Application of computed tomography to investigate strain...

Application of computedtomography to investigate

strain fields caused by conepenetration in sands

27 February, 2004

Jeroen van Nes

• Drilling• Sampling and laboratory testing• Penetration testing• Geophysics• etc.

CPT : Relatively cheap, fast, easy and reliable.

CPT field data geotechnical soil parameters

Planning Construction

Site investigation

Intro

Application of computed tomography to investigate strain fields caused by cone penetration in sands

Intro


OBJECTIVEto investigate if the relationships betweencone parameters and geotechnicalparameters can be improved by studyingstrain fields caused by cone penetration insands.

Presentation overview

• Background information

• Tests Descriptions

• Test results

• Conclusions

• Applications & Recommendations

Introduction ConclusionsTestresults

Testsdescriptions

Backgroundinformation


Applications &Recommendation

CPTApplication of computed tomography to investigate strain fields caused by cone penetration in sands

The CPT probe• long tube• cone tip.

Pushed into the ground

Resistance/Friction

Conclusions (kind of soil & Some engineering properties)

Key parameters: Relative density, effective stress level and compressibility


Testsdescriptions



Deformation Types

Shearing

– dilation

– compaction

High compressive stresses

– grain crushing/compaction

• Not-influenced area



Testsdescriptions



•Medical background

Difference in X-Ray intensity

Attenuation of X-ray beam

Attenuation value of every pixel

Absolute density


CT scanner (1)


Testsdescriptions



CT scanner (2)3D representation

MOVIE



Testsdescriptions



Crossections

CT scanner (3)Application of computed tomography to investigate strain fields caused by cone penetration in sands


Testsdescriptions



Sand sample (1)Application of computed tomography to investigate strain fields caused by cone penetration in sands

Sand: Baskarpsand

• Fine sand• Well sorted• No contamination• >80 % SiO2 (Quartzitic)


Testsdescriptions



A

B

D

C

H

Preparation: Pluviation

• Homogenous density• Different densities

1,481,501,521,541,561,581,601,621,641,661,681,70

0 20 40 60 80 100 120height [cm]

dens

ity [g

/cm

3]

set1set2Lineair (all)



Sand sample (2)

Testsdescriptions


Calibration chamber

Scale effects• Boundary effects (cone size diameter - sand sample diameter)• Grain size effects (cone size diameter - mean grain size diameter)

Probe• Standard: diameter 35.7 mm• Miniature cone: diameter 6 mm

Introduction




Testsdescriptions


CPT-tests (1)

CPT-tests with different

• Horizontal Swelling

• Penetration Resistance




Testsdescriptions


02468

101214161820

0 20 40 60 80 100 120Penetration depth (mm)

Res

ista

nce

(MP

a)

DenseModerate denseLoose

Different sanddensities

High repetability ofmeasured resistance



CPT-tests (2)

Testsdescriptions

Scanned CPT testsTest 1: Reference testTest 2: Lower confining pressureTest 3: Lower densityTest 4: Rigid boundaries


Test Density [g/cm3] Confining pressure [KPa] BoundryTest 1 1,61 78-82 Flexible membraneTest 2 1,61 38-42 Flexible membraneTest 3 1,54 78-42 Flexible membraneTest 4 1,61 78-82 Rigid alumium mould

Test at multiple stages



Testsdescriptions


Density-Attenuation relation

Estimation sand density

Relation is determined forBaskarp sand, 100 mm.

y = 9 7 3 ,4 1 x - 7 7 7 ,6 5R2 = 0 ,9 9 1 7

6 5 0 ,0

7 0 0 ,0

7 5 0 ,0

8 0 0 ,0

8 5 0 ,0

9 0 0 ,0

1 ,4 5 1 ,5 1 ,5 5 1 ,6 1 ,6 5 1 ,7

S a n d d e n s ity [g /c m3]

Hou

nsfi

eld

unit

val

ues

[H

M eas urem entpoints

Sand grain size < resolution of theCT-scanner

Sand sample scanned with multipledensities




Testsdescriptions


Processing of the data

Processing programs• Amira:

• Q-win




Testsdescriptions


Absolute density

Homogenity• marker lines• Density variation

Loosening

Densification


Introduction ConclusionsTest

resultsTests

descriptionsApplications &

RecommendationBackgroundinformation

• 100 mm cone penetration


Density-changes (1)

Introduction ConclusionsTestsdescriptions


Testresults

Reference test Lower confiningpressure

Lower sanddensity

Rigidboundaries


• 6 mm cone penetration

Density-changes (2)Application of computed tomography to investigate strain fields caused by cone penetration in sands



Testresults

Reference test Lower confiningpressure

Lower sanddensity

Rigidboundaries


Measuring the sizes of the dilating areasApplication of computed tomography to investigate strain fields caused by cone penetration in sands


Testsdescriptions

Theoreticalbackground




Testresults

6 mm under the cone tip

-0,3-0,2

-0,10

0,1

0,20,3

-60 -40 -20 0 20 40 60

Horizontal distance (mm)

Den

sity

cha

nge

(g/c

m3)1,4

1,5

1,6

1,7

1,8

1,9

-60 -40 -20 0 20 40 60


Den

sity

[g/c

m3]

NOPEN PEN

At the cone tip

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

-60 -40 -20 0 20 40 60

Horizontal distance (mm)Den

sity

cha

nge

(g/c

m3)

1,4

1,5

1,6

1,7

1,8

1,9

-60 -40 -20 0 20 40 60Horizontal distance (mm)

Den

sity

[g/c

m3] NOPEN PEN

Above cone

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

-60 -10 40

Horizontal distance (mm)Den

sity

cha

nge

(g/c

m3)1,4

1,5

1,6

1,7

1,8

1,9

-60 -40 -20 0 20 40 60


Den

sity

[g/c

m3]

NOPEN PEN

1,1

1,3

1,5

1,7

1,9

0 20 40 60 80 100 120

Vertical distance (mm)

Den

sity

(g/c

m3)

Pen No Pen


Dilating areas

• Inside: dilating areas

• Outside: No changes & Compression

III III

IV


IV



Testresults


• Sand sample created by pluviation are not perfect homogenous.

• CT is a suitable technique to investigate the deformations in sand by a CPT test

• The degree of dilation and the extend of the dilating zones are dependent on thetest conditions

– Large difference: change of density.

– Small difference: change of boundaries & confining pressure.

Conclusions (I)



Testsdescriptions



Conclusions (II)

• 3 Different deformation zones are believed to be created by the conepenetration test


A B C

Introduction Testresults

Testsdescriptions

Applications &RecommendationConclusionsBackground

information

Conclusions (III)

No failure structures– Large failure plains– Shear bands

Difference• Large failure structures• Defined zones


Introduction Testresults

Testsdescriptions

Applications &RecommendationConclusionsBackground

information

• Better understanding of the CPT test

• Validation models– DEM (Distinct Element Models)– Cavity expansion theory

• Validation of other tests– pressuremeter test

Applications



Testsdescriptions



Recommendations

Test setup• Loading system around CT scanner.• a smaller cone.• a larger calibration chamber.

Processing• Replace interpretation by a computer program


Testsdescriptions



QuestionsApplication of computed tomography to investigate strain fields caused by cone penetration in sands


Testsdescriptions



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