Pile Supported embankment

Hello @everyone,
How can we simulate the staged construction of embankment over soft soil? I have gone through the example application of FLAC 2D but it mentions about adding the layers of smaller thickness but how can we provide time gap between one layer and the next one as in the real construction? I am trying to do this to verify the settlement of a real case study.

The time between addition of subsequent loading (i.e. adding new layers) does not matter if solve to equilibrium after applying each load. The only thing I can think of that would be time dependent is if you are using a creep constitutive model.

so what are we suppose to do to get the settlement values in certain time interval?

@Newuser - I am not sure if I understand your question. Here is an example that might better explain layered embankment loading - Pile-Supported Highway Embankment — Itasca Software 9.0 documentation

If you are asking for time-dependent settlement values you should run a consolidation analysis:


I am not quite sure if this is the answer to your question.
In FLAC2D Example, Embankment supported by Piles, it is assumed that almost all the loading are carried by the end bearing pile. The total settlement may be estimated by assuming a low value of Fluid Bulk Modulus Kf and time calculation is neglected. But if the bearing capacity of the piles is insufficient to carry the weight of embankment , leading to a relatively large displacement at the piles top . The soft soil will subject to a great portion of the embankment weight and we will need to discuss the full consolidation process.

A. Use the equation below to estimate the consolidation time :

tc = Lc^2 / K* (K+4G/3)

tc= time needed for completing consolidation process
Lc = Longest path for water to flow out the model
K* =mobility coefficient in (m^2/Pa-sec)≈1×10^4 K (m/s) ; K is conductivity
K = Bulk Modulus of Soil
G = Shear Modulus of Soil

B. Procedures :
(1). Use above equation to roughly estimate the time required
(2). Use a real value of Kf (≈1e9 )
(3). Run standard coupling of fluid and mech scheme , set time-total equal to tc
(4). Monitoring the pore pressure and displacement vs time-total to see how much time it need to actually converge. This also give you the settlement vs time.

  • The entire run will require a longer time to reach convergence. You might want to set few check points and save the files for those check points.

(C) The following are part of an example :

zone fluid property permeability 1e-12 ; K* = 1e-12
model fluid active on
zone gridpoint initialize velocity (0,0)
zone gridpoint initialize displacement (0,0)
; — set mechanical limits —
model mechanical substep 100
; — mech and fluid run continuously taking turn.
model mechanical follower on
model fluid substep 50
; — histories —
model history fluid time-total
zone history displacement-y position 0,10
zone history displacement-y position 1,10
zone history displacement-y position 2,10
zone history pore-pressure position 2,9
zone history pore-pressure position 5,5
zone history pore-pressure position 10,7
zone gridpoint initialize ratio-target 1e-3 range position-x 4 21
; — solve to 3,000,000 sec for the fist check points —
model solve fluid time-total 3.0e6 or mechanical convergence 10

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I need to run a drained analysis. I have attached the data files.

My problem is:

When I run the drained analysis, the pore pressure is increasing instead of decreasing and thus my settlement is not increasing.

I have followed the documentation guide to provide the boundary condition to facilitate drainage through the top of the model but it looks like it is not happening. Would you kindly check my data file and solve this issue?

In the DLAC 3D example (Emb on a cam clay, FLAC 3D), the water table is at the surface. And they applied the equivalent surcharge instead of staged lift construction at the top of soft soil. The initial fluid-tension is set to -1e20 and zone face apply pore-pressure 0 range group ‘Top’.

My case : water table at z=-2. So how do I need to simulate this situation in my staged construction where the top would be embankment top but pore-pressure 0 needs to set at 2 m below ground.

ini.dat (1.6 KB)
unsupportedcondition.dat (4.0 KB)
I cannot upload (.sav file for my model) here. If you can see the data file suggest me how to simulate this situation, it would be great.

Hello @cheng @jscic
In the FLAC 2D example application “Pile supported Highway Embankment”,
the simulation is done for undrained case with water table at the ground surface.
I need to run the simulation in FLAC 3D for the similar case but with water at depth z= -2 and for drained condition.

I followed another example application “Emb loading in Cam Clay” where the drained condition is allowed. In this example, the whole embankment load is provided as an equivalent surcharge and water table is at the surface. So, In drained condition, water will flow out of the model from the top of the ground surface with “Zone face apply pore-pressure zero range group “Top”” command…
However, In my case, the embankment load will be provided in several lifts. So When I have take the water out from the model which is at z = -2 m, (2m below existing ground), is the command "ZOne face apply pore-pressure position-z -2 fine ? I also tried 'Zone face apply pore-pressure range position-z -2 5 fine? This is because, at this state my model will have 5 m lift on top of the existing ground surface. The thing I am not sure about my drained condition working properly is that, my pore-pressure is increasing instead of decreasing at drained condition?

Any insights on this will be greatly appreciated. Thanks

Hello @Newuser,
Yes, the zone face apply pore-pressure 0 range ... command is fine. If you need to set the apply condition on a non-surface zone face, you can null that zone above the apply range. See the 2nd Note in the zone apply documentation.