Interface stiffness factor: Embedded beam row

Question

Hi all, 
 
 Can I know the source from which these interface equations are obtained?

Thanks,

Dennis Waterman · Accepted Answer

Dear Nitharshan, 
 1) It's almost correct ..... you're using the absolute displacement of the pile, but that should of course be the relative displacement, that is relative to the soil. Because it's the difference between pile and soil displacement that causes the friction. 
 2) For embedded beams the R_inter parameter is not used, because the ISF's already take care of the interface reduction. That is why the equation says "G_soil" and not "G_i". 
 Hence, the difference you find is probably because you use the absolute displacement instead of the relative displacement. 
 With kind regards, 
 Dennis Waterman

Stefanos Papavasileiou · Answer

Dear Nitharshan, 
 I am quoting from the Reference manual (page 298 of PLAXIS 2D 2023.1): 
 Note that the default values of the interface stiffness factors are valid for bored piles which are loaded statically in the axial direction and behaviour of the surrounding soil is modelled using the Hardening Soil model with small-strain stiffness. The phreatic level is assumed to be located at the ground surface. These values should be modified if the conditions in the model are different from the ones assumed to derive the default values.

Faseel Khan · Answer

The default values have been derived by PLAXIS as part of the validation (Sluis, 2012)
Sluis, J.J.M. (2012). Validation of Embedded Pile Row in PLAXIS 2D, Master of Science thesis, Delft University of Technology

Dennis Waterman · Answer

Dear Nitarshan, 
 Q1). The equations to do so are actually given in the validation article below figure 2. 
 Q2). Think physics ... how can one limit the capacity of a spring? Well, by connecting one end to some kind of sliding construction with a certain maximum friction. When the spring force exceeds the maximum friction of the slider, the slider will start sliding and thus the spring force cannot increase anymore. Hence, the "slider" is what limits the skin friction to tau_max. 
 Q3). First of all, the installation effects of bored piles are rather small (compared to driven piles) and testing has shown that the installation effects also don't last very long. Hence, within months after installing bored piles the bearing capacity of bored piles are what one could expect as if no installation effects would have taken place. So assuming installation effects are minor, the default situation is based on the most suitable allround model for soil, which is HSsmall, and based also on what is a common situation in which pile foundations are used. Those are often soft clay or peat areas with high groundwater level. Therefore a groundwater level at soil surface was chosen. Q4) Unfortunately the plots are not on the same scale. The right plot has a skin friction at the foot that is more than 2x higher than the left plot, but clearly it's not plotted as 2 times higher. So I think on the same scale the plots would actually not be that different, apart from the fact that in the right one the skin friction increases with depth for the lower 25% of the pile. Just based on the plot there is not much to conclude. The skin friction depends on the relative displacement between pile and soil and is limited by the skin resistance. The skin resistance is different in both cases because of the different ISF values. So maybe the skin friction was redistributed in both cases - you could check that by checking mobilised skin resistance. But as said, just from two plot it's impossible to say why the distributions are different. 
 With kind regards, 
 Dennis Waterman

Interface stiffness factor: Embedded beam row

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