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