Size effect on modelling Plaxis 2d

Question

Hi, 
 
 Why when the model size have effect on the results even for huge dimensions? known that no deformation is happening below 4B of the footing. 
 Actually I am trying to do sensitive Analysis for my then I used 100*100 model size just to see if there are any effects in the Load-settlement results 
 I found that with every-time I make my model bigger it gives me higher values !! even I tried 1000*1000 and as well have affects 
 (I am simulating other's work, and my results is the same as them at (X=20 Y=10) but when I started playing with model size things changed. 
 Model/ learning to find the ultimate bearing capacity of strip footing on sand.

Dennis Waterman · Accepted Answer

Dear Raied, 
 There are more factors to take into account: 
 1) Mesh refinement. If you increase the model, do you also refine the mesh? Note that the mesh generation option "Very coarse/coarse/medium/fine/very fine" assures a certain amount of elements. If you increase the geometry size and you generate the mesh with the same options, you get (approximately) the same amount of elements, hence the elements will be bigger. And bigger elements give a higher bearing capacity 3) Elastic vs elastoplastic behaviour. For elastic models like Mohr-Coulomb the influence zone of loading is heavily overestimated. Therefore it is recommended to use an elasto-plastic model for soil behaviour, like HS, SS, HSsmall etc. 
 2) Small strain stiffness. At a certain distance from the project area the induced (shear) strains are very small and therefore the soil will behave stiffer. This should be taken into account by either using a soil model that automatically does that (like HSsmall) or the user has to manually apply stiffer material sets further aways from the project area. Though the effect of small strain stiffness is more significant for deformations than for bearing capacity, there is still some effect on bearing capacity. With kind regards, 
 Dennis Waterman

Faseel Khan · Answer

Footings spread out the load as shown in the pressure bulb diagram which shows that different geometries of footings will spread out the load differently. At a depth of 4B, for a continuous footing the load is about 10% of the load at the surface whereas for a square footing it is less than 5% but it is not 0%. The zone of soil within which the vertical stress is significant and causes significant deformation of the soil, affecting the safety and stability of the structure, is known as pressure bulb. The soil outside the pressure bulb is assumed to have negligible stresses but is NOT zero. The deeper we go below the footing, the lesser the load value becomes because it gets spread out to a larger area and so the pressure (Force/Area) becomes very small but it is not zero which is also what you see in the pressure bulb diagram as shown below.

Size effect on modelling Plaxis 2d

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