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THE USE OF CPT AND CPTU FOR SOILCHARACTERIZATION

Claudia MEISINA Department of Earth and Environment Sciences

University of Pavia , [email protected]

WORKSHOPCPT AND ITS USE FOR THE GEOTECHNICAL

INVESTIGATIONS

New Delhi, 4th April 2012

OUTLINE1. Introduction

2. Application of CPT and CPTu

3. General factors affecting interpretation of CPT and CPTu data

4. Stratigraphic profiling

5. Soil classification methods

6. Lithotype and stratigraphic boundaries identification examples insome italian soil types

1. Conclusions

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1. INTRODUCTION

The geological models provide an understanding of

the geological processes which made the site

(geological materials, geological structure and the

ancient and active geological processes in the

area)

The engineering geological model can be used in

the characterization of a site for engineering

purposes. The engineering geological model can

be achieved through the identification of the

stratigraphic units and the spatial reconstruction of

the lithological variability; generally this can be

done through geognostic surveys (boreholes,

trench pits, etc).

complementary tools for stratigraphic investigations

Lithotype identification

Identification of stratigraphic boundaries Lithological variations

reconstruction of the stratigraphic profile stratigraphic correlations

CPT/CPTU measurements provided a high-resolution data set suitable for3D modeling of subsoil.

Continuous measurements of soil parameters (qc, fs, u)

Measurement repeatibility

Possibility of investigating a soil volume greater than that of

laboratory samples

1. INTRODUCTION

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Contrasting response of four

different alluvial facies associations(fluvial channel, crevasse splay,

levee and floodplain deposits)

to cone penetration. A palaeosol,

marking the transgressive surface

(TS) is indicated by anomalous fs

and u values.

Interpreted geological cross-

section, showing how

CPTU profiles can be usedfor recognition of major

stratigraphic discontinuities

and mapping of sedimentary

bodies.

Amorosi & Marchi, 1999

NWSE stratigraphiccross-section showingthe six stratigraphicunits identified in thestudy area (from base totop, Grv: Pleistocenelowstand gravels, Snd:

transgressive sands, U1, U2,

U3 and U4 Holocene

sediment bodies).

Correlation betweenboreholes andCPT/CPTUprofiles is shown.

Lafuerza et al., 2005

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2. APPLICATION OF CPT AND CPTu(ISO 22476-1)

G: profiling and material identificationwith low associated uncertainty level

G*: indicative profiling and materialidentification with high associateduncertainty level

H: interpretation in terms of design

with low associated uncertainty level

H*: indicative interpretation in termsof design with high associateduncertainty level

The selection of the type of CPT/CPTu is related to the type of soil and to the accuracy

which is a function of the intended use of the data.

USE OF CPT AND CPTu

A: homogeneously bedded soils with verysoft to stiff clays and silts (typicallyqc

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APPLICATIONCLASSES

PENETRATIONTEST TYPE

MEASURED

PARAMETERS

Allowable minimum

accuracy (a)SOIL INTERPRETATION

3CPTu

CPT

Cone resistance

Sleeve friction

Pore pressure

Inclination

Penetration length

200 kPa or 5%

25 kPa or 15%

50 kPa or 5%

5

0.2 m or 2%

A

B

C

D

G

G, H*

G, H

G, H

4 CPTCone resistance

Sleeve friction

Penetration length

500 kPa or 5%

50 kPa or 5%

0.2 m or 1%

A

B

C

D

G*

G*

G*

G*

3: evaluation of mixed bedded soil profiles with to soft to dense soils, in terms ofprofiling and material identification. Interpretation in terms of engineering

properties for very stiff to hard and dense to very dense layers. For stiff clays orsilts and loose sands only an indicative interpretation can be given. Penetrometertype depends on project requirements.

4: indicative profiling and material identification for mixed bedded soil profiles withsoft to very stiff or loose to dense layers. No appreciation in terms of engineeringparameters can be given. Tests are to be performed with CPTe

3. GENERAL FACTORS AFFECTINGINTERPRETATION OF CPT AND CPTu

Equipment design

In situ stresses

Compressibility, cementation and particle size

Stratigraphy

Before analysing any CPT/CPTu data, it is important to realize and account forthe potential errors that each element of data may contain

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Equipment design

The three major areas of cone design that influenceinterpretation are:

1. Unequal area effects.

2. Piezometer location, size and saturation.

3. Accuracy of measurements.

most significant in soft, normally consolidated, fine-grained soils.

sand are little influenced

In situ stresses

stress (geologic) history of the deposit is of great importance inCPT/ CPTu interpretation

Compressibility, cementation and particle size

The compressibility of soils can significantly influence qc and fs.

Highly compressible sands low cone resistance and highfriction ratio values.

Cementation between particles reduces compressibility andthereby increases the cone resistance.

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Stratigraphy

The transition from one layer to another will not necessarily be registered asa sharp change.

The cone resistance is influenced by the material ahead and behind thepenetrating cone. Hence the cone will start to sense a change in materialtype before it reaches the new material and will continue to sense a materialeven when it has entered a new material. Therefore, the CPT/CPTu will notalways identify the correct transition in thinly interbedded materials.

The distance over which the cone senses an interface increases withmaterial stiffness.

soft materials diameter of the sphere of influence

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5. SOIL CLASSIFICATION

A soil classification system provides ameans of grouping soils according totheir engineering behavior.

The conventional method for determininga soil type is by laboratory classificationof samples retrieved from a borehole(e.g. USCS).

If a continuous, or nearly continuous,

subsurface profile is desired, the conepenetration test (CPT(CPTu) providestime and cost savings over traditionalmethods of sampling and testing.

Begemann (1965)

Schmertmann (1978)

Searle (1979)

Douglas & Olsen (1981)

Robertson et al. (1986)

Robertson (1990, 2009, 2010)

Eslami & Fellenius (1997)

A number of classification methods arereported to predict soil type from eitherCPT or/both CPTu dataCharts that link cone parameters to soiltype

5.1 SOIL CLASSIFICATION CHARTS

Begemann (1965)

the classification chart formechanical cone penetration tests

is based on 250 different data,relating to Dutch soils.

The qc is on the y-axis and thesleeve friction fs on the x-axis.

The lines (passing through theorigin), which subdivide the map infields, allowing us to identify thesoil, were obtained on the basis ofthe weight percentage of particleswith a diameter less than 16 mm

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5.1. SOIL CLASSIFICATION CHARTSSchmertmann (1978)

the graph uses the Begemann databaseand a series of mechanical conepenetration tests carried out in Florida.

On the y-axis qc is plotted on alogarithmic scale, whereas the frictionratio Rf = (fs/qc)*100 is plotted on the x-axis on a linear scale.

Qualitative indications about density of

sands (increasing with qc) and stiffnessof clays (increasing with fs) are alsogiven.

the method is not so accurate for low qcvalues

5.1. SOIL CLASSIFICATION CHARTSSearle (1979)

the classification chart represents thecone resistance qc (MPa) on the y-axis in logarithmic scale, and on the

x-axis Rf in the same scale.

The Searle method, like theSchmertmann method, providesadditional indications, such as thedensity of sands and stiffness of finesoils.

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5.1. SOIL CLASSIFICATION CHARTS

qt = qc + u2*(1-An/Ac)

The chart could be used in real-time to evaluate soil type during and immediately after the

CPTU, since it only requires the basic CPTU measurements.


the use of both charts can lead to different indications

qt=1MPa; Rf = 4%; Bq = 0.1

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Zone Soil Behaviour Type (SBT)

1 Sensitive fine-grained

2 Clay - organic soil

3 Clays: clay to silty clay

4 Silt mixtures: clayey silt & silty clay

5 Sand mixtures: silty sand to sandy silt

6 Sands: clean sands to silty sands

7 Dense sand to gravelly sand

8 Stiff sand to clayey sand*

9 Stiff fine-grained*

* Overconsolidated or cemented

Robertson (2010) provides an update of the chart in terms of dimensionless cone resistance, (qc/pa),

where pa = atmospheric pressure (pa = 1 bar = 100 kPa = 0.1 MPa) and Rf (in percent), both on log

scales to expand the portion where Rf < 1%. The number of soil behaviour types has also been reduced

to 9 to match the Robertson (1990) chart.


Robertson (1990)

The author proposed

using normalized cone

parameters qt, Rf and

Bq, to take into

account the influence

that the lithostatic

pressure may exert at

great depths

The chart can be used

for depths of more

than 30 m from ground

level.

The normalization of

the parameters

requires also some

input of soil unit weight

and groundwater

conditions (use of the

chart during post-

processing).

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Eslami & Fellenius (1997, 2000)

the classification chart is based on a database

containing soils taken from 20 sites from various

parts of the world.

The database does not include cases of

cemented soils or very stiff clays.

qE (effective cone resistance) = (qt u2).

In dense sandy soils qE only differs marginally

from qt; whereas in the case of fine grained soils

qt and qE could assume very different values.

The authors split the classification chart into a

series of fields, corresponding to the various

lithotypes the Canadian Foundation Engineering

Manual (Canadian Geotechnical Society, 1985).

5.3. SOIL CLASSIFICATION CHARTS -limitations

the correlations were established on soils coming from geological contextsthat might be different than the examined soils.

The geologicalgeotechnical conditions (lithotype, degree of alteration,cementation, consolidation, etc) of soil used to find the correlations shouldbe carefully analyzed to verify their applicability to the studied soil;

The soil classification boundaries, defining soil classification zones, werelargely subjectively determined (Cai et al., 2011)

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The classifications methodshave some limits:

the application of Begemann(1965) classification chart isdifficult for values where qc< 5MPa and fs

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the CPT and CPTU-based charts were predictive of soil behaviourtype (SBT), since the cone responds to the in-situ mechanicalbehaviour of the soil and not directly to soil classification criteriabased on grain-size distribution and soil plasticity (e.g. Unified SoilClassification System, USCS*).stress history,

macro fabric

void ratio

water content

good agreement between USCS-based classification and CPTU-

based SBT, except for mixed soils (i.e. sand mixtures and siltmixtures);

* The USCS classification system is also based on remolded soil conditionsrather than in situ conditions

CPT/CPTU response SBT

60% sand40% fines

silty sand (sand-silt mixtures) or clayeysand (sand-clay mixtures)

USCS

high plasticity

the soil behaviour may be more

controlled by the clay and the CPTu-

based SBT will reflect this behaviour

and will predict a more clay-like

behaviour, such as clayey silt to siltyclay

low plasticitysoil behaviour will be controlled more by

the sand and the CPTu-based SBT

would predict a more sand-like soil type,

such as silty sand to sandy silt

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5.4. INNOVATIVE METHODSfuzzy logic (Zhang and Tumay, 1999),

the artificial neural networks (Kurup and Griffin,

2006)

probabilistic approaches (Jung et al., 2008)

Approach based on statisticsand probability. It estimates theprobability of sand, silt, andclay in investigated soils.

Kurup et al, 2010

6 Lithotype and stratigraphic boundaries

identification examples in some italian soil types

A - peaty deposits;B - alluvial - lacustrine deposits, extremelyheterogeneous in terms of depth and area;C- terraced alluvial deposits;D - terraced alluvial deposits in the PoValley, mainly sandy;E - recent alluvial deposits of the River Po;F - alluvial fan deposits;G H - ancient terraced alluvial depositssouth of the River Po;I - estuarine - marine deposits

DATABASE

CPT, CPTu data, approximately 6-23 mdeep, from 11 different Italian sites,belonging to different geological contexts

were collected from published reports orobtained from tests

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6.1. Lithotype and stratigraphic boundaries

identification - methodTEST EQUIPMENT

Tests were carried out with a Paganipenetrometer (TG 63-100, TG 63-200,TG 73-200) (Pagani, 2009). The testequipment consists of 60 cone, with a10 cm2 base area and a 150 cm2 frictionsleeve located above the cone. Thefilter position for pore pressuremeasurements is behind the cone tip(u2). CPTu were carried out at constantspeed of 2 cm/s. The pushingequipment consists of hydraulic jackingand reaction system mounted on aheavy lorry with screw anchors. Thethrust capacity is of 100 to 200 kN. Thefield data acquisition system includesanalogue to digital converters. Thepiezocone provides values of coneresistance, sleeve friction and porepressure every 1 cm.


identification - method

Borehole-logs

Laboratoryinvestigation

Penetration tests

Soil profiles

Classification tests

Direct shear tests

Oedometer tests

Triaxial tests

Different fluidsfor filter saturation

Different periodsof the year

Wet

Dry

Glycerine

Silicon oilsCPTU classification

charts

CPT-CPTuclassification

charts

% of success

Nof intervalscorrectlyclassified

in a lithologicalclass/

total nofintervals of thatlithological class

Comparison betweenCPT/CPTU

and borehole logs

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identification results CPTBegemann chart (1965) Schmertmann chart (1978) Searle chart (1979)

Site

Peat(1)

Sand/

gravellysand(6)

Clay/

loam(

3)

Siltysand(5)

Clay,siltandsand(4)

Organicclaysandmixedsoils

(1)

Sand(6)

Siltyand

sandyclays(3)

Siltsandclayeysands(4)

Silt-sandmixtures(5)

Peat(1)

Sand(9)

Clayeysilt(5)

Clayeysandysilt(6)

Clayeysiltysand(7)

Siltysand(8)

Sand(10)

A 78 0 - - - 78 45 - - - 0 9 - - - - -

B 67 0 0 0 12 78 0 0 14 12 0 - 17 28 14 0 0

E - - - 0 6 - - - 8 6 - - 0 13 - 0 -

Percentage of success for CPT (A = organic soils, B = lacustrine soils, E = alluvial soils)

CPT interpretation charts usually identify organic soils (78% of rate ofsuccess) but they show unsatisfying results for mixed silty soils (0-28%)

6.2. Lithotype and stratigraphic boundariesidentification results CPT

1 - Peat 1 - Peat

2 - Clay 2 - Peaty clay

3 - Clay/Loam 3 - Clay

4 - Silt, Clay, Sand 4 - Silty clay5 - Silty sand 5 - Clayey silt

6 - Sand/Gravelly sand 6 - Clayey sandy silt

7 - Clayey silty sand

8 - Silty sand9 - Sand

10 - Gravell sand

11 - Sandy gravel12 - Gravel

5 - Sandy silt

6 - Sand

1 - Organic clays and mixed soils

2- Inorganic clays

3 - Sandy and silty clays

4 - Clayey sands and silts

Begemann, 1965

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7

SBT

Depth(m)

CPT borehole

Schmertmann, 1978

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7

SBT

Depth(m)

CPT borehole

Searle, 1978

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10 11 12

SBT

Depth(m)

CPT borehole

while thepeats andorganic clays

have a highsuccess rateof correctidentification,the otherclasses(mainly silts)are hardlyever identifiedcorrectly

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6.2. Lithotype and stratigraphic boundariesidentification results CPT

the Begemann method and in particular theSchmertmann method gave good success rates inthe case of soft clays, organic clays or sands.

the Searle method has lower success rate. However,the lithotypes are in general classified as adjacent

or similar and so the misinterpretation observed for

such a method are, in practice, acceptable. Theinteresting aspect of the Searle method is that it isbased on a significantly greater number of classes


identification results CPTUSBT B C F-w F-d G H-s H-g

IL

Robertsonetal.chart(1986)

2- Organic soil 100

3- Clay 100 100

3 e 4 - Clay and Silty clay 100 46 85 100 51 10

5- Clayey silt to silty clay 0 0 10 12

5 & 6 21 52

6- Sandy silt to clayey silt 0 0 117- Silty sand to sandy silt 0 16 10

8- Sand to silty sand 0 0 10

9- Sand 100

Robertsonchart(1990) 2-Organic soils and peat 60

3-Clays (clay to silty clay) 51 100 0 100 85 100 100

4-Silt mixtures (silty clay to clayey silt) 0 0 04 & 5 10 44 95-Sand mixtures (sandy silt to silty sand) 0 9 0 0

6-Sand (silty sand to clean sand) 0 100

9-Very stiff, fine-grained soil 68

CPTU percentage of success (C, G, H, L = alluvial soils,F = alluvial fan soils, I = marine, littoral soils; w = wetperiod; d = dry period; s = sil icon oil; g = glycerin).

Robertson et al. (1986) chart

correctly identify 100% of

organic soils, clays and sands,

whereas most of intermediatesoils (such as clayey sil t and

sandy silt) are not recognized,

with percentages of success

that range from 50% to 0%;

Robertson chart (1990) shows

results comparable to the

previous chart

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identification results

u (kPa)

0

2

4

6

8

10

12

-50 150 350 550

u2(kPa)

u0(kPa)

qc (MPa)

0

2

4

6

8

10

12

0 5 10 15

depth(m)

Rf (%)

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Grain size (%)

0

2

4

6

8

10

12

0 20 40 60 80

clay si lt sand gravel

Silt with clay and

peat

Silt and sand

Alternating sandy

silt and

clayey silt

Clayey silt

Atterberg limits

0

2

4

6

8

10

12

0 50 100

IP Wp Wl

Silt and clay, clayey

silt with peat

Clayey silt with

sand

Sand

water table

All classification methods allow to detect stratigraphical boundaries

Olocenic superficial deposits of the plain between Altopascio and Bientina. Soil profile and geotechnicalcharacteristics. qc: cone resistance; u0: in-si tu pore pressure; u2: pore pressure measured at cone base; fs:

sleeve friction; Rf: friction ratio (fs/qc*100) IP: plastic index; Wp: plastic limit; Wl: liquid limit

extreme granulometric and lithological heterogeneity


1- Sensitive fine-grained soil

2- Organic soil

3- Clay

4- Silty clay to clay

5- Clayey silt to silty clay

6- Sandy silt to clayey silt

7- Silty sand to sandy silt

8- Sand to silty sand

9- Sand

10- Sand to gravelly sand

11- Very st iff fine-grained soil

12- Overconsolidated or cemented sand to clayey sand

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10 11 12

Depth(m)

SBT


0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10 11 12

SBT

Robertson et al. (1986)Filtered data (A=1; D=0,5)

Silt with clay andpeat

Silt and clay, clayeysilt with peat

Alternating sandy siltand clayey silt

Clayey silt

Clayey silt withsand

Sand

water table

Silt and sand

Filtering methodologies canbe applied to qc, u and fs

values

BOREHOLE

CPTU testsrevealed

decimetric

levels of

sandy silt/silty

sands

intercalation

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7. Factors that influence CPT and CPTU

interpretation

The use of CPT and CPTU for the identification of lithotypes andstratigraphical boundaries is sometimes complicated byseveral constraints:

1. the minimum layer thickness that can be detected by penetrationresistance

2. the presence of soils made up of different grain size (e.g.gravelly clay),

3. the presence of partially saturated soils

4. the presence of mixed soils (i.e. sand mixtures, silt mixtures)

5. the repeatability of the tests in different climatic conditions.

1. the minimum layer thickness that can be detected bypenetration resistance

The detected thickness depends on the relative stiffness of twocontiguous layers

the penetration resistance of a soft layer (clay) below a rigid layer (densesand) is fully mobilized even for thicknesses of 1-2 diameters,

a thickness of 10-20 diameters is needed to fully mobilize the resistance of a

rigid layer underneath a soft one.

(Vreugdenhil et al. (1994), Ahmadi and Robertson (2005))

landfill

silty clay

clayey silt

sandy silt

sandy clay

water table

0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5 6 7 8 9 10 11 12

SBT

Robertson et al., 1986

0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5 6 7 8 9

SBT

Robertson, 1990

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2.the presence of soils made up of different grain size (e.g.gravelly clay),

the classes of soils proposed by the various authors indicate a gradual transitionfrom fine to coarse - grained soils. Soil made up of very different grain size (e.g.gravelly clay) can not be interpreted correctly

the inclusions can distort the soil interpretation by causing sharp reductions in pore-water pressure (pwp) that temporarily impair the performance of the cone sensor,when the cone sensor is located on the cone shoulder. These rapid reductions inpwp are caused by the inclusion being pushed aside by the cone, thus creating localsuctions adjacent to the pwp sensor (Ramsey, 2010).

3. the presence of partially saturated soils

u (kPa)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-50 150 350

CPTu1(grease)

CPTu 2(siliconoil)

qc (Mpa)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10

dep

th(m)

CPTu1 (grease)

CPTu 2 (siliconoil)

Rf (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20

CPTu1(grease)

CPTu 2(siliconoil)

grain size (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60 80

clay silt sand gravel

Atterberg limits (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40

IP Wp Wl Wn

clayey silt

sandy silt

clayey silt

sandy silt

clayey silt

sandy silt

silty sand

water table (9.5 m)

Olocenic deposits of the River Po in Calendasco (Piacenza, Northern Italy).Clayey silts and sandy silts (CL) with sandy intercalations down to a variable depth ofbetween 8.6 and 6.6 m. At greater depths there is a gravelly layer. The water table is 9.5meters below ground surface.Two CPTU tests were performed by saturating the tip with grease (CPTU1) and siliconoil (CPTU2).

increase of qc is not correlated to a lithologic change

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1- Sensitive fine-grained soil 1- Sensitive fine-grained soil

2- Organic soil 2-Organic soils and peat

3- Clay 3-Clays (clay to silty clay)

4- Silty clay to clay 4-Silt mixtures (silty clay to clayey silt)

5- Clayey silt to silty clay 5-Sand mixtures (sandy silt to sil.sand)

6- Sandy silt to clayey silt 6-Sand (silty sand to clean sand)

7- Silty sand to sandy silt 7-Sand to gravelly sand

8- Sand to silty sand 8-Sand - Clayey sand to very stiff sand

9- Sand 9-Very stiff, fine-grained, overconsolidated or

10- Sand to gravelly sand cemented soil

11- Very stiff fine-grained soil

12- Overconsolidated or cemented sand to clayey

sand

Clayey silt

sandy silt

clayey silt

sandy silt

clayey silt

sandy silt

sand

0,0

1,0

2,0

3,0

4,0

0 1 2 3 4 5 6 7 8 9 10 11 12

Depth(m)

SBT


0,0

1,0

2,0

3,0

4,0

0 1 2 3 4 5 6 7 8 9

SBTn

Robertson, 1990

The clayey silts between 0.6 mand 2.1 m are distributed in

numerous fields.

The superficial silty clay layer is

identified as sandy silt/silty sand

by Robertson et al. (1986) and

as sand by Robertson (1990).

The variability in the

interpretation of the layer from 0cm to 2.10 m and the

overestimation of soil grain size

can be explained by the

presence of a partially saturated

layer, which leads to an increase

of the resistances, particularly

evident in the classification

obtained with the Robertson

method (1990).

The CPTU tests carried out

using different saturation fluids

do not show any significant

variations in stratigraphic

interpretation

Calendasco. Comparison between stratigraphical profile of the

borehole and those obtained through CPTu tests. SBT: soil

behavior (in black: CPTU1, in red: CPTU2)

4. the presence of mixed soils (i.e. sand mixtures, siltmixtures)

The CPT and CPTU test typically shear fine-grained materials in anundrained manner and coarse-grained materials in a drained manner.

1- Sensitive fine-grained soil 1- Sensitive fine-grained soil 1-Sensitive - collapsible clay

2- Organic soil 2-Organic soils and peat and silt

3- Clay 3-Clays (clay to silty clay) 2-Clay and silt

4- Silty clay to clay 4-Silt mixtures (silty clay to clayey silt) 3-Silty clay and clayey silt

5- Clayey silt to silty clay 5-Sand mixtures (sandy silt to sil.sand) 4-Sandy silt and silty sand6- Sandy silt to clayey silt 6-Sand (silty sand to clean sand) 5-Sand and sandy gravel

7- Silty sand to sandy silt 7-Sand to gravelly sand

8- Sand to silty sand 8-Sand - Clayey sand to very stiff sand

9- Sand 9-Very stiff, fine-grained, overconsolidated or

10- Sand to gravelly sand cemented soil

11- Very stiff fine-grained soil

12- Overconsolidated or cemented sand to clayeysand

Robertson et al. 1986

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8 9 10 11 12

SBT

Depth(m)

Robertson 1990

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

27.5

30

0 1 2 3 4 5 6 7 8 9 10

SBT

Eslami e Fellenius 1997

SBT

Sand

Clay

Clayey silt -

silty sand/sandy

silt

The success rates aregood for saturatedhomogeneous soils,particularly for soft clayor organic soils.

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Groundwatertable

Landfill

qc (MPa)

0

2

4

6

8

10

12

14

16

18

20

22

0 10 20 30 40

Borehole-log n2

Clays and silts with algae

Sand with c lay, silt, gravel

"Panchina"

Silt and sand

Silt with gravel , sand, c lay

Silt and clay

Sand with gravel

Gravel and rounded

pebbles

Fine sand and silt

Depth(m)

u2 (kPa)

0

2

4

6

8

10

12

14

16

18

20

22

-50 450 950

fs (kPa)

0

2

4

6

8

10

12

14

16

18

20

22

0 100 200 300

Alternances of clay,

clay mixtures and

sand mixtures

Clay mix. and sand mix.SandSand/Sand mixturesClay mixtures

Sand

Clay

Sand

Interpretation with

Robertson chart

(1990)

Landfill

SITE 2: LIVORNO COASTAL PLAIN

The response to conventional CPT/CPTU of intermediate soils in partially drainedconditions (Jaeger et al, 2010).For silty clays or soft silty sands the classification charts mis-classify the soil type.Intermediate soils tend to be much more difficult to differentiate (Ramsey, 2010; LoPresti et al., 2010).

5. the repeatability of the tests in different climaticconditions

u (kPa)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-50 50 150 250

CPTu 1 (wetperiod)

CPTu 2 (dryperiod)

u0 (kPa)

qc (Mpa)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2 4 6 8

depth(m)

CPTu 1 (wetperiod)

CPTu 2 (dryperiod)

Rf (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15

CPTu 1(wetperiod)

CPTu 2(dryperiod)

grain size (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60

clay silt sand gravel

Atterberg limits (%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60

IP Wp Wl Wn

man-madedeposits

silty clay

clayey silt

sandy silt

silty clay

water table

Qc and fs depend on the in situ conditions, which are related to the climaticconditions of the period when the tests are carried out

alluvial fan of the Scuropasso Stream (an Apennine right tributary of the River Po), inthe province of Pavia (Northern Italy)The higher penetrometric resistance values in the dry period down to a depth of almost 3.0 m, are o be

attributed to higher values of the effective stresses as an effect of the partial saturation in the dry period. The

different trend of qc in the two periods, also confirmed by the Rf friction ratio, seems to show the thickness of

the soil, which is sensitive to the variations of moisture content as a result of the climate (active zone).

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landfill

silty clay

clayey silt

sandy silt

sandy clay

water table

0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5 6 7 8 9 10 11 12

SBT


0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5 6 7 8 9

SBT

Robertson, 1990

Scuropasso Stream alluvial fan. Comparison between stratigraphical profile of the borehole and those

obtained through CPTU tests. SBT: soil behavior; in red: CPTU1; in black: CPTU2.

oman-made deposits (0-0.75 m) are distributed in numerous fields in relation to the heterogeneity of the

material.

oIn the Robertson (1990) classification the soils that go down to a depth of 1.60 m fall into fields with

very stiff soil (fields 8 and 9).

oWith respect to the test carried out in June we can note a variation in the classification between 0.75

and 2.30 m due to an increase in the resistances, connected to de-saturation.

8. CONCLUSIONS CPT and CPTU parameters can be used to provide an estimate of soil behavior type

(SBT) that may not always agree with traditional soil classifications based on grain

size distribution and soil plasticity.

The considered classification charts correctly identify the lithotypes in the case of

homogeneous saturated deposits.

The success rate is predominantly good for soft or organic clays and for sands, whileit drops quite notably for the intermediate soils (silts, clayey and sandy silts and fine

sands with silt) and for soils made up of very different grain size (e.g. gravelly clay).

For the CPTs, the Begemann method and in particular the Schmertmann method gave

good success rates in the case of soft clays, organic clays or sands. The Searle

method has lower success rate. However, the lithotypes are in general classified as

adjacent or similar and so the misinterpretation observed for such a method are, in

practice, acceptable. The interesting aspect of the Searle method is that it is based on

a significantly greater number of classes. All the considered methods correctly

identified the stratigraphic boundaries.

CPTU gave a better estimation of the soil profile with respect to CPT. For some

interpretation methods, data filtering greatly enhanced the ability to accurately predict

soil profile. In some case it seems that there are problems with detecting thin layers

even when using CPTU. All the considered methods correctly identify the stratigraphic

boundaries.

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The presence of a shallow partially saturated crust (especially in the case offine - grained soils) led to over estimation of the soil grain size. Suchmisinterpretation is emphasized when using the Robertson (1990) method.

The results underline that the considered interpretation methodologiesdepend very closely on the geological conditions of the soils, on which theseclassifications were established, and hence cannot be regarded as totallyreliable. Moreover penetration tests always need a calibration by means ofstratigraphic logs from boreholes.

The stratigraphic logging and classification based on CPT and CPTU datarequires knowledge about the geological history and soil genesis to allow fora proper interpretation. Nevertheless, the CPT and CPTU can be used withconfidence when supported by all the other tests and information at ourdisposal from the site investigation.

CPT/CPTU tests can be used for subsurface stratigraphic correlations andthey can significantly help in the identification of engineering geological unitsand in the construction of the engineering geological model of a site. Theycan define local situations which require detailed studies.

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