GROUND IMPROVEMENT BY STONE COLUMNS

GROUNDIMPROVEMENT

Defined as the “the process of enhancing the quality of

soil.”

Ground improvement mainly refers to the improvement of soil layers

The ground improvement techniques applied are tools used by the geotechnical engineer for "fixing” the problems of poor ground.

When a poor ground existed at the project site, for instance, the early builder was faced with the following questions:

replaced with a more suitable material?

Should the weak ground be bypassed laterallyor vertically ?

Should the design be changed to reflect theground’s limitation?

With the development of ground improvement, the new questions facing the current builders are:

Should the problematic ground at the project site be fixed instead of bypassed?

What are the critical issues that influence the successful application of a specific fixing tool?

Which fixing tool should be used for improving the soil conditions?

Why do we choose “STONE COLUMN” out of

so many “Ground Improvement

Techniques” available?

They act as vertical drains.

They also mitigate the potential for Liquefaction.

They have negligible demerits which can be

easily overcome by suitable techniques.

Significance Of Stone-Columns in INDIA

Method of installing stone column

(RAMMING) does not require any skilled

labour-any layman can do the job.

Its installation is economically very feasible-

no high cost is required to do the execution.

India has significant deposits of soft clays.

What are basically stone columns??

STONE COLUMNS are vertical columns of

compacted aggregate are formed through

the soils to be improved These columns

result in considerable vertical load

carrying capacity and improved shear

resistance in the soil mass.

What Soils are Suitable for installation of

STONE COLUMNS?

Any soil type that does not respond to

vibration alone is a candidate for stone

columns. These soils include silty and

clayey sands, silts, clays, and some

layered soils where damping of vibrations

occurs.

Fines content more than 12 % and/or clay

content of more than 2 % cannot be

compacted by Vibro Compaction.

Zone C silty sand with 12 % to 20 % fines is

treated with Stone Columns.

Top Feed Stone Columns were not “invented”

but rather evolved from the Vibro

Compaction process.

Zone D contains the silts and clays-stone

columns.

Areas of Application of Stone Columns

To improve the stability of embankments and natural

slopes

To increase the bearing capacity of a site to make it

possible to use shallow foundation on the soil

Reduction of total and differential settlements.

Reduction of liquefaction potential of cohesionless

soil.

To increase the time rate of settlement.

Permits constructions on fills.

Prevents earthquake induced lateral spreading.

Benefits of Vibroflotation

Stone Columns are a technical and potentially economical alternatives to deep foundation

Stone Columns are more economical than the removal and replacement of deep poor bearing soil on a large site

Very useful where infrastructure does not permit high vibration technique such as dynamic compaction, deep blasting or piling

Where time is critical to project start-up site improvement can be achieved quicker by vibroflotation than by preloading the soils.

Stone Columns provide a vertical drainage path for excess pore water pressure dissipation

With Vibroflotation differential settlements are often in the order of 10%to 15% of total settlement

Installation methods of stone columns

VIBRO-COMPACTION METHOD.

Wet, Top Feed Method (Replacement and Displacement)

Dry, Bottom Feed Method (Displacement)

RAMMING (INDEGENEOUS METHOD).

Vibro-replacement This refers to the wet, top

feed process in which water is used to aid the penetration of the poker vibrator into the ground.

Part of the in-situ soil is washed away due to the jetting action of the water thereby creating a long hole.

The soil that is washed away is replaced with compacted stones to form stone columns

Vibro-displacement (Top Fed)

This refers to the dry, top or bottom feed process

No water is used.

As vibrating Poker is inserted, in-situ soil is displaced and laterally compacted.

The drilled hole is filled with compacted gravel.

Vibro-displacement (bottom fed)

Relative advantages and disadvantages

VIBROREPLACEMENT Faster and usually

less costly.

Achieves better results in silty sands above ground water.

Furnishes better results.

VIBRODISPLACEMENTReduces the effort

of sludge handling with clayey soils.

Ideal for contaminated soils where the contaminants shall remain at depth.

RAMMING :

Ramming is an indigenous method. It can described as the

“METHOD IN WHICH DENSIFICATION OF SOIL MASS IS ACHIEVED MANUALLY WITHOUT ANY SKILLED LABOUR”

SIGNIFICANCE OF RAMMING

No skilled labour required.

No big initial investment required- it’s highly feasible economically.

Easy to follow the steps, no qualification required.

Lastly, this method got its birth in INDIA, so can be indigenously followed.

DESIGN OF STONE COLUMNSNecessary Information:Soil investigation data: Bore logs in-situ

tests results Nature of soil, soil profiles etc.,

Ground water level and its condition.

Layout of the structure: Foundation system, loading pattern and intensity as determined by structural analysis.

Sufficient information of structures existing nearby.

IMPORTANT FEATURES OF STONE COLUMN TREATMENT

Influence of soil type: Suitable for loose sandy soils including silty or clayey sands.(7 to 50kpa). Not suitable for sensitive clays & silts. (sensitivity>4).

Influence of construction Methodology.

Treatment depth.

Area of treatment.

Termination.

BASIC DESIGN PARAMETERS

Stone column diameter, DPattern Equilateral Triangular pattern Square patternSpacing Equivalent DiameterReplacement ratioStress concentration factor(n)

Pattern:

Equivalent Diameter

The equivalent circle has an effective diameter (De) which is given by the equation:

De= 1.05 S for an equivalent triangular pattern.

=1.13 S for a square pattern

S=spacing of the stone columns.

Replacement Ratio (as)

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

Where As= Area of stone

column.

Ag=Area of ground surrounding the column.

A=Total area within the cell.

as=0.907 (D/S)2

Failure Modes of Stone Columns

Four Basic Failure Modes of Stone Columns are:

General shear failure.

Local shear failure.

Bulging failure.

Failure by sliding.

The modes of failure of Stone Columns depend uponthe following parameters:Type of Stone Column (End-bearing or

Free Floating).

Type of Loading on columns.

Passive resistance of tributary clay.

DESIGN CONCIDERATIONS

GENERAL

ADJASCENT STRUCTURE

ULTIMATE LOAD CARRING CAPACITY

ENVIRONMENTAL FACTORS

LOAD TEST RESULTS

FACTOR OF SAFETY