the patterns are shown in Fig. 7 and Fig. 8

3) Spacing: The design of stone columns should be site specific and no precise guidelines

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

spacing may broadly ranges from 2 to 3 depending upon the site conditions, loading

pattern, column factors, the installation technique, settlement tolerances, etc. For large

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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 foundation.

4) Replacement ratio: To quantify the amount of soil replaced by the stone, the term

replacement ratio, as is used.

as = 0.907 (D/S)2

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

commonly employed equilateral triangular pattern.

5) Stress Concentration factor: The stress concentration factor, n, due to externally

applied load σ, is defined as the ratio of average stress in the stone column σs ,to the stress

σg, in the soil within the unit cell,

n=σs/ σg

The value of n generally lies between 2.5 and 5 at the ground surface

6) Stress: By assuming a triaxial state of stress in the stone column and both the column

and the surrounding soil at failure, the ultimate vertical stress σ1, which the stone column

can take, may be determined from the following equation:

σ1/ σ3 = (1+sin φs) /(1-sin φs)

where

σ3 = lateral confining stress mobilized by the surrounding soil to resist the bulging of the

stone column

φs = angle of internal friction of the stone column

σ1/ σ3 =coefficient of passive earth pressure of the stone column.

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7) Settlement: Consolidation settlement of the composite (treated) soil S, is given by:

S= mvσgH

where

mv = Coefficient of volume change

σg = Vertical stress in surrounding ground

H =Thickness of treated soil

8) Aggregate: Crushed stone or gravel which is chemically inert, devoid of organic matter,

hard etc are used for constructing the aggregate column. Well graded stones of 75mm to

2mm may be used.

Fig. 7 Triangular pattern of arrangement Fig. 8 Square pattern of arrangement

8. FUNCTIONS AND USES

Stone column improves the shear strength of the subsoil to increase the bearing capacity. It improves the stiffness of subsoil to decrease settlements. It has the ability to carry very high loads since columns are ductile. It is more economical than piling. Rapid consolidation of subsoil is facilitated in stone column.

Immediate increase of shear strength and friction angle of treated soil occurs. There

is no waiting period after installation unlike PVD. Embankment construction can begin soon after installation. When installed in a uniform grid pattern it ‘homogenizes’ variable soil properties, thereby reducing the potential for differential settlement.

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9. DISADVANTAGES

Sensitive clays do not adequately regain shear strength. Due to this, ground

improvement by stone column cannot be achieved in clays with sensitivity greater

than 4. Stone columns when installed at a distance of less than 3.66m can cause high

lateral pressures and displacement of adjacent structures. Severe cracks could be

the stone column site due to the vibrations of 30-50Hz.

Stone column installation in extremely cohesive clays and silts is suitable only if

preloading facility is available, especially for storage tank construction.

10.CONCLUSION

The crude form of stone column seen during the construction of Taj Mahal has been

improvised greatly and is being made a technically viable option for ground

improvement considering it’s cost effectiveness in the recent past.

The major conclusions arrived at are as follows:

Stone columns improves the bearing capacity and reduces the settlement of weak

soil strata.

Owing to rapid consolidation due to the accelerated dissipation of excess pore water

pressure into the drainage path formed by stone columns, construction can be

started quickly.

Thorough subsoil investigation from borelogs supplemented by penetration tests and

other insitu test results should be strictly carried out before designing the stone

column.

Stone columns when installed at a distance of 4.87m or more eliminates the damage

caused by vibrations.

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