Introduction to CPT. Peter K. Robertson. Santiago, Chile. CPT in Geotechnical Practice. July, 2014

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1 Introduction to CPT Peter K. Robertson CPT in Geotechnical Practice Santiago, Chile July, 2014

2 Ground Investigation To investigate ground and groundwater conditions in and around site consistent with project requirements Nature, sequence and variability of strata Groundwater conditions Physical, chemical and mechanical characteristics of strata Field work designed to test and evaluate geologic model

3 Flexibility Ground conditions often complex Site investigation should be conducted as operations of discovery (geologic framework) Work should be varied in light of new information Observational approach needed

improved links between field and office Important to

4 Communication Communication important for flexible ground investigation Modern tele-communications have improved links between field and office Important to give field personnel adequate background information

5 Major areas of uncertainty Natural variability Complex soil behavior Limitations of in-situ testing Difficulties obtaining truly undisturbed samples Limitations of laboratory testing

6 In-situ Testing ADVANTAGES Large volume of tested soil Layering, fabric, spatial variability maintained Testing of difficult to sample soils Testing of soils in natural environment Reduced costs and time LIMITATIONS Complex and poorly controlled boundary conditions Uncontrolled drainage conditions Soil disturbance Non-uniform strain fields Deformation and failure modes different than design problem Limited identification of soil nature

7 Applicability of major in-situ tests (first published by Mitchell et al, 1978) After Robertson 2012

8 Requirements for a Good Penetration Test Reliable, operator independent measurements Examples: CPT, CPTu, SCPT, DMT, SDMT Repeatable disturbance of surrounding soil Examples: CPT, CPTu, SCPT, DMT, SDMT Measurement of more than one independent variable Example: CPTu, SCPT, SDMT Where s the SPT?

9 Standard Penetration Test (SPT) ADVANTAGES: Simple and rugged equipment Low equipment cost Easily available Soil sample obtained LIMITATIONS: Disturbed sample (index tests only) High variability and uncertainty Crude number (N value) Discontinuous

10 SPT Corrections (N 1 ) 60 = N C ER C N C R C B C S N = Measured SPT C ER = Energy correction (0.4 to 1.4) C N = Overburden correction (σ vo ) -0.5 C R = Rod length correction (0.7 to 1.0) C B = Borehole diameter correction (1.0 to 1.2) C S = Sampler correction (1.0 to 1.2) Borehole disturbance also a major factor

11 Influence of Energy on SPT Measured SPT N = 12 If ER i = 40% N 60 = 8 If ER i = 80% N 60 = 16 If SPT measured at depth of 4m ER i could be as low as 20% Then N 60 = 4 Hence, 4 or 16?

12 Common reasons given to do SPT Other (better) tests are not locally available demand/require better tests, then contractors will make the investment SPT gives soil sample take samples with direct push equipment SPT is less expensive SPT very expensive based on a per data point CPT not possible in local soils Push CPT with drill-rig & drill out hard layers

13 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

Cone Penetration Test (CPT) ADVANTAGES: Fast and continuous profiling Repeatable and reliable data Economical and productive Strong theoretical basis for

14 Cone Penetration Test (CPT) ADVANTAGES: Fast and continuous profiling Repeatable and reliable data Economical and productive Strong theoretical basis for interpretation Additional sensors LIMITATIONS: High capital investment Skilled operators No soil sample (during CPT) Penetration restricted in gravels/cemented layers

15 Discrete CPT Soil Sampling Direct-Push Piston-Type Sampler Single-Tube System 300mm long x 28mm diameter sample Small disturbed sample (index testing only)

16 Basic Cone Parameters Sleeve Friction f s = load / 2 rh Pore Pressure u 2 Tip Resistance q c = load / r 2 Robertson, 2013

17 2 cm 2 10 cm2 15 cm 2 40 cm 2 Cone Penetrometer Sizes ASTM Standard

18 Example CPT Trucks/track Mayne, 2010

19 Special CPT Vehicles After Mayne, 2010

20 CPT with a Drill Rig

21 Get SPT N60 from CPT Jefferies and Davies (1993) The best way to obtain an SPT N value is to do CPT and convert qc/n60 = 8.5 (1 Ic/4.6) Ic = (( log Qt)2 + (log Fr )2)0.5

22 SPT-CPT Correlations SBT Ic

23 Example SPT-CPT comparison SPT Zero values Tailings deposit

24 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

26 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

27 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.

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