Interface stiffness factor: Embedded beam row

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.

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.

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

Thanks both for reply.

If the modelled pile in Plaxis 2D not satisfied the criteria for this interface (for eg: MC model, phreatic surface below 2m from GL etc.), how to derive the interface parameters for the embedded beam row? 

Thanks. 

In general, if I were a designer I would try to calibrate them based on some pile tests. In the end, the behaviour of the embedded beams as a pile group should show a similar response as in a real case. But since we are doing modelling some testing and even trial & error are required as part of the job.

The decision on what material properties to use depends on the geotechnical engineer and the data that there are available.

Noted, thanks. 

Q1) Let's say I have the pile load vs settlement curve from there we can obtain the stiffness (which is RS, RN, kF). So in order to get ISF (which is the input in Plaxis) how we can use that value (for cases like homogenous soil and different soil stratum)?

Q2) In the following paper, what are they referred to as a plastic slider in the lateral direction?

Q3) Based on this we can know that the bored pile is modeled as an embedded beam don't capture the effect of the installation of the jacking pile and the lateral direction is not good. Why HSS model and phreatic at ground level were used for this?

Stefanos Papavasileiou said:

I am quoting from the Reference manual (page 298 of PLAXIS 2D 2023.1):

Q4) I modeled the embedded beam row pile with different ISF (The force application is same so when ISF increases then deflection decreases). The result from the checking is shown below. When see the skin friction plot the ISF Rs,RN,kF 0.2,0.2,2 base skin friction is lesser than that of ISF Rs,RN,kF 3,3,30 (Since total load application of the load on the pile is same in both cases => area of the skin friction plot is same). All cases the base resistance is 0. Can I know the reason of this changes in the skin friction plot. 

Thanks. 

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