stone columns - an overview(ground improvement)

Stone Columns: An Overviewby

S.V. Abhishek & V. Tarachand

Department of Civil EngineeringCollege of Engineering (A)

Andhra UniversityVisakhapatnam

Stone ColumnsAmongst various techniques for improving in-

situ ground conditions, stone columns are probably the most versatile, due to their ability to perform a variety of important geotechnical functions.

Origin Germany (1950s) In India, the use of stone columns began in the

early 1970s.Load bearing columns of well compacted coarse

aggregate installed in the ground to serve various purposes such as reinforcement, densification and drainage.

Applicable Soil TypesSoft, Non-Compactible, Weak SoilsGranular Soils with High Fines

Content (in excess of 15%)Organic SoilsMarine/Alluvial ClaysLiquefiable SoilsWaste FillsReclaimed Fly Ash/Pond Ash Ponds

Functions Improve the bearing capacity of weak soils. Carry high shear stresses by acting as stiff

elements and hence increase the stability of embankments founded on soft ground.

Facilitate radial drainage (by acting as vertical drains) and dissipate rapidly the excess pore water pressure leading to acceleration of consolidation process and reduced post-construction settlements.

Mitigate the potential for liquefaction and damage by preventing build up of high pore pressure, providing a drainage path and increasing the strength and stiffness of the ground.

Due to high angle of internal friction and stiffness of stone column when compared to that of in-situ weak soil, majority of applied load is transferred to stone column.

As a result, less load is transferred to surrounding weak soil which leads to reduction in settlement.

Installation Patterns

Area of Influence = (√3/2)S2 Area of Influence = S2

De = 1.05S De = 1.13S

Time required for consolidation is directly proportional to square of the drainage path.

Load Carrying Mechanism Lateral earth pressure/radial confining stress against bulging from surrounding soil.

Surface resistance or frictional resistance developed between the column material and surrounding weak soil acting upwards within the critical length.

Passive resistance mobilized by column material.

Load carrying capacity of stone columns = 100 to 400 kN.

Note: End bearing is not considered in estimation of load carrying capacity because load carrying mechanism is local perimeter shear.

Estimation of Load Carrying CapacityAssumed that foundation loads are carried only

by the stone columns with no contribution from the intermediate ground.

Hughes & Withers (1974)where,

qa = allowable bearing capacity of stone column

Kp = coefficient of passive earth pressure = tan2(45+φ/2)

c = cohesion of soil

σr’ = average effective radial stress over a depth of ‘4d’ where ‘d’ is the diameter of the column

F.S. = factor of safety = 1.5 to 3.0

.S.F

)c4(Kq

'rp

a

Settlement ControlStone columns should extend through weak

soil to harder firm strata to control settlements.

Provision of stone columns does not reduce the entire consolidation settlement. The reduction depends on the spacing of stone columns (generally 2.0 to 3.0 m c/c over the site).

Maximum percentage reduction of settlement is 75%.

Drainage Function of Stone ColumnsLoad carrying capacity of stone columns is

generated by the top section of the column which extends to about 4 times the diameter of the stone column.

The length below 4d allows for radial drainage and acceleration of settlements.

To retain continuity of drainage path, it is necessary to provide a 150 mm thick drainage blanket on top of the stone columns.

Failure Mechanism (IS: 15284 Part 1 – 2003)

Installation Techniques

1. Rammed Stone Column Technique

2. Vibro-ReplacementWet Top-Feed Method

Dry Bottom-Feed Method

Rammed Stone Column by Cased Borehole Method (Datye and Nagaraju, 1975)

Vibro-Replacement

Extension Tube

Coupling

Air or WaterSupplyMotor

Excentric Weight

Tip

The Depth Vibrator Principle:

Depth Vibrator (Courtesy of Keller Group)

Top-Feed Vibrator

Bottom-Feed Vibrator

Air Chamber and Lock

Extension Tube

Flexible Coupling

Electric Motor

Stone Feeder Pipe

Eccentric Weight

Wet Top-Feed Method(Courtesy of Keller Group)

Dry Bottom-Feed Method with Leader Supported Rig (Courtesy of Keller Group)

Vibrocat (Courtesy of Keller Group)

Dry Bottom-Feed Crane Hung System (Courtesy of Keller Group)

Quality Control - Production Monitoring

On-board M4 Computer: Depth, Compaction, Length of Pull, Re-penetration, Increase in Power Consumption, Column Diameter

Stone Column after Installation

Post-Installation Quality Control (Load Test)

Applications in Civil EngineeringRail and Road EmbankmentsBridge Approaches and AbutmentsOffshore Bridge AbutmentsAirport Runways and TaxiwaysStorage Tanks (LNG, Crude Oil, LPG etc)Power Plants

Ipoh-Rawang Electrified Double Track(Malaysia)

Ipoh-Rawang Electrified Double Track(Malaysia)

KVMRT Kajang Maintenance Depot(Malaysia)

Bridge Approaches and Abutments

Putrajaya, Malaysia

Putrajaya Bridge Approach Embankment

RL +12.0

Top of Bridge Deck

Bored piles

Water Lev. RL +21.5

Pile cap

EEmmbbaannkkmmeenntt

R.C. Structure

Stone columns Stone columns

RL +32.0

Offshore Bridge Abutments

Johor Bahru, Malaysia

Airport Runways and Taxiways

Alor Setar Airport Project, Malaysia

Storage Tanks

Hazira LNG Terminal, Gujarat

Case History of Highway Embankment on Stone Column Treated Ground in Queensland,

Australia (Oh et al. 2007)

Subsoil Properties

Untreated SC @ 3m c/c

SC @ 2m c/c

520 mm495 mm

390 mm

Lateral Displacement Profiles

Conclusions Stone Columns are one of the most versatile

techniques for engineering the ground. They can be installed to improve a variety of

ground conditions through several variants of the technique such as rammed stone columns and vibro-replacement (wet top-feed and dry bottom-feed methods).

The in-situ ground is improved by reinforcement, densification and drainage functions performed by the stone columns.

From the case history, the embankment treated with stone columns spaced at 2 m centre to centre experienced the least settlement and lateral displacement when compared to the other cases.

Acknowledgments

Prof. M.R. MadhavProfessor Emeritus, JNTU

Visiting Professor, IIT Hyderabad

Dr. V.R. RajuManaging Director

Keller Asia

Earthquake Rose

Thank You