1/18/2013

1

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

1/18/2013

2

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

Robertson, 2012

1/18/2013

3

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

Robertson, 2012

1/18/2013

4

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

Robertson, 2012

1/18/2013

5

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

1/18/2013

6

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

1/18/2013

7

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

1/18/2013

8

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

1/18/2013

9

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)

Robertson, 2012

1/18/2013

10

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

Robertson, 2012

1/18/2013

11

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

Robertson, 2012

1/18/2013

12

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

1/18/2013

13

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)

1/18/2013

14

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

1/18/2013

15

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

1/18/2013

16

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

1/18/2013

17

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

1/18/2013

18

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

1/18/2013

19

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

Robertson, 2012

1/18/2013

20

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

1/18/2013

21

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

Robertson, 2012

1/18/2013

22

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)

1/18/2013

23

Example Interpretation

Soft NC clay overlying very stiff OC silty clay

Located in 5-star Mandarin Oriental Hotel