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Pressuremeter testing

Fernando SchnaidFederal University of Rio Grande do Sul, Brazil

Short course:Pressuremeter

Hydraulic hose

Conducting hose

Standardcone rods

Cone rodadaptor

Amplifierhousing

Contractionring

Pressuremetermodule

Contraction ringConnector

Piezocone

43,7 mm

645

705

625

Pushhead

Control unit

Introduction

Testing equipment

Theorectical background

Cavity expansion theory

Interpretation

Standard methods

Curve fitting technique

Advanced analysis

Large strain analysis

Non-textbook materials

Unsaturated soil conditions

References (text books)The pressuremeter and foundation engineering. F.Baguelin, J.F. Jezequel & D.H.Shields. Book Trans. Tech. Publ. Series (1978)Pressuremeter Testing: methods and interpretation. R.J. Mair & D.M. Wood. CIRIA Report (1987)Pressuremeters in Geotechnical Design.B. Clarke. Blackie (1995) Cavity Expansion Methods in Geomechanics. H.S.Yu. Kluwer Academic Publishers (2000)In Situ Testing in Geomechanics. F. Schnaid. Taylor & Francis (2008)

Equipment and proceduresEquipment and procedures

Drilling fluiddrilling fluid return

Drilling fluid

pore-pressurecell

soil removal

cutter

Penetration

shoe

Testing

rubber

electricaland gas

feelermembrane

DefinitionThe pressuremeter is a cylindrical probe that has an expandable flexible membrane designed to apply a uniform pressure to the walls of a borehole.

ISSMFE (Amar et al, 1991)

Uniqueness In situ stress-strain measurementCavity expansion theory (ideally modeled as an expanding cavity in an elastic-plastic continuum).

The pressuremeter probe

Different installation techniquesPrebored: PBPM

Menard probe, MPT

Self - boring: SBPM

Pushed-in: PIPcone-pressuremeter (CPMT)

Prebored pressuremeter (PBP)

Designed to be lowered in a pre-bored holeMeasuring system

Volume displacement (Menard type)Radial displacement

Menard probe: 3 independent cells (centre cell + 2 guard cells)

EquipmentTypical results

Menard pressuremeter probegas

ground level

guard cell

measuringcell

probe

burette pressure gauge

Typical pressuremeter test result

Self-boring pressuremeter (SBPM)Designed to minimise disturbance to the surrounding soilType

PAF (pressiomètre autoforeur) – Jezequel et al (1973)Camkometer – Wroth (1973)

Measuring systemDisplacements

(3 instrumented arms: 120o spacing: centre membrane)Pressure transducer

driven pressurecutter (position and rotation)drilling fluid (pressure)

Self-boring pressuremeter test

Self-boring pressuremeter test

Drilling fluiddrilling fluid return

Drilling fluid

pore-pressurecell

soil removal

cutter

Penetration

shoe

Testing

rubber

electricaland gas

feelermembrane

Typical SBP test result (loops removed)

300

300

300

0,03

Cavity stra in

Pres

sure

(kPa

)

0,030,03

Lift-off pressure

70

60

50

40

30

20

10

0

0.040.030.020.01 0.00-0.01

Displacement (mm)

Tota

l pre

ssur

e (k

Pa)

arm 3arm 2arm 1

PoUncertainties related to the assessment of the in situ horizontal stress

Pushed-in pressuremeter

Soil around the probe is completely disturbed during penetrationCone-pressuremeter (CPMT)

Full-displacement tool (100% volume strain)Measuring system: Volume or radial displacementInterpretation:

More complex Large strain analysisUnloading portion of the pressuremeter curve

Typical cone-pressuremeter test result

CalibrationsMUST BE PERFORMED: Before & after test3 groups of calibrations (Clarke, 1995):

Pressure and displacement measuring system

Conventional procedures Membrane stiffness

Inflate the membrane in airsensitive to temperaturecalibration cycles required (stress or strain controlled)Compliance of the system (volume changes)

Procedures

Stress controlled or strain controlled testsPre-bored devices: stress controlled

Increments of pressure are specified Typically 15 to 20 increments

Self-boring (computer control systems)Strain controlled: few readings at the initial stiff soil responseStress controlled: problems around the onset of yieldCompromise: tests becomes strain controlled after having started by equal pressure increments

Procedures

Clays: strain rates 1%/minfully undrained expansion

Unload-reload cyclesallow for creep strains to cease before cycle

recognise the dependency of modulus on strain amplitude

Note: test requires highly specialised operator skills and site supervision.

20

Theoretical interpretation (examples)Undrained analysis

a) Gibson e Anderson (1961) (su, G, σho)b) Palmer (1972) (su = ƒ (ε), G)c) Jefferies (1988) (su, G, σho) Curve fitting techniqued) Yu & Collins (1998) (su, G, σho) OC Clays

Drained analysis (pure frictional materials)a) Vésic (1972) (PL = ƒ (φ´, σ´ho, Δ))b) Hughes et al (1977) (φ´= ƒ (s, φcv, ψ))c) Robertson e Hughes (1986) (φ´= ƒ (s, φ´cv, ψ) d) Houlsby et al (1986) (φ´, φ´cv, ψ, G, σ´ho) unloading e) Manassero (1989) ( σ1/σ3 x γ (p.c.) ← σr, εθ , φ´cv)

21

Principlesthe analysis of problems involving axially symmetric loading was introduced by Timoshenko and Goodier (1951). enables simulation of expansion of an infinite long cylindrical cavity (length is much greater than radius) the surrounding material is subjected to plane strain deformation, with no deformation in the direction (assumed vertical) parallel to the axis of the cavity. in the definition of the problem, the radial σ´r, circumferential σ´θ and axial σ´z stresses are all principal stresses. The axial (vertical) stress is considered to be the intermediate stress and plane strain conditions in the axial direction are assumed.

22

Expansion of cylindrical

cavity

Tensile circunferential strainεθ = y/r

Radial strainεr = δy/ δr

Circunferential strain at cavity wall (only measured variable)

εc = (r-r0)/ r0

23

Elastic ground

Elastic theory

in

24

Elastic ground

σ´ σ´

σ´r

Po σ´z

σ´θ

o ao r

a

25

R GA C E F

G’A’ C’ E’ F’

Rδ

2θσσ −r

G Su

θEEr −G’

F’

E’ C’ A’

2θσσ −r

Su

θEEr −G’

F’

E’ C’ A’

hoσ

2θσσ +r

Elastic-Plastic response: clay

26

Elasto-Plastic response: sand

1sin φ

ACE

F

O

2θσσ −r

S’=2

'' θσσ +r

hoσF

Plastic

Elastic

A C E

R

r1

27

Cohesionless soilsFailure is governed by a Mohr-Coulomb criterion

. Shear: sand dilates or contracts (φ´≠ φ´cv) Rowe´s stress-dilation theory .

ψ = mobilised angle of dilation (assumed to be constant)

. The onset of yielding p-u0= σ´h0 (1+sinφ´)

φ−φ+

=σσ

θ sin1sin1

,

,r

⎟⎟⎠

⎞⎜⎜⎝

⎛ψ−ψ+

⎟⎟⎠

⎞⎜⎜⎝

⎛

φ−φ+

=φ−φ+

sin1sin1

sin1sin1

sin1sin1

,cv

,cv

,

,

ψγ−=υ sinc

28

Cohesionless soilsHughes et al (1977): after yielding loge (p-uo) = S loge (εc+c/2)+const Plot loge (p-uo) x loge (εc+c/2) ⇒ slope S

. Parameters

( )( ),

,

sin1sinsin1Sφ+

φψ+=

( ) ,cvsin1S1

Ssinφ−+

=φ

( ) cvsin1SSsin φ−+=ψ

29

Unloading analysisMathematics: extension of the loading analysis

1sin φ2

θσσ −r

S’=2

'' θσσ +rGF2

E2

E1

D1C1

A1

D2

A2B2 C2

B1

F

Elastic

A B E

R

r1

r2

C D

30

Unloading analysis

Jefferies (1988)

amax and Pmax are the radius and pressure at the end of the loading stageP versus slope equal 2 times Su

⎟⎟⎠

⎞⎜⎜⎝

⎛−−⎟⎟

⎠

⎞⎜⎜⎝

⎛+−=

max

maxmax ln2ln12

aa

aa

SSGSPP u

uu

⎟⎟⎠

⎞⎜⎜⎝

⎛−−

max

maxlna

aa

a

InterpretationMethods: Limitations of Cavity Expansion

1. Probe not vertical2. Vertical stresses are not intermediate stresses3. Anisotropy and non-homogeneity4. Soil not a continuum – discontinuities5. Partial drainage6. Ground properties are test rate dependent7. Cavity may not expand as a cylinder8. Installation effects

After Clarke, 1995

Interpretation (standard methods)1. HORIZONTAL STRESSa) Lift-off pressure

Problems: inclination, movement of the body, compliance of the system.

SBPM technique in SAND: is disturbance inevitable?(Windle, 76; Fahey, 82; Wroth,,84, Fahey & Randolph, 84)PMP: hard to define datum (plastic effects during unloading)

b) Methods based on shear strengthMarshland and Randolph (77): stiff clays

Forces consistence ⇒ p(yield) ≈ po + Su

Lift-off pressure: Typical test

Interpretation2. SHEAR MODULUS

Unload-reload loopsExpand membrane: elastic-plastic boundary on undisturbed soilStress cycle will be elastic

Non-linear soil responseMeasured G should account for the relevant stress and strain levels acting around the probe (e.g Bellotti et al, 89)Pre-failure deformation properties (after Tatsuoka, Jardine)

dVdpVG

ddp

21G

c

⋅=

ε⋅=

Illustration of a typical pressuremeter curve

Typical unload-reload loop

1

2Gur

1. Wait for creep strains to cease

2. Average slope or consider non-linear response

Unload-reload loop - clay

Unload-reload loop - sand

Non-linearity

Usual plot G/G0 x γ ⇒G0 from seismic tests

Modulus degradation:

Tatsuoka & Shibuya (1992)

Interpretation3. UNDRAINED SHEAR STRENGTHa) Slope of p: ln (ΔV/V) curve

All conditions previously discussed should be met: Undisturbed, homogeneous mass, elastic-perfectly plastic

Plastic part of the pressuremeter loading curve: straight line when results are plotted in log scale as total cavity pressure against volumetric strain

Note: Pressuremeter tends to overestimate predicted Su values L/D effects should be taken into account

⎟⎠⎞

⎜⎝⎛ Δ

+=VVSu lnlimψψ

SBP test in clayafter Wroth (72)

Cavity strain (%)

Cav

ity p

ress

ure

(kP

a)

SBP test in clay after Ghionna et al (72)

Interpretation

4. ANGLES OF SHEARING AND DILATIONa) Slope of ln (p-u0): ln εc curve

Note: Reference datum should be carefully selected

( )

( )( ) cv

cv

´sin1sssin´sin1s1

s´sin

´sin1´sinsin1s

φ−+=ψφ−+

=φ

φ+φψ+

=

SBP test in sandafter Wroth (72)

Circumferential strain (%)Cav

ity p

ress

ure

(kP

a)

SBP test in sandafter Wroth (72)Cavity strain (%)

P-u

0(k

Pa)

Curve fitting approach

the parameters that produce an analytical curve which satisfactorily adjusts to the experimental results are representative of the soil behaviour and compatible with other in situ test results.The analytical methods should be implemented in mathematical packages.The danger is that different combinations of parameter values can produce an equally good fit of experimental data.Introduce software and the fitting process

48

Interpretation: advanced analysis for Napoles

1. Large strain analysis 2. Unsaturated soil conditions3. Cemented materials

49

Collapse potential: unsaturated soil mechanics

OEDOMETRIC TEST PRESSUREMETER TEST PLATE LOAD TEST

σ v

Hi H f

constant diameter

ri r f σ r

H =

con

stan

t

σ v

Δ H

unknownfield stress

(H i = ?)

Colapso material: Equador

0.9 1.0 1.1 1.2 1.3 1.4r/ro

0

100

200

300

400

500

600

700

800

900

1000

1100

pres

são

(kPa

)

EPN5 (s = 45 kPa) - 1998

EPN6 (s = 40 kPa) - 1996EPI4 (s = 0) - 1998EPI5 (s = 0) - 1997EPI6 (s = 0) - 1997

52

Unsaturated Soil conditions

0 25 50 75 100 125time (min)

-60

-50

-40

-30

-20

-10

0po

re w

ater

pre

ssur

e (k

Pa)

37 kPa (1m)

42 kPa (2m)

50 kPa (3m)

53

Unsaturated Soil conditions

0 100 200 300 400 500 600 700 800injected volume (cm³)

0

200

400

600

800

1000

1200

1400ca

vity

pre

ssur

e (k

Pa)

0

5

10

15

20

25

30

35

40

45

50

suct

ion

(kPa

)

constant watercontent curve

saturatedcurve

constant w.c. (tensiometer at 30 cm)

constant w.c. (tensiometer at 60 cm)

saturated(tensiometer at 30 cm)

0 10 20 30 40 50 60 70 80 90tempo (horas)

-50-45-40-35-30-25-20-15-10-505

10

poro

-pre

ssão

(kPa

)

sucção = 39 kPa

início da inundação (10:08 h)

final da inundação (14:08 h)

tendência denova equalização

período do ensaio

( ) 'cosc'sen1PP of φ⋅+φ+=Yield stress

'sen1'cosc2

'sen1'sen1

r φφ

φφσσ θ +

⋅−

+−

⋅=Stress state

( )c cu u

a b u ua w

a w= ′ +

−

+ −Cohesion intercept

avr u

3p −

++= θσσσ [ ]2

r2

v2

vr )()()(21q θθ σσσσσσ −+−+−=

( ) pppM

qps

2

2

0 ++

=Yield function

css 'cotcp φ−=−( )

1K2K3

'cos3K

1K2K4pqM

o

o2/1

cs2

2o

o2o

+⎥⎥⎦

⎤

⎢⎢⎣

⎡−

+−== φ

Analysis

Constant suction during shear

56

Unsaturated Soil conditions

0.9 1.0 1.1 1.2 1.3r/ro

0

200

400

600

800

1000

cavi

ty p

ress

ure

(kPa

)

YG - 2 m depth

predicted curves ( = 43 , = 15 , Po = 60 kPa)

experimental curve (s = 43 kPa)

experimental curve (s = 0)unsaturated curvec = 20 kPa = 0.24G = 5.5 MPa

φ ψo o

saturated curvec = 1 kPa = 0.3G = 3.0 MPa

ν

ν

57

ps po

q

p

Mη

η

> M < M

q 1

p1

rtrajetória pressiométrica elásticaPressuremeter stress path

58

-1 0 1 2 3p/Po

0

1

2

q/Po

M

s/Po = 2

s/Po = 1

s/Po = 0.5

s/Po = 0

CSL

-1 0 1 2 3p/Po

0

1

2

s/Po

LC

59

60

Cohesive frictional soils

(φ´, φ´cv,ψ, c´, G, σ´ho)Suggested approach: curve fitting technique

Analysis: fully draineda) Yu e Houlsby (1991,1995) b) Mantaras & Schnaid (2003) &

Schanaid & Mantaras (2004)

Futai et al, 2004Futai et al, 2004

Gens & Nova, 1993Gens & Nova, 1993

⎟⎠

⎞⎜⎝

⎛ φ+

πσ

+⎟⎠

⎞⎜⎝

⎛ φ+

π=

εε

−σσ

2´

4tan.

´´c.2

2´

4tan

dd

1

1.´´ f

3

f2

1

v3

1

Schnaid & Mantaras, 2002, 2003Schnaid & Mantaras, 2002, 2003

Flow rule- Rowe, 1962Flow rule- Rowe, 1962

Structuration and destructuration during shear

Structuration and destructuration during shear

65

Stress distribution Domínio elástico Domínio plástico

o2

2o

r Pr

.bα1

Y1)α(P

σ +⎥⎦⎤

⎢⎣⎡

++−

=αα1

α1α

or .r.b

1αα

1-αY1)α(P2

α1Yσ

−−

+⎥⎦⎤

⎢⎣⎡ +−

+−

=

2

2o

oθ r

.bα1

Y1)α(P

Pσ⎥⎦⎤

⎢⎣⎡

++−

−= αα1

α1α

o

θ .rα

.b1α

α1-α

Y1)α(P2.

α-1Yσ

−

−

+⎥⎦⎤

⎢⎣⎡ +−

+=

σ

Elástico σrp

Plástico σre

Po

σθp σθ

e

o r = a r = b raio

Pressuremeter as a trial value boundary problem:Pressuremeter as a trial value boundary problem:

G, σho, ν, φ, ψ, c

67

Case Study: Hong KongResidual granite

0 100 200 300 400 500 600 700 800p` (kPa)

0

100

200

300

400

500

q (k

Pa)

z = 30 m

z = 32 m

z = 36 m

= 33.4 => ´ = 41.3

= 30.4 => ´ = 36

68

Case Study: Hong KongSBP at 29.6m depth

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14Cavity strain

0

200

400

600

800

1000

Pres

sure

[kPa

]

arm 1

arm 2

arm 3

Average

Analytic simulation

69

Case Study: Hong KongSBP at 30.6m depth

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14cavity strain

0

200

400

600

800

1000

pres

sure

[kPa

]

arm 1

arm 2

arm 3

Average

Analytic simulation

70

Case Study: Hong Kongfriction angle

28

30

32

34

36

38

40

Dep

th [m

]

20 25 30 35 40 45 50' (degrees)

SBPM Hughes et al (1977)

SBPM Yu & Houlsby (1991), loading

SBPM Analytic simulation , loading

SBPM Houlsby et al (1986)

SBPM Analytic simulation, unloading

Lab. TX

DMT (Marchetti,1997)

71

Concluding remarks1. There is still an enormous application to pressuremeter tests in

non-text book materials, which will require developments on equipment, testing procedures and interpretation

2. The purpose if this last session is to stimulate the discussion and development of methods to interpreted data obtained from tests in residual soil and unsaturated materials.

3. Analysis of pressuremeter data through a curve fitting technique is proposed. As the theoretical framework of interpretation is becoming more sophisticated this approach becomes increasingly attractive.

4. Interpretation of pressuremeter data requires engineering judgment, regardless the method of interpretation that adopted.