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Drained and Undrained
Analysis
1
Analysis
Prof. Minna KarstunenUniversity of Strathclyde
Thanks to Dennis Waterman, Antonio Gens, Marcelo Sanchez & Helmut Schweiger
Outline
• Drained / undrained conditions
• Modelling undrained behaviour with Plaxis
• Influence of constitutive model and parameters – Influence of dilatancy
2
– Influence of dilatancy
– Undrained behaviour with Mohr-Coulomb Model
– Undrained behaviour with Hardening Soil Model
• Summary
Nicoll
Highway, Highway,
Singapore
Drained / Undrained Conditions
In undrained conditions, no water movementtakes place and, therefore, excess pore pressuresbuild up
∆u ≠ 0, ∆σ ≠ ∆σ'∆u ≠ 0, ∆σ ≠ ∆σ'
In drained conditions, no excess pore pressuresbuild up
∆u = 0, ∆σ = ∆σ'
Drained / Undrained Conditions
• Drained analysis appropriate when
– Permeability is high
– Rate of loading is low
– Short term behaviour is not of interest for – Short term behaviour is not of interest for
problem considered
• Undrained analysis appropriate when
– Permeability is low and rate of loading is high
– Short term behaviour has to be assessed
Drained / Undrained Conditions
Suggestion by Vermeer & Meier (1998) for deep excavations:
T < 0.10 (U < 10%) use undrained analysis
T > 0.40 (U > 70%) use drained analysis
tDγ
EkT
2
w
oed====
k = Permeability
Eoed = Oedometric modulus = 1/mv
γw = Unit weight of water
D = Drainage length
t = Construction time
T = Dimensionless time factor
U = Degree of consolidation
Undrained Behaviour
Implications of undrained soil behaviour:
– Excess pore pressures are generated
– No volume change
In fact small volumetric strains develop because a In fact small volumetric strains develop because a
finite (but high) bulk modulus of water is introduced
in the finite element formulation
– Predicted undrained shear strength depends
on soil model used
– Assumption of dilatancy angle has serious
effect on results
Triaxial test (NC)Typical results from drained (left) and undrained (right) triaxial tests on normally
consolidated soils (from Atkinson & Bransby, 1978)
Triaxial test (OC)Typical results from drained (left) and undrained (right) triaxial tests on overconsolidated soils
Stress Paths in Undrained Triaxial Test – NC / OC
1 3 2
t−
=σ σ
1 3
1 3
2
2
2
's
s
′ ′+
′ ′+
=
=
σ σ
σ σ
Strength Parameters According to MC
� Mohr-Coulomb parameters in terms of effective stress
tancτ σ ϕ′ ′ ′= +ϕ′
τ
c′ σ ′
11 33 sin ; sin 2 tan tan2
c c′ ′ ′ ′ + ′ ′ ′= + = + ′ ′
− σ σϕ
σ
ϕ ϕ
σ
In terms of principal effective stresses
Strength Parameters According to MC
� Mohr-Coulomb parameters in terms of total stresses
� Only undrained conditions!
τ
uC
total stressestancτ σ ϕ′ ′ ′= +
� Soil behaves as if it was cohesive
� : undrained shear strength
� only changes if drainage occurs (no change if undrained conditions prevail)
( )u u
c s=
1 3
2 F
σ σ−
,σ σ′
uC
u-C
1 3
2 F
σ σ′ ′−
Effective stresses
uc
ucτ =
What is the critical case: drained or
undrained?
NC
OC
Loading
Unloading
t
NC
OC
note that for soils in general:
•level of safety against failure is lower for short term (undrained) conditions for loading problems (e.g. embankment)
•level of safety against failure is lower for long term (drained) conditions for unloading problems (e.g. excavations)
however …
Unloading
s, s’
What is the critical case: drained or
undrained?t
NC
OC
Loading
•For very soft NC soil, factor of safety against failure may be lower for short term (undrained) conditions for unloading problems (e.g. excavations)
•For very stiff OC soil, factor of safety against failure may be lower for short term (undrained) conditions for loading problems (e.g. embankment)
s, s’
Unloading
FE Modelling of Undrained
Behaviour
What Plaxis does when specifying
type of material behaviour: undrained ?
The issue:
•Constitutive equations are formulated in terms of σ’•Constitutive equations are formulated in terms of σ’
•We need to compute D
εσ ∆=∆ '' D
D∆σ = ∆ε
FE Modelling of Undrained Behavior
Principle of effective stress �
with
since the strains are the same in each phase,
fσσσ ∆+∆=∆ '
[ ]Twwwf ppp 000∆∆∆=∆σ
εσ ∆=∆ ff D33
33
00
01ef KD =
n
KK
f
e ≅
εσ ∆=∆ '' D
' ' ( ' )f f fD D D D D∆σ + ∆σ = ∆σ = ∆ε = ∆ε + ∆ε = + ∆ε
pore fluid stiffness, related to thebulk modulus of pore fluid (water) Kf
fDDD += '
33 n
� We need D D∆σ = ∆ε
FE Modelling of Undrained Behavior
� Example: linear elastic model + plane strain
∆εDσ∆ ′=′
4 2 2' ' ' 0
3 3 3
2 4 2' ' ' 0
3 3 3
2 2 4' ' ' 0
exx xx
e
yy yy
ezz zz
K G K G K G
K G K G K G
K G K G K G
+ − −
′σ ε ′σ ε− + − = ′ σ ε − − +′σ γ
& &
& &
& &
)1(2)21(3 ν+=
ν−=
EG
EK fDDD += '
'G G=
2 2 4' ' ' 0
3 3 3
0 0 0
exy xy
K G K G K G
G
− − +′σ γ
& &
4 2 20
3 3 3
2 4 20
3 3 3
2 2 40
3 3 3
0 0 0
exx xx
e
yy yy
ezz zz
exy xy
K G K G K G
K G K G K G
K G K G K G
G
+ − −
σ ε σ ε− + − = σ ε − − +σ γ
& &
& &
& &
& &
=∆σ D∆ε
FE Modelling of Undrained Behavior� Example: linear elastic model + plane strain
4 2 2' ' ' 0
3 3 3 02 4 2
0' ' ' 0 3 3 3
0
e e e
e e e
K G K G K G
K K K
K K KK G K G K GD
K K K
+ − −
− + − = +
fDDD += '
02 2 4
' ' ' 0 0 0 0 03 3 3
0 0 0
e e eK K K
K G K G K G
G
− − +
4 2 2' ' ' 0
3 3 3
2 4 2' ' ' 0
3 3 3
2 2 4' ' ' 0
3 3 3
0 0 0
e e e
e e e
e e e
K G K K G K K G K
K G K K G K K G KD
K G K K G K K G K
G
+ + − + − +
− + + + − + = − + − + + +
FE Modeling of Undrained Behavior� Example: linear elastic model + plane strain
fDDD += '
4 2 2' ' ' 0
3 3 3
2 4 2' ' ' 0
' 3 3 3
2 2 4' ' ' 0
3 3 3
0 0 0
e e e
e e e
e
e e e
K G K K G K K G K
K G K K G K K G KD D D
K G K K G K K G K
G
+ + − + − +
− + + + − + = + = − + − + + + 0 0 0 G
4 2 20
3 3 3
2 4 20
3 3 3
2 2 40
3 3 3
0 0 0
K G K G K G
K G K G K GD
K G K G K G
G
+ − −
− + − = − − + ' eK K K= +
4 4'
3 3e
K G K K G+ + = +
FE Modelling of Undrained
Behavior
All the above (which is valid for any soil (or model) for which the principle of effective stress applies) can be easily combined with the FEM
• instead of specifying the components of D, specify D'‚ and Ke
• when calculating stresses,
fDDD += '
εσ ∆=∆ '' D
f e vKσ ε∆ = ∆
fσσσ ∆+∆=∆ '
Undrained Behaviour with PLAXIS
A value must be set for Ke
• real value of Ke = Kw/n ~ 1•106 kPa (for n = 0.5)
• in fact, the pore-fluid is assigned a bulk modulus that is substantially larger than that of the soil skeleton (which ensures that during undrained loading the volumetric strains are very small)are very small)
(((( ))))(((( ))))
(((( ))))u
u
u
uw
total
213
1G2
213
E
n
K'KK
νννν−−−−
νννν++++====
νννν−−−−====++++====
PLAXIS automatically adds stiffness of water when undrained material type is chosen using the following approximation:
Undrained Behaviour with PLAXIS
uu
(((( ))))(((( ))))(((( ))))'1213
1'EK
u
u
totalνννν++++νννν−−−−
νννν++++==== assuming νu = 0.495
• Note: this procedure gives reasonable results only for ν' < 0.35 !
• Note: in Version 8 B-value can be entered explicitly for undrained materials
Undrained Behaviour with PLAXIS
Skempton’s B parameter (undrained conditions)
( )w
w
w
KK'
KB
K
K'BBB
+=
+
=σ∆+σ∆+σ∆=∆=∆n
n
ppw ;
1
1 ;
3
1321
therefore, if Kw
is very large (compared to K’), B is very close to 1
Undrained Behaviour with PLAXIS
Method A (analysis in terms of effective stresses):type of material behaviour: undrainedeffective strength parameters (c', ϕ', ψ‘)effective stiffness parameters (E50', ν‘)
Method B (analysis in terms of effective stresses):type of material behaviour: undrainedtype of material behaviour: undrainedtotal strength parameters (c = cu, ϕ = 0, ψ = 0)effective stiffness parameters (E50', ν‘)
Method C (analysis in terms of total stresses):type of material behaviour: drainedtotal strength parameters (MC: c = cu, ϕ = 0, ψ = 0)total stiffness parameters (MC: Eu, νu = 0.495)
Undrained Behaviour with PLAXIS
(Method A)
• Analysis in terms of effective stress
• Type of material behaviour: undrained
• u changes (excess pore water pressures generated)
• Constitutive equations are formulated in terms of σ’
' 'D∆σ = ∆ε' 'D∆σ = ∆ε
In the case of Mohr Coulomb model:
effective strength parameters c’, ϕ’, ψeffective stiffness parameters E50', ν'
• the total stiffness matrix is computed as: fDDD += '
Undrained Behaviour with PLAXIS
(Method A)
u
uf
t
ESPTSP
c
– single set of parameters in terms of effective stress (undrained, drained, consolidation analysis consistent)
– realistic prediction of pore pressures (if model is appropriate)
– the undrained analysis can be followed by a consolidation analysis (correct pore pressures, correct drained parameters)
– cu is a consequence of the model, not an input parameter!!
s, s’
ESP
uc
( ) )''('' ; )''(''1
0 3123122 σ∆+σ∆ν=σ∆σ∆+σ∆ν−σ∆′
==ε∆E
For plane strain: the undrained effective stress path rises vertically
• In the case of the Mohr-Coulomb model (in plane strain), it is easy to compute cu analytically
Undrained Strength for Method A
Linear Elasticity
( ) ( ) 0)'1(''3
1 '''
3
1'0' 0
'
'31321 ≈ν+σ∆+σ∆=σ∆+σ∆+σ∆=∆⇒≈∆⇒≈
∆=ε∆ pp
K
pv
( ) 0'''2
1 31 ≈∆=σ∆+σ∆ s
Effective Stress
Path, ESP B’
a’ =
c’c
osφ’
sinφ’( )1 3
2t
σ − σ=
Plane strain: effective stress path rises vertically
Undrained Strength for Method A
uc
A’
a’ =
c’c
os
( ) ( )1 3 1 3' '
' , 2 2
s sσ + σ σ + σ
= =
{ }
{ }0 0
0 0
1'cos ' 'sin ' 'cos ' sin '
2
'cos ' 1 1 sin ' ,
' ' 2
u o vo ho
u ho
v v vo
c c s c
c cK K
′ ′= + = + +
′= + + =
′
φ φ φ σ σ φ
σφφ
σ σ σ
uc
,o o
t s′
t
Undrained Strength for Method A
• The Mohr Coulomb model in terms of effective stresses OVERESTIMATES the undrained shear strength of soft clays!
s, s’
u realc
uMCc
Undrained Behaviour with PLAXIS
Method A (analysis in terms of effective stresses):type of material behaviour: undrainedeffective strength parameters (c', ϕ', ψ‘)effective stiffness parameters (E50', ν‘)
Method B (analysis in terms of effective stresses):type of material behaviour: undrainedtype of material behaviour: undrainedtotal strength parameters (c = cu, ϕ = 0, ψ = 0)effective stiffness parameters (E50', ν‘)
Method C (analysis in terms of total stresses):type of material behaviour: drainedtotal strength parameters (MC: c = cu, ϕ = 0, ψ = 0)total stiffness parameters (MC: Eu, νu = 0.495)
Undrained Behaviour with PLAXIS(Method B)
• analysis in terms of effective stress• type of material behaviour: undrained • u changes
• constitutive equations are formulated in terms of σ’ (but
strength in total stresses!)
εσ ∆=∆ D
total strength parameters c = cu, ϕ = 0, ψ = 0effective stiffness parameters E50', ν'
fDDD += '
' 'D∆σ = ∆ε
Undrained Behaviour with PLAXIS(Method B)
3 ; 0.495
2 1u u
EE ν
ν
′= =
′+
Resulting undrained stiffness parameters
t
Undrained Behaviour with PLAXIS(Method B)
s, s’
uc
– parameters in terms of total stress and effective stress
– prediction of pore pressures (generally unrealistic)
– the undrained analysis should not be followed by a consolidation analysis (pore pressures unrealistic)
– cu is an input parameter!!
ESP TSP
Undrained Behaviour with PLAXIS
Method A (analysis in terms of effective stresses):type of material behaviour: undrainedeffective strength parameters (c', ϕ', ψ‘)effective stiffness parameters (E50', ν‘)
Method B (analysis in terms of effective stresses):type of material behaviour: undrainedtype of material behaviour: undrainedtotal strength parameters (c = cu, ϕ = 0, ψ = 0)effective stiffness parameters (E50', ν‘)
Method C (analysis in terms of total stresses):type of material behaviour: drainedtotal strength parameters (MC: c = cu, ϕ = 0, ψ = 0)total stiffness parameters (MC: Eu, νu = 0.495)
Undrained Behaviour with PLAXIS
(Method C)
TSP=ESP
t
uc
– Parameters in terms of total stress
– No prediction of pore pressures (only total stresses are obtained)
– The undrained analysis can not be followed by a consolidation analysis
– cu is an input parameter!!
s, s’
Undrained Behaviour with PLAXIS
(Method C)
• Analysis in terms of total stress• Type of material behaviour: drained (in spite of modelling an undrained case)
• Porewater pressure does not change (because it is not calculated)
• Constitutive equations are formulated in terms of σ• Constitutive equations are formulated in terms of σ
εσ ∆=∆ D
total strength parameters c = cu, ϕ = 0, ψ = 0total stiffness parameters Eu, νu = 0.495
Undrained Shear Strength from
Advanced Models
� Although it is possible, in a few simple cases, to obtain
an analytical expression for cu, it is advisable to perform
a numerical “laboratory” test to check the value of
undrained shear strength actually supplied by the model
� It is important to perform the numerical “laboratory” test
under the same condition as in the analysis
� Plane strain, triaxial, simple shear
� Correct initial stresses
� Compression, extension, simple shear
� Not all cu values are achievable with a particular model
Influence of Dilatancy on
Undrained Shear Strength
If we set then, negative volumetric plastic deformations occur at failure:
0>ψ
0
e p
v v v∆ε = ∆ε + ∆ε
∆ε ≈
(elastic-plastic behavior)
(undrained conditions)
Therefore, in undrained analysis, dilatancy, , must be set to zero!
0v
∆ε ≈ (undrained conditions)
result: unlimited increase of q (or t), i.e. infinite strength!!
0 0 ' ' 0
At failure: ' 0
sin 0
p e e
v v vp K
q M p q
t s t
∆ < ⇒ ∆ > ⇒∆ = ∆ >
∆ = ∆ ⇒ ∆ >
′ ′∆ = ∆ ⇒ ∆ >
ε ε ε
ϕ
ψ
Comparison MC-HS (influence ψ)
200
225
250
275
300
Simulation of undrained triaxial compression test – MC / HS model - q vs ε1
ε1 [%]
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00
q [kN
/m2]
0
25
50
75
100
125
150
175
MC non dil
MC dil
HS_1 non dil
HS_1 dil
Comparison MC-HS (influence ψ)
175
200
225
250
275
300
MC non dil
MC dil
HS_1 non dil
HS_1 dil
total stress path
Simulation of undrained triaxial compression test – MC / HS model - q vs p´
p' [kN/m2]
0.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00 225.00 250.00
q [
kN
/m2]
0
25
50
75
100
125
150
175
Comparison MC-HS (influence ψ)e
xce
ss p
ore
pre
ssu
re [kN
/m2]
60
70
80
90
100
MC non dil
MC dil
HS_1 non dil
HS_1 dil
Simulation of undrained triaxial compression test – MC / HS model - ∆pw vs ε1
ε1 [%]
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00
exce
ss p
ore
pre
ssu
re [kN
/m
-20
-10
0
10
20
30
40
50
Summary
• FEM analysis of undrained conditions can be performed in effectivestresses and with effective stiffness and strength parameters (Method A)
• Method A must be used:
– if consolidation/long term analysis are required
– advanced soil models are adopted
• undrained shear strength is a result of the constitutive model
• care must be taken with the choice of the value for dilatancy angle
• Methods B (and C) provide alternative ways to analyze undrained problems but:
– the constitutive model does not generally represent the true soil
behaviour (before failure)
– potentially useful for stability problems in undrained conditions
(specification of undrained shear strength is straightforward)