Ground Improvement by Vibro Stone Column

Desh Raj Sonyok

Saheem Ahmed

CE 596 Soil Improvement

Term paper – Fall 2006

Civil Engineering Department

New Mexico State University

Importance of Vibro Replacement

• Reduce settlement of soil

• Improve bearing capacity

• Improve shear strength of soil

• Liquefaction mitigation

Soils Suitable for the Vibro Replacement

Construction Procedures of the Vibro Replacement Method

Analysis on Vibro Replacement Stone Column

• Few mathematical analysis available for the vibro replacement stone column

• Priebe (1995) proposed an equation for the improvement factor as a function of area replacement ratio. He further proposed a design chart for the improvement factor which is much more common in practice.

Preibe’s (1995) Equation

• Improvement factor equation (Preibe, '1995)

• Settlement after the improvement

( )

−

−−+= 11..4

5.10 AAK

AA

A

An

cac

cc

( )

−

−−+

=∞

114

51

AA.K.

AA

A

AD

dpS

cac

ccs

( )2452cac tanK φ−°=

Priebe (1995) Design Chart

Analysis and Comparison

• Stone Column to Prevent Settlement in Ireland

Measured Settlements

Soil Parameters and SettlementBefore After Befor

eAfte

r

Depth (m) Soil type N value

E (MPa)

N value

E (MPa)

s (mm) s (mm)

Recorded s (mm)

Estimated s (mm) *

0.0-3.5 SP 10 10 18 18 31 18 40 - 70 167

3.5-6.0 SP 2 2 20 20 120 12

6.0-7.0 ML/CL

4 4 6 6 17 11

7.0-8.5 GW 6 12 10 20 12 7

8.5-11.0 GW 30 60 30 60 5 5

11.0-14.0 CI 27 38 27 38 6 6

14.0-16.0 GW 35 70 35 70 3 3

Stone Column to Prevent Settlement in South Africa

Settlement Measurements

Calculate Settlement

Structural load: 500 kPa – 700 kPaGrid type: triangularAverage length: 8 m –14 mDiameter: 740 mm

Zone Spacing (m) A (m2) A/AC Settlement (mm)

1 1.5 1.94 6.5 90

Settlement for the Vibro Stone Column Equation– Proposed Method

cs

ss AA

Aa

+=

0s e1

ea

+∆=

c

snσσ

=

( )scss a.a. −σ+σ=σ 1

( ) σµ=σ

−+

=σ .a1n1

1c

sc

( ) ( )[ ]

σ∆µ+−+−

=0

c0

0ss

c

P

.Plog

ea1a1

HCS

σ∆µ=∆

σ∆µ=∆

σ∆µ=ε

..H.bH

..V.bV

..bd

ss

ss

ssv

Average stress over the unit cell

Stress concentration on the surrounding soil

Settlement equation of the surrounding soil

If compressibility of the stone column is bs,

Settlement of the stone column

• The proposed equation gives an estimation of the settlement of the surrounding soil and the column alone. The final settlement of the site will be definitely with in the limit proposed above.

Limitation of the equation

• The proposed equation is applicable to the homogeneous soil with clay to be improved. The equation to be modified for the layered soils

• This method assumes constant compressibility index before and after treatment

Conclusions• The primary purpose of this method is to improve the

settlement characteristics of the soft soil, increase load bearing capacity, and shear strength

• Estimation of settlement for the design of vibro stone column proposed by Priebe is found to be more conservative

• A rational method of estimating settlement in a homogenous soil is proposed based on the basic concept of composite material and soil mechanics.

• Result of this preliminary work is yet to validate numerically and in the field.

• Detailed research in future is necessary to check its validity and applicability.

Thank You