Although the 'Geo-Install' workshops have shown significant advances towards modelling installation, many of these methods are currently not available in commercial packages.

*Cavity Expansion*
A common method used to model both stone column and pile installation is cavity expansion. This is the process of expanding a cylindrical cavity of zero thickness in a radial direction until the radius of the cavity equals the radius of the pile or SC. This is an effective method for modelling the excess pore pressures, in particular, generated during pile/SC installation.

*Interface Strength Reduction Factor*
PLAXIS allows for an input of an 'interface strength reduction factor' in order to take into account a reduction in soil strength arising from installation. Typical values range between a value of 0.6 to 1.0 depending on the particular soil type. This strength reduction factor can only be applied when using interface elements, however, and thus may not be applicable to SCs. During the installation of SCs, the stone and soil being treated become interlocked. Thus modelling SCs using interface elements may over-predict punching of the stone columns in the soil.

In PLAXIS 3D, the user can select between volume piles and embedded piles to model a pile foundation. Embedded piles comprise of beam elements and have special interfaces. Thus strength reduction at the pile-soil interface can not be modelled using embedded piles.The differences between these two types of pile will be covered in another post in more detail.

## 67 comments:

Hi I need to model a column in soft clays to study the phenomenon of bulging

Because it is stone columns you are using I would recommend that you do not use interface elements. You should use symmetry (where you model half the stone column) so you can easily see any bulging of your stone column. You can create a half column in the pile designer and select user defined pile.

In the calculation phase, I would suggest you use the initial K0 procedure, then the second phase would be converting the soil within your pile area to a material with the properties of stone. Final stage would be a UDL on the surface of your soil model or on a pad footing supported by the stone columns.

What issue in particular do you wish to know more about?

Hi, could you describe me about the modelling stone column in plaxis 3D foundation, shall a use massive circular pile instead of SC, and later define the material of massive pile by stone column properties? and a wanna calculate the bulging failure, for the calculation the first phase is k0 and second phase is consolidation analysis?

Yes, I suggest you use massive circular pile (without interfaces to simulate the interlocking between the stone and soil). The massive circular pile should obviously be the same diameter as the SC you wish to model. You can set the depth of the SC using workplanes.

1st phase: K0

2nd phase: Change soil properties within massive circular pile to properties of stone (this is done in calculations, not in the soil model!!)

3rd phase: Turn "on" udl on soil surface, i.e. double click on hatched lines and select load you wish to apply e.g. 100kpa (you must have selected a udl on the top of your soil model in the 'model' window beforehand, indicated by hatched lines).

4th phase: consolidation using 'ultimate time' or 'minimum excess pore pressure'. (note:phases 1-3 should be 'plastic' as opposed to consolidation phases)

Make sure to refine mesh around stone column. Bulging will also depend on vertical refinement. Depending on the length/diameter ration of your SC, your SC will have bulging or punching failure.

Does this help?

yes thank you, how can i obtain the result of punching failure of stone column from output results?and also bulging?

There should be a noticeable transition from punching failure to bulging failure and vice versa. For punching failure, there should be a significant relative displacement between the stone column and the soil.

For bulging failure, you can select 'plastic points' in the output window to see if the soil is in the plastic state around the SC. A significant number of plastic points around the column relates to bulging failure.

My main area of interest is in piling but I know there is currently no literature on how to define bulging/punching failure for stone columns but I know of research that will be published in the near future that will be of help

for 2nd phase: Change soil properties, shall i change just the properties in initial workplane which the top of sc is placed?or should change the material at the end level of sc?

You change soil properties on top workplane. PLAXIS then converts the soil between the top workplane and the next workplane to stone. Make sure the 2nd workplane is located at the bottom of the stone column you wish to install

how can i hide the surrounding soil in the output result to show the position of sc? i want to show them in the output by 3D shape

You should use symmetry i.e. model half of the stone column!

Each boundary in the model is a symmetry boundary. In pile designer, you can choose a user defined pile. Select the top right quadrant as a circular arc and the bottom right quadrant as 'corner'. This should create half of a stone column. Position this directly on the boundary, symmetry will then duplicate this the far side. It si the exact same as modelling a full SC in the middle of you model. When you go to output this way, you can get a nice cross-section view of your SC with stresses, displacements etc.

it means i have to defined a workplane at half of sc?

No you define workplane for full length of SC like normal

but i want show the vertical displacement of a row of SC! and when i use cross section the data are given by soil element numbers, is it possible to show vertical displacement in X-direction or Y-direction?

sorry in X-direction and Z-direction

can i use dl on the piles?

Yes use dl on head of volume piles, point load on heads of embedded piles

Hello, i need to model vibro concrete column supported by RE wall so can advice how to model?

sorry, i need help on plaxis 2D, to model RE wall supported by VCC (Vibro concrete column) can advice me, how to model in PLAXIS 2D?

Hello sir, I am modelling the unit cell in Plaxis 3D, but when the geometry settings is limited to only unit cell, then the plate, through which I am transferring the stress, faces re-bouncing.In other words, there will be two stress values corresponding to one displacement value.

When validating previous papers through Plaxis 3D, it is not able to predict the behaviour in lower cohesion values.

Hello,

I am running Plaxis 3d on window 7. Whenever I click to generate 2D mesh, it brings up a blank output window without the mesh. What could be wrong? Pls advice.

Myself or my colleagues have not come across this problem.

The only thing I can suggest is uninstalling the software and installing it again. Have you tried that??

Myself or my colleagues have not come across this problem.

The only thing I can suggest is uninstalling the software and installing it again. Have you tried that??

Hi. would you please describe me about modeling deep soil columns in soft soil wit plx3d foundation.

columns should be:

1.massive pile?

2.vertical beam?

3.embedded pile?

You should use massive pile. Whether you use interface elements or not is up to you.

Vertical beam is a structural element.

Embedded pile is a beam element which is a simplification of volume piles (i.e. massive pile)

HI,Thnx a lot. the tank that I am modeling these DSM columns, is small.(200mm *300mm* 300mm),when i use massive pile with 20mm diameter, in meshing the program doesnt respond? I have changed meshing size but it doesnt work.

but I dont have this problem with embedded pile.

20mm diameter...is that correct? or do you mean 200mm dia?

If you do mean 20mm...hat is very small!!! But 200mm should be ok!!

Have you tried using symmetry and modelling just half the column?? You can go to pile designer, create half of a pile and place it on the symmetry boundary. This is the same as a full pile in the middle of your computational domain. You should try this before resorting to embedded piles. Embedded piles are a significant simplification and the bearing capacity is an input parameter. i.e. you specify the capacity, it is not a result of FE calculations

thnx. but actually I am modelling my physical model that contains 4, 6 and 8 group deep soil mix columns with 20 ,25 and 50mm diameter. its a stabilization of soil box with small columns diameters, so I think i have some problems with size effects? when I model with embedded pile I dont have this problem.....so a embedded pile with no intraction can play the same as massive pile?

(thnx for your consideration)

Ok, I see now. I have never attempted to simulate model pile tests. Your diameter is probably too small. Perhaps embedded pile is the best way forward for you. In my research I have compared results determined by embedded piles and volume (massive) piles to measured field data. Embedded piles are obviously a simplification but still give good predictions. So try proceeding with the embedded piles. I would be very interested to hear how you get on with this

thanks a lot. I will do it with embedded piles and i will tell you the results...

Hello,

I read some papers about possibility of using Plaxis 2D (I use V8) instead of 3D for modeling problems like embankments supported by piles or pile-raft foundations.

How can I take into account out-of-plane distance between piles (modeled as plates)? Is it enough to reduce EA and EI parameters?

Are predictions of this method accurate enough compared to 3D calculations (settlement, bending moments)? Can they be used for design purposes?

The out-of-plane piles will have to be represented by an annulus ring i.e. a hollow cylinder. Be careful though, finite element modelling is itself a simplification, a 2-D analysis of a complex problem can be an over-simplification. 2-D analyses should only be used for small groups. It is hard to say how they will compare to 3D predictions, the only way to know for sure is to check it for yourself although you probably do not have the 3D software.

I suggest you read the paper by McCabe and Lehane (2006) "predictions of pile group response using a simplified non-linear finite element model". They replicate a small pile group load test using 2D FE modelling with good agreement to field data.

...and as for design...I would not have much confidence in 2D results if a problem has complex geometry

Thanks. I was afraid of over-simplification with that approach.

Another question then. If I use Hardening Soil model, I have to use E50 from drained triaxial test. Is it possible to use value from undrained triaxial test or some correlation between value from undrained and drained test?

Yes, you can choose to enter undrained parameters or drained parameters, I suggest you have a read through the manual, it explains it quite clearly.

There are correlations between undrained and drained stiffness using Poisson's ratio, probably also in the manual

I have carried out field loading tests on isolated stone column on layered compressible soil and wish to use the 3-D Plaxis to validate the field results and determine displacement, how do I go about this.

Ndu.

Hi, I'm working on my thesis about piles which have a range of diameters. I have some problems about Plaxis 3D Foundation. I used volume piles and applied lateral loads. How can I find the bearing capacity of pile groups? can I find it directly on Plaxis program? And how can I reach the bearing capacity of each pile in pile groups?

Moreover I want to find the resistance of soil along the piles. How can I find this?

Thank you for your help.

Hello.

Apologies for this late reply, I have been busy with my own thesis!

PLAXIS 3D Foundation does not give you a direct answer if that is what you mean. All plaxis will do is give you the load displacement response of your piles. Therefore you must use your own engineering judgement to distinguish when your single piles and pile groups have failed.

as regards each individual pile, you may have to consider the proportion of the total pile group load taken by a particular pile and plot against pile displacement and again decide yourself at what point the pile fails.

I am not 100% sure how to find the resistance of the soil along the piles, perhaps look at the compressive forces in the pile with depth?

Or maybe even look at the compressive stresses in the soil directly in front of the pile?

I hope this helps. My research is on axially loaded pile groups so I am not as experienced with horizontally loaded pile groups. Let me know how you get on!

Thank you for your help. With my supervisior, I decided to find failed load which pile has with a procedure. The procedure says, the maximum horizontal displacement should be %2 of your diameter of piles. So I calculated the maximum bearing capacity of each pile and drew the load displacement curves and decided the failed loads.

I have also found how can I reach the resistance of the soil along the piles. To reach it, first select the interfaces which on piles and after that choose the normal stress, then you can see the normal stress on your piles which caused by soil.

What do you think about piles which created with beams and created with volume piles? Which one is the best?

I hope my researches can help everyone who study with horizontally loaded pile groups.

Your comment is very much appreciated....very helpful for other readers researching pile group behaviour under lateral load.

Volume piles are the best option. Embedded piles use beam elements and assume a Linear Elastic zone around the beam to simulate the volume of a pile. Also, the pile capacity is an input parameter when using embedded piles.

That said, however, I compared volume pile predictions to embedded pile predictions when looking at axially-loaded pile groups with satisfactory agreement. The reference to that paper is:

*Sheil, B.B. & McCabe, B.A. (2012) Predictions of friction pile group response using embedded piles in PLAXIS. The 3rd International Conference on New Developments in Soil Mechanics and Geotechnical Engineering, 28-30 June 2012, Near East University, Nicosia, North Cyprus

Thank you for your help. I 'll use your opinion about piles in my thesis. But could you explain me how can i reach your article? I can't find this on net or on my universities library.

Hi dear friends,

A single pile was failed in certain point load which I modeled in PLAXIS 2D,but this pile (with the same definition) isn't failed in PLAXIS 3D.

Where is the problem?

thanks

PLAXIS 2-D models large strain much better than 3-D. For example, that's why cavity expansion is modelled using 2D software rather than 3-D.

One other thing to check is your mesh refinement in PLAXIS 3-D. For your 2-D mesh you may have a lot of elements over the length of your pile whereas in 3-D you may have fewer elements which will make a difference

I modeled small scale pile,so according to your suggestion,it is better to use plaxis 3D instead of 2D,yes?

thank you.

I would use plaxis 2-d if I was modelling a single pile

I also agree with B.Sheil BE Koohyar.

Hi, I have some problems about Plaxis 3D Foundation. I used volume piles and applied lateral loads. But the loads which carried by pile groups and pile cap didn't equal to loads which I applied. So I think some of the loads were lost with friction between pile cap and soil. Therefore, I want to find the shear stress under the pile cap. But I can't find it because Plaxis didn't allow to put the interfaces under the pile cap. Do you know another way to find the shear stress (Tz-x ) under the pile cap?

Thank you for your help.

First of all...how did you find the load taken by the piles?

I do not think there is a way to predict exactly how much shear stress the cap is transmitting to the soil. one way around this, however, is the following:

1) Carry out the analyses like you have done and calculate the loads in the pile

2) Carry out a similar analysis but this time make sure the cap is NOT in contact with the ground, i.e. excavate the soil (to a depth of, say, 0.5m) uniformly below the pile cap. It is important to keep everything the same so you can make comparisons to your original analysis. So keep the pile lengths below ground level the same. One thing - the excavation of the soil below the pile cap may now cause your soil to be more overconsolidated than your original analysis. So I recommend that for the top 0.5m of soil you intend to excavate use a 0 kN/m3 unit weight so it does not have any impact on the soil stresses.

Now compare the loads between these two analyses - if they are the same then your cap is not transmitting load to the ground surface. If they are not the same then the difference is the contribution of the cap.

Hope this helps!

First of all...how did you find the load taken by the piles?

I do not think there is a way to predict exactly how much shear stress the cap is transmitting to the soil. one way around this, however, is the following:

1) Carry out the analyses like you have done and calculate the loads in the pile

2) Carry out a similar analysis but this time make sure the cap is NOT in contact with the ground, i.e. excavate the soil (to a depth of, say, 0.5m) uniformly below the pile cap. It is important to keep everything the same so you can make comparisons to your original analysis. So keep the pile lengths below ground level the same. One thing - the excavation of the soil below the pile cap may now cause your soil to be more overconsolidated than your original analysis. So I recommend that for the top 0.5m of soil you intend to excavate use a 0 kN/m3 unit weight so it does not have any impact on the soil stresses.

Now compare the loads between these two analyses - if they are the same then your cap is not transmitting load to the ground surface. If they are not the same then the difference is the contribution of the cap.

Hope this helps!

Thank you I will try your suggestion. I didn't calculate the load taken by the piles. What's your suggestion to calculate it?

Firstly, I want to explain my research to you. I made pile groups (2x2) with 15m lenghts. And I put the pile cap which had 0.5m thickness. I applied the horizontal load (3600 kN) to the side of pile cap. As result, I calculated the normal stress on the pile's caps' side which against the load I applied and I calculated the shear forces which happened on the piles' heads.

And then, I wanted to calculate shear stress caused by the friction under the pile cap. I will try it with your sugestion.

Finally, I want to check if sum of the calculated normal stress, shear forces on the piles' heads and shear stress caused by frictions equal to the horizontal load which I applied to the pile cap. If it won't be equal to eachother, I must put the dummy soil under the pile cap for prevent the shear stress caused by frictions. So I can prevent the lost load. Therefore, piles will take all horizontal loads which I applied.

I want to know about the model of stone column and its material properties in Soft clay and stiff clay used in PLAXIS 2D software.

could we do impact assessment of a load on the soil in plaxis? if so could you please suggest a method...

can we give impact load on soil in plaxis? if so could you rcommend a method?

hi, could u describe me about modelling the effects of lateral movement on bridge abutment using plaxis 2d? and also need to do the analysis on single pile n group pile beneath the abutment.do they any specific phase to be analyzed first?-first time using this software..appreciates if u could gelp me,,thanks,,

You need a 3-D version (3-D or 3-D Foundation).

None of what you described can be accurately considered using a 2-D analysis

is it possible to model under reamed piles in plaxis??

Yes it is possible in PLAXIS 2-D to model single cylindrical underreamed pile. Not possible in PLAXIS 3D Foundation.

I am not sure about PLAXIS 3D

I am doing a study on retaining wall with varying backfill strengthened conditions using plaxis 2D and 3D. Can anyone suggest me where to obtain tutorials related to this area??

Hi friends.

Please tell me how can one group of stone columns in plaxis 2D models do and how to understand the rupture What is the model?

Thank you

Post a Comment