Application of computed tomography to investigate strain...

transcript

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

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

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

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

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]

ity [g

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

101214161820

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

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]

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

0,20,3

-60 -40 -20 0 20 40 60

Horizontal distance (mm)

m3)1,4

-60 -40 -20 0 20 40 60

NOPEN PEN

At the cone tip

-60 -40 -20 0 20 40 60

Horizontal distance (mm)Den

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

m3] NOPEN PEN

Above cone

-60 -10 40

Horizontal distance (mm)Den

m3)1,4

-60 -40 -20 0 20 40 60

NOPEN PEN

0 20 40 60 80 100 120

Vertical distance (mm)

Pen No Pen

Dilating areas

• Inside: dilating areas

• Outside: No changes & Compression

III III

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

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

Application of computed tomography to investigate strain...

Documents