CPTu ppt

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CPTU Derived Soil Engineering Parameters for CLAY

1. Key Aspects of Clay Soil Behavior

2. Important engineering design parameters

3. Background and application of CPTU correlations for estimation of design parameters

4. Applied to Case Studies in follow-on lecture.

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Basic Soil Behavior - CLAY1-D Consolidation

Most Important Parameter:Yield stress = σ'vy ≡ σ'p ≡ p'cAlso known as:- Preconsolidation stress- Maximum past pressure

Key Aspects:1. Compressibility (RR and CR)2. Yield stress (σ'p)3. Coefficient of consolidation (cv)4. Hydraulic conductivity (kv)5. Horizontal stress (σ'h0 or K0)

3/50

Basic Soil Behavior - CLAY

Most Important Parameter:Undrained shear strength = su

Undrained Shear Strength

Key Aspects:1. Shear induced pore

pressures2. Effect of OCR3. Anisotropy4. Rate effects

4/50

General Aspects of CPTU Testing in Clay

1. Penetration is generally undrained and therefore excess pore pressures will be generated.

2. Cone resistance and sleeve friction (if relevant) should be corrected using the measured pore pressures.

3. The measured pore pressures can also be used directly for interpretation in terms of soil design parameters.

5/50

Interpretation of CPTU data in clay1. State Parameters = In situ state of stress

and stress history

2. Strength parameters

3. Deformation characteristics

4. Flow and consolidation characteristics

5. In situ pore pressure6/50

In Situ State Parameters

1. Soil Unit weight: γw for computation of in situ vertical effective stress (σ'v0)

2. Stress historyσ'p and OCR = σ'p/σ'v0

3. In situ horizontal effective stressσ'h0 = K0σ'v0

2

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Estimation of Soil Unit Weight

[Larsson and Mulabdic 1991]

Iterative procedure

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Estimation of Soil Unit Weight

19.01220.51120.01019.5919.0818.5718.0618.0518.0417.5312.5217.51

ApproximateUnit Weight

(kN/m3)Zone

[Robertson et al. 1986]

Note: 1 kN/m3 ≈ 6.36 pcf

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Stress History: OCR = σ'p/σ'v0

Estimation of Stress History (OCR or σ'p) can be based on:

• Direct correlation with CPTU data

• Pore pressure differential via dual element piezocone

• Indirect correlation via undrained shear strength

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CPTU Stress History CorrelationsWroth (1984), Mayne(1991) and others proposed theoretical basis (cavity expansion; critical state soil mechanics) for the following potential correlations between CPTU data and σ'p or OCR:

σ'p = = f(Δu1 or Δu2)σ'p = f(qt - σv0)σ'p = f(qt - u2)

OCR = f(Bq= Δu2/(qt - σvo))OCR = f(Qt = (qt - σvo)/ σ'v0))OCR = f((qt - u2)/ σ'v0)

Most Common:

σ'p = k(qt – σv0)

or

OCR = k[(qt – σv0)/σ'v0]

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CPTU Stress History Correlations

0

2

2

2

0.2

6

10

14

18

22

26

30

34

4

4

0.4 6

6

0.6 8

8

0.8 10

10

F*

= f

/ (q

)t

tt

vo-σ

Δσ

u/

'1

vo

B =

(u-u

)/ (

q-

)q

2o

tσ v

o(q

- )

/ '

tv

σ

σo

vo

1.0 12

12

1.2 14

14

1.4 16

16

1

1

1

1

5

5

5

5

10

Overconsolidation ratio, OCR

10

10

10

20

20

20

20

X

X

X

X

X

X

[Lunne, et al. 1989]

Legend:

TrollBrageHaltenbankenHagaRioVancouverCowdenBrent CrossOnsøy

X

EmmerstadDrammen lean clayDrammen plastic clay

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CPTU Stress History CorrelationsComprehensive study initially by Chen and Mayne (1996) with later updates (e.g., Mayne 2005):

σ'p = 0.47(Δu1) = 0.53(Δu2)

σ'p = 0.33(qt - σv0)

σ'p = 0.60(qt - u2)

Most common

Note: values listed above are from best fit regressions; there is a sizable range in all values, e.g., k ranges from 0.2 to 0.5 for σ'p = k(qt – σv0)

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Example - CPTU Stress History

CorrelationBoston Blue Clay Site –Newbury, MA.

σ'p values obtained from Constant Rate of Strain (CRS) Consolidation tests conducted on high quality Sherbrooke Block samples

Stress (kPa)0 200 400 600 800

Dep

th (m

)

2

4

6

8

10

12

14

Fill

σ'v0

σ'p Block Samples

σ'p(CPTU) = 0.3(qnet)

Boston Blue Clay

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CPTU Stress History CorrelationsData from NGI Block Sample Database(Karlsrud et al. 2005)

- Laboratory tests conducted on high quality undisturbed block samples (e.g., Sherbrooke Block Sampler) → sample quality can have a significant influence on σ'p

- Soft to medium stiff clayssu(CAUC) = 15 – 150 kPa; OCR = 1.2 – 6.3;Ip = 10 – 50 %; St = 3 - 200

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Vertical Effective Stress, σ'v (kPa)

10 100 1000

Ver

tical

Stra

in ε

v (%

)

0

5

10

15

20

25

Free Piston regular tubeFixed Piston - special tube76 mm SPT samplerSherbrooke Block

Depth = 7.4 m

Importance of Sample Quality – Boston Blue Clay

Used 4 sampling methods

1. Poor: SPT sampler

2. Fair: Standard 76 mm thin walled tube sampler (with free or fixed piston)

3. Good: Fixed piston sampler in mudded borehole using modified 76 mm diameter thin walled tube

4. Best: Sherbrooke Block Sampler

CRS Tests

σ'v0

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1 102 3 4 5 6 7 8 9OCR

0.0

0.2

0.4

0.6

0.8

1.0

1.2B q

St <= 15St > 15

St <= 150.88 - 0.51 log OCR

St > 151.15 - 0.67 log OCR NGI Block Sample

Database

OCR = f(Bq)

[Karlsrud et al. 2005]

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1 102 3 4 5 6 7 8 9OCR

0.0

2.0

4.0

6.0

8.0

10.0

(u2-

u 0)/σ

' v0

St <= 15St > 15

St > 15:2.5 + 6 log OCR

St <= 15:2.4 + 8 log OCR

NGI Block Sample Database

OCR = f(Δu2/σ'v0)


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1 102 3 4 5 6 7 8 9OCR

0

4

8

12

16

20

Qt

St <= 15St > 15

St > 15: OCR = (Qt/2)1.11

St <= 15: OCR = (Qt/3)1.2

NGI Block Sample Database

OCR = f(Qt)


4

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Ove

rcon

solid

atio

n ra

tio, O

CR

Pore pressure difference, PPD= (u -u )/u1 2 o

Robertson et al. (1986)

Levadoux and Baligh (1980)

Roy et al. (1982)

Sully (1986)

0 1 2 3 4 5 6 7 8 9

u1u2

uo

OCR= 0.66 + 1.43 (PPD)

a) 10

9

8

7

6

5

4

3

2

1

0

From pore pressure data using dual element piezocone

PPD = (u1 – u2)/u0

[Sully et al.,1988]20/50

K0 – OCR Relationship for ClaysFor simple case of loading followed by unloading, K0 increases with increasing OCR such that:

K0,OC = K0,NC(OCR)n

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In Situ Horizontal Effective Stress There are currently no reliable methods for determining the in situ horizontal effective stress, σ'h0 = K0(σ'v0) from CPTU data

For approximate (preliminary) estimates consider correlations based on:• OCR via CPTU correlations for OCR or su• Measured pore pressure difference

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K0-OCR-PI Relationship

[Brooker and Ireland 1965]

Need values for Plasticity Index (PI) and OCR.

Determine OCR from 1) CPTU correlations or via 2) undrained shear strength correlation (next slide)

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NGI Relationship among OCR-su/σ'v0-K0-PI

From Basic Soil Behavior

su/σ'v0 = S(OCR)m

K0,OC = K0,NC(OCR)n

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Estimate K0 from Dual Element Piezocone

[Sully and Campanella 1991]

Difference between u1and u2 increases with increasing OCR → K0also increases with increasing OCR, hence positive correlation between (u1 – u2)/σ'v0and K0.

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Shear Strength of ClaysFor most design problems in clays (especially loading) the critical failure condition is undrained.

1. Undrained Shear strength su (= cu)

2. Remolded undrained shear strength (sur) or Sensitivity, St = su/sur

Note: 1kPa = 20.9 psf26/50

Notes Regarding Undrained Shear Strength1. The undrained shear strength is not unique.

2. The in situ undrained shear strength depends on many factors with the most important being: mode of shear failure, soil anisotropy, strain rate and stress history.

3. Therefore su required for analysis depends on the design problem.

4. Measured CPTU data are also influenced by such factors as anisotropy and rate effects.

5. The CPTU cannot directly measure su and therefore CPTU interpretation of su relies on a combination of theory and empirical correlations

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Theoretical Interpretation CPTU in Clay1. Existing theories for interpretation of su from CPTU data involve several simplifications and assumptions. Therefore existing theories must be "calibrated" against measured data

2. Most important to use realistic and reliable soil data from high quality tests conducted on high quality samples

3. At NGI – key reference is to use su from Anisotropically consolidated triaxial compression (CAUC) tests conducted on high quality undisturbed samples. A secondary reference is to use the average su(ave) [or mobilized for stability problems] = 1/3[su(CAUC) = su(DSS) + su(CAUE)]

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Undrained Shear Strength Anisotropy

Plasticity Index PI (%)

0 20 40 60 80 100

s u/ σ' vc

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Triaxial Compression (TC): qf

Direct Simple Shear (DSS): τh

Triaxial Extension (TE): qf

TC

DSS

TE

[Ladd 1991, Ladd and DeGroot 2003]

σ1f (δ = 0°)

TC

σ1f(δ = 90°)

TE

σ1f (δ = 45 ± 15°)

DSS

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Undrained Shear Strength from CPTU DataTheories for interpretation:1. Bearing capacity2. Cavity expansion3. Strain path methods

All result in a relationship of the form:qt = Ncsu + σ0, where σ0 could = σv0, σh0, σm0

In practice most common to use:qt = Nktsu + σv0, for which theoretically Nkt = 9 to 18.

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Undrained Shear Strength from CPTU Data

The empirical approaches available for interpretation of su from CPT/CPTU data can be grouped under 3 main categories:

1. su estimation using "total" cone resistance

2. su estimation using "effective" cone resistance

3. su estimation using excess pore pressure

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Undrained Shear Strength from CPTU Data

su = qnet/Nkt = (qt – σv0)/Nkt

su = Δu/NΔu = (u2 – u0)/NΔu

su = qe/Nke = (qt – u2)/Nke

Most Common

Often used

Seldom used

Need empirical correlation factors Nkt, NΔu, or Nke factors as correlated to a specific measure of undrained shear strength, e.g., su(CAUC) or su(ave)

32/50

CPTU su Cone Factors

[Aas et al.1986]

su(Lab) = su(ave) =

1/3[su(CAUC) + su(DSS) + su(CAUE)]

su(CAUC)

Note: Nkt for su(CAUC) < Nkt for su(ave)

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CPTU su Cone Factors – Karlsrud et al. (2005)

Axial Strain (%)

0 2 4 6 8 10

She

ar S

tress

(kPa

)

0

10

20

30

4014 m

p' (kPa)

10 20 30 40 50 60 70 80

Block76 mm Tube54 mm Tube

Block and tube samplesof Onsøy, Norway clayPI = 30 to 40

CAUC Recompression tests

Update of CPTU su cone factors using NGI high quality block sample database. Derived cone factors as function: OCR, Sensitivity (St) and Plasticity Index (Ip)

34/50


1 102 3 4 5 6 7 8 9OCR

0

2

4

6

8

10

12

14

16N

kt

St <= 15St > 15


su = (qt – σv0)/Nkt

35/50


1 102 3 4 5 6 7 8 9OCR

0

2

4

6

8

10

NΔu

St <= 15St > 15


su = (u2 – u0)/NΔu

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0 10 20 30 40 50Ip (%)

0

2

4

6

8

10

NΔ

u OCR 1-2 St<=15OCR 1-2 St>15OCR 2-4 St<=15OCR 2-4 St>15OCR > 4 St<=15OCR>4 St>15

OCR 1-2OCR 2-4OCR > 4; St <= 15

( )( )


7

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0 0.2 0.4 0.6 0.8 1 1.2Bq

0

2

4

6

8

10

Nke

St <= 15St > 15


su = (qt – u2)/Nke

38/50

CPTU su Cone Factors – Karlsrud et al. (2005)Best fit regression lines to plotted data for su(CAUC)

0.17211.5 – 9.05Bq≤ 15

Nke 12.5 – 11.0Bq> 15

NΔu

Nkt

ConeFactor

9.8 – 4.5logOCR> 150.128

6.9 – 4.0logOCR + 0.07Ip≤ 15

8.5 + 2.5logOCR> 15 0.1977.8 + 2.5logOCR + 0.082Ip≤15

Standard Deviation

Regression EquationSensitivitySt

Best relationship (statistically) = NΔu. Note: NΔu correlation uses direct measurement (u2) and does not require use of qt which must be corrected for overburden stress in other correlations.

39/50

Updated NGI NΔu, CAUC Cone Factor for St ≤ 15

Plotted for Range OCR = 1 to 10 and Ip = 10 to 80

2

4

6

8

10

12

14

24

68

10

10203040506070

NΔ u

OCR

Plasticity Index (Ip)

4 6 8 10 12 14


High = 12.5@ OCR = 1 and Ip = 80

Low = 3.6@ OCR = 10 and Ip = 10

40/50

su from CPTU via CPTU-σ'p correlationsFor a given element of soil, the preconsolidation stress σ'p is essentially unique whereas su which is strongly dependent on method of measurement and is therefore not unique.

Alternative procedure to estimate su is first determine σ'p (and hence OCR) from the CPTU data, then use established laboratory (e.g., CAUC, DSS) or in situ (e.g., FVT) relationships between su and σ'p (or OCR) for a particular mode of su shear.

Examples:SHANSEP Equation (Ladd 1991)su/σ'v0 = S(OCR)m, with S = su/σ'v0 at OCR = 1e.g., su(DSS)/σ'v0 = 0.23(OCR)0.8

su(mob) = 0.22σ'p Mesri (1975)

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Remoulded Undrained Shear Strength sur

Comparison between UUC triaxial test data on remolded samples with CPTU friction sleeve data for Offshore California site

[Quiros and Young 1988]

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Remoulded Undrained Shear Strength sur

Comparison of laboratory measurements of remolded undrained shear strength with sleeve friction from CPTU tests for Ormen Lange area offshore Norway.

80

82

84

86

88

90

92

94

96

98

100

0 50 100 150 200 250 300 350 400

Re moulde d strength in R2, kPa

Dep

th b

elow

sea

bed

, m

UU(rem )FC(rem)CPTs in R2 19_2 &20"Intact" rings hear residual [Kvalstad et al. 2004]

8

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Undrained Shear Strength Sensitivity, St

Relationship between Sensitivity and CPTU Rffor two sites in Norway

[Rad and Lunne 1986]

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Deformation Parameters1. Constrained Modulus – for 1-D compression, M2. Undrained Young's Modulus, Eu3. Small strain shear modulus, Gmax

Two approaches for use of CPT/CPTU data to estimate deformation parameters:

1. Indirect methods that require an estimate of another parameter such as undrained shear strength su.

2. Direct methods that relate cone resistance directly to modulus.

45/50

Example of Direct Correlation between CPTU and Gmax

Mayne and Rix (1993)

Estimation of small strain shear modulus Gmax for clays from CPT qc data + estimate e.

Note: Gmax is anisotropic + in the context of CPT/CPTU testing, better to measure directly down hole with seismic cone (= Gvh)

46/50

Consolidation and Hydraulic ConductivityMeasurement: dissipation of penetration pore pressures during pause in penetration. Can be u1 or u2. Ideally measure until Δu = 0 but time depends on ch and kh.

Derived Soil Properties:1. Coefficient of Consolidation, ch

2. Hydraulic Conductivity (= permeability), kh

Since the dissipation is radial, ch and kh are derived. Some clays can have highly anisotropic consolidation and flow parameters (e.g., varved clays) – need to use published anisotropy ratios to estimate kv and cv.

47/50

CPTU Normalized Dissipation Curvesxxx Bothkennar, UK (= soft clay)

Dissipation Tests at 15 m depth

Typically plot:U = Δu/Δui as function twhich for the u2 position =(u2 – u0)/(ui – u0)whereu0 = in situ pore pressure before penetration, andui = u2 at t = 0

U50

t50

48/50

Theory for CPTU derived ch and kh

Terzaghi Theory: cv = (TH2)/t

Torstensson (1975, 1977) suggested use time at 50% dissipation and for CPTU geometry thus,

ch = (T50/t50)r2

Hence for 10 cm2 cone, ch = 0.00153/t50 [m2/s]

Terzaghi Theory: kh = chγwmh

Determine ch from dissipation test + need estimate mh= coefficient of volume change, which can be correlated to qc or qt

ch

kh

9

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Coefficient of Consolidation

Houlsby and Teh (1988, 1991): Strain Path Theory and Finite Element Analysis

For u1 or u2 and 10 cm2 or 15 cm2

cones. Uses t50 + requires Rigidity Index, Ir = G/su [Ir tends to decrease with increasing OCR and Ip]

ch = (T*50)r2(Ir)1/2/t50

T*50 = 0.118 for u1= 0.245 for u2

50/50

Example ch – Boston Blue Clay (Newbury, MA)

10 cm2, u2 Piezocone

t50 = 1750 s, a = 1.78 cm

T*50 = 0.245, Ir ≈ 100

ch = 0.0044 cm2/s

Note: if u0 unknown and cannot assume hydrostatic then must run full dissipation → can be very time consuming.

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Recommendations - CPTU Derived Soil Engineering Parameters for CLAY

1. Do not eliminate sampling and laboratory testing2. Verify reliability of results and that undrained conditions prevail3. With increasing experience modify correlations for local conditions

Good CPTU Interpretation methods exist for:• Soil Unit Weight (γw)• Stress History: OCR or σ'p• Undrained Shear Strength for su(CAUC) and su(ave)• Small strain shear modulus (Gmax)• Coefficient of Consolidation (ch)

Approximate estimates can be made from CPTU data for:1. In Situ horizontal effective stress (σ'h0 or K0)2. Remolded undrained shear strength (sur) or Sensitivity (St)3. Hydraulic Conductivity (kh)

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