GROUND IMPROVEMENT

BY

STONE COLUMNS

Prof. A.K.Choudhary

Professor & Ex Head

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 in normally consolidated clay. The test results indicated that ultimate capacity of stone column was governed primarily by the maximum radial reaction of the soil against the bulging and the extend of vertical movement in the stone column was 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 the stone 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 to use on confined sites or in environmentally sensitive 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

qu = NΦ ( σr +4Cu)

Where

qu = ultimate strength

N Φ =tan2(θ) =tan2(45 +Φ/2)

σr = effective radial stress.

K =effective earth pressure coefficient.

= effective vertical stress generally taken 4times the dia. Of stone columns.

C u = undrained cohension of immediate surrounding soil.

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, industrial and residential buildings.

To reduce settlement in areas of landfill.

To reduce the risk of liquefaction under roadways,

airport runways, embankments, bridges and tunnels.

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!!!!!!!