10/1/2012

1

Gregg Drilling & Testing, Inc.

Site Investigation Experts

Introduction toCone Penetration Testing

Peter K. Robertson

Webinar

2012

Robertson, 2012

History of CPT

• First developed in 1930’s as mechanical cone

• Electric cones developed in 1960’s

• Primary device for off-shore investigations since1970’s

• Major advancements since 1970:

– Pore pressure measurements

– More reliable load cells & electronics

– Addition of seismic for shear wave velocity

– Additional sensors for environmental applications

– Significant increase in documented case histories

Robertson, 2012

10/1/2012

2

Basic Cone Parameters

Sleeve Frictionfs = load/2rh

Pore Pressureu2

Tip Resistanceqc = load/ r

2

Robertson, 2012

Role of CPT

CPT has three main applications:

• Determine sub-surface stratigraphy and identifymaterials present,

• Estimate soil parameters

• Provide results for direct geotechnical design

Primary role is soil profiling and can be supplemented

by samples, other in-situ tests and laboratory testing

Robertson, 2012

10/1/2012

3

GOOD Precedent & local experience POOR

SIMPLE Design objectives COMPLEX

LOW Level of geotechnical risk HIGH

LOW Potential for cost savings HIGH

Traditional Methods Advanced Methods

What level of sophistication isappropriate for site investigation

& analyses?

Simplified Complex

Robertson, 2012

Robertson, 2012

10/1/2012

4

Advantages of CPT

Advantages over traditional combination ofboring, sampling and other testing

• Fast (2 cm/sec = 1.2m/min ~4 ft/min)

• Continuous or near continuous data

• Repeatable and reliable data

• Cost savings

Robertson, 2012

UDtube

CasedBoreholes

SPT: N60

VST: su, St

CHT:Vs, Vp

SOFTCLAY

FIRMSAND

CONVENTIONAL DRILLING& SAMPLING

DIRECT-PUSHTECHNOLOGY

DropHammer

SCPTùqt

fs

u2

t50

Vs

Oscilloscope

PMT: E’Packer: kvh

Lab

old new After Mayne, 2010

10/1/2012

5

Discrete CPT Soil Sampling

CPT (Piston-Type) Sampler

• Single-Tube System

• 30cm (12”) long by 25mm (1 ”) diameter

Robertson, 2012

Example CPT Trucks/track Mayne, 2010

10/1/2012

6

Special CPT VehiclesAfter Mayne, 2010

CPT with a Drill Rig

Robertson, 2012

10/1/2012

7

Portable CPTRamset Limited Access

Remote Locations

Robertson, 2012

Safety

• Improved safety using push-in methods

– No hammer or rotating parts

– Similar safety precautions compared to direct pushequipment (pinch points, clamps)

• No cuttings for disposal

– Significant cost savings

– Reduced contact with possible contamination

• Lower visibility and public exposure withenclosed trucks

Robertson, 2012

10/1/2012

8

ConePenetrometer

Sizes

2 cm2

10 cm2

15 cm2

40 cm2

ASTM Standard

Robertson, 2012

CPT Sensors

Since development of electric cones - many new sensorsadded:

• Pore pressure (u)

• Inclination (i)

• Seismic (Vs, Vp)

• Vision (camera)

• Geo-environmental sensors

– ph, electrical, fluorescence (LIF & UVIF), manyothers……...

Robertson, 2012

10/1/2012

9

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

Robertson, 2012

CPTu InterpretationSoil Type

– Soil behavior type (SBT)

In-situ State

– Relative density (Dr) or State Parameter (y) and OCR

Strength

– Peak friction angle (f’) 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

10/1/2012

10

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

4

5

6

7

8

9

10 12

11

2

Con

eR

esis

tanc

e(b

ar)q

t

CPT SBT basedon in-situ soil

behavior - not thesame as

classificationbased on

Atterberg Limitsand grain sizecarried out on

disturbedsamples

After Robertson & Campanella, 1986

SANDS

CLAYS

MIXED SOILS

Robertson, 2012

CPT Data Presentation

Example CPTu Plot Robertson, 2012

10/1/2012

11

CPT- Normalized SBT Chartq

-t

vo

s

s' v

o

0.11

100

10

1000

1 10

Normalized Friction Ratio

1

23

4

5

6

7 8

9j '

Normalized Classification Chart

fsx 100%

q-t vos

Nor

mal

ized

Con

eR

esis

tanc

e

Zone Normailzed Soil Behavior Type

123456789

sensitive fine grainedorganic materialclay to silty clay

clayey silt to silty claysilty sand to sandy silt

clean sands to silty sandsgravelly sand to sand

very stiff sand to clayey sandvery stiff fine grained

CLAYSUndrained

SANDSDrained

MIXED SOILSPartially drained

After Robertson, 1990 Robertson, 2012

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 &

amount/type of fines

Increasing compressibility

SANDS

CLAYS

Robertson, 2012

10/1/2012

12

RepeatabilityRobertson, 2012

Theoretical solutions for CPT• Most widely used theories:

– Bearing capacity methods (BCM)

– Cavity expansion methods (CEM)

– Strain path methods (SPM)

– Finite element methods (FEM)

– Discrete element methods (DEM)

• Combinations:

– SPM-FEM (e.g. Teh & Houslby, 1991)

– CEM-SPM (e.g. Yu & Whittle, 1999)

– CEM-FEM (e.g. Abu-Farsakh et al., 2003)

– CEM-BCM (e.g. Salgado et al., 1997)

Robertson, 2012

10/1/2012

13

Theory for CPT

• Challenges:

– Major assumptions needed for:

• Geometry & boundary conditions

• Soil behavior

• Drainage conditions

• Real soil behavior very complex

• Semi-empirical correlations still dominate, butsupported by theory

Robertson, 2012

Schematic of soil loading around cone

Generalized stress-strain relationship

Robertson, 2012

10/1/2012

14

Transition zoneCPT data in

‘transition’ when coneis moving from one soil

type to another whenthere is significantdifference in soilstiffness/strength

CPT data withintransition zone will be

misinterpreted

In interlayered depositsthis can result in

excessive conservatismAhmadi & Robertson, 2005

Robertson, 2012

Perceived applicability of CPT forDeriving Soil Parameters

Initial stateparameter

StrengthParameters

DeformationCharacteristics*

FlowCharact.

SoilType

γ/Dr ψ Ko OCR St su Φ’ E M Go k ch

Clay 3-4 2 1-2 2-3 1-2 4 2-3 2-3 2-3 2-3 2-3

Sand 2-3 2-3 5 4-5 2-3 2-3 2-3 2-3 3 3-4

Applicability rating: 1 high reliability, 2 high to moderate, 3 moderate, 4 moderate to low,5 low reliability.

* Improved when using SCPT

Robertson, 2012

10/1/2012

15

Stress History: OCR

Wroth (1984), Mayne (1991) and othersproposed theoretical solutions (based on cavityexpansion & critical state soil mechanics):

σ’p = f(qt - σvo)* OCR = f [(qt - σvo)/ σ’vo]*

σ’p = f(Du) OCR = f [Du/(qt - σvo)]

σ’p = f(qt –u2) OCR = f [(qt –u2)/ σ’vo]

* Most CommonRobertson, 2012

OCR = 0.33 Qt

(When OCR < 4)

Qt = (qt - σvo)/ σ’vo

Alternate based onhigh quality block

samples:(OCR < 10 & St < 15)

OCR = 0.25 (Qt)1.25

Correlation between Qt and OCR

(Kulhawy & Mayne, 1990)

Robertson, 2012

10/1/2012

16

Strength Parameters - Clay

Undrained strength ratio as afunction of direction of loading

Jamiolkowski et al., 1985 & Ladd, 1991

Robertson, 2012

su = qt – σvo

Nkt 10 < Nkt < 16

Nkt With sensitivity

Nkt With PI & OCR

For soft clays (based on excess pore pressure, Δu):

su = Δu = u – uo

NΔu NΔu

7 < NΔu < 10

Undrained Shear Strength, su

Robertson, 2012

10/1/2012

17

Undrained shear strength, su

CSSM & Empirical observations (Ladd, 1991):

(su/s’vo)ave = 0.22 (OCR)0.8

OCR = 0.25 (Qt)1.25

Combined: (su/s’vo)ave = Qt/14

Hence, Nkt ~ 14

Robertson, 2012

Undrained Shear Strength - CPT

Recent experience from high quality samples show:(Low, 2009)

Cone Factor, Nkt

Average undrained shear strength 11.5 to 15.5su,ave = 1/3 (suTC + suTE + suSS)

Mean 14

Values will vary somewhat with plasticity & sensitivity of claySwedish experience suggests:

Nkt = (13.4 + 6.65 wL)

Robertson, 2012

10/1/2012

18

Estimation ofGround Water

Table from CPTDissipation Tests

Robertson, 2012

Example pore pressure dissipation

Piezo-Dissipations at Evergreen, North Carolina

0

100

200

300

400

500

600

700

800

900

1000

0.01 0.1 1 10 100

Time (minutes)

Me

asu

red

u2

(kP

a)

Dissipation Record at 4.2 m

Groundwater Table at 0.4 m

u0 = (4.2 - 0.4m)*9.8 kN/m3 = 37 kPa

at 50% consolidation:

u = ½(829 + 37) = 433 kPa

t50 = 7 minutes

u2 during CPTu

Extrapolation

ch = T50 · r2

t50

Where:T50 is the

theoretical timefactor, t50 is themeasure time,

and r is theradius of the

probeAfter Mayne, 2010

Robertson, 2012

10/1/2012

19

Pore pressure dissipation in stiff clay

Depth = 8.47 m

50

100

150

0.1 1 10 100 1000

Time (minutes)

Meas

ure

du

2(k

Pa

) Measured u2

Hydrostatic u0

Pred CE-MCC

Dilatory Field Data

Fitted

Analytical

Solution

After Cruz & Mayne 2006)Robertson, 2012

Laboratory ch values and CPTu results

After Robertson et al., 1992

Theoretical solutions

0.001

0.01

0.1

1

10

0.001 0.01 0.1 1 10

ch from Piezocone Dissipation (cm2/min)

Me

as

ure

dL

ab

cv

(cm

/2/m

in)

Amherst Crust

Brent Cross

Cowden

Madingley

Raquette River

St. Lawrence Seaway

Strong Pit

Taranto

Bothkennar Soft Clay

Canon's Park

Drammen soft clay

McDonald's Farm soft clay

Onsoy soft clay

Porto Tolle soft clay

Rio de Janeiro soft clay

Saint Alban soft clay

1:1 Line

cvh = coefficient of consolidation

Robertson, 2012

10/1/2012

20

Permeability from CPTBased on theoryvia dissipation

test, t50

kh = (ch gw)/M

where:M is the 1-D constrained

modulusgw is the unit weight of

water, in compatible units.M can be estimated from

Qtn

Increasing M

Parez & Fauriel, 1988

Undrained

50 kPa

100 kPa

Robertson, 2012

Flow Characteristics from CPTU

• Uncertainties– Initial distribution of u (OCR > 4)

– Soil non-homogeneity (stratigraphy)

– Soil macrofabric

– Influence of cv

– Filter element clogging/smearing

• Very useful to evaluate

Approximate flow characteristics for finegrained soils

Robertson, 2012

10/1/2012

21

Seismic CPT

• >25 years experience (1983)

• Simple, reliable, and inexpensive

• Direct measure of soil stiffness

– Small strain value, Go = ρ·Vs2

• Typically 1 meter intervals

• Combines qc and Vs profile in same soil

Robertson, 2012

SCPT Equipment and Procedures

After Rice, 1985

Cone Penetrometer Shear Wave Traces

DT DD

DD

DTVs=

Robertson, 2012

10/1/2012

22

Seismic CPTRobertson, 2012

SCPT

• Shear wave velocity a useful fundamentalparameter

• SCPT very useful since it provides both CPT dataand Vs in one profile

• Potential to evaluate ‘unusual’ soils

• Settlement calculations based on Vs

Robertson, 2012

10/1/2012

23

In-situ Testing and GeotechnicalDesign

DIRECT METHODS INDIRECT METHODS

In-situ Test Results

Soil Model

Solution of Complex BVP

Design Parameters

Geotechnical Design

In-situ Test Results

Geotechnical Design

Pre

vio

us

Pe

rfo

rma

nce

Of

Co

nstr

uctio

n

Robertson, 2012

Perceived Applicability

PileDesign

BearingCapacity

Settlement* CompactionControl

Lique-faction

Sand 1-2 1-2 2-3 1-2 1-2

Clay 1-2 1-2 3-4 3-4 2-3

IntermediateSoils

1-2 2-3 3-4 2-3 2-3

Reliability rating: 1 = High, 2 = High to Moderate, 3 = Moderate,4 = Moderate to Low, 5 = Low

* Higher when using SCPT

Robertson, 2012

10/1/2012

24

Summary

• CPT can be a fast, reliable and cost effectivemeans to evaluate soil profile, geotechnicalparameters, groundwater conditions andpreliminary geotechnical design.

• Suitable for a wide range of soils, except fordense gravels and hard rock.

Robertson, 2012

Software Development

• PC based data acquisition systems

• Digital data

• Real-time interpretation

• Cell-phone for data transmission

• Color presentation

– Soil profile

– Interpretation parameters

• Interpretation software (e.g. CPeT-IT)

Robertson, 2012

10/1/2012

25

Example CPTInterpretation

Software

CPeT-IThttp://www.geologismiki.gr/

Robertson, 2012

Example Plots

Robertson, 2012

10/1/2012

26

Normalized plots

Robertson, 2012

SBT charts

Non-normalized Normalized

Updated Robertson 2010Robertson, 2012

10/1/2012

27

Estimated parameters (1)

Robertson, 2012

Estimated parameters (2)

Robertson, 2012

10/1/2012

28

Questions?

Robertson, 2012