Since the two dimensions of the cap are comparable to each other, 3D modeling should be used. Plane stress and plane strain assumptions can be used provided that the one dimension of the problem is much larger than the other (theoretically equal to infinity).

I hope this helps,

George

Thanks George, I thought the same, maybe a simplified equivalent axi-symmetric model of one pile (a quarter of the cap), only considering vertical loads and no moments, could work in this case.

Basically if you would like to simulate the behaviour of a group of piles, 3-D finite element modelling may provide a tool for you to understand the behaviour of a group of piles. Plane strain model is only suitable for understanding of the behaviour of a single pile. But a simple plane strain model could be a good start in this case. Then you need to think about how to incorporate the effect of group pile during the modelling.

The mentioned problem is 3D in nature and 2D models will not give satisfactory results

The saving in computational cost achieved through axisymmetric approximation of square group of piles is not worth the resulting error, which could be as high as 20%, depending on the geometry and layout

This depends on what you are after and what is the nature of the loading.  For a pile founded on rock and if it is only vertical load on your piles, as long as you get the axial stiffness correctly represented in 2D, you are not too far off as most displacement is elastic compression of the piles. No matter what you do, the actual distribution of load in each pile is not going to be correct. For a MSc project, as long as the author is aware of the pit falls and understand the limitations of his 2D simplification then I reckon it is acceptable.

Plane strain mode of analysis is ok for a single pile. U ned to go for 3D numerical modelling approaches for analysing behaviour of roup of piles.

Dear Cristian,

In my opinion, this problem can be modeled  in Plane strain with little limitations. You can model the pile group using Embedded Pile/Beam Row element, you can find it in the new version of  Plaxis 2D suite. This element consider the spacing between piles  in the out of plane direction and consider the 2D/3D equivalence  (check links below for this element validation and verification examples).

Moreover, ITASCA FLAC 2D can model pile element in plane strain analysis considering the spacing between them in the out of plane direction under "2D/3D Equivalence " section and field name called "Z spacing".

Through these links you can find comparisons between 3D Pile and 2D Embedded Pile row in different study cases.

Finally, check this attached file "Embankment on Rigid Inclusions Webinar", this about using embedded beam row element in modelling rigid concrete inclusions under embankments. The tutor used a simple trick to adjust the load transfer at pile top to match the hand calculated values.

Regards,

http://kb.plaxis.nl/sites/kb.plaxis.nl/files/kb-publications/Iss34%20Art2%20-%20Validation%20of%20the%20Embeded%20Pile%20Row%20in%20PLAXIS%202D.pdf

http://www.jip-ts.co.jp/plaxis/verifications/20_casestudy_embedded_pile_row_nov2012_0.pdf

Plane strain model may be used for modeling piles. You may follow the following paper in this regard. However, 3D modelling is always a better option.