GROUND IMPROVEMENT

BY

STONE COLUMNS

Department of Civil Engineering,

NIT, Jamshedpur

INTRODUCTION

Soil improvement technique have been

developed rapidly in the past several yrs.

because on various occasions it is noted that

the local soil by nature, unable to bear the

proposed structure. Use of stone column is one

of such technique.

HISTORICAL BACKGROUND

first stone column was used in ancient Egypt in 2600 BC.

In experimental approach, Hughes and Withers

(1974) carried out series of model tests innormally consolidated clay. The test resultsindicated that ultimate capacity of stone columnwas governed primarily by the maximum radialreaction of the soil against the bulging and theextend of vertical movement in the stone columnwas limited to about 4 times the diameter

PRINCIPLE AND MECHANISM

Crushed stone is laterally compacted against

surrounding soil

Dense stone , aid in further transmitting

vibrational energy into the soil causing

densification

Stone column in situ soil form an integrated

system having low compressibility & high shear

strength.

Stone column act as vertical drains ,excess pore

water can rapidly dissipate.

TEST BEFORE STONE CLOUMN

Standard Penetration Testing (SPT)

Cone Penetrometer Testing (CPT)

Dilatometer Testing (DMT)

Load Testing

Shear Wave Velocity Profiling

EQUIPMENTS & MATERIALS

crawler crane

vibrating probe

Rammer

Bailer and casing

Hooper

Crushed coarse aggregates.

Water

CONSTRUCTION TECHNIQUE

TWO METHODS:-

Vibratory Technique using vibrofloat

• Wet top feed method

• Dry bottom feed method.

Rammed Stone Column Technique

STEPS INVOLVED

WET, TOP FEED METHOD (REPLACEMENT AND

DISPLACEMENT)

Jetting water is used to remove soft material,stabilize the probe hole, and ensure that thestone backfill reaches the tip of the vibrator.

This is the most commonly used and most cost-efficient of the deep vibratory methods.

However, handling of the spoil generated by the

process may make this method more difficult touse on confined sites or in environmentallysensitive areas.

WET TOP-FEED METHOD PROCESS

DRY, BOTTOM FEED METHOD (DISPLACEMENT)

This technique uses the same vibrator probes as

standard Vibro-Replacement Stone Columns, but with

the addition of a hopper and supply tube to feed the

stone backfill directly to the tip of the vibrator. Bottom Feed Vibro-Replacement is a completely dry

operation where the vibrator remains in the ground

during the construction process. The elimination of flushing water in turn eliminates the

generation of spoil, extending the range of sites that can

be treated. Treatment is possible up to a depth of 80 feet and is not

inhibited by the presence of groundwater.

DRY BOTTOM-FEED METHOD PROCESS

2. RAMMED STONE COLUMN TECHNIQUE :-

In this technique the granular fill is introduced into a

pre-bored hole and compacted by a heavy rammer.

The hole is made by using normal bored piling rig

with winch bailer and casing.

Sometimes mixtures of stone aggregate and sand

generally in the proportion of 2:1 are used as

backfill material.

Gravel backfill of aggregate size 75 mm to 2 mm is

generally recommended.

SPECIFICATION

Diameter of stone column ranges from 0.5 to

0.75 m

Spacing depends upon(1.5 to 3m)

Characteristic Of untreated ground

soundness of stone column

settlement tolerence.

Weight of rammer varies from 2 – 3 tonnes

falling from height 1- 1.5m

MONITORING

Ramming –

compaction should be done at 1.5-2.0m of

column

settlement should be < 17mm for 10

blows,each imparting 20j

INSTALLATION

BEHAVIOUR OF STONE COLUMN

CRITICAL LENGTH OF STONE COLUMN

q = N ( σ +4C )u Φ r u

Where

q = ultimate strengthu

N =tan (θ) =tan (45 +Φ/2)Φ2 2

σ = effective radial stress.r

K =effective earth pressure coefficient.

= effective vertical stress generally taken

4times the dia. Of stone columns.C = undrained cohension of immediate

surrounding soil.u

REPRESENTATION OF SETLLEMENT

REDUCTION:

BENEFITS:

Increases in bearing capacity

Increase in shear strength

Reduction in foundation settlement.

Mitigation of liquifaction.

Cost and time savings over other conventional

systems

It can be applied close to existing structure.

APPLICATIONS

To improve soils prior to construction of

foundations and slabs for commercial, industrialand residential buildings.

To reduce settlement in areas of landfill.

To reduce the risk of liquefaction under roadways,

airport runways, embankments, bridges andtunnels.

As an alternative support system to deep

foundations (piling).

PROBLEMS ENCOUNTERED:-

If the soil is cohesive , the disturbance reduces

the strength of the surrounding soil, resulting in

no densification due to formation of cavity.

Sensitive clays do not adequately regain shear

strength, due to high sensitivity after stone

column.

CASE STUDY

Vibro Stone Columns to support high reinforced soil walls for a highway project

THANK YOU!!!!!!!