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# Unsaturated Soils - Science topic

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Hi,
I am trying to model heat and mass transfer in three phases (soil, water & air) porous media. I want to know what equation should I use for the mass transfer rate or mass flux (between water phase and air phase) due to the evaporation of water or condensation of vapors in porous media voids.
There are two class of liquid-vapour mass transfer models available (limited to pore-scale simulation):
#1 First type calculates the mass flux based on KINETIC THEORY, and its derivative models such as Hertz-Knudsen, Schrage-Mills, Lee etc. can be used for large problem domains.
#2 Second type is called ENERGY-JUMP CONDITION, uses thermal resistance at the liquid-vapour interface to calculate the heat flux, which finally calculates the mass flux.
I recommend you to follow this review work:
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This topic is created to be a place for sharing reliable open-sources that contain quality free courses, webinars and short educational videos in the field of geotechnical engineering.
Zew Zealand Geomechanics Society Recorded Webinars.
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It is exciting to see that this year’s Nobel Prize for physics goes to complex system studies. Fingering flow (wetting front instability) in unsaturated soils is a typical complex-system problem. The complex system is partially characterized by emergence and adaptation. For highly non-linear unsaturated flow, the emergent pattern is the fingering, and the corresponding adaptation principle is the optimality, such as the minimization of global flow resistance. Based on these ideas, I have mathematically demonstrated that the relatively permeability is a function of both saturation and water flux, while the traditional theory considers the relative permeability as a function of saturation only. The work was supported by experimental results and documented in a recent book
One key issue in applying the complex-system framework to unsaturated flow is to find a physical principle to describe the adaptation. To do that, does anyone have a more general principle than the minimization of the global flow resistance?
Please find the attached files. Regards.
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Two major flow mechanisms for preferential flow in unsaturated zone are the existence of macro pores or structures and wetting front stability (fingering flow). Do you have any field evidence or theoretical arguments to tell which of the two mechanisms is more important?
The occurrence and development of figuring flow (wetting front instability) in unsaturated zone are very much like those of turbulence flow in pipes. The environment disturbance may determine transition from laminar flow to turbulent flow. However, once fully developed, the turbulent flow is not significantly impacted by the disturbance any longer. Along the same line, the heterogeneity may impact the initial stage of fingering flow, but not the fully developed fingering flow that is mainly determined by the nonlinear nature of the unsaturated flow.
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Does geo studio software consider the capillary rise in the soil? (for modeling in SEEP/W and CTRAN/W)
Mahsa,
I think geo studio software is considering capillary effect in the analysis..the capillary effect in a partially saturated soil condition, the zone within the depth of capillary there are negative pore water pressure. I think this effect can be incorporated in the software.. I suggest you to read from the software manual..I attached herewith software manual for your reference..all the best in your research..
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Hi Ahmed,
I am trying to program an unsaturated soil model based on the double stress principle into ABAQUS. Do you do something like that? it is possible to use UMAT to achieve the unsaturated soil model? If you did some jobs related to this, would you like to discuss it with me?
Thank you very much!!
Regards,
Xingyi
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How can one do slope stability analysis in FEM or FDM for unsaturated soil conditions and fully saturated conditions?
Dear Pandit,
In order to model the unsaturated soil behaviour, we have to model it hydromechanically. It means that first, we have to model the hydraulic behaviour of soil in which the soil-water characteristic curve and unsaturated hydraulic is vital to be found. Then, must include the SWCC and unsaturated hydraulic conductivity to simulate the soil behaviour mechanically. For doing this, we can use ABAQUS, Plaxis, GeoStudio package, and ... For more details, you can refer to follows papers:
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We know that the void ratio of soils is affected by increasing temperature. However, the experimental data involved in this topic is rare. From a theoretical aspect, how can we estimate the change in void ratio with temperature, especially unsaturated soils.
I would like to appreciate it if you can provide a calculation method or experimental data on this topic through papers you know.
Dear Tuan
The question comes down to asking if there is a soil theory that deals with the thermodynamics of unsaturated soil. There is none officially recognized, nor is there any defining the soil according to its organization in solid, liquid and gas (air) phases. You must know that voids in soils are occupied by air and water and that only water is playing a role on the swelling-shrinkage of a soil. Temperature impacts the density of water (the relation is known), that is negligible compared to the thermodynamical link between the solid and liquid phases that is responsible of the swelling-shrinkage process of soils by water.
See "Modeling the soil system: Bridging the gap between pedology and soil–water physics" on my Research Gate space or "Hydrostructural Pedology, New Scientific Discipline Allowing for Physical Modelling of 'Green Water' Dynamics in the Soil-Plant-Atmosphere System" DOI:10.9734/JAERI/2018/43822
Regards
E.B.
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Certain Casagrande piezometers or Vibrating Wire piezometers have a high air entry filter made of porous ceramic. The reason for using a high air entry filter, typically 1 to 3 bars is to prevent gas pressure present in some soil to penetrate through the filter, so that only water pressure will be measured.
So, I’m looking at a formula to convert the air entry value of the ceramic, say 1 bar, to an equivalent Darcy coefficient of permeability in cm/sec?
The Air entre value is a pressure AEV given in kPa at a specific volumetric water content θ.
The pressure is defined as AEV= Force (N)/Area (mm2).
The permeability K= distance (cm)/time (Second)
Based on these relations, it is impossible to establish a mathematical correlation between (AEV) and (K). However, a correlation can be established based on experiments by measuring the AEV and K on duplicate specimens for a specific soil material.
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I'm aware of Hydrus and Netpath, but are there others? I am highly interested in water stable isotopes in variably saturated and non-equilibrium flow (double porosity/permeability). Any hints would be deeply appreciated.
stable Isotopes of Water Vapor in the Vadose Zone: A Review of Measurement and Modeling Techniques
Keir Soderberg et al..
First published: 07 September 2012
Abstract
The stable isotopes of soil water vapor are useful tracers of hydrologic processes occurring in the vadose zone. The measurement of soil water vapor isotopic composition (δ18O, δ2H) is challenging due to difficulties inherent in sampling the vadose zone airspace in situ. Historically, these parameters have therefore been modeled, as opposed to directly measured, and typically soil water vapor is treated as being in isotopic equilibrium with liquid soil water. We reviewed the measurement and modeling of soil water vapor isotopes, with implications for studies of the soil–plant–atmosphere continuum. We also investigated a case study with in situ measurements from a soil profile in a semiarid African savanna, which supports the assumption of liquid–vapor isotopic equilibrium. A contribution of this work is to introduce the effect of soil water potential (Ѱ) on kinetic fractionation during soil evaporation within the Craig–Gordon modeling framework. Including Ѱ in these calculations becomes important for relatively dry soils (Ѱ < −10 MPa). Additionally, we assert that the recent development of laser‐based isotope analytical systems may allow regular in situ measurement of the vadose zone isotopic composition of water in the vapor phase. Wet soils pose particular sampling difficulties, and novel techniques are being developed to address these issues.
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Shear resistance of unsaturated soils
Thanks you very much.
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Hello,
My thesis is about unsaturated soils and determining the stress-deformation of the embedded stone columns. The soil sample is silty sand without plasticity and it's categorized as SM in the unified soil classification system.
I have studied some papers about Barcelona basic model but I think that this constitutive model is suitable for fine soils like clays and not for sands.
My main question is how can I model unsaturated soils with the variable modulus of elasticity in PLAXIS 3D
Sarvat Gull
Thanks. It was helpful for me
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I am going to solve Richards equation for unsaturated zone using finite difference method. I read in several papers that van Genuchten method is used to estimate hydraulic properties. In van Genuchten formula we can estimate theta and K. what is the difference between the theta  obtained by van Genuchten formula and the theta  obtained by Richards equation. The other question is that in solving Richards equation we assume an initial h. in the next step h should be estimated so that we can calculate  using this equation. How it can be updated in each step.
Shahriar Shahrokhabadi y Zoubair Boulahia , Vahid Moosavi necesito una ayuda en mi simulación por "Simulink" la ecuación de richards ,
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TDR can be used to measure non-aqueous liquids in saturated soils, why not to measure aqueous liquids in unsaturated soils.
Dear Sir,
Time-domain reflectometry TDR is sensitive to the moisture content in the soil mass and measure the volumetric water content
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there are so many methods to predict the strength of unsaturated soil but for low suction,is there any methods to predict the strength of unsaturated soil for high suction?
Dear Sir,
Conventional soil tests such as Oedometer and triaxial tests can be used to indicate the strength of unsaturated soil BUT under the condition of the suction theory.
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In the consolidation theory of unsaturated soils proposed by Fredlund, the excess pore-water and -air pressures will dissipate with time, and will eventually return to their values prior to loading. We all know that the pore water pressure in unsaturated soils is often negative. If the pore water pressure is still negative after the surcharge is applied, will the excess pore water pressure caused by the surcharge dissipate? I am so confused. Can anyone explain this?
Yazgan Kırkayak I would be happy to share my data after the experiment.
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regards. participate in my hot discussion about international provisions and unsaturated soils . come and challenge
The most important textbooks on unsaturated soil are:
1- Fredlund, D. G. , Rahardjo, H., (1993), "Soil Mechanics for Unsaturated Soils", John Wiley & Sons.
2- Lu, N. and Likos, W. J., (2004), "Unsaturated Soil Mechanics", Published by John Wiley & Sons, Inc., Hoboken, New Jersey.
3. Ng, C. W. W. and Menzies, B. (2007), " Advanced Unsaturated Soil Mechanics and Engineering", by Taylor & Francis.
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i am looking for an applied topic about unsaturated soil and continue the experimental tests with traxial cyclic sample like swcc and comparision the data .i like your idea in this regard .and waiting for discussion
You probably want to consider the effect of the humidity in the weather on the suction and triaxial cyclic tests as well.
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I am developing a soil column experiment to study antibiotic transport and attenuation. Most works in the literature perform a saturation with a CaCl2 solution in the soil before performing the leaching experiment. I would like to know the reason for this saturation with CaCl2. I have not found any work that explains the reason for this saturation.
I would be very grateful for any help.
In sorption and leaching exprimnets one mostly uses 0.01 M CaCl2 (see also OECD guidelines). This is because a) soil never contains simply water but always a solution of different salts. b) Agricultural soils typically have soil pH >4.5. Under that conditions Ca ions largely dominate the solution. As a consequence of a) and b) 0.01 M CaCl2 is a perfect surrogate of soil solution.
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What is the degree of credibility lies on the analysis of stability of slopes containing unsaturated soils? Can liquefaction and other detrimental conditions be induced when seismic waves travel through an unsaturated media ?
in general, the liquefaction risk is reduced by decreasing the degree of saturation or increasing the level of suction. However, depending on the amount of induced excess pore pressure, there is always the possibility of the degree of saturation reduction.
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with what kinds of softwares we can evaluate the deformation and volume change of unsaturated soil caused by change of matric suction?
GeoStudio software
or SoilVision
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Now, I am now working on soil erosion testing, submerged jet test (JET). I wonder that erosion strength of soil may be depended on water saturation in soil mass, related with unsaturated flow. My idea is measuring some variable of unsaturated soil during the JET test. Which equipment (e.g. several type of suction meters) I can use during the test? Thank you in advance.
Calcium losses lead to water Erosion. It has an impact on the soil structure
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I recently measured the surface tension using the pendant drop method. What I found was that the surface tension decreases with time for pure water and salt water. I also noticed that this kind of thing reported by other publications. But I haven't found any paper discussed the reason for this phenomenon. Does any one know why? Thanks.
Yes , it does reduce with time due to presence of air as surfacetant...
Abstract:An improved method has been developed for the determination of surface and interfacial tensions from primary drop shape data. In addition, wetting angles of sessile drops may be determined. The method has been built around a commercial pendant drop instrument and an IBM-compatible PC with a frame grabber card. In order to differentiate the drop profile, a filter routine using a local threshold and interpolation technique has been developed that is combined with an edge-tracing algorithm. The program for calculation of surface tension is divided into two parts. The first part is based on the traditional optical method and uses inflection of the drop profile. By means of several polynomial interpolations and curve fitting of theoretical profiles, the form factor β and surface tension γ are determined. The second part of the calculation utilizes the above values as a first estimate and then performs a further optimalization of γ by comparison between experimental and theoretical Young—Laplace profiles. With a PC AT with a 80287 mathematical coprocessor the measurements take about 5 s and the reproducibility is typically 0.01–0.03 mN/m for a wide range of known liquids.
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ASTM code for unsaturated soils.
Soil property also plays a major  role and how for it is influencing depends. Further on the other conditions ex: testing procedure,method  also should be accounted.
you can look into following literature.
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Currently, I am working on unsaturated soil mechanics applied to slope stability, I read about  ICFEP (general code for geotechnical analysis with a wide range of capabilities) developed by Potts & Zdravkovic and their team.
Is anyone working with ICFEP?, I am looking for experiences using this program.
I will try to help soon
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I did a literature review and it is not still clear to me if GPR can let me measure relative water content in the depth of soils. I am going to evaluate the change in water content in the unsaturated part of the soil (vadose zone).
you my take alook on the attached article. I hope it will help
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As it is seen in literature, the ADE with a (dS/dt) is used to model a reactive transport. However, how it can be modeled if nitrate as one solute is stuck by another solute (e.g. nanoparticles)? in this case part of nitrate is adsorbed by soil particles and another part by nano.
It goes even beyond: Nitrate adsorbed to nano may be adsorbed to soil because nano adsorbs to soil. I know just one code capable of modelling co-transport and co-sorption. (You may consult: Totsche K.U., Knabner P. & Kögel-Knabner I. 1996. The modelling of reactive solute transport with sorption to mobile and immobile sorbents - Part II: Model discussion and numerical simulation. Water Resour. Res. 32, 1623-1632.). Kai Totsche is a Professor at Jena University in Germany
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Hi !!
I am working on Stability of unsaturated soil slopes due to infiltration, but I have convergence and numerical oscillation problems … This oscillated pore-water pressure distributions do not appear realistic, as they indicate drying of the soil, which is unlikely with an influx boundary condition.
Has anyone had similar problems?... Is there a relationship between the element size and the time step, that allows me to solve these problems?
The truth is that so far I'm working on the infiltration analysis and later I will start with the stability analysis , so your answer will be very useful to me in the future.
In the picture attached is observed a little better the problem: The initial distribution of suctions (step 0) is observed and variation in suction when a rain of 0.1 m / day at the soil surface is applied; the problem is that over time the suction increases rather than decreases. I read some articles about it and I think it can be a problem of numerical oscillation.
I have understood that SEEP is excellent for flow modeling, but now I only have PLAXIS 3D to develop my thesis.
I am Spinning around the matter and so far I don´t know how to resolve it, so I appreciate any help you can give me...The models are three-dimensional
Thank you again
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Need good quality tensiometer. If it is capable of continuous datalogging then will be better. Should be inexpensive.
Hi there,
In case if you are still looking for a tensiometer, it may be better to buy a transducer. Due to the small range of the suction that you have. The transducer with 1 bar to -1 bar range is good for your case I guess. You can use it for you case study. The cost is also reasonable.
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I atempted to reproduce the model described in the paper from Hamdhan and Schweiger, 2011 (Hamdhan, I. N., & Schweiger, H. F. (2011). Slope stability analysis of unsaturated soil with fully coupled flow-deformation analysis. IAMG publication Salzburg, Austria) but I meet some difficulties: The Factor of safety values I obtain are systematically higher than the one reported in the paper and the saturation and suction distributions are also quite different. Please find attached the plots of the FOS I obtained vs the FOS from Hamdhan and Schweiger (2011), and the suction distribution I obtained after a simulation of 60 hour of 10mm/h rainfall. There seems to be a problem with the water load, and suction is much higher (or saturation is much lower) in my results than in the paper.
Does anyone have any idea of why I have such different results? Do you think about an option I could have missed?
Bachir, thank you very much for your answer. Indeed we both used a saturated fundation with zero suction. I contacted the support service of PLAXIS and it seems that the difference between my results and those described in the paper might simply be due to the different PLAXIS 2D versions we used (2015 and 2010 versions, respectively). To be confirmed...
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Hello
What's the typical   rate of  speed for the unconfined  compression  test and UU  triaxial  test ?
Thank you
Following the german standard 18136 (unconfined compression test)  you have to use 1% of the initial specimen height (in mm) per minute for the deformation rate. For soils which are improved with a binder agent you should use 0,4% of the initial speciment height.
According to DIN18137-2 you use 1% of the initial specimen height per minute as deformation rate for the triax UU test.
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The hysteresis phenomenon is very important for unsaturated soils. We have different models to simulate the hysteresis. But how to get the permeability coefficient on the scanning curve? By integration?
Integrate the SWCC to get the permeability
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Previously, i read an article mentioned that FLAC can model such problem's by using THOGH code. Does anyone know about this modeling and THOGH-FLAC it self?
And, which software's can consider unsaturated condition?
Yes there are finite element codes that can deal with the unsaturated conditions.
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I am using comsol weak form to build a coupled mechanical-flow model. However, when I simulate consolidation problem of unsaturated soil, Numerical oscillation is observed. It is unavailable to change the size of element and time step. I wish to know how to solve the problem.
This behaviour can be observed for rounded and sub rounded quartz sand experimentally. Numerically,  oscillation can be obtained when using Mohr-Coulomb constitutive model with an angle of dialatancy less than the internal friction angle. The intensity of the oscillations depends on 1. increasing internal friction angle and 2. increasing mesh refinement.  This behaviour is due to the apparent softening exhibited in shear bands by materials with a non-associated flow rule. So, try to use different mesh refinements or different internal friction angle values in your modelling, if possible.
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Hi all,
Can I estimate the soil water characteristic curve of well graded sand with a degree of compaction of 90% and sandy silt with a degree of compaction of 90% using an empirical method. I have the following information about the two soils:
1- Liquid limit and plasticity index.
2- Optimum moisture content.
3- Maximum dry density.
I know there are more than one equation available in literature, but I have a very limited information about the two soils. It would be very helpful if someone can help me with this.
Thank you
You can estimate the SWCC from particle size distribution curve. For more info, see Fredlund's publications
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Hi,
I have model the reinforced soil for the saturated condition using ABAQUS without any issues. But as a part of my research I need to model the problem for unsaturated (partially saturated ) soil. I find it difficult to find guidelines or materials need to go ahead with this. If some one can give me some ways to start with, that would be great.
Hi Shran,
I assume that you want to model the mechanical behavior of the grain skeleton under partially saturated conditions, in contrast to fully saturated drained or dry conditions. If this is the case, the proper and most accurate way would be to employ a three-phase mixture theory for the soil (grain skeleton + water + gas), like in the reference (see link).
However, you may just want to consider a kind of "extra cohesion" for your grain skeleton representing the partial saturation conditions. In my opinion, the easiest way to implement this would be to subtract a small "capillary pressure" from the diagonal components of the stress tensor (with compressive stress taken as negative) before you integrate in time your material model for saturated drained resp. dry soil. After integration you have to add the same amount of pressure again before passing over the updated stress tensor to ABAQUS. Hope this helps.
Regards, Daniel
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Can anybody suggest how to model different wetting-drying path in FORTRAN for Unsaturated soil?
What is the reason for using Fortran?
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Dear all,
I am new to unsaturated soil mechanics, I am working on rainfall induced landslides.
Modeling of soil slopes involves modeling of unsaturated part of soil slope. Now I have a SWCC plot please tell me how to get the van Genuchten parameters. Procedures for determining van Genuchten parameters (Alpha, n, m...).
Plot the grain-size distribution (GSD) data and determine whether the soils are well graded or gap graded. From the GSD curve you could determine the soil particle shape factor alpha which ranged from 1.1 for fine-textured soils to 2.5 for coarse-textured soils (Arya et al 1999). Now did you estimate SWCC empirically or obtain from experiment ? Are the SWCCs adsorption or desorption curves ? If experimentally, then you can deduce n (rate of desaturation of the soil as suction exceeds the air-entry value) and may require finding the intermediate suction which could be obtained by subtracting the suction at air-entry value point on SWCC (the first point of inflection on SWCC ) from soil residual suction on SWCC (the second inflexion point where SWCC curve is about becoming flat and parallel to the horizontal axis). This intermediate suction is associated with desaturation region of SWCC and could be inserted into van Genuchten equation and subsequently solved to obtain n. This would only be possible after obtaining m from GSD curve fitting. You may also find the attached articles useful.
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Hi everyone I am wondering is there a software for modeling the settlement of a unsaturated soil in 3D.I already know that Geo-studio can model unsaturated soils in 2D and that FLAC3D just can't model unsaturated soils and PLAXIS 3D have some problems modeling unsaturated soils.
If you are willing to model the contact laws between particles, or you want to do some fundamental studies I suggest you to use DEM software such as PFC 3D, or EDEM.
otherwise, PLAXIS package are built based on some assumptions that are still debatable.
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One argued with me that solute transported by the capillary rise as advection. Can it really be? My counter argument was for media like clayey soils, the velocity of the capillary rise is much slower. So according to J=CV equation, in such media, dispersion should be dominant.
What do you think?
Andre is right. It is all a matter of the physical parameters of the problem. Capillary rise usually occurs due to a pressure jump on the interface, however the intensity of the Péclet number will inform which physical mechanism is more prominent: diffusion or advection.
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I'm looking to potentially validate an analytical solution for the SWRC for monodisperse glass bead media; however I am having difficulty finding published experimental data including curves from which I can compare my results. They must have almost no polydispersity if possible, which is where it is difficult to find papers.
Thanks!
If you could send me your e-meil address, I can send you a paper explaining the importance of such curves in practical terms.  Jan (cermak@mendelu.cz)
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Hello, I am doing some simulation of the deformation of saturated and unsaturated soils. I need the normal consolidation test data for different soils both at saturated and unsaturated state (including the soil property like particle size D50). I am digitizing some from the literature, but that would cause errors. I shall be highly obliged if anyone can share me this kind of test data or recommend some papers. I will cite your paper if the data is published already.
Thanks.
Thanks for the suggestion, Prof. Rogers.
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Dear all
how can i predict elastic modulus of unsaturated soils such as shear modulus at small strain?
Hi dear
you can read my paper then i will wait for your questions.
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Spud-cans and Mud-mats are two common  types of the jack-up foundations in offshore regions. Some times these types of foundations are located on gassy soils and unsaturated soil mechanic approach should be implemented for assessing the bearing capacity and general stability.
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As we know water in soil, especially fine grained soil which has been shown to be able to sustain very high tension. So, I would like to know the  plausible explanation for the high water tension (much greater than 100 kPa pressure, commonly consider cavitation pressure) that has been directly measured in soils
100kPa is the cavitation pressure for pure, free water. In pore spaces, meniscus curvature and adsorption of water onto particle surfaces means that the water is no longer free nor, potentially, pure, so its cavitation pressure is far higher. In very small pore spaces, however, it is unlikely that traditional 'curved' menisci dominate the water structure as these pores are tens of water molecules in size - it is therefore likely that water is completely adsorbed in these pores.
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Suction meassurement in unsaturated soil
In the field the best method is the use of tensiometers. However, it is necessary to verify the type, size, and the quality of the porous cup. As indicated by Dr. Berti you can also use the soil water retention curves, which can be determined directly in the field or in the lab. To evaluate the swrc you can utilize suction tables and Richard's pressure chambers and the gravimetric method to measure the soil water content. In the field it is necessary to use tensiometers plus devices to measure the soil water content (tdr, neutron gauges, etc).
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I am studying the unsaturated soil compression, but I found the degree of saturation of soil did not change with the vertical loading 10Kpa. I think maybe I did not set the right the water level. Could you give me some advice about the water level setting?
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