Panel Discussion on Education

Unsaturated Soil Mechanics instruction: Guiding the non-specialist instructor
A panel discussion with panelists Sandra Houston, John McCartney and Alessandro Tarantino, moderated by Marina Pantazidou (a TC106-TC306 collaboration) and organized during the 8th International Conference on Unsaturated Soils.

GOAL FOR THE UNSAT2023 PANEL ON EDUCATION: Answer potential questions by non-specialists

What follows is the perspective –in the form of questions– phrased by a non-specialist, Marina Pantazidou, after being tutored by a specialist, Michael Bardanis. This collaboration was crucial: without the specialist’s involvement, the non-specialist would not have the necessary stamina to translate bewilderment about issues in Unsaturated Soil Mechanics (UNSAT SM) into questions. The goal of the collaboration was to involve panelists in order to produce as a team an UNSAT Soil Mechanics Q & A collection that will be helpful to Soil Mechanics instructors willing to include some elements of UNSAT Soil Mechanics in their courses.

The questions are grouped under topics typically addressed in an introductory course in Soil Mechanics/Geotechnical Engineering. A few more advanced questions are also included in order to give a broader perspective. Panelists providing answers have been asked to keep in mind the different aims of instructors and, if possible, provide answers suitable for each aim. A gradation of three such aims –all modest–  is described below:

  1. Instructor aims to remove from her notes/lectures something that is “plainly wrong” (unacceptable) from an UNSAT SM perspective. Here we focus on students not having to unlearn something before expanding their own UNSAT SM knowledge in a subsequent course.
  2. Instructor aims to clarify that a statement or an equation in her notes is –from an UNSAT SM perspective– a simplification, and state the range of validity of the statement/equation. The focus here is on alerting the students that they are taught about only part of the picture: they learn principles and calculations suitable for only subsets of reality (e.g. only saturated soils). Note: Aim (II) includes aim (I).
  3. Instructor aims to create “receptors” for future knowledge about UNSAT SM. This is the most ambitious of the three modest aims: when students see something in a more advanced geotechnical engineering course, they will recognize that it fits with what they have seen in the introductory course.


    GROUP OF QUESTIONS A: Soil profile above the water table
  1. What is the distribution of pore water pressure above the water table in the field? In a soil column?
  2. May we talk of an “equilibrium” water pressure distribution in the field other than hydrostatic?
  3. Is it reasonable to assume that equilibrium conditions are rare in the field for low permeability soils?

  4. GROUP OF QUESTIONS B: “Effective stress” or in general “variables we need to keep track of in order to describe and predict soil behavior”
  5. From the perspective of an UNSAT SM specialist, can the existing alternative expressions for effective stress that include both air pressure and water pressure be excusable for explaining concepts in an introductory course? [Example: Bishop’s Equation: σ' = σ – ua + χ (ua - uw)]
  6. What is really the “claim to prediction” of effective stress? (Textbooks do not agree: see Appendix on last page.) If we have excused other description/prediction failures of the effective stress for saturated soils, then why are we so strict for the failure of effective stress to describe/predict states in unsaturated soils?

  7. GROUP OF QUESTIONS C: Sandcastles & other demonstrations for effective stress (e.g. Elton, 2001)
  8. Is it wrong to tell students that sand castles stand due to higher effective stress? Should we say instead that the castle stands due to higher shear strength?
  9. Is it wrong to tell students that vacuum-packed coffee is strong due to higher effective stress?

  10. GROUP OF QUESTIONS D: Shear stress
  11. In my course, I teach the Mohr-Coulomb criterion for shear strength (τ¢ = c¢ + σ¢ tanφ¢). I see that in UNSAT SM there are a few alternative expressions, e.g. Jaksa (2020) gives the following two:
    I do not know what to make of this variety, so I prefer say nothing about unsaturated strength to my students. Do I have a simple better alternative than say nothing?
  12. Is there any simple demonstration based on principles (not on examples, e.g. sand castles) of suction contributing to shear strength?

  14. I get the impression that the answer to the question “which shear strength equation to use” is contested within the UNSAT SM community (see also Question No 8). Is this question a main issue for UNSAT SM? If not, perhaps we could withhold judgement and focus on more important/applied issues?
  15. Which geotechnical problems require UNSAT SM for the analysis that will produce their solution?
  16. Are there any applications of UNSAT SM in the field? Has the UNSAT SM community recorded some case studies?

  18. After so many years of being taught and teaching about effective stress, it is hard to give it up. Do I understand correctly that its main failure for UNSAT soils is in predicting volume change? Or deformation? Or both?
  19. Please give me some carefully selected annotated results, ideally both in the lab and the field, showing the inability of effective stress to predict … (whatever is the answer to Question 13 above) and the successful prediction of a suitable UNSAT SM approach.

Elton, D. (2001). Soils Magic, Geotechnical Special Publication 114, ASCE, Reston, Virginia.
Jaksa, M.B. (2020). Reflections on some contemporary aspects of Geotechnical Engineering Education – From critical state to virtual immersion, 2nd John Burland Lecture, Proceedings of the ISSMGE Int. Conf. Geotechnical Engineering Education GEE 2020, Athens, Greece, June 23-25, (accessed March 25, 2023).

APPENDIX – what do we read about effective stress in current textbooks (Atkinson, 2007; Briaud, 2013; Budhu, 2011; Powrie, 2014).
Atkinson (2007: p 70) quotes Terzaghi’s (1936) strong definition: “all measurable effects of a change of stress, such as compression, distortion, change of shearing resistance, are due exclusively to change in effective stress”. Then on page 71 he comments: “No conclusive evidence has yet been found that invalidates Terzaghi’s original postulate, at least for saturated soils at normal levels of engineering stress, and the principle of effective stress is accepted as an axiom in Soil Mechanics”.
Briaud (2013: p 252) says only that the effective stress “is one of the most important parameters to know when dealing with soils”.
Budhu (2011: p 152) says in bold italics “Deformations of soils are a function of effective stresses, not total stresses.”
Powrie (2014: p 22) states in his usual succinct style “It is the effective stress which controls the volume and strength of soil.”

Atkinson, J. (2007). The mechanics of soils and foundations, 2nd Ed. (1st Ed. 1993), Taylor and Francis, Oxon, UK.
Briaud, J.-L. (2013). Geotechnical Engineering: Unsaturated and saturated soils, John Wiley & Sons, Inc., Hoboken, New Jersey.
Budhu, M.(2011). Soil Mechanics and Foundations, 3rd Ed. (1st Ed. 2000), John Wiley & Sons, Inc., Hoboken, New Jersey.
Powrie, W. (2014). Soil Mechanics Concepts and Applications, 3rd Ed. (1st Ed. 1996), CRC Press, Taylor & Francis Group, Boca Raton, Florida.

Marina Pantazidou,
ISSMGE TC306 Chair
March 27, 2023