basic cone parameters - basecampsites.com
TRANSCRIPT
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Gregg Drilling & Testing, Inc.
Site Investigation Experts
Cone Penetration TestingInterpretation of Soil
Parameters -Fine-grained soils
Dr. Peter K. RobertsonWebinar #3
Jan. 2013
Robertson, 2012
Basic Cone Parameters
Sleeve Frictionfs = load/2rh
Pore Pressureu2
Tip Resistanceqc = load/ r
2
Robertson, 2012
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Soil Parameters
What you can get from the CPT in
fine-grained soils
such as: clay, silty clay, clayey silt,silt?
Robertson, 2012
Perceived applicability of CPT forDeriving Soil Parameters
Initial stateparameter
StrengthParameters
DeformationCharacteristics*
FlowCharact.
SoilType
γ ψ Ko OCR St su Φ’ E,G M Go k ch
Fine-grained
2-3 2-3 1 2-3 1-2 4 2-3 2-3 2-3 2-3 2-3
Coarse-grained
2-3 2 4-5 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
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Soil Behaviour Type (SBT)Q
t=
(qt-
svo
)/s
’ vo
Fr = 100[fs/(qt-svo)]
Fine-grained
soils
Coarse -grained
soils
Robertson (1990)
Fine-grained soilsessentially plot in
SBT zones 1, 2, 3, 4and 9 on the
normalized SBT chartby Robertson (1990)
Approx. Ic > 2.60
Robertson, 2012
Generalized CPT Soil Behaviour Type
A
B C
D
CPT Soil Behaviour
A: Drained-dilative
B: Drained-contractive
C: Undrained-dilative
D: Undrained-contractive
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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 (Teh & Houslby, 1991)
– CEM-SPM (Yu & Whittle, 1999)
– CEM-FEM (Abu-Farsakh et al., 2003)
– CEM-BCM (Salgado et al., 1997)Robertson, 2012
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
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Factors affecting CPT interpretation
• Geology & geologic history
– In-situ stresses (importance of horizontal stresses)
– Soil compressibility (mineralogy)
– Cementation
– Particle size (e.g. gravel size)
– Stratigraphy/layering
CPT should be interpreted within ageologic context
Geologic Context
• Most semi-empirical correlations are based oncase histories in ‘well behaved’ soils
– Mostly normally to lightly overconsolidated
– Relatively young (Holocene to Pleistocene-age)
– Silica based (quartz)
– Sedimentary soils
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Schematic of soil loading around cone
Robertson, 2012
Generalized stress-strain relationship
Tip resistance, qt,controlled more by
peak strength
Sleeve friction, fs,controlled more byremolded strength
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
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Theoretical solution for OCR
Hybrid SCE-CSSM theory (Mayne, 1991)
OCR = σ’p/σ’vo
OCR = 2[ (2/M Qt)/(4/3(lnIr +1) +2.57)]1/L
Assume: L = 0.8, Ir ~ 100 to 300, M = 1.1
OCR = 0.25 (Qt)1.25
Where: Qt = (qt – svo)/s’vo
For OCR < 4 & St < 15: OCR ~ 0.33 Qt
Robertson, 2012
Data fromMayne, 2006
OCR = 1/3 Qt1
OCR = 0.25 (Qt)1.25
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Importance ofSample Quality
High quality samples tend toproduce higher OCR, strength
and sensitivity values
Lunne et al, 2006
Robertson, 2012
Updated OCR Correlation
Robertson, 2012
0.26(Qt)1.2
0.46(Qt)1.1
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Strength Parameters - Clay
Undrained strength ratio as afunction of direction of loading
Jamiolkowski et al., 1985 & Ladd, 1991
Robertson, 2012
Normally consolidated
su = qt – σvo
Nkt 10 < Nkt < 20
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 (cu)
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Undrained Shear Strength - CPT
After The & Houslby, 1987
su = qt – σvo
Nkt
Nkt ~ 10 to 18
su based on FVT
Robertson, 2012
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
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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
fssu(remolded)
Sensitivity, St
St = su (peak) ~ 7/ Fr
su(remolded)
su(peak) = qt – σvo
Nkt
Soil Sensitivity from CPT
su(r) /s’vo = fs/s’vo = Fr*Qt /100
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Examples of su(r) from CPT fs
New OrleansMayne, 2008
Burswood, PerthLow, 2009
Scoggins DamFarrar et al , 2008
Contours of OCR & Sensitivity (St)
OCR
St
OCR = 5
OCR = 2
Qtn controlled by OCR(peak shear strength)
Fr controlled by St
(remolded shearstrength)
su(r) /s’vo = fs/s’vo
su(r) /s’vo = Fr*Qt /100
OCR = 25
OCR = 1
Robertson, 2012
OCR = 10
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Schneider et al (2008) chartVariation of OCR
and Bq on Schneideret al (2008) chart
Qt = (qt-svo)/s’vo
Bq = Du2/ (qt-svo)Du2/s’vo = Qt Bq
Normalized porepressure (either Bq
or Du2/s’vo ) noteffective to estimate
OCR
Example – Bothkennar, UKHight et al., 2003
Holocene-age, estuarine clayey silt(NC to LOC)
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Example – Onsoy, NorwayLunne et al., 2003
Holocene-age, marine clay(NC to LOC)
Example – Cowden, UKPowell & Butcher, 2003
Pleistocene-age, glacial stony clay till(HOC)
Su(PLT)
Ko = 0.5(OCR)0.5
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Estimation of 1-D ConstrainedModulus, (M)
M = 1/ mv = dsv / de (in units of stress)
Cc = 2.3 (1+e0) s'vo / M
Where mv = equivalent oedometer coefficient of compressibility.dsv = change in vertical stressde = change in vertical straine0 = initial void ratioCc = Compression index
Robertson, 2012
Constrained
Modulus, M(Mayne, 2006)
M = aM (qt – sv)
1 < aM < 20
Depending on soiltype and stresshistory (OCR)
M
M = 1
M = 20
Robertson, 2012
M
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1-D Constrained modulus, MM = aM (qt - svo)
when Ic > 2.2 use:
aM = Qtn when Qtn < 14
aM = 14 when Qtn > 14
when Ic < 2.2 use:
aM = 0.02 [10 (0.55Ic + 1.68)]
Note: when Ic > 2.2 (clays)Cc = 2.3(1+eo)/Km
after Robertson, 2009
Robertson, 2012
Comparison between Lab and CPT M
M = aM (qt - svo)
aM = Qtn when Qtn < 14aM = 14 when Qtn > 14
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Venice lagoon Load Test
Circular tank106 kPa load~60 months
Most vol.strain in
softer siltyunits
SBT Method for PermeabilityEstimated permeability based on SBT
SBTn SBT Permeability (m/sec) SBT Ic
1 Sensitive fine-grained 3x10-10 to 3x 10-8 NA2 Organic soils - clay 1x10-10 to 1x 10-8 Ic > 3.603 Clay 1x10-10 to 1x 10-9 2.95 < Ic < 3.604 Silt mixtures 3x10-9 to 1x 10-7 2.60 < Ic < 2.955 Sand mixtures 1x10-7 to 1x 10-5 2.05 < Ic < 2.606 Sand 1x10-5 to 1x 10-3 1.31 < Ic < 2.057 Sand to gravelly sand 1x 10-3 to 1 Ic < 1.318 Very dense/stiff soil* 1x 10-8 to 1x10-3 NA9 Very stiff fine-grained 1x 10-9 to 1x10-7 NA
After Lunne et al, 1997Robertson, 2012
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Update on k from CPT via SBT Ic
Robertson, 2012
SBT Ic
k, m/sRange suggested byLunne et al, 1997
CPTu Dissipation Tests
Robertson, 2012
uo = Hwater = 62.37 feet
Test depth, Dcone = 90.22 feet
Depth to piezometric surface, Dwater = 27.9 feet
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Equilibrium Piezometric Pressure
1 psi = 2.306 ft of water
u0u0
u0u0
t100 ~ 33 mins
t100 ~ 13 mins
t100 ~ 4 mins
t100 ~ 10 mins
Flow Characteristics from CPTU
Theory:
• Simple uncoupled solutions provide accuratepredictions
• Dissipation controlled by horizontal ch
• Initial distribution of excess pore pressures havemajor influence on process
• Consolidation predominantly in recompression modeespecially for times less than 50%
• Rigidity index (IR = G/su) important
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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
Average laboratory ch values and CPTu results
After Robertson et al., 1992
undrainedpenetration
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Rate effects - drainage
Dimensionless Velocity, V = v D / cv
(v = penetration rate; D = cone diameter; cv = coefficient of consolidation)
Undrained when V > 1 [i.e. cv < 7x10-4 m2/s (7 cm2/min); t50 > 1 min]
Robertson, 2012
Undrained
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 flowcharacteristics for fine grained soils
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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
Summary
• CPT interpretation should be done within ageology framework (i.e. understand thegeology)
• CPT can provide good estimate of a widerange of geotechnical parameters in most fine-grained soils
• Best to view parameters as a profile (i.e.maintain the stratigraphy and variability)