CPT - a useful tool in

geotechnical investigations

• Quick review what we measure

• Development of CPT in Norway

• New ISO standards

• Interpretation of the results

• Advantages of add on sensors

• Future challenges

• Conclusions

Basic case with CPT

36 mm diameter probe (10 cm2)

Example CPTU profile

Cone resistance, qc, MPa

Sleeve friction, fs, kPa

Pore pressure, u2, kPa 0

5

10

15

20

25

30

35

0 10 20

qt (MPa)

De

pth

BG

S (

m)

0

5

10

15

20

25

30

35

0 300 600

fs (kPa)

0

5

10

15

20

25

30

35

-100 200 500

u2 (kPa)

0

5

10

15

20

25

30

35

0.5 1

DEM, e

60o

qc

fs

u2

DEM

In addition frequently measure inclination, i

Quick review of CPT in Norway (1/2)

• Up to 1971 only used a few times on land; main methods

for in situ sounding are vane tests and pressure/rotary

sounding

• 1971/1972 soil investigations starts in North Sea –

essential part from very beginning

• From 1972 many R&D projects on performance and

interpretation of CPT/CPTU

NGI – Fugro CPT study in Scandinavian Clays 1973

Correlations developed were important for foundation design of platforms in North Sea

Quick review of CPT in Norway (2/2)

• Status 2013 on land: CPTU has completely replaced the

vane tests and is now used quite extensively on important

projects

10 -10 Norwegian companies offers CPT/CPTU

All use Swedish equipment without cable

ENVI Memocone

Geotech with acoustic transfer of data

• Status 2013 offshore: No Norwegian companies offer

CPT, but NGI and other parties are strongly involved in

planning, specification and interpretation

New ISO standards

ISO 22476-1 (2012) Electrical CPTU

Completed December 2006

(Finally !!) Publisert 2012

ISO - DIS 19901 Part 8: Marine Soil Investigations

Draft out for hearing among ISO countries

Expected to be published 2014

(Statoil already use this standard)

Main elements of new Eurocode/ISO

standard on CPT/CPTU ISO 22476-1 (2012)

• Equipment

• Procedures

• Corrections

• Other aspects

Application classes Corrections Zero readings before and after tests Pore pressure response

Corrections described in ISO standard:

Penetration length and penetration depth

ISO standard gives formulae for how to correct for inclination

For pore pressure effect on qc: well known For inclination; have to consider:

0 10 20 30 40 50Cone resistance MPa

No Correction for Slope

70

60

50

40

30

20

10

0

De

pth

(m

)

0 10 20 30 40 50Cone resistance MPa

With Correction for Slope

70

60

50

40

30

20

10

0

De

pth

(m

)

0 5 10 15 20 25Slope (degr)

70

60

50

40

30

20

10

0

De

pth

(m

)

slope

slope EW

slope NS

Soil Profiles with and without

correction

From Mandy Korff, Deltares

Example of depth correction

Correction of 3 m at 70 m depth

CPT Truck CPT Cone

Example Non-vertical CPT

Importance of saturation on measured pore pressure response

Bad Medium Good

Misleading results will be obtained if the filter and its measuring system is not fully saturated. Errors will then also occur in the calculation of qt

Use of results in projects

• Layering

• Soil classification

• Udrained shear strength of clays

• Relative density of sand

• Deformation and flow parameters

• Pile bearing capacity

• Monitoring soil improvements

• Liquesfaction potential

Example North Sea – clay and sand layers

Measured parameters

Example North Sea – clay and sand layers

Clay

Sand

Hydrostatic pp

Clay

Gravel

20

15

10

5

0

0 5 10 15 20

Cone Resistance, qc (MPa)

Sleeve Friction, fs (MPa)

Dep

th (

m)

0 5 10

Estimated Soil TypeFriction Ratio

Rf (%)

0.000 0.125 0.250 0.375 0.500

17.9

15.4Very stiff to hard

sandy CLAY

Stiff to very stiff

sandy CLAY

7.2

13.1

11.2

1.5

0.7

Dense to very Dense

SAND and GRAVEL

Medium Dense

to dense

SAND and GRAVEL

Very Soft

Organic CLAY

Loose SAND

Soft CLAY

0.1 1 10 -0.4 0 0.4 0.8 1.2

1000

100

10

1

1000

100

10

1

Qt Qt

1

1

Increasing sensitivity 3

4

5

6

7

9

8

2

Increasing OCR, age

Increasing OCR, agecementation

Norm

ally co

nsolidated

1

3

4

5

6

7

2

uo

vo qt

u

Zone Soil behaviour type

1. Sensitive, fine grained

2. Organic soils-peats

3. Clays-clay to silty clay

Zone Soil behaviour type

4. Silt mixtures clayey silt to silty clay

5. Sand mixtures; silty sand to sand silty

6. Sands; clean sands to silty sands

Zone Soil behaviour type

7. Gravelly sand to sand

8. Very stiff sand to clayey sand

9. Very stiff fine grained

Normalized soil behaviour classification chart

Robertson,1990

Fr Bq

Example of use of soil classification chart

for Oslo airport

Sandven et al., 1998

Sensitive quick clays

Udrained shear strength of clay

Two examples:

• Soft lightly OC clay (in south Norway)

• Stiff OC clay (offshore Norway)

su = cu

su also used in new ISO standard on marine soil investigations

New railway link Oslo to south Norway

New double track route

Results of standard soil boring with 54 mm composite piston sample Given in tender documents

Initial solution in tender documents given by client

60 m

50

40

30

20

Bedrock

50

57

Fill Geotextile

Concrete piles

Very expensive solution

CPTU-Results

+42,8

0 m

5

10

15

1

New CPTs proposed by NGI were carried out

Undrained shear strength profile (suCAUC),

Nykirke Railway Track

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

suA (kPa)

Dyb

de

(m)

Basert på Nkt Basert på NDu

Styrkeprofil anbud Nc leire - SuA = 0,3*p0'Based on Nkt

Shear strength profile, tender

Based on NDu

Nc clay - suA=0,3*po’

Using N-factors to get su

CAUC from CPTU

Original design strength given in tender documents

suCAUC

From qt

From u

suCAUC is undrained shear strength

measured in triaxial compression test

N u = u/suCAUC = 8

u = u – uo

Nkt= (qt - vo)/suCAUC = 10

Based on NGI data base

Nykirke Railway Track, solution based

on new su profile

Prefabricated vertical drains

Excavation residues / berm

Moraine

Rockfill

3 meters of preload

GeotekstileGravel ( 0 - 200 )

Bedrock

Settlement gauges

Piezometers

Clay

New less expensive solution could be found

Use 3 m preload and prefabricated vertical drains to get rid of settlements

Block sampling

with Sherbrooke

sampler

Block sample cleaned and wrapped in plastic cling film

0 2 4 6 8 10 12 14 0

20

40

60

80

100

Blokk

54mm

Block

54 mm

0 40 80 120 160 200 0

40

80

120

54mm

Block

54 mm

Effective normal stress, ( 1’ + 3’)/2 (kPa)

Axial deformation, (%)

Sh

ear s

tress (

kP

a)

Sh

ear s

tress (

kP

a)

Z = 10.1 m

Z = 9.5 m

Results of CAUC triaxial tests on new block samples compared with old 54 mm piston samples

New CAUC

tests confirmed

interpreted su

values from

CPTUs

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

suA (kPa)D

ybd

e (m

)

Basert på Nkt Basert på NDu

Blokkprøver Valgt profil SuA

54 mm prøver, øvre grense estimert

Based on Nkt

Block sample, CAUC tests

54 mm samples, estimated upper limit

Based on Ndu New SuA profile used

Final design shear strength profile

Case history Nykirke railway track

Upgraded shear strength profile resulted

in possible change in technical solution

Total cost savings of about USD 1.2 mill or 25 % of total

Contract cost

Example: oil and gas

field development

Åsgard in Norwegian

Sea

Åsgard 240 – 310 m water depth

Subsea structures with floating production & storage

Anchoring of floaters

Seabed structures

Foundations for:

– Wellheads

– Control and production units

– Manifolds, tie-ins, end

termination for pipelines

– Riser support

– Protection systems

Pipelines

Risers

Picard et al., 2007Offshore Nigeria

Picard et al., 2007

Picard et al., 2007Offshore Nigeria

Soil investigation

carried out over

several years

Profile BB’ from CPTUs

Typical soil profile from Borehole with CPTU and sampling

Nkt = (qt - vo)/suCAUC

Nkt in OC clays • We still do not have a rational scheme for determining Nkt

factors in OC clays offshore Norway

• If we do not have any local or regional experience we

tend to use Nkt = 15 -20

• In most projects we can adjust as we get more data – as

in shown example

• One problem is difficulty of knowing effect of sample

disturbance plus effects of fissures and inhomogenieties;

ie uncertainties in reference su

Should we operate with operational shear strength like Marsland in UK?

From Marsland and Powell (1988)

Nkt values in stiff OC UK clays

Marsland used 865 mm dia plates to derive operational shear strength as reference su and hence Nkt factors depends on degree of fissuring

Relative density

• As input to correlations to other parameters such as friction

angle, , and various deformation moduli, M, E, Gmax

• Building in of specimens for triaxial, DSS , oedometer and

resonant column tests

In spite of many limitations relative density, Dr, is still a very important parameter in characterizing sandy soils

Large Calibration Chamber

ENEL Italy

Sand sample :

-diam. = 1.22 m

-hight = 1.5 m

Tests in large calibration chambers (CC) in period ca 1970 – 1990 especially in Italia Main background for correlations we use today.

Calibration Chamber Tests

Grain size distribution curves of some sands used

Correlations to Dr and other parameters are mainly based on CC tests on Ticino and Hokksund sands

% P

assin

g

Relative density, Dr (ID)

e = in situ void ratio = volume of voids/volume of

solids

emax = max. void ratio (loosest state)

emin = min. void ratio (denses state)

minee

eeID

max

maxDr

According to ASTM standard

qc, vo’,Dr relationship for Ticino NC sand

After Baldi et al.(1986)

Similar correlations developed for OC sands Then replace vo’ with meano’ Where meano’ = 1/3 (1+2 Ko) vo’

Need to evaluate Ko

Drammen sand : Results of 2 CPTUs

qc = 2.1 MPa

3.0

7.0

vo,kPa qc,MPa Dr,%

60 2.1 27

100 3.0 28

200 7.0 43

Example estimating Dr in sand

McDonald’s Farm, Vancouver, Canada

Limitations of qc, vo’,Dr correlations

• Only valid for the type of sands used in CC

• Fine to medium uniform, mainly quartz sand

• Unaged/uncemented

• vo’ > 50 kPa

• Not valid for silty soils

• Not valid for compressible sand,

eg calcareous sand

• Uncertain at shallow depth( < 3 – 5 m)

Some tentative corrections

Relative density in silty sands

Cone resistance in silty sand is lower than in clean sands. Relative density will be too low if using standard correlations

There are no well established method to correct for silt content But for predicting liquefaction potential correlations have been developed based on extensive R&D in US

Since liquefaction potential is to a large extent dependant on in situ density and stress which also control cone resistance we may use the correction in lack of other methods

Diagram for correcting qc for liquefaction analyses

qc1= (qc/pa)(pa/ vo’)0.5

pa = reference stress = 100 kPa

Work out qc1 from basic CPT data, then qc1,corr = qc1 + qc1

Then use qc,corr in Baldi’s diagram to estimate Dr

Soil description

40

30

20

10

0

Dep

th b

elo

w s

eab

ed,

m

CP

TU

0 10 20 30 40 50

Cone resistanceqt, MPa

-0.5 0 0.5 1 1.5

Pore pressureu, MPa

0 1 2 3 4

Friction ratioRf %

-1 -0.5 0 0.5 1

Pore pressureratio, Bq, %

End of PCPT 33.3 m

Very dense SAND

Very soft sandy CLAY

Soft to firm CLAY

Medium dense becoming loose siltySAND 17.10-17.40 Stiff CLAY

Firm becoming very hard gravellyCLAY

hydrostaticline

Case in Irish Sea – wind farm development

Typical seafloor CPTU profile

Upper sand layer

0.002 0.02 0.2 2 20

Grain size, mm

0

10

20

30

40

50

60

70

80

90

100

%P

assin

g

A1/A1A

A8/A8A

C7

D1/D1A

D4

E6-P9-B1

SUB1

PF1-Batch-1

PF1-Batch-2

ISO Standard Sieves .075 .125 .25 .5 1 2 4 8 1619 31.5 63

3"US Standard Sieves 200 100 50 30 16 8 4 3/8" 3/4" 1.5"

SILT SAND GRAVELCLAY Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse

0.006 0.06 0.6 6 60

Compare with CC sands

Quite similar to CC sands: Ticino and Hokksund. Use Baldi et al (1985)correlation

% P

assin

g

Soil description

40

30

20

10

0

De

pth

be

low

seab

ed

, m

0 10 20 30 40 50

Cone resistanceqt, MPa

-0.5 0 0.5 1 1.5

Pore pressureu2, MPa

0 1 2 3 4

Friction ratioRf %

-1 -0.5 0 0.5 1Pore pressure

ratio, Bq, %

0 20 40 60 80 100120140

Relative density,Dr, %

Very dense SAND

Very soft sandy CLAY

Soft to firm CLAY

Medium dense becoming loose silty

SAND

Firm becoming very hard gravelly

CLAY

Dr interpreted using correlation

presented by Baldi et al. (1986)and using K0 = 0.5 and 1.0.

hydrostaticline

Example from Windfarm in Irish Sea

Questionable due to high silt content

Dr in lower, silty sand

0.002 0.02 0.2 2 20

Grain size, mm

0

10

20

30

40

50

60

70

80

90

100

%P

assin

g

A1/A1A

A8/A8A

C7

D1/D1A

D4

E6

SUB1

PF2-Batch-1

PF2-Batch-2

ISO Standard Sieves .075 .125 .25 .5 1 2 4 8 1619 31.5 63

3"US Standard Sieves 200 100 50 30 16 8 4 3/8" 3/4" 1.5"

SILT SAND GRAVELCLAY Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse

0.006 0.06 0.6 6 60

Sand very different from CC sands, Correlations have to be used with caution, if at all

Fines content 10 – 45 %

% P

assin

g

CPT’10

25

20

15

10

5

0

De

pth

belo

w s

ea

bed

, m

0 20 40 60 80 100 120

Relative density, Dr, %

Without correction

With correction for 35% fines

Calculated from undisturbed samples

Baldi et al. (1986) Dr correlation using K0 = 0.4.

qc correction assuming 35% fines

Lab Dr values from meas. on "undisturbed samples"

Very denseSAND

Medium dense todense silty SAND

to sandy SILT

Example from Wind farm in Irish Sea

Corrected using liquefaction correction chart

Relative density in compressible sands

High compressibility gives lower cone resistance compared to a low compressibility sand

Using correlations for low to medium compressibility sands will give too low Dr

Calcareous, highly compressible sands have been tested in calibration chambers in Italy and Oxford, UK

0 10 20 30 40 50 60 70 80 90 100

Dr (Original)

0

5

10

15

20

((q

c/p

a)/

(v' /p

a)0

.5)0

.5Toyora sand

Quiou sand, dry

Quiou sand, saturated

Hokksund, Italy

Ticino, Ismes

Ticino, ENEL

DogsBay, OU

Hokksund, OU

LeightonBuzzard,OU

Average low to med compressibility

Average high compressibility

Average curve medium to low compressible sands

Average curve highly compressible sands

For highly compressible sands we can tentatively use ratio between these average lines to estimate Dr

Evaluate compressibility from oedometer tests

ORI9 500µm-4x

Quntifying compressibility based on oedometer tests

Calcareous Central America sand

Quartz sands

Calcareous sands

Dr- Baldi et al corr

Dr – Compr sand corr

53 % 76 %

45 % 64 %

30 % 38 %

0 10 20 30 40 50 60 70 80 90 100

Dr (Original)

0

5

10

15

20

((q

c/p

a)/

(v' /p

a)0

.5)0

.5

Toyora sand

Quiou sand, dry

Quiou sand, saturated

Hokksund, Italy

Ticino, Ismes

Ticino, ENEL

DogsBay, OU

Hokksund, OU

LeightonBuzzard,OU

Average low to med compressibility

Average high compressibility

Example Central America Calcarous compressible sand

qc, MPa

fs, MPa

Rf, %

Effects of shallow depths on CPT interpretation

Correlations by Baldi et al. and others really valid for vo’ > 50 kPa – corresponding to about 5 m depth; ie uncertain 0 – 2/3 m below sea floor

For shallower depth new approach has been suggested by Emerson et al. (2008)

Mainly an issue offshore in connection with pipeline/seabed structures

Effects of shallow depths on CPT interpretation

Emerson et al. (2008)

zcrit = critical depth qst = «limit» value of qc

Analysis of an in situ CPT profile according to Emerson et al.(2008).

Definition of critical depth and qst

Relative density as function of cone resistance at critical depth qst

Emerson et al.(2008)

So far this approach is valid for NC sands

Dr %

20 40 60 80 100

qst (M

Pa)

qst

Tentative approaches for dealing with silty sands, highly compressible sands and shallow depth need further development, but are the best we can do now Alternative route for future may be to use additional sensors like nuclear density measurements

Corrections to relative density, Dr

Additional sensors – for improved interpretation

CPTU Cone penetration test with pressure measurement

DMT Flat Plate Dilatometer

PMT/ CPT Pressuremeter Test with CPT

MOSTAP SAMPLER

Detector

NDP GC Neutron density probe

Source

Lead shield

ERCPT Electrical resistivity CPTU

Electrodes Wenner Array

Seismic Source

SCPTU SDMT Seismic CPT Or Seismic DMT

Geophones or Accelerometers

Seismic cone - Illustrate by land based system

Measure shear wave velocity as function of depth Geophone

From Paul Mayne

Use of SCPT for identifying aged/cememented sands

Eslaamizaad and Robertson, 1997

qc1 = (qc/pa)(pa/ v0’)0.5

qc1

Go/q

c

High compressibility

Low compressibility

Increasing cementation and/or aging

Small strain shear modulus Gmax = ρVs

2

ρ = soil density

Slide 67

Other applications for the seismic cone:

• Static and dynamic design parameters

• Soil liquefaction assessment

• Quantitative sample quality evaluation

• Ground truth for geophysical methods

• Vs enhance CPTU interpretation

Gamma Cone Penetrometer

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0 10.0 11.0

12.0

13.0

14.0

15.0

qc (MPa)0 2 4 6 8 10 12 14 16 18 20 fs (kPa)0 50 100 150 200 250 300 350 400 450 500

0 2 4 6 8

Friction ratio, Rf

0 5 10 15 20 25 30 35 40

Natural Gamma (radiation collision counts per period)

Courtesy of Gardline

Chalk

Future challenges

• More widely use of add on sensors: Seismic cone,

nuclear density probe +

• Better correlations for ”non- textbook” soils

Heavily OC fissured clays

Silty sand/silt

Calcareous sands

Chalk

Peat

Etc

• More reliable sleeve friction readings

Summary and conclusions

• CPT/CPTU has an essential role in offshore soil investigations; and is

also gaining in use on land

• Best tool available for stratification and very useful for assessing soil

behaviour

• In soft clay undrained shear strength should be computed both from qt

and u

• No universal scheme exist for Nkt factors in stiff OC clays in North Sea

– local correlations still required

• Relative density correlations are strictly valid for medium to fine,

uniform, silica sands – but tentative corrections for other sands may be

used

• Potential for enhanced interpretation by using add - on sensors

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