Basic Cone Parameters - Gregg Drilling & Testing, Inc ... · 12/14/2012 7 Proposed common SBT zones...

12/14/2012

1

Gregg Drilling & Testing, Inc.Site Investigation Experts

Cone Penetration TestingInterpretation -Soil Type

Dr. Peter K. RobertsonWebinar #2Dec., 2012

Robertson, 2012

Basic Cone ParametersSleeve Frictionfs = load/2 rh

Pore Pressureu2

Tip Resistanceqc = load/ r 2

Robertson, 2012

12/14/2012

2

Advantages of CPT

Advantages over traditional combination ofboring, sampling and other testing

• Fast• Continuous or near continuous data• Repeatable and reliable data• Cost savings

Robertson, 2012

Unequal End Area Effects on qc

qt = qc + u2(1-a)

a = 0.60 to 0.85

a = tip net area ratio~ An/Ac

In sands: qt = qc

In very soft clays:correction to qt is important

Cones should have high net area ratioa > 0.8

12/14/2012

3

CPTu InterpretationSoil Type– Soil behavior type (SBT) & stratigraphy

In-situ State– Relative density (Dr) or State Parameter ( and OCR

Strength– Peak friction angle (’) and undrained strength (su)

Stiffness/compressibility– Shear (Go), Young’s (E’) and 1-D constrained (M)

Consolidation/permeability– Coeff of consolidation (cv) and permeability (k)

Robertson, 2012

Role of CPT

CPT has three main applications:• Determine sub-surface stratigraphy and identifymaterials present (Soil Type – SBT)

• Estimate soil parameters• Provide results for direct geotechnical design

Primary role is soil profiling and can be supplementedby samples, other in-situ tests and laboratory testing

Robertson, 2012

12/14/2012

4

First CPT ‘soil classification’ chart

Begemann (1965) type mechanical cone

Increasing friction ratioRf = fs/qc

Soil Behaviour Type (SBT) ChartZone Soil behaviour type

1234

5678

9101112

Sensitive fine grainedOrganic materialClaySilty Clay to clay

Clayey silt to silty claySandy silt to clayey siltSilty sand to sandy siltSand to silty sand

SandGravelly sand to sandVery stiff fine grained*Sand to clayey sand*

*Overconsolidated or cemented

SBTRobertson& Campanella, 1986

Robertson, 2012

= 100 (fs/qt)

Sand

Clay

Note: 1 tsf ~ 0.1 MPa

(qt/pa)

10

100

1000

1

12/14/2012

5

CPT - Soil Behavior Type (SBT)Non-Normalized Classification Chart

Friction Ratio (%), Rf

1000

10

1

0 1 2 3 4 5 6 7 8

100

3

1

45

6

7

8

9

10 12

11

2

Cone

Resis

tanc

e(ba

r)q t

Robertson & Campanella, 1986

CPT SBT based on in-situ soil behavior(strength, stiffness,compressibility) - not

the same asclassification basedAtterberg Limits andgrain size carried outon disturbed samples

Robertson, 2012

Sand

Clay

Note: 1 bar ~ 1 tsf ~ 0.1 MPa

Soil Samples with CPT equipmentCPT Piston-Type Sampler• Single-Tube System

• 300mm long x 28mm diameter sample

Robertson, 2012

12/14/2012

6

CPT Data Presentation

Example CPTu Plot Robertson, 2012

Normalized SBTn Charts

Robertson, 1990

Normalized CPT Parameters

SAND

CLAY

Mixed Soils

12/14/2012

7

Proposed common SBT zones

SBT zoneRobertson et al (1986)

SBTn zoneRobertson (1990)

Proposed commonSBT description

1 1 Sensitive fine-grained2 2 Clay: organic soil3 3 Clays: clay to silty clay

4 & 5 4 Silt mixtures: clayey silt & silty clay6 & 7 5 Sand mixtures: silty sand to sandy silt8 6 Sands: clean sand to silty sand

9 & 10 7 Dense sand to gravelly sand12 8 Stiff sand to clayey sand*11 9 Stiff fine-grained*

Robertson, 2010Robertson, 2012

Why Friction Ratio (fs/qt)?

Normally to OC consolidated clay, friction ratio, Fr ~ constantF = 100(fs/’vo)

Fr = 0.1%

Fr = 10%

NC NC

OC OC

Qt = (qt-vo)/’vo Qt = (qt-vo)/’vo

Fr = 100[fs/(qt-vo)]

12/14/2012

8

Tumay Fuzzy-logic SBT

Appliesfuzzylogic toSBT

Robertson, 2012

Probability of Soil Type(not % grain size)

Schneider et al (2008) chart

Combines normalized cone resistance (Q) and excess pore pressure (u/’v)Good offshore – less effective on-shore, where saturation can not be assured

Robertson, 2012

(log scale) u2/’vou2/’vo

Qt = (qt-vo)/’vo

Example NC Clay

12/14/2012

9

Schneider et al (2008) chart

Variation of OCRand Bq on Schneideret al (2008) chart

Qt = (qt-vo)/’vo

Bq = u2/ (qt-vo)

u2/’vo = Qt Bq

Compare pore pressure charts

Schneider et al, 2008 Modified Robertson (1990)

NC clay NC clay

12/14/2012

10

Generalized CPT Soil Behaviour Type

A

B C

D

CPT Soil Behaviour

A: Drained-dilative

B: Drained-contractive

C: Undrained-dilative

D: Undrained-contractive

CPT SBT Index, Ic

Soil Behavior TypeIndex, Ic

Ic = [(3.47 – log Q)2 + (log F+1.22)2]0.5

Function primarily ofSoil Compressibility

Compressibility linked tosoil plasticity

Increasing compressibility

SANDS

CLAYS

Robertson, 2012

12/14/2012

11

SBT from CPTPlasticity Index asfunction of SBT Ic

Boundary betweensand-like and clay-like

soils is PI ~ 10

When Ic < 2.6095% samples NP84% with PI < 12%

Data from Cetin & Ozan, 2009

Non Plastic

Plastic

Ic = 2.60

Robertson, 2012

SBT from CPTFines content asfunction of SBT Ic

No uniquecorrelation

Also a function ofplasticity of fines

Coarse grained

Fine grainedData from Cetin & Ozan, 2009

Robertson, 2012

12/14/2012

12

SBT from CPT

Low Plastic High Plastic

Clay like

Sand like

Idriss & Boulanger, 2008

Transitionregion

Ic ~ 2.60

Robertson, 2012

SBT from CPT• Soil behavior influenced by fines content andplasticity of fine– e.g. soils with small amount of high plastic fines behavemore like a clay and soils with large amount of low plasticfines behave more like sand

• CPT Soil Behavior Type Index Ic capturescombined influence of fines content andplasticity of fines

Fines content alone can not capture correct soilresponse

Robertson, 2012

12/14/2012

13

CPT Normalization• Early normalization based on theory for clays

Qt = (qt – v) / 'v

• Recently normalization based on soil type,density and stress level

Qtn = [(qt – v)/pa] (pa/'v)n

Where:(qt – v)/pa = dimensionless net cone resistance,(pa/'v)n = stress normalization factorn = stress exponent that varies with soil type, density & stress levelpa = atmospheric pressure in same units as qt and v

Robertson, 2012

CPT NormalizationStrength envelope in sandysoils is curved, curvaturedepends on:

• Density• Stress level

CPT penetration resistance adirect measure of shearstrength

Stress exponent varies withdensity and stress level

Robertson, 2012

12/14/2012

14

CPT NormalizationQtn = [(qt – v)/pa] (pa/'vo)n

n = 0.381 (Ic) + 0.05 ('vo/pa) – 0.15

If stress normalization correct - no needfor additional stress level corrections

(e.g. Ks)

Robertson, 2012

Compare stress normalization

Robertson, 2012

after Cetin & Ozan, 2009

12/14/2012

15

Example CPT - UBC Fraser River

Clean Sand

NC Clay

Organic SILT

silty sand

Fraser River Delta, Vancouver, BC (UBC)Campanella & Robertson, 1983

Holocene-age deltaic deposit

Example CPT - UBC Fraser River

Clean Sand

NC Clay

Organic SILT

silty sand


Fraser River Delta, Vancouver, BC (UBC)Campanella & Robertson, 1983

12/14/2012

16

Example CPT – Amherst UMassConnecticut Valley Lacustrine Varved Clay

DeGroot & Lutenegger, 2003

Clay Crust

Varved Clay

Dissipation tests (not varves)Late Pleistocene-age lacustrine deposit

Example CPT – Amherst UMassConnecticut Valley Lacustrine Varved Clay

DeGroot & Lutenegger, 2003

Clay Crust

Varved Clay


12/14/2012

17

Example CPT – Venice LagoonVenice Lagoon, Treporti Test Site

Simonini et al, 2003

Samples

Sand

Silt

SandSiltSand

Interbedded Silt

Holocene/Pleistocene-age lagoon deposit

Example CPT – Venice LagoonVenice Lagoon, Treporti Test Site

Simonini et al, 2003

Samples

Sand

Silt

SandSiltSand

Interbedded Silt


12/14/2012

18

Example CPTMixed soil profile

San Francisco Bay area, USA

Soft Clay

Stiff Clay

Dense Sand

Holocene/Pleistocene-age deposit

Example CPTMixed soil profile

San Francisco Bay area, USA

Soft Clay

Stiff Clay

Dense Sand

12/14/2012

19

How deep can you push the CPT?Depends on:• amount of reactionpush force

• amount of rodfriction

With 15 cm2 cone(10cm2 push rods) and200 kN (20 tons)reaction – canpenetrate soil withSPT (N)60 > 100

Example CPT – Mine TailingsDeep Mine TailingsSouthwest, USA

Interbedded sand/silt tailings

Very young, hydraulically placed tailings

12/14/2012

20

Example CPT – Mine TailingsDeep Mine TailingsSouthwest, USA


Interbedded sand/silt tailings

Example CPT – Soft RockVery stiff soil – soft rockNewport Beach, CA, USA

Stiff Silt

Weak Sandstone/SiltstoneSPT (N)60 ~100

Pleistocene/Tertiary-age deposits

12/14/2012

21

Example CPT – Soft RockVery stiff soil – soft rockNewport Beach, CA, USA

Stiff Silt

Weak Sandstone/SiltstoneSPT (N)60 ~100


New SoftwareCPeT-IT

http://www.geologismiki.gr/

12/14/2012

22

Summary• CPT is a fast, reliable method to determine soilstratigraphy and soil type in a cost effectivemanner.

• CPT-based Soil Behaviour Type (SBT) chartsavailable to estimate soil type, based on eithertip-friction and/or tip-pore pressuremeasurements

• CPT equipment can be used to take smalldiameter push-in soil samples to verify soiltype

Basic Cone Parameters - Gregg Drilling & Testing, Inc ... · 12/14/2012 7 Proposed common SBT zones...

Documents

Transcript of Basic Cone Parameters - Gregg Drilling & Testing, Inc ... · 12/14/2012 7 Proposed common SBT zones...