cpt - a useful tool in geotechnical investigations · for in situ sounding ... • as input to...
TRANSCRIPT
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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