Geosynthetics - Science topic

In SEEP/W (GEOSTUDIO series software), the addition of geosynthetic materials and the analysis of drainage characteristics are realized through the coupling of material constitutive model and seepage boundary conditions. First, it is necessary to create a new material type in the material library, select the "anisotropic permeability medium" model, and input the horizontal permeability coefficient (\(k_h\) and vertical permeability coefficient (\(k_v\)) of the geosynthetic material, which can be obtained by ASTM D4716 and other standard tests. For drainage geocomposite materials (such as drainage plates), it is necessary to define its water conductivity (T), that is, the penetration flow per unit width, calculated as \(T = k_h \cdot t\), where t is the thickness of the material. In the model construction phase, the geosynthetic material is modeled geometrically as an independent Region to ensure that its Interface units with the surrounding soil are correctly divided. To simulate the filtration function of geotextiles, the filter criterion can be enabled in the material properties and the seepage behavior can be constrained by the parameters of equivalent aperture (\(O_{95}\)) and gradient ratio (GR). For unsaturated seepage analysis, it is necessary to further define the relationship curve between volume water content function (\ (\theta\)) and matric suction (s), which is usually fitted by Van Genuchten model or Fredlund-Xing model. Drainage characteristics are calculated in conjunction with boundary conditions: a "free drainage" boundary (with a water head of 0) is applied to the drainage outlet of the geosynthetic material, and a "Drainage Element" is installed within it to simulate centralized drainage. Through steady state seepage analysis, the water head distribution and seepage flow inside the material can be obtained to verify whether its drainage capacity meets the design requirements (such as the safety factor of no less than 3 specified in ASTM D7931 standard). For transient analysis, the effect of material compressibility (volume compression coefficient \(m_v\)) on the drainage rate should be considered, especially in the consolidation process of soft soil foundation. The mesh encryption strategy should be paid attention to in numerical simulation, and second-order elements should be used at the interface between geosynthetic materials and soil to improve the calculation accuracy. Parameter sensitivity analysis shows that the anisotropy ratio of permeability coefficient (\(k_h/k_v\)) has a significant effect on the drainage path, and it is recommended to determine the reasonable value through laboratory test or inversion analysis. In addition, it is necessary to verify the convergence of the model to ensure that the seepage field satisfies the law of conservation of mass, and the typical error is controlled within 1%. Through the above method, the drainage efficiency of geosynthetic materials under complex seepage conditions can be evaluated systematically, which provides a reliable basis for engineering design.

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Dear Suresh,

Geocell is two types, one is perforation and other is without. Since it is honeycomb structure, high density aggregates to be filled inside the cells upto the thickness of the cell (mostly 100mm height). it works like mat foundation. Load distribution is equal, less chances of differential settlement. Due to the high confinement of the geocell and closed packing the settlement very much reduces.

Additionally provide the geogrid at the base of the geocell, it works like a basal reinforcement that load laterally spread. Hence total vertical settlement will reduce considerably. Rest all comes under design, amount of load comes from superstructure, quality of HDPE (geocell material), height of the cell (thickness of the geocells to be designed), joint connection & connecting material and original clay soil undrained shear strength.

surely it will reduce the settlement at higher level and bearing capacity improves based on the general shear failure model.

Dear colleagues,

Thank you for your very informative answers.

You can find the answer to this question through the attached file.

Best regards

Jochen Hack Thank you. But "embedded region" cannot be assigned for certain applications like encasement, where the contact properties govern the behaviour.

For a small scale experiment setup, just fold the longitudinal axis very carefully. In field formwork in needed similar in the case of wraparound walls.

Dear Azm Shaher Al-Homoud ,

I have 3 conference proceedings.

Typically below 40% of the load at failure (failing load at quasi-static test).

Embedded beam element or shell element

Dear Naga Linda,

The below-mentioned literature may be useful.

Don't know the exact answer for stone column but for if geosynthetic is geotextile than, most commonly 0.7-0.75 for soil-geotextile (Rint).

If geogrid than it depends on median grain size to minimum aperture size of geogrid. If D50<dmin, you may assume full interlocking (holtz and lee 2002) based on original work by jewell (1985).

Hello Yusuf Alhowaidi

Join here the pdf file, Pages 239-247 from pdf (or 251-259 from paper)

Thanks for the guidance! I'll check it out.

Thanks for your useful information William F. Hansen . Actually, I need an idea. If you use these values ( C, Phi of C. C. Block) in any one of your research work, then, let me know.

My pleasure

1-about the lateral slab. How did the soil under it resist to a loading of 420 KN ?. i think that the only the soil cann't resist tath great the loading.( is the facing blocked ? )

The geosynthetic element will not work if you want to model a deep mixed column. What you could do is following...

1) If you use plaxis 3d you should define column elements as volume fe.

2) If you use plaxis 2d it is more difficult but you could use a smeared region with improved soil simulating the region with deep mixed columns with spacing. In this case you must put more effort to define this soil parameters for which some correlation curves do exist.

I feel both PLAXIS3D and FLAC3D is good in simulating your problem but PLAXIS3D is very easy to use as compared to FLAC3D. PLAXIS3D based literature related to your problem is available, so i recommend you to collect those paper and use effectively in your work..All the best in your research..

The shear bonding between geogrid and pavement is depends on aperture size of the grid is true. I feel you need to conduct shear test with varying aperture size of grid with pavement material and find out which one is giving better response. You even vary the aperture size and also various geosynthetic material can be used for the test. all the best..

Your question is actually not clear. Are you struggling with finding the modulus of elasticity of sand or struggling with inputting these values in plaxis? You usually need bulk modulus, shear modulus, cohesion (if any), density, friction angle, dilation angle etc. of sand and elastic modulus of geosynthetic, its poisson ratio. You may also need the interface parameters, such as stiffness per unit area, soil-geosynthetic interface friction angle, etc. Typical values for these parameters are available in the literature and if you are simulating the real problems, then you have to carry out tests to obtain these parameters.

The activation starts at the moment when what you call "bulging" is large enough. It could start immediately after load application, radial pre-compression iz zero, see above. The point is: activation starts at the moment when diameter of granular fill reaches the nominal diameter of geosynthetic encasemnt as produced, e.g. 70 cm or whatever. Is this a proper answer? In a homogenious soft soils the largest bulging occurs in the upperst part down to a depth in the range of 2 to 3 D.In layered soils it can start in the softest soil layer in depth.

Hi Prashanthi

Linear elastic model is fine. Be careful about choosing the elastic modulus. The strain range should not exceed the elastic limit from your tension test. Poisson's ratio can be generally taken in the range of 0.3-0.4.

Madhavi

Please have a look at the following articles:

1.Almeida, M.S.S., Hosseinpour, I., Riccio, M., and Alexiew, D. (2015). “Behavior of Geotextile-Encased Granular Columns Supporting Test Embankment on Soft Deposit.” Journal of Geotechnical and Geoenvironmental Engineering, 141(3), 04014116.

2.Mohapatra, S.R., Rajagopal, K., and Sharma, J. S. (2016). “Large direct shear load test on geosynthetic encased granular columns.” Geotextiles and Geomembranes 44(3), 396-405.

Thank you for your suggestion. The code we are using is a .p file in matlab which we cannot view or change. Hence the presence of scatteredInterpolant function is unsure. Kindly suggest any other method to overcome the problem.

you may create surface to surface cohesive interaction without creating mesh part.

i hope it helps

Cohesive surface is a surface. Cohesive element may be a line element.

please check the properties given for surface. the surface properties are different from those of the bulk material.

Read this report 

It gives complete information on geogrid modeling in reinforced walls.

FHWA (2009) Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes – Volume I, and Volume II.

Orientation of fibres mainly governs the strength improvement than the tensile strength of the fibres. Attached file gives more details on this aspect. However, dense sands tend to impose higher tensile stresses on fibres, which leads to higher confinement and strength improvement. If the fibres cannot withstand the tension, they break and hence could not achieve the improvement effect.

Small strain - or small displacement - refers  to the case where we assume that changes after a displacement is so small that the geometry is virtually unchanged. Large displacements would invalidate assumptions like the famous "linear-elastic deformation" one. In cases of small strain, we are indeed talking about linear equations governing the relation between force and displacements. And we take the viewpoint that the material is not deformed once the stress has been removed.

Large strain/displacement is, basically, everything else, but common to them are that they lead to a permanent deformation of the material (that is, deformations that remain after the stresses are removed).

Geogrids and geocells are not alternatives to each other in all situations. The choice of the reinforcement depends on the subgrade modulus as well as the design traffic (msa). Geogrids are cheaper than geocells and easy to lay, but if the required improvement in strength and modulus is very high, you need multiple layers of geogrids placed at regular intervals over considerable depth below the pavement, whereas a layer of geocells filled with aggregate works out to be economical. Geocells are the speedy solution over emergencies, whereas geogrids though simple and easy to install, are not applicable over softer subgrades. Moreover, geocells provide higher lateral confinement and beam effect, thus reducing the thickness of pavement significantly. For many geotechnical problems, savings in time and assured performance of the solution play vital role than economical aspects. 

Thanks for the information. Do you have experience with that model ? does it apply also for landfill covers ? I think we will have to continue with a catchment model.

This is valuable information Jan. Thank you. In Brazil, we also specify cationic bitumen emulsions for geogrids (instead of asphalt cement), in the rate of 0.4 - 0.6 l/m² (residual). However, the rate specification is still very empirical, as well as the specification of the geogrid type (polymeric or fiber glass, composite or not, tensile properties...). See attached the Brazilian specification (in Portuguese).