Drained or Undrained: Is That the Question? Amin Rismanchian Nick Ramsey Feb. 2014

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 In this presentation first we define the drained and undrained behaviour in the soil. Then in the first section of the presentation 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. In the second section of the presentation, a test programme to study the effect of drainage on breakout of pipelines is explained and its results are going to be presented. It is followed by a brief discussion on the flaws of the current methods and finally we have the conclusion of the whole presentation. Date

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)

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

Twilight Zone: Partially Drained Soil Type 1a SILTS and low Ir CLAYS 1b CLAYS 2 Essentially drained SANDS 3 Transitional soils 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?? Refer to Schneider et al. (2008) for the boundary lines.

Twilight Zone: Partially Drained Soil Type 1a SILTS and low Ir CLAYS 1b CLAYS 2 Essentially drained SANDS 3 Transitional soils 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?? Refer to Schneider et al. (2008) for the boundary lines.

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

Drainage is a Function of the Normalised Velocity Drained penetration Partially drained penetration Undrained penetration Spool CPT Twilight zone Spudcan penetration Penetration resistance Pipeline penetration Ian Finnie introduced the normalised velocity of vd/cv 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, in this example when the CPT is in the partially drained zone. In this case, the soil strength registered by a T-bar, which usually has a higher diameter, is lower than what measured by CPT. Moreover, depending on the size and laying condition of pipelines, the soil may behave undrained or partially drained and may have significantly lower shear strength. The same happens for spudcans.

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) Range of uncertainty: Depends on the application. Sometimes very high. Range of uncertainty: Up to 1.5 times Range of uncertainty: 100 to 1,000 times But it can be decreased! Twitch test: Randolph and Hope (2004) conducted penetration tests with various penetration rates as plotted in the figure. It is noted that with decreasing the penetration rate, first the penetration resistance (i.e. soil resistance) is decreased with the decrease in the viscous effect. Then the penetration resistance (shear strength) is increased with further decrease of penetration rate until it the soil shows fully drained behaviour at which the penetration resistance does not decrease further. In this figure the soil behaviour is partially drained at 0.1<V<10. At this range of velocities still excess pore pressure is generated. Therefore, adopting the results of the tests in this region as undrained will over estimate the undrained shear strength. Likewise is adopting results of tests with penetration rates V>100 as undrainded condition. Reviewing the parameters affecting the drainage and the methods for assessing them are listed in this slide. It should be pointed out that the sampling process, transportation of them and sample preparation will affect the results of the consolidation tests specifically in sensitive soils. (Randolph and Hope 2004)

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  Date

Why Drainage is the Question Dilatant soil (e.g. silty SAND/sandy SILT in NWS) Lateral equivalent friction factor, H/W′ Short breakout duration; impermeable soil Twilight zone Undrained behaviour Long breakout duration; permeable soil ‘Snap’ buckling Drained behaviour Slow thermally- induced buckling Normalised time, T = cvt/D2 Significant difference in lateral equivalent friction factor dependent on breakout duration

Why Drainage is the Question More permeable soil (or longer breakout duration) Dilatant soil (e.g. silty SAND/sandy SILT in NWS) Lateral equivalent friction factor, H/W′ Short breakout duration; impermeable soil Undrained behaviour Long breakout duration; permeable soil ‘Snap’ buckling Drained behaviour Slow thermally- induced feed-in Normalised time, T = cvt/D2 Insignificant difference in lateral equivalent friction factor dependent on breakout duration

Why Drainage is the Question Less permeable soil (or shorter breakout duration) Dilatant soil (e.g. silty SAND/sandy SILT in NWS) Lateral equivalent friction factor, H/W′ Short breakout duration; impermeable soil Undrained behaviour Long breakout duration; permeable soil ‘Snap’ buckling Drained behaviour Slow thermally- induced feed-in Normalised time, T = cvt/D2 Insignificant difference in lateral equivalent friction factor dependent on breakout duration

High Uncertainty Lateral equivalent friction factor, H/W′ UB UB UB BE LB Undrained behaviour P50 Possible values Drained behaviour Best estimate LB LB Uncertainty Normalised time, T = cvt/D2 Probability

Reducing the Uncertainty Reasonable definition of volume change behaviour and velocities  Narrowing of uncertainty Lateral equivalent friction factor, H/W′ H/W′ UB UB UB BE LB P50 Undrained behaviour LB Possible values Drained behaviour Best estimate LB Uncertainty Normalised time, T = cvt/D2 Probability

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. There is not a fixed rule to say whether drained or undrained are conservative. It depends on the problem. Sometimes, for example, the pipe is expected not to break out. In this case the drained condition discussed above provides the conservative estimations. However, sometimes the pipe is expected to breakout (e.g. in case of lateral buckling design). In this case the undrained condition provides the conservative estimations. Mechanical properties of soil in drained and undrained conditions are needed for producing the yield envelopes and to have an estimation of their hardening law and pipe penetration resistance in various drainage conditions. It is noted that the drained parameters are derived from undrained test results or reverse. This is not going to lead into accurate estimations of soil behaviour.

Thank You Acknowledgments First author: Prof. David White, Dr. Fraser Bransby and other colleagues at Fugro. 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.