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Www.fugro.com © Fugro 2013 Amin Rismanchian Nick Ramsey Feb. 2014 Drained or Undrained: Is That the Question?

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Presentation on theme: "Www.fugro.com © Fugro 2013 Amin Rismanchian Nick Ramsey Feb. 2014 Drained or Undrained: Is That the Question?"— Presentation transcript:

1 © Fugro 2013 Amin Rismanchian Nick Ramsey Feb Drained or Undrained: Is That the Question?

2 Date 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

3 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)

4 Example: Cone Penetration Test, Offshore WA Is this test indicating undrained/drained or partially drained conditions in the surficial soils?

5 Twilight Zone: Partially Drained ZoneSoil Type 1aSILTS and low I r CLAYS 1bCLAYS 2Essentially drained SANDS 3Transitional soils Refer to Schneider et al. (2008) for the boundary lines.

6 Twilight Zone: Partially Drained ZoneSoil Type 1aSILTS and low I r CLAYS 1bCLAYS 2Essentially drained SANDS 3Transitional soils Refer to Schneider et al. (2008) for the boundary lines.

7 Example: Cone Penetration Test, Offshore WA Twilight zone (partially drained) Fully drained Fully undrained

8 Drainage is a Function of the Normalised Velocity CPT Spudcan penetration Pipeline penetration Partially drained penetration Undrained penetration Penetration resistance Spool Drained penetration Twilight zone

9 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 times But it can be decreased! Range of uncertainty: Depends on the application. Sometimes very high.

10 Date 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

11 Twilight zone Lateral equivalent friction factor, H/W Normalised time, T = c v t/D 2 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

12 Lateral equivalent friction factor, H/W Normalised time, T = c v t/D 2 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

13 Lateral equivalent friction factor, H/W Normalised time, T = c v t/D 2 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)

14 Drained behaviour Undrained behaviour Best estimate LB UB Possible values UB LB P 50 Probability Uncertainty UB LB BE H/W Lateral equivalent friction factor, H/W Normalised time, T = c v t/D 2 High Uncertainty

15 Best estimate LB UB Possible values UB LB P 50 Probability Uncertainty 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 = c v t/D 2 Drained behaviour Undrained behaviour Reducing the Uncertainty

16 Conclusions and Discussions Every soil type can behave drained/partially-drained/undrained depending on: –Rate or duration of loading –Drainage length –Coefficient of consolidation (c v ) 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 c v ) There is no means of being conservative or unconservative.

17 Thank You All materials and content contained within this presentation remain the property of Fugro (© Fugro 2013) and may not be copied, reproduced, distributed or displayed without Fugro's express written permission. All third party information featured within the presentation remains the intellectual property of their respective originators. First author: –Prof. David White, Dr. Fraser Bransby and other colleagues at Fugro. Acknowledgments


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