Unsaturated Soils - Science topic

Hi Arvind Kumar ,

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:

Zew Zealand Geomechanics Society Recorded Webinars.

Please find the attached files. Regards.

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.

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..

You can refer to the following paper:

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:

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.

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.

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.

Thanks you very much.

Thanks. It was helpful for me

Shahriar Shahrokhabadi y Zoubair Boulahia , Vahid Moosavi necesito una ayuda en mi simulación por "Simulink" la ecuación de richards ,

Yazgan Kırkayak I would be happy to share my data after the experiment.

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.

You probably want to consider the effect of the humidity in the weather on the suction and triaxial cyclic tests as well.

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.

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.

GeoStudio software

or SoilVision

Calcium losses lead to water Erosion. It has an impact on the soil structure

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.

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.

I will try to help soon

you my take alook on the attached article. I hope it will help

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

 Hi Bestun and Hossein, thank you very much for your answer:

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

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.

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...

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.

Integrate the SWCC to get the permeability

Yes there are finite element codes that can deal with the unsaturated conditions.

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.

You can estimate the SWCC from particle size distribution curve. For more info, see Fredlund's publications

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

What is the reason for using Fortran?

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.

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.

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.

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)

Thanks for the suggestion, Prof. Rogers.

Hi dear

you can read my paper then i will wait for your questions.

My Dear Amir Akbari Garakani; Here are few links on Spud-cans modelling may be useful to you Regards

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.

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).

HI LEI JU; Please go through the following links may be useful to you.

Regards

You just need to applied cyclic loading in your boundary condition and use different possible options of cyclic loading to implement you cyclic loading curve (Amplitude and Time).

Have you tried applying the correction term in Fredlund and Xing (1994) when fitting the curve? It might improve your fit to the high suction data range. Be aware that its use might mean that "a" in FX does not necessarily equal "1/a" in VG - it's not something that I have tried so I can't be sure.