Introduction to CPT

Peter K. Robertson

CPT in Geotechnical Practice

Santiago, Chile

July, 2014

Ground InvestigationTo investigate ground and groundwater

conditions in and around site consistent withproject requirements

• Nature, sequence and variability of strata

• Groundwater conditions

• Physical, chemical and mechanical characteristics of strata

Field work designed to test and evaluategeologic model

Flexibility

• Ground conditions often complex

• Site investigation should be conducted asoperations of discovery (geologic framework)

• Work should be varied in light of newinformation

• Observational approach needed

Communication

• Communication important for flexible groundinvestigation

• Modern tele-communications have improvedlinks between field and office

• Important to give field personnel adequatebackground information

Major areas of uncertainty

• Natural variability

• Complex soil behavior

• Limitations of in-situ testing

• Difficulties obtaining truly undisturbedsamples

• Limitations of laboratory testing

In-situ Testing

• ADVANTAGES– Large volume of tested

soil

– Layering, fabric, spatialvariability maintained

– Testing of difficult tosample soils

– Testing of soils in naturalenvironment

– Reduced costs and time

• LIMITATIONS– Complex and poorly

controlled boundaryconditions

– Uncontrolled drainageconditions

– Soil disturbance

– Non-uniform strain fields

– Deformation and failuremodes different thandesign problem

– Limited identification ofsoil nature

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

After Robertson 2012

Requirements for a Good PenetrationTest

• 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 independentvariable– Example: CPTu, SCPT, SDMT

Where’s the SPT?

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

SPT Corrections

(N1)60 = N CER CN CR CB CS

N = Measured SPT

CER = Energy correction (0.4 to 1.4)

CN = Overburden correction (σ’vo)-0.5

CR = Rod length correction (0.7 to 1.0)

CB = Borehole diameter correction (1.0 to 1.2)

CS = Sampler correction (1.0 to 1.2)

Borehole disturbance also a major factor

Influence of Energy on SPT

Measured SPT N = 12

If ERi = 40% N60 = 8

If ERi = 80% N60 = 16

If SPT measured at depth of 4m

ERi could be as low as 20%

Then N60 = 4 Hence, 4 or 16?

Common reasons given to do SPT

• Other (better) tests are not locally available

– demand/require better tests, then contractors willmake 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

History of CPT

• First developed in 1930’s as mechanical cone

• Electric cones developed in 1960’s

• Primary device for off-shore investigations since1970’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 interpretation• Additional sensors

LIMITATIONS:• High capital investment• Skilled operators• No soil sample (during CPT)• Penetration restricted in gravels/cemented

layers

Discrete CPT Soil Sampling

Direct-Push Piston-Type Sampler

• Single-Tube System

• 300mm long x 28mm diameter sample

• Small disturbed sample (index testing only)

Basic Cone Parameters

Sleeve Frictionfs = load/2rh

Pore Pressureu2

Tip Resistanceqc = load/ r

2

Robertson, 2013

ConePenetrometer

Sizes

2 cm2

10 cm2

15 cm2

40 cm2

ASTM Standard

Example CPT Trucks/track Mayne, 2010

Special CPT VehiclesAfter Mayne, 2010

CPT with a Drill Rig

Get SPT N60 from CPT

Jefferies and Davies (1993) –“The best way to obtain an SPT N value is to doCPT and convert”

qc/N60 = 8.5 (1 – Ic/4.6)

Ic = ((3.47 - log Qt)2 + (log Fr + 1.22)2)0.5

SPT-CPT Correlations

SBT Ic

Example SPT-CPT comparison

Tailings deposit

SPT

Zero values

GOOD Precedent & local experience POOR

SIMPLE Design objectives COMPLEX

LOW Level of geotechnical risk HIGH

LOW Potential for cost savings HIGH

Traditional Methods Advanced Methods

What level of sophistication isappropriate for site investigation

& analyses?

Simplified Complex

UDtube

CasedBoreholes

SPT: N60

VST: su, St

CHT:Vs, Vp

SOFTCLAY

FIRMSAND

CONVENTIONAL DRILLING& SAMPLING

DIRECT-PUSHTECHNOLOGY

DropHammer

SCPTùqt

fs

u2

t50

Vs

Oscilloscope

PMT: E’Packer: kvh

Lab

old new After Mayne, 2010

Summary

• CPT can be a fast, reliable and cost effectivemeans to evaluate soil profile, geotechnicalparameters, groundwater conditions andpreliminary geotechnical design.

• Suitable for a wide range of soils, except fordense gravels and hard rock.