Fernando Schnaid - Antonino Di...
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Pressuremeter testing
Fernando SchnaidFederal University of Rio Grande do Sul, Brazil
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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
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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)
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Equipment and proceduresEquipment and procedures
Drilling fluiddrilling fluid return
Drilling fluid
pore-pressurecell
soil removal
cutter
Penetration
shoe
Testing
rubber
electricaland gas
feelermembrane
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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).
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The pressuremeter probe
Different installation techniquesPrebored: PBPM
Menard probe, MPT
Self - boring: SBPM
Pushed-in: PIPcone-pressuremeter (CPMT)
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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
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Menard pressuremeter probegas
ground level
guard cell
measuringcell
probe
burette pressure gauge
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Typical pressuremeter test result
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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
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driven pressurecutter (position and rotation)drilling fluid (pressure)
Self-boring pressuremeter test
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Self-boring pressuremeter test
Drilling fluiddrilling fluid return
Drilling fluid
pore-pressurecell
soil removal
cutter
Penetration
shoe
Testing
rubber
electricaland gas
feelermembrane
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Typical SBP test result (loops removed)
300
300
300
0,03
Cavity stra in
Pres
sure
(kPa
)
0,030,03
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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
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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
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Typical cone-pressuremeter test result
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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)
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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
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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.
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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)
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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.
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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
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Elastic ground
Elastic theory
in
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Elastic ground
σ´ σ´
σ´r
Po σ´z
σ´θ
o ao r
a
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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
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Elasto-Plastic response: sand
1sin φ
ACE
F
O
2θσσ −r
S’=2
'' θσσ +r
hoσF
Plastic
Elastic
A C E
R
r1
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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
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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
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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
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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
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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
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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
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Lift-off pressure: Typical test
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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⋅=
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Illustration of a typical pressuremeter curve
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Typical unload-reload loop
1
2Gur
1. Wait for creep strains to cease
2. Average slope or consider non-linear response
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Unload-reload loop - clay
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Unload-reload loop - sand
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Non-linearity
Usual plot G/G0 x γ ⇒G0 from seismic tests
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Modulus degradation:
Tatsuoka & Shibuya (1992)
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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
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SBP test in clayafter Wroth (72)
Cavity strain (%)
Cav
ity p
ress
ure
(kP
a)
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SBP test in clay after Ghionna et al (72)
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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+
=
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SBP test in sandafter Wroth (72)
Circumferential strain (%)Cav
ity p
ress
ure
(kP
a)
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SBP test in sandafter Wroth (72)Cavity strain (%)
P-u
0(k
Pa)
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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
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Interpretation: advanced analysis for Napoles
1. Large strain analysis 2. Unsaturated soil conditions3. Cemented materials
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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 = ?)
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Colapso material: Equador
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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
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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)
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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)
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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
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( ) '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
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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
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ps po
q
p
Mη
η
> M < M
q 1
p1
rtrajetória pressiométrica elásticaPressuremeter stress path
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-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
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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)
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Futai et al, 2004Futai et al, 2004
Gens & Nova, 1993Gens & Nova, 1993
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⎟⎠
⎞⎜⎝
⎛ 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
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Structuration and destructuration during shear
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Structuration and destructuration during shear
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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
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Pressuremeter as a trial value boundary problem:Pressuremeter as a trial value boundary problem:
G, σho, ν, φ, ψ, c
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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
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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
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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
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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)
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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.