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Less mobility in late 2019, and 2020 due to covid :-) staying at home, working and attending conferences online and visit colleagues also virtually. Less CO2 footprint thanks to less travel and saving time and cost; perhaps not that bad!

October 2020:  Our group had two lectures in European Conference on Unsaturated Soils (EUNSAT 2020) and one in InterPore 2020 which were held, both, fully online.

Summer 2019: Lectures in Pedofract seminar (Spain) and InterPore 2019 (Valencia, Spain).

Late August and September of 2018, I had a couple of university visits. One day research visit to Hamburg University of Technology, Paris Tech., and then Delft University of Technology on August 28, 27, and 31, respectively.

A lecture at Utrecht University on our ongoing research at Shiraz University on Biological Soil Stabilization on August 30, 2018, and then I have been visiting Porous Media Research Group of Bergen University (Norway) where I had a lecture on September 10th titled "Suction stress and effective stress in unsaturated porous media:the role of fluid-fluid interfaces and the importance of dynamic condition"

Meanwhile, I could have very short side visits (cultural/city seeing moments), during the trip (to Germany, France and Norway). Here, I have added some photos of these trips.

I could visit Panthéon and the burial place of Marie Curie.

Here some photos:

• • Recently I came across an interesting article on the work input for unsaturated porous media titled 

Work input for unsaturated soils considering interfacial effects

Of course on this topic, there are not only this one in the literature, you may find a few other classic papers [see for instance these: A, B, C , ...].

This recent article by colleagues from  Institute of Rock and Soil Mechanics, Chinese Academy of Science and University of Beijing Jiaotong has this interesting feature that it furthermore brings another pointer to the need to account for the presence and amount of air-water interfacial area for formulating the effective stress and its parameter. To me, regardless of the formulation you have, the need to characterize this variable is clearly felt not only for modeling the mechanical behavior of unsaturated soils but also for other geoenvironmental problems from evaporation in porous media [e.g. 1] to dissolution of contamination  (heat and mass transfer in unsaturated porous media in general) [2,3] to remobilization of colloids and viruses in wetting and drying cycles and so forth. So, the inclusion of specific air-water interfacial area is not only for mitigating the hysteresis in modeling unsaturated soils but also it would be almost a must when one wants to arrive at a comprehensive framework for hydro-mechanical models addressing geoenvironmental issues (flow, deformation, transport and phase change, i.e., fully coupled problems). The physics of a complex system containing multiple fluids and interfaces needs macroscopic (continuum scale) representatives of interfaces (their amount). It is not yet disclosed, if some macro-scale variables accounting for their shapes would also be needed; that would for sure bring further complexity. Yet the need to stay enough detailed/deep but be practical is also felt.

• • As of August 9th, 2018 our paper titled 

 Theoretical and experimental investigations on the role of transient effects in the water retention behaviour of unsaturated granular soils

 jointly prepared together with Hamburg University of Technology colleagues is among most downloaded papers of the Journal of Geomechanics for Energy and the Environment

Congratulations to all! (Marius, Tom, Jurgen and Majid)

• • I had a visit to the geotechnics group (Department of mining and geology at the university of Belgrade) during March, 2018. I had also a lecture there on the role of interfaces and transient condition on the mechanical of behavior of unsaturated soils. From the questions I received after the lecture, it seems to me that now it is time for our unsaturated soil community to write more on why the principles of unsaturated soil mechanics are required to be used in engineering practice. The engineers ask here and there << Do we need to use unsaturated soil mechanics?>>.  The effect and the implications of simply using the assumptions/principles of saturated/classic soil mechanics and the influence of not resorting to the unsaturated soil mechanics have to be demonstrated via different practical examples, namely, design and analysis of dams, slope stability, and so on. The need for a book which addresses, more in-depth and elaborately, practical cases is now felt more and more.

  • Recently, I came across the paper titled "Model for Predicting the Tensile Strength of Unsaturated Cohesionless Soils" co-authored by Mr. Penghai Yin, and Professor Sai Vanapalli of the University of Ottawa (An early/draft version of the paper can be accessed via research-gate of Penghai, here). They have incorporated/formulated the effect of specific air-water interfacial area in tensile strength of unsaturated soils. This recent paper and "almost" recent thesis by Jindal Prateek (2016) (please see the section 2.2.1 herein) all point to the importance of accounting for the effect of interfaces on the mechanical behavior of unsaturated soils.  Mr. Yin and Prof. Vanapalli followed the same line of thought we had followed in our theoretical work on the effect of interfaces on the hydromechanical behavior of unsaturated soils. There was also the work by Prof. Likos titled "Effective Stress in Unsaturated Soil: Accounting for Surface Tension and Interfacial Area" addressing the effect of interfaces. Historically speaking, various respected pioneers in unsaturated soil mechanics (Prof. Fredlund, late Prof. Blight, ...) have had notes on the role of air-water interfacial areas (the so-called contractile skin) on the mechanical behavior of unsaturated soils but there has not been much attention (theoretical, and numerical studies) focused on their role until such recent works. Among recent numerical studies, one can also refer to various recent publications by Prof. Wan and his colleagues, in particular Dr. Jérôme Duriez (now at IRSTEA); you may see this one, and this one for instance.

  • Our paper with colleagues of Institute of Geotechnical Engineering and Construction Management (Hamburg University of Technology) has, now, been accepted. This paper aims to address the effect of non-equilibrium condition on the state of stress and flow in unsaturated soils. The paper presents the results of soil-water retention experiments on the unsaturated soil samples. Among various findings, we demonstrated that suction stress curve at non-equilibrium condition can be different from the "equilibrium suction stress curve". Such results can be of considerable importance for analyzing the stability of unsaturated slopes and unsaturated soil mechanics in practice, i.e., cases such as rainfall-induced landslides. Please read more at the this link.

  • Professor Khalili and Dr. Khoshghalb of UNSW had precious notes on our work on a thermodynamic bridge between the effective stress formulation and soil water retention formula. Among various valuables notes they had offered, there was their point that one should approach the determination of air entry value in deformable porous media (soils etc) with care. This link would guide you to their major publication on this issue. Their comments and our reply can be accessed here at this link and this one.

  • We organized a workshop on " Advances in modeling flow and deformation in unsaturated porous media" which is supported by Darcy Center for Porous Media as well as International Society for Porous Media (InterPore). The workshop website is available at:

https://www.interpore.org/events/workshops/advances-in-modeling-flow-and-deformation-in-unsaturated-porous-media 

I think we were quite successful to gather respected and distinguished colleagues from different countries (Norway, Scotland, Swiss, France, Germany, Netherlands, and so on). Every once in a while such workshops are very useful to be organized as they indeed boost the exchange of ideas.

• • Our paper titled "Bridging Effective Stress and Soil Water Retention Equations in Deforming Unsaturated Porous Media: A Thermodynamic Approach" co-authored with Professor Jacques Huyghe (Bernal chair Professor at the University of Limerick, Ireland), me and Prof. Hassanizadeh (Utrecht University, NL) is now published and is online at this link. We consider the dependency of soil water retention curve on the stress level by taking into account the void ratio dependency of the air entry value (i.e., we use the relationships of soil water retention curve established by Professors Tarantino and Domenico Gallipoli for deforming unsaturated media (for an idea of the starting point please see, for instance, this sample paper here), we plug in everything in the mixture theory and we arrive at a relationship for the effective stress parameter [*] which can be considered of the same category of that of Professors Khalili and Khabbaz  (please see their paper here). In fact In our paper, we have offered a theoretical way to establish such a relationship by resorting to the mathematical symmetry of the strain energy function of the mixture and its derivatives. A plus for our relationship for the effective stress parameter proposed in this paper can be that all its parameters are, in fact, those parameters of the "retention curve of a deformable porous medium". Therefore, no need to find new parameters. 

  [*] If you are not a soil scientist, a geotechnical engineer, a hydrogeologist and/or  you are not in any relevant discipline, you can find out more about the term "effective stress" here at this wikipedia link; good also to see these videos: link1 (the lecture on the effective stress prepared by Dr Phil Renforth), and link2 (A video prepared by Prof. Burland on the effect of water on the soil strength). You would, therefore and consequently, note that, in our case, we would like to find the effective stress relationship for the case where more than one liquid lives inside our soil sample or in general in our porous material : for instance, when air and water both co-exist there (e.g., an earth slope which is getting wet by the rain water entering the slope).

• • I was invited as one of the jury members of the PhD thesis of Caroline Chalak at Université Grenoble Alps (Update on March 2017: Caroline thesis is now available online under this link). The thesis title was multi-phase and multi-material interactions in granular media and other jury members were Professor Cambou (Ecole Centrale de Lyon), Prof. Millet (Université de la Rochelle), Prof. Jean-Yves Delenne (Université de Montpellier 2) and Professor Selvadurai (Université de McGill). I was delighted to be invited and also be able to meet other experts which I knew only by name before. The thesis was consisted of two main parts. The first basically included the interaction between fluid phase and solid skeleton in an attempt to examine a micromechanical based determination of effective stress. The second part was aiming at modeling compaction of a proppant region using discrete element method. I think the use of discrete element method in modeling various phenomena of interest in environmental and energy geotechnics will grow as we see lots of interesting examples of its use nowadays.  

  • My new paper with Grenoble colleagues is now online. We examined the applicability of Love–Weber stress as a measure of effective stress. We built a conceptual model for the free energy change of a system of grains and water bridges and we determined each energy term using a discrete element method (a grain-scale model). Finally, we inspected the thermodynamic admissibility of Love–Weber stress as a measure of effective stress. The presented work is novel for the reason that many use Love–Weber stress as a measure of effective stress, heuristically, and without any rigorous mathematical proof (better to say without considering its thermodynamic admissibility). Here, in this work, we examine where such a stress measure is the same as its "thermodynamic twin" (where it is thermodynamically admissible) and where (in which packing), it differs from the simulated values obtained from the joint discrete element-thermodynamic analysis. Furthermore, we give an updated version of Morrow conceptualization of a water bridge-grain system. In our updated version, grains' movements (and energy associated with it) are also taken into account. 

Please read our joint paper with Grenoble colleague here: 

Partially saturated media: from DEM simulation to thermodynamic interpretation 

Caroline Chalak, Bruno Chareyre, Ehsan Nikooee & Felix Darve 

Update (July 2017):  Almost one year after our article: Colleagues of the University of Calgary, namely, Jérôme Duriez, Richard Wan and Mehdi Pouragha have also found another evidence from their numerical simulation that the resulting tensor from Love-Weber formulation cannot be universally considered as a replacement for the effective stress tensor; you may have a look at their abstract/paper here.

• • Our editorial of the special issue in vadose zone hydrology on the principle of effective stress in variably saturated porous media has been among most read-papers of this journal for almost a year; as of August 30, 2016, it is and has been within the items listed in most reads but this statistics changes dynamically and monthly, still very good news! You may read more about history behind this special issue here.

  •  I was honored by receiving "Interpore Rosette 2015" (Annual Award of International Society for Porous Media in 2015).