01. Intro To Cpt.pdf

10/1/2012

Gregg Drilling & Testing, Inc. Site Investigation Experts

Introduction to Cone 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 since 1970’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

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Basic Cone Parameters Sleeve Friction fs = load/2rh

Pore Pressure u2

Tip Resistance qc = load/ r 2

Robertson, 2012

Role of CPT CPT has three main applications: • Determine sub-surface stratigraphy and identify materials 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

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What level of sophistication is appropriate for site investigation & analyses? GOOD SIMPLE LOW LOW

Precedent & local experience Design objectives Level of geotechnical risk Potential for cost savings

POOR COMPLEX HIGH HIGH

Traditional Methods

Advanced Methods

Simplified

Complex Robertson, 2012

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Advantages of CPT Advantages over traditional combination of boring, 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

DIRECT-PUSH TECHNOLOGY

CONVENTIONAL DRILLING & SAMPLING Lab

UD tube

Oscilloscope Drop Hammer

SCPTù qt fs u2 t50 Vs

Cased Boreholes

FIRM SAND

CHT: Vs, Vp SPT: N60 VST: su, St PMT: E’ Packer: kvh

SOFT CLAY old

new

After Mayne, 2010

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Discrete CPT Soil Sampling CPT (Piston-Type) Sampler • Single-Tube System

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

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Example CPT Trucks/track

Mayne, 2010

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Special CPT Vehicles

After Mayne, 2010

CPT with a Drill Rig

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Portable CPT Ramset Limited Access

Remote Locations

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Safety • Improved safety using push-in methods – No hammer or rotating parts – Similar safety precautions compared to direct push equipment (pinch points, clamps)

• No cuttings for disposal – Significant cost savings – Reduced contact with possible contamination

• Lower visibility and public exposure with enclosed trucks Robertson, 2012

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15 10

40 cm2 cm2

cm2

Cone Penetrometer Sizes

2 cm2

ASTM Standard Robertson, 2012

CPT Sensors Since development of electric cones - many new sensors added: • • • • •

Pore pressure (u) Inclination (i) Seismic (Vs, Vp) Vision (camera) Geo-environmental sensors – ph, electrical, fluorescence (LIF & UVIF), many others……...

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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 ratio a > 0.8 Robertson, 2012

CPTu Interpretation Soil 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

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CPT - Soil Behavior Type (SBT) Non-Normalized Classification Chart 1000

10

12 11

9 SANDS 8

Cone Resistance (bar) qt

100

7 6 5

MIXED SOILS

4

10

3 CLAYS 1 2 1 0

1

2

3

4

5

6

7

8

CPT SBT based on in-situ soil behavior - not the same as classification based on Atterberg Limits and grain size carried out on disturbed samples

Friction Ratio (%), Rf

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After Robertson & Campanella, 1986

CPT Data Presentation

Example CPTu Plot

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CPT- Normalized SBT Chart Normalized Classification Chart 1000

qt - svo

7

8

s 'v o

j' 100

Zone Normailzed Soil Behavior Type

9

1 2 3 4 5 6 7 8 9

SANDS

Normalized Cone Resistance

Drained 6 5

MIXED SOILS Partially drained

10 4

CLAYS

sensitive fine grained organic material clay to silty clay clayey silt to silty clay silty sand to sandy silt clean sands to silty sands gravelly sand to sand very stiff sand to clayey sand very stiff fine grained

Undrained

1

3

2 1 0.1

1

Normalized Friction Ratio

10 fs q-t

s

x 100% vo

Robertson, 2012

After Robertson, 1990

CPT SBT Index, Ic Soil Behavior Type Index, Ic SANDS

Ic = [(3.47 – log Q)2 + (log F+1.22)2]0.5

Function primarily of Soil Compressibility Increasing compressibility

CLAYS

Compressibility linked to soil plasticity & amount/type of fines Robertson, 2012

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Robertson, 2012

Repeatability

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

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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, but supported by theory Robertson, 2012

Schematic of soil loading around cone Generalized stress-strain relationship

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Transition zone CPT data in ‘transition’ when cone is moving from one soil type to another when there is significant difference in soil stiffness/strength CPT data within transition zone will be misinterpreted In interlayered deposits this can result in excessive conservatism

Ahmadi & Robertson, 2005

Perceived applicability of CPT for Deriving Soil Parameters Initial state parameter Soil Type

γ/Dr

Clay

3-4

Sand

2-3

ψ

2-3

Strength Parameters

Deformation Characteristics*

Flow Charact.

Ko

OCR

St

su

Φ’

E

M

Go

k

ch

2

1-2

2-3

1-2

4

2-3

2-3

2-3

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

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Stress History: OCR Wroth (1984), Mayne (1991) and others proposed theoretical solutions (based on cavity expansion & critical state soil mechanics): σ’p = f(qt - σvo)* σ’p = f(Du) σ’p = f(qt –u2)

OCR = f [(qt - σvo)/ σ’vo]* OCR = f [Du/(qt - σvo)] OCR = f [(qt –u2)/ σ’vo]

* Most Common Robertson, 2012

Correlation between Qt and OCR (Kulhawy & Mayne, 1990)

OCR = 0.33 Qt (When OCR < 4)

Qt = (qt - σvo)/ σ’vo

Alternate based on high quality block samples: (OCR < 10 & St < 15) OCR = 0.25 (Qt)1.25

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Strength Parameters - Clay

Undrained strength ratio as a function of direction of loading Jamiolkowski et al., 1985 & Ladd, 1991

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Undrained Shear Strength, su su = qt – σvo Nkt Nkt

With sensitivity

Nkt

With PI & OCR

10 < Nkt < 16

For soft clays (based on excess pore pressure, Δu): su = Δu = u – uo NΔu NΔu

7 < NΔu < 10 Robertson, 2012

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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 su,ave = 1/3 (suTC + suTE + suSS)

11.5 to 15.5

Mean 14 Values will vary somewhat with plasticity & sensitivity of clay Swedish experience suggests: Nkt = (13.4 + 6.65 wL) Robertson, 2012

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Estimation of Ground Water Table from CPT Dissipation Tests

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Example pore pressure dissipation Piezo-Dissipations at Evergreen, North Carolina 1000

u2 during CPTu

Measured u 2 (kPa)

900

ch = T50 · r2 t50

Dissipation Record at 4.2 m

800 700 600 500

at 50% consolidation: u = ½(829 + 37) = 433 kPa

400

Extrapolation

300 200 100

Groundwater Table at 0.4 m u0 = (4.2 - 0.4m)*9.8 kN/m 3 = 37 kPa

0 0.01

0.1

1

Time (minutes) After Mayne, 2010

10

t50 = 7 minutes

100

Where: T50 is the theoretical time factor, t50 is the measure time, and r is the radius of the probe Robertson, 2012

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Pore pressure dissipation in stiff clay Depth = 8.47 m

Measured u2 (kPa)

150 Measured u2 Hydrostatic u0 Pred CE-MCC 100 Fitted Analytical Solution

Dilatory Field Data

50 0.1

1

10

100

1000

Time (minutes) After Cruz & Mayne 2006)

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Laboratory ch values and CPTu results Theoretical solutions

M easured Lab c v (cm / 2 /m in)

10

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

cvh = coefficient of consolidation 1

0.1

Onsoy soft clay Porto Tolle soft clay Rio de Janeiro soft clay Saint Alban soft clay 1:1 Line

0.01

0.001 0.001

0.01

0.1

1

10

ch from Piezocone Dissipation (cm2/min)

After Robertson et al., 1992

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Permeability from CPT Parez & Fauriel, 1988 50 kPa

Based on theory via dissipation test, t50

100 kPa

kh = (ch gw)/M

Undrained

Increasing M

where: M is the 1-D constrained modulus gw is the unit weight of water, in compatible units. M can be estimated from Qtn 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 fine grained soils Robertson, 2012

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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 Cone Penetrometer

Shear Wave Traces

DT DD

Vs= Robertson, 2012

DD DT

After Rice, 1985

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Robertson, 2012

Seismic CPT

SCPT • Shear wave velocity a useful fundamental parameter • SCPT very useful since it provides both CPT data and Vs in one profile • Potential to evaluate ‘unusual’ soils • Settlement calculations based on Vs

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In-situ Testing and Geotechnical Design INDIRECT METHODS

DIRECT METHODS

In-situ Test Results

Of Construction

Previous Performance

In-situ Test Results

Soil Model Solution of Complex BVP Design Parameters

Geotechnical Design

Geotechnical Design

Robertson, 2012

Perceived Applicability Pile Design

Bearing Capacity

Settlement* Compaction Control

Liquefaction

Sand

1-2

1-2

2-3

1-2

1-2

Clay

1-2

1-2

3-4

3-4

2-3

Intermediate Soils

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

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Summary • CPT can be a fast, reliable and cost effective means to evaluate soil profile, geotechnical parameters, groundwater conditions and preliminary geotechnical design. • Suitable for a wide range of soils, except for dense 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

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Example CPT Interpretation Software

CPeT-IT http://www.geologismiki.gr/

Robertson, 2012

Example Plots

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Normalized plots

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SBT charts

Non-normalized

Normalized Updated Robertson 2010 Robertson, 2012

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Estimated parameters (1)

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Estimated parameters (2)

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Questions?

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