drained or undrained is that the question
TRANSCRIPT
www.fugro.com© Fugro 2013
Amin RismanchianNick Ramsey
Feb. 2014
Drained or Undrained:Is That the Question?
www.fugro.comDate
What I am going to talk about
What do I mean by “Drained” and “Undrained”?
Relevant soil parameters, and methods of assessing these parameters
Drained vs. undrained breakout resistance
Flaws of current methods
Conclusions
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What do I mean by “drained” and “undrained”?
“Fully Drained is the condition under which water is able to flow into or out of a mass of soil in the length of time that the soil is subjected to some change in load.”
“Fully Undrained is the condition under which there is no flow of water into or out of a mass of soil in the length of time that the soil is subjected to some change in load. Changes in load cause changes in pore water pressure, because the water cannot move into or out in response to the tendency of volume change.”
Partially drained is the “twilight zone” between fully drained and fully undrained behaviour.
(Duncan and Wright 2005)
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Example: Cone Penetration Test, Offshore WA
Is this test indicating undrained/drained or partially drained conditions in the surficial soils?
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
2
2.5
3
Cone penetration resistance, qc (MPa)
Dept
h be
low
mud
line
(m)
-0.05 -0.025 0 0.025 0.05Pore pressure (MPa)
Generated pore pressure, u2
Hydrostatic pore pressure, uo
This slide is an example of a CPT performed in NWS Australia. Studying the excess pore pressures graph shows that both negative and positive excess pore pressures have been generated during this test, indicating an undrained condition based on the definition. However, we are not sure if it was really undrained or partially drained. Also we do not know how much the viscous effect contributed in the penetration resistance.
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Twilight Zone: Partially Drained
Zone Soil Type
1a SILTS and low Ir CLAYS
1b CLAYS
2 Essentially drained SANDS
3 Transitional soils
-2 -1.5 -1 -0.5 0 0.5 1 1.5 21
10
100
1000
Δu/σ′vo (-)
Qt (
-)
2
3 1a
1b
Refer to Schneider et al. (2008) for the boundary lines.
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Twilight Zone: Partially Drained
Zone Soil Type
1a SILTS and low Ir CLAYS
1b CLAYS
2 Essentially drained SANDS
3 Transitional soils
-2 -1.5 -1 -0.5 0 0.5 1 1.5 21
10
100
1000
Δu/σ′vo (-)
Qt (
-)
2
3 1a
1b
Refer to Schneider et al. (2008) for the boundary lines.
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Twilight Zone: Partially Drained
Zone Soil Type
1a SILTS and low Ir CLAYS
1b CLAYS
2 Essentially drained SANDS
3 Transitional soils
-2 -1.5 -1 -0.5 0 0.5 1 1.5 21
10
100
1000
Δu/σ′vo (-)
Qt (
-)
2
3 1a
1b
Refer to Schneider et al. (2008) for the boundary lines.
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-2 -1.5 -1 -0.5 0 0.5 1 1.5 21
10
100
1000
Δu/σ′vo
Qt
2
3 1a
1b
Twilight Zone: Partially Drained
Zone Soil Type
1a SILTS and low Ir CLAYS
1b CLAYS
2 Essentially drained SANDS
3 Transitional soils
Refer to Schneider et al. (2008) for the boundary lines.
When we plot the results of the CPT example from the previous slide on Schnider et al. (2008) chart, it is noted that the behaviour of this soil generally was partially drained. Can we use the results of this CPT in the design of drained or undrained cases?
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Example: Cone Penetration Test, Offshore WA
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
2
2.5
3
Cone penetration resistance, qc (MPa)
Dept
h be
low
mud
line
(m)
-0.05 -0.025 0 0.025 0.05Pore pressure (MPa)
Generated pore pressure, u2
Hydrostatic pore pressure, uo
Twilight zone (partially drained)
Fully drained
Fully undrained
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Drainage is a Function of the Normalised Velocity
CPT
Spudcan penetration
Pipeline penetration
Partially drained penetration
Undrained penetration
Pen
etra
tion
resi
stan
ceSpool
Drained penetration
Twilight zone
Normalised velocity of vd/cv is used to assess the drainage behaviour of soil. Where v is the penetration rate, d is the diameter of the penetrometer and cv is the coefficient of consolidation. Randolph and hope (2004) and Schneider et al. (2007) among other researchers showed that for V>30-100 the behaviour is undrained and for V<0.01-0.03 the behaviour is drained.Therefore, for the same soil penetration resistance of a spudcan can be lower than the recorded resistance by a CPT.
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Parameters Affecting Drainage Behaviour of Soil
Rate and duration of loading/shearing– Estimated from
installation/operation conditions Drainage length
– Estimated from the geometry of the problem
Coefficient of consolidation– Laboratory methods
• Rowe cell• CRS• Indirectly from permeability
– Estimated from in-situ tests:• Dissipation tests• Twitch tests• Parkable piezoprobe
(Chatterjee et al. 2014)(Randolph and Hope 2004)
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Parameters Affecting Drainage Behaviour of Soil
Rate and duration of loading/shearing– Estimated from
installation/operation conditions Drainage length
– Estimated from the geometry of the problem
Coefficient of consolidation– Laboratory methods
• Rowe cell• CRS• Indirectly from permeability
– Estimated from in-situ tests:• Dissipation tests• Twitch tests• Parkable piezoprobe
(Chatterjee et al. 2014)(Randolph and Hope 2004)
Range of uncertainty: Up to 1.5 times
Range of uncertainty: 100 to 1,000 timesBut it can be decreased!
Range of uncertainty: Depends on the application. Sometimes very high.
www.fugro.comDate
What I am going to talk about
What do I mean by “Drained” and “Undrained”?
Relevant soil parameters, and methods of assessing these parameters
Drained vs. undrained breakout resistance
Flaws of current methods
Conclusions
www.fugro.com
Twilight zone Lateral equivalent friction factor, H/W′
Normalised time, T = cvt/D2
Short breakout duration; impermeable soil
Long breakout duration; permeable soil
Drained behaviour
Undrained behaviour
Slow thermally- induced buckling
‘Snap’ buckling
Significant difference in lateral equivalent friction factor dependent on breakout duration
Dilatant soil (e.g. silty SAND/sandy
SILT in NWS)
Why Drainage is the Question
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Lateral equivalent friction factor, H/W′
Normalised time, T = cvt/D2
Short breakout duration; impermeable soil
Long breakout duration; permeable soil
Drained behaviour
Undrained behaviour
Slow thermally- induced feed-in
‘Snap’ buckling
Dilatant soil (e.g. silty SAND/sandy
SILT in NWS)
Why Drainage is the Question
More permeable soil (or longer breakout duration)
Insignificant difference in lateral equivalent friction factor dependent on breakout duration
www.fugro.com
Lateral equivalent friction factor, H/W′
Normalised time, T = cvt/D2
Short breakout duration; impermeable soil
Long breakout duration; permeable soil
Drained behaviour
Undrained behaviour
Slow thermally- induced feed-in
‘Snap’ buckling
Dilatant soil (e.g. silty SAND/sandy
SILT in NWS)
Why Drainage is the Question
Insignificant difference in lateral equivalent friction factor dependent on breakout duration
Less permeable soil (or shorter breakout duration)
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Drained behaviour
Undrained behaviour
Best estimate
LB
UB
Possible values
UB
LB
P50
Probability
Uncertainty
UB
LB
BE
H/W′Lateral equivalent friction factor, H/W′
Normalised time, T = cvt/D2
High Uncertainty
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Best estimate
LB
UB
Possible values
UB
LB
P50
ProbabilityUncertainty
UB
LB
BE
H/W′
Reasonable definition of volume change behaviour and velocities Narrowing of uncertainty
Lateral equivalent friction factor, H/W′
Normalised time, T = cvt/D2
Drained behaviour
Undrained behaviour
Reducing the Uncertainty
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Conclusions and Discussions Every soil type can behave drained/partially-drained/undrained
depending on:– Rate or duration of loading– Drainage length– Coefficient of consolidation (cv)
Site investigations should be specifically targeted to suit the field events and design requirements
In specific soils (e.g. silty sands/sandy silts) both drained and undrained behaviours should be checked
Narrowing down the range of the above parameters, significantly reduces uncertainties (e.g. by in-situ estimation of cv)
There is no means of being conservative or unconservative.
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Thank You
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First author:– Prof. David White, Dr. Fraser Bransby and other
colleagues at Fugro.
Acknowledgments