Long Term Stability - Drained vs Undrained
Long Term Stability - Drained vs Undrained
(OP)
Hello,
I have a question regarding use of undrained shear strength of fine-grained materials for the long term stability assessment. I have found a few sources that provide a clear guideline on using drained strengths and pore water pressures corresponding to steady seepage conditions for long-term stability analyses. However some advocate that for poorly drained materials undrained failure mode is still possible even in the long term and hence the analysis should be conducted using undrained shear strength. I can't find anything published supporting this approach and would appreciate if you could share your experience on this topic.
Thank you in advance
CEMAB
I have a question regarding use of undrained shear strength of fine-grained materials for the long term stability assessment. I have found a few sources that provide a clear guideline on using drained strengths and pore water pressures corresponding to steady seepage conditions for long-term stability analyses. However some advocate that for poorly drained materials undrained failure mode is still possible even in the long term and hence the analysis should be conducted using undrained shear strength. I can't find anything published supporting this approach and would appreciate if you could share your experience on this topic.
Thank you in advance
CEMAB
RE: Long Term Stability - Drained vs Undrained
RE: Long Term Stability - Drained vs Undrained
1. What are the details of the analysis?
2. Are you adding material to an existing slope?
3. Are you cutting material from a slope?
4. Are you constructing a water retaining structure?
These questions to the point of how the load/stress on/in the slope are changing. If there is no change in stress and has been no change in stress for a long time, then drained strengths are going to control becuase any drainage has already occured. However, if you are doing something to increase the stress in the soil, then either drained or undrained strength may control depending on the specifics of the problem.
Mike Lambert
RE: Long Term Stability - Drained vs Undrained
Thanks. Your questions relate to short term (changes) conditions, but I specifically asked about the long term stability assessment where pore water pressure corresponds to steady seepage conditions or in your words “there has been no change in stress for a long time”.
RE: Long Term Stability - Drained vs Undrained
Thanks for your response. I still don’t get it both formally and mentally [dazed]. The US Society on Dams “Strength of Materials for Embankment Dams” (http://www.ussdams.org/07materials.PDF), USACE Publications such as “Stability of Earth and Rock-Fill Dams”, Duncan, J., & Wright, S. (2005) “Soil Strength and Slope Stability” all direct to use drained shear strength for the long term stability assessment. Are they all incorrect or not conservative? Moreover, something tells me that once drainage occurred and there is no excess pore water pressure, the initiation of the slope failure must be in drained manner. Can this be the real reason behind the drained strength approach?
RE: Long Term Stability - Drained vs Undrained
I think you need to post information on what kind of structure you are looking at and the history of the structure. Your questions are general enough that there can be many different answers depending on the assumptions someone makes.
Mike Lambert
RE: Long Term Stability - Drained vs Undrained
RE: Long Term Stability - Drained vs Undrained
I appreciate your answer and thanks for the reference. Just clarify a few points. Here is the definition given by Duncan and Wright “Soil Strength and Slope Stability” (2005) on short term conditions: “Short term refers to conditions during or following construction—the time immediately following the change in load.” The long term conditions described as: “After a period of time, the clay foundation would reach a drained condition, and the drained analysis would be performed because long term and drained conditions carry exactly the same meaning. Both of these terms refer to the condition where drainage equilibrium has been reached and there are no excess pore pressures due to external loads.” This is how I see the definition of the long term conditions as well. In other words: If the slope didn’t fail in a short term following an introduction of any change in load/stress and drainage equilibrium is reached after a while, the slope is under the "long-term" conditions and should be checked for failure in a drained manner.
RE: Long Term Stability - Drained vs Undrained
Another point to consider is whether you are discussing first time "failure" or subsequent where the residual strength may be at play.
http://howland.co.uk/Articles/The%20natural%20proc...
http://www-civ.eng.cam.ac.uk/geotech_new/people/bo...
RE: Long Term Stability - Drained vs Undrained
RE: Long Term Stability - Drained vs Undrained
Now think of an embankment, say a levee, that has been in place for many years so that all pore-water pressures from consolidation and shear have dissipated under existing conditions (long-term condition). A flood comes along and rapidly loads the levee and foundation. If the levee and foundation are normally to lightly overconsolidated they can still develop pore-water pressure due to shear and so the shear strength would be based on undrained response related to pre-flood effective stresses. This example is a long term condition that is suddenly loaded, but what if you don't have a change in load, can you have an undrained soil response? Probably not, and you would design using drained strengths. Steady state seepage conditions (no excess pore-water pressures associated with compression or shear) is synonimous with drained shear strengths. Now, you may want to look at the assumption of no loading closely to be sure that assumption is correct as slope instability can happen fast so that shear induced pore-water pressures may not dissipate (think of static liquifaction of mine tailings that have a collapsing structure).