CE 5101 Lecture 9 –CE 5101 Lecture 9 –Consolidation MonitoringConsolidation Monitoring

OCT 2010

Prof Harry Tan

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OutlineOutline

• Consolidation Monitoring Principles

• Preconsolidation Pressure

• Strength Increase

• Settlement Records– Hyperbolic Method– Asaoka Method

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Preloading and Surcharge Preloading and Surcharge The Problem: Slow and large settlements

Solution: Preloading reduces long-term settlements

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PVD and Surcharge PVD and Surcharge The Problem: Slow and large settlements

Solution: PVD and Surcharge accelerates settlements. This will reduce amount of residual settlements remaining after surcharge is removed

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How do we know if consolidation has been How do we know if consolidation has been completedcompleted

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• Measure Pc after surcharge is removed• Take undisturbed thin wall samples, do Oedometer test to determine Pc• If Pc > Po+Pf, then we have adequate preloading of the ground

How do we know if consolidation has been How do we know if consolidation has been completedcompleted

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• Alternatively, we can also measure the Undrained shear strength of soft clay after surcharge removal• Undrained shear strength can be measured in undisturbed triaxial samples with UU compression test to get cu value. It can also be measured insitu with filed vane shear test • Typical cu/Pc ratio is a function of clay drained friction angle and is about: 0.2 to 0.3 for Singapore Marine Clay depending on how cu is determined. For practical design we use cu/P = 0.25• So we obtain Cu/0.25 = Pc value of the soft clay after preloading

FV: Field Vane ShearTC: Triaxial compression Cu/P=0.3

TE: Triaxial Extension Cu/P=0.2

DSS: Direct Simple Shear Cu/P=0.25

FV:Field Vane

PM: Pressuremeter

Use Settlement RecordsUse Settlement Records

Most practical method in many cases

• Easy and cheap to measure

• Less prone to error

• Can monitor many locations of the site

• Easy to analyse by:– Hyperbolic Method– Asaoka Method

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Hyperbolic MethodHyperbolic MethodSee papers by Tan SA in 1993 to 1996• Basis of Method – Terzaghi Theory give U vs Tv plot for

1D compression with vertical flow only• The method was extended by TanSA to Barron Theory

and Carrilo Theory for application to Vertical Drains 1D compression under combined vertical and horizontal flow

• The method compares very well with the established Asaoka Method

• It has been validated with many case history data• It is now widely used in industry for determining when

desired consolidation is achieved

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Hyperbolic Method – Terzaghi TheoryHyperbolic Method – Terzaghi Theory

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• Replot using Tv/U vs Tv• Linear section between U=0.5 and U=0.9 and slope is 0.824• Equation of linear section is:

vv TU

T

• Radial lines through U50 and U90 give slopes of 1/0.5 and 1/0.9

• From linear plot; Ultimate primary settlement is obtained when Tv is infinity• When Tv is very large, equation reduces• So we get ultimate settlement from the inverse of slope

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UorU

Hyperbolic Method – Extend to Vertical DrainsHyperbolic Method – Extend to Vertical Drains

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Barron and Hansbo theory for PVD

Carrilo Theory for combined Uv and Uh

Hyperbolic Method – Extend to Vertical Drainsa are slopes in Theoretical Plots

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5.0

150 9.0

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Hyperbolic Method – Extend to Vertical DrainsHyperbolic Method – Extend to Vertical Drains

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For real field data, we can identify from the relationships, the slopes of:

9050 ,, SSSi90

90

50

50

SSSi

Hyperbolic Method – What is a for PVD

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Typical real problems, a is between 0.74 and 0.76, use 0.75 as a very good estimate for many cases

Hyperbolic Method – Ska Edeby Data

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• Area I Sand Drains of 180mm diam with triangular spacing of 2.2, 1.5 and 0.9m• Area II Sand Drains of 180mm diam with triangular spacing of 1.5m• Area IV NO Sand Drains

Hyperbolic Method – Ska Edeby Data

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Hyperbolic Method – Ska Edeby Area I

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Hyperbolic Method – Ska Edeby Area IV

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Hyperbolic Method – Advantage of Method

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Hyperbolic Method – Advantage of Method

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Hyperbolic Method – Validation

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Hyperbolic Method – Validation

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Hyperbolic Method – Validation

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Hyperbolic Method – Validation

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• 1/Si over-predicts primary settlements, but may be OK when you include secondary compression

Asaoka Method (1978)

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Asaoka Method (1978)

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Hyperbolic vs Asaoka Method (Tan 1996)

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Hyperbolic vs Asaoka Method (Tan 1996)

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Procedure to Use Hyperbolic Method

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Van Oord – Lekir Coast Malacca Reclamation

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Van Oord – Lekir Coast Malacca Reclamation

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