Drained or Undrained :Is That the Question
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In this presentation first we define the drained and undrained behaviour in the soil. Then the parameters causing the soil to behave drained or undrained are elaborated. It is followed by a short discussoin on the methods of measuring these parameters and how the uncertainty in design can be reduced by a well planned site investigation. These slides were presented at AOG 2014. Source: http://www.diversifiedexhibitions.com.au/~public/aog/conference-pdfs/adapting_optimising_challenging_Seabeds/Challenging-seabeds-Rismancian-Ramsey-230pm.pps
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Drained or Undrained :Is That the Question
- 1. www.fugro.com© Fugro 2013 Amin Rismanchian Nick Ramsey Feb. 2014 Drained or Undrained: Is That the Question?
- 2. 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
- 3. www.fugro.com 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. www.fugro.com Example: Cone Penetration Test, Offshore WA Is this test indicating undrained/drained or partially drained conditions in the surficial soils? 0 0.5 1 1.5 2 2.5 3 0 0.5 1 Depthbelowmudline(m) Cone penetration resistance, qc (MPa) -0.05 -0.025 0 0.025 0.05 Pore 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.
- 5. www.fugro.com Twilight Zone: Partially Drained Zone Soil Type 1a SILTS and low Ir CLAYS 1b CLAYS 2 Essentially drained SANDS 3 Transitional soils 1 10 100 1000 -2 -1 0 1 2 Qt(-) Δu/σ′vo (-) 2 3 1a 1b Refer to Schneider et al. (2008) for the boundary lines.
- 6. www.fugro.com Twilight Zone: Partially Drained Zone Soil Type 1a SILTS and low Ir CLAYS 1b CLAYS 2 Essentially drained SANDS 3 Transitional soils 1 10 100 1000 -2 -1 0 1 2 Qt(-) Δu/σ′vo (-) 2 3 1a 1b Refer to Schneider et al. (2008) for the boundary lines.
- 7. www.fugro.com Twilight Zone: Partially Drained Zone Soil Type 1a SILTS and low Ir CLAYS 1b CLAYS 2 Essentially drained SANDS 3 Transitional soils 1 10 100 1000 -2 -1 0 1 2 Qt(-) Δu/σ′vo (-) 2 3 1a 1b Refer to Schneider et al. (2008) for the boundary lines.
- 8. www.fugro.com 1 10 100 1000 -2 -1 0 1 2 Qt Δu/σ′vo 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?
- 9. www.fugro.com Example: Cone Penetration Test, Offshore WA 0 0.5 1 1.5 2 2.5 3 0 0.5 1 Depthbelowmudline(m) Cone penetration resistance, qc (MPa) -0.05 -0.025 0 0.025 0.05 Pore pressure (MPa) Generated pore pressure, u2 Hydrostatic pore pressure, uo Twilight zone (partially drained) Fully drained Fully undrained
- 10. www.fugro.com Drainage is a Function of the Normalised Velocity CPT Spudcan penetration Pipeline penetration Partially drained penetration Undrained penetrationPenetrationresistanceSpool 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.
- 11. www.fugro.com 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)
- 12. www.fugro.com 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.
- 13. 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
- 14. 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
- 15. 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 More permeable soil (or longer breakout duration) Insignificant difference in lateral equivalent friction factor dependent on breakout duration
- 16. 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)
- 17. www.fugro.com 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
- 18. www.fugro.com Best estimate LB UB Possible values UB LB P50 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 = cvt/D2 Drained behaviour Undrained behaviour Reducing the Uncertainty
- 19. www.fugro.com 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.
- 20. www.fugro.com 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