Introduction to Cone Penetration Testing

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Job Peters
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Robertson Webinar 2012 History of CPT First developed in 1930 s as mechanical cone

1970 s Major advancements since 1970: Pore pressure measurements More reliable load

1 Gregg Drilling & Testing, Inc. Site Investigation Experts Introduction to Cone Penetration Testing Peter K. Robertson Webinar 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 1

Basic Cone Parameters Sleeve Friction f s = load / 2 rh Pore Pressure u 2 Tip Resistance q c = load / r 2 Role of CPT CPT has three main applications: Determine sub-surface stratigraphy and identify

2 Basic Cone Parameters Sleeve Friction f s = load / 2 rh Pore Pressure u 2 Tip Resistance q c = load / r 2 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 2

3 What level of sophistication is appropriate for site investigation & analyses? GOOD Precedent & local experience POOR SIMPLE Design objectives COMPLEX LOW Level of geotechnical risk HIGH LOW Potential for cost savings HIGH Traditional Methods Simplified Advanced Methods Complex 3

Advantages of CPT Advantages over traditional combination of boring,

2m/min ~4 ft/min) Continuous or near continuous data Repeatable and

DIRECT-PUSH TECHNOLOGY UD tube Oscilloscope Drop Hammer Cased Boreholes

4 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 CONVENTIONAL DRILLING & SAMPLING Lab DIRECT-PUSH TECHNOLOGY UD tube Oscilloscope Drop Hammer Cased Boreholes CHT: V s, V p FIRM SAND SCPTù q t f su2 t 50 V s SPT: N 60 VST: s u, S t SOFT CLAY PMT: E Packer: k vh old new After Mayne,

5 Discrete CPT Soil Sampling CPT (Piston-Type) Sampler Single-Tube System 30cm (12 ) long by 25mm (1 ) diameter Example CPT Trucks/track Mayne,

6 Special CPT Vehicles After Mayne, 2010 CPT with a Drill Rig 6

equipment (pinch points, clamps) No cuttings for disposal Significant cost savings Reduced

7 Ramset Limited Access Portable CPT Remote Locations 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 7

pressure (u) Inclination (i) Seismic (V s, V p ) Vision (camera)

8 2 cm 2 10 cm2 15 cm 2 40 cm 2 Cone Penetrometer Sizes ASTM Standard CPT Sensors Since development of electric cones - many new sensors added: Pore pressure (u) Inclination (i) Seismic (V s, V p ) Vision (camera) Geo-environmental sensors ph, electrical, fluorescence (LIF & UVIF), many others... 8

9 Unequal End Area Effects on q c q t = q c + u 2 (1-a) a = 0.60 to 0.85 a = tip net area ratio ~ A n /A c In sands: q t = q c In very soft clays: correction to q t is important Cones should have high net area ratio a > 0.8 Soil Type CPTu Interpretation Soil behavior type (SBT) In-situ State Relative density (D r ) or State Parameter (y) and OCR Strength Peak friction angle (f ) and undrained strength (s u ) Stiffness/compressibility Shear (G o ), Young s (E ) and 1-D constrained (M) Consolidation/permeability Coeff of consolidation (c v ) and permeability (k) 9

10 CPT - Soil Behavior Type (SBT) Cone Resistance (bar) q t Non-Normalized Classification Chart SANDS MIXED SOILS 4 3 CLAYS 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 (%), R f After Robertson & Campanella, 1986 CPT Data Presentation Example CPTu Plot 10

11 CPT- Normalized SBT Chart 1000 Normalized Classification Chart 7 8 q t - v o s ' vo s Normalized Cone Resistance j ' SANDS Drained MIXED SOILS Partially drained 4 CLAYS 3 9 Undrained 2 Zone Normailzed Soil Behavior Type 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 Normalized Friction Ratio f s x 100% q- s t vo After Robertson, 1990 CPT SBT Index, I c SANDS Soil Behavior Type Index, I c Ic = [(3.47 log Q) 2 + (log F+1.22) 2 ] 0.5 Increasing compressibility CLAYS Function primarily of Soil Compressibility Compressibility linked to soil plasticity & amount/type of fines 11

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)

12 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) 12

13 Challenges: Theory for CPT 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 Schematic of soil loading around cone Generalized stress-strain relationship 13

14 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 Ahmadi & Robertson, 2005 In interlayered deposits this can result in excessive conservatism Perceived applicability of CPT for Deriving Soil Parameters Initial state parameter Strength Parameters Deformation Characteristics* Flow Charact. Soil Type γ/d r ψ K o OCR S t s u Φ E M G o k c h Clay Sand Applicability rating: 1 high reliability, 2 high to moderate, 3 moderate, 4 moderate to low, 5 low reliability. * Improved when using SCPT 14

15 Stress History: OCR Wroth (1984), Mayne (1991) and others proposed theoretical solutions (based on cavity expansion & critical state soil mechanics): σ p = f(q t - σ vo )* OCR = f [(q t - σ vo )/ σ vo ]* σ p = f(du) OCR = f [Du/(q t - σ vo )] σ p = f(q t u 2 ) OCR = f [(q t u 2 )/ σ vo ] * Most Common Correlation between Q t and OCR (Kulhawy & Mayne, 1990) OCR = 0.33 Q t (When OCR < 4) Q t = (q t - σ vo )/ σ vo Alternate based on high quality block samples: (OCR < 10 & S t < 15) OCR = 0.25 (Q t )

16 Strength Parameters - Clay Undrained strength ratio as a function of direction of loading Jamiolkowski et al., 1985 & Ladd, 1991 Undrained Shear Strength, s u s u = q t σ vo N kt 10 < N kt < 16 N kt N kt With sensitivity With PI & OCR For soft clays (based on excess pore pressure, Δu): s u = Δu = u u o N Δu N Δu 7 < N Δu < 10 16

17 10/1/2012 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 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 = ( wl) 17

10/1/2012 Estimation of Ground Water Table from CPT Dissipation Tests Example pore pressure dissipation Piezo-Dissipations at Evergreen, North Carolina 1000 u2 during CPTu Measured u 2 (kpa) 900 ch =

18 10/1/2012 Estimation of Ground Water Table from CPT Dissipation Tests 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 at 50% consolidation: u = ½( ) = 433 kpa 400 Extrapolation Groundwater Table at 0.4 m u0 = ( m)*9.8 kn/m 3 = 37 kpa Time (minutes) After Mayne, t50 = 7 minutes 100 Where: T50 is the theoretical time factor, t50 is the measure time, and r is the radius of the probe 18

19 10/1/2012 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 Time (minutes) After Cruz & Mayne 2006) 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 Onsoy soft clay Porto Tolle soft clay Rio de Janeiro soft clay Saint Alban soft clay 1:1 Line ch from Piezocone Dissipation (cm2/min) After Robertson et al.,

20 10/1/2012 Permeability from CPT Parez & Fauriel, kpa Based on theory via dissipation test, t 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 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 20

21 10/1/2012 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 SCPT Equipment and Procedures Cone Penetrometer Shear Wave Traces DT DD Vs= DD DT After Rice,

22 10/1/2012 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 22

23 10/1/2012 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 Perceived Applicability Pile Design Bearing Capacity Settlement* Compaction Control Liquefaction Sand Clay Intermediate Soils Reliability rating: 1 = High, 2 = High to Moderate, 3 = Moderate, 4 = Moderate to Low, 5 = Low * Higher when using SCPT 23

24 10/1/2012 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. 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) 24

25 10/1/2012 Example CPT Interpretation Software CPeT-IT Example Plots 25

26 10/1/2012 Normalized plots SBT charts Non-normalized Normalized Updated Robertson

27 10/1/2012 Estimated parameters (1) Estimated parameters (2) 27

28 10/1/2012 Questions? 28

CPT Data Interpretation Theory Manual

CPT Data Interpretation Theory Manual 2016 Rocscience Inc. Table of Contents 1 Introduction... 3 2 Soil Parameter Interpretation... 5 3 Soil Profiling... 11 3.1 Non-Normalized SBT Charts... 11 3.2 Normalized