Interface modeling in Pile-supported embankment

I want to model a pile supporting an embankment load (i.e. most part of the embankment load is transferred to the piles and remaining small part to the soft soil). I am building a unit cell model right now. My pile will have both skin friction and at the same time end-bearing capacity as well. I am seeking advice/ insight on how to apply interface properties to the pile at the top and bottom. Is it necessary to anchor at the top as well? I am following FLAC 2D (Pile supported highway embankment) documented example where the pile is anchored at both the top and bottom.

In my case, the top (head) of the pile is extended to a small distance inside the first lift. So I am confused how should I apply interface properties? The soft soils where the pile shaft lie do not have friction angle but the 1st lift where the top is extended has both friction and cohesion… Any insights on this will be highly appreciated?

Hello @Newuser,
It’s entirely up to you if you want to define pile coupling properties at the tops of the piles. In the example you mentioned it limited the coupling properties to the top and bottom of the piles using the range keyword. You can do the same thing to give different pile properties in different soil conditions.
Here are some additional examples regarding piles structural elements in FLAC2D:
Pile Examples

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The interface of pile and surrounding soils in Pile elements is represented by coupling springs. Two directions of springs are used, i.e., normal and shear directions. Each direction has 3 properties to be assigned : stiffness、cohesion and friction, namely coupling-stiffness-shear、coupling-stiffness-normal …。

If the pile is subjected only vertical force then you can assign the 3 spring in the normal direction equal to 0 ( actually you do nothing about them since the default is 0)。Therefore, all you need to do is to assign 3 spring properties in shear direction. However, these springs as above mentioned are to specify for skin friction only (Note that : when the pile is created the links of these springs are created automatically ). The end-bearing spring (link) must be created manually and a link conditions must be addressed for end-bearing spring.

Spring in Shear direction for skin friction can be estimated based on static formula、SPT、CPT or a pile loading test. Some reduction may be applied to the properties of cohesion and friction of spring when considering the pile types、installation method. The spring stiffness can be estimated by

Ks=10 ×max[ ( K+4/3 G ) / (∆Zmin ) ]

The link of end-bearing spring can be specified based on the real conditions. In the example, the pile top is embedded in the foamed concrete, 0.05 m, which may allow the movement in y direction and rotation so we set them “ Free” , and in the x direction (axial), we set it “Rigid” . At the toe, we allow the movement in y and rotation,so they are set “Free” , but in x direction we may apply a normal-yield model when the force at end node is reaching the strength level it will follow the normal-yield force-displacement law and presenting a large displacement. Therefore we need to provide the parameter for this model. The parameters are Stiffness、Area and Yield-compression. Area is normally set to 1 m^2 ; yield-compression is the ultimate end bearing capacity of the pile which can be estimated by static formula、SPT、CPT or a pile loading test. Stiffness is the critical issue. In my opinion, since in the example, the load of pile is not reaching the end bearing capacity (the most important point of this design), the settlement of embankment is governed by the compression of the soil underneath the pile toe and within the elastic range. Therefore, the stiffness in the normal-yield model would decide the settlement of embankment. Since the pile is stiffer than the soil so I will use the “real” soil stiffness for this parameter :
〖 Ks=max[ ( K+4/3 G ) / (∆Zmin ) ]
The following are the codes I wrote for this part, for your references, I am not guarantee that this is absolutely correct since there is no monitoring data to justify this process.

; Create New Link at Pile Top and Toe by Using Normal-Yield Model
[stiff= 2.02e7]
[ycomp = 2.5e5]

; Pile Top
struct link delete range pos-y -0.6 0.05
struct link create target zone group ‘top’ range pos-y -0.6 0.05
struct link attach x=rigid y=free rotation=free …
range group ‘top’

; Pile Toe
struct link delete range pos-y -12.8 -12.2
struct link create target zone group ‘Bottom’ range pos-y -12.8 -12.2
struct link attach x=normal-yield y=free rotation=free …
range group ‘Bottom’
struct link prop x stiffness [stiff] area 1 yield-comp [ycomp] …
range group ‘Bottom’

The maximum displacement is about 3.8 cm。 Note : the data file in the example misplaced the 600 and 640。

Thank you very much…