Pore pressure curve - consolidation

FLAC3D
1

Hello all. I am a beginner in FLAC3D and am working on the consolidation of a unit cell model.
when I trying to plot the pore pressure against time, as soon as I activate the fluid i.e. model fluid active on, the pore pressure is expected to decrease. But in my case it is increasing initially to a certain value and then starts to decrease.
I am unaware, what’s wrong with my code or the inputs I have given.
I have also attached my code and the pore pressure variation. Kindly let me know where and what is wrong.

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Thanks in advance.

2

To receive quicker and more convenient assistance, could you please share the actual scripts (so they can be copied and pasted to reproduce the issue) rather than a picture of the scripts?

3

Sure.

Datafile1:
model new
model large-strain off
model title ‘Instantaneous pore pressures generated under an applied load’
model configure fluid
zone create cylinder point 0 (0,0,0) point 1 (1.275,0,0) point 2 (0,10,0) point 3 (0,0,1.275) size 5 10 5
; — mechanical model —
zone cmodel assign elastic
zone property young 3e6 poisson 0.3
zone face skin
zone face apply velocity-z 0 range group ‘bottom’
zone face apply velocity-x 0 range group ‘west’
zone face apply velocity-x 0 range group ‘east’
zone face apply velocity-z 0 range group ‘east’
zone face apply velocity (0,0,0) range group “south”
; — apply load slowly —
fish define ramp
ramp = math.min(1,global.step/1000)
end
zone face apply stress-normal = -0.3e6 fish ramp …
range position-y 10
; — fluid flow model —
zone fluid cmodel assign isotropic
zone gridpoint initialize fluid-modulus 9e8
; — pore pressure fixed at zero at the surface —
zone gridpoint fix pore-pressure 0 range position-y 10
; — settings —
model fluid active off
; — timing —
[global t0 = time.clock/100]
model solve convergence 1
[(time.clock/100)-t0]
model save ‘SE_INST_6’
program return

Datafile2:
model restore ‘SE_INST_6’
; — turn on fluid flow model —
zone fluid property permeability 1e-12
model fluid active on
zone gridpoint initialize velocity (0,0,0)
zone gridpoint initialize displacement (0,0,0)
; — set mechanical limits —
model mechanical substep 100
model mechanical follower on
model fluid substep 10
; — histories —
model history fluid time-total
zone history pore-pressure position (0,0,0)
zone history pore-pressure position (0,0,1.275)

zone history displacement-y position (0,10,0)
zone history displacement-y position (0,10,1.275)
; — solve to 3,000,000 sec —
[global t0 = time.clock/100]
model solve fluid time-total 3.0e8 or mechanical convergence 10
[(time.clock/100)-t0]
model save ‘SE_END_6’
program return

The obtained pore pressure curve is below.

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I am unsure why the pore pressure is being constant and then increasing initially. Ideally it is supposed to decrease right?
Where and what do I have to modify in the code to set my problem right?

Thanks in advance.

4

The reason is that you used a logarithmic scale on the x-axis. If we use a normal time scale, we will observe reasonable results (I did not run it completely):

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5

If running to the end, the plot looks

b
b3800×1951 155 KB

It is absolutely numerically reasonable.

6

Hello cheng.

For the above problem, if I have to apply an incremental loading(i.e. 20 kPa for first 2 hrs, then 40 kPa for the next 4 hrs and 60 kPa for the next 8 hrs), how should the code be writtern?

I could not find any example code for the incremental loading for the consolidation. Can you help me with this?

7

You can use the command “zone face apply stress-normal xx” on the same faces at different times, each time it will overwrite the previous apply.

8

zone face apply stress-normal 20e3 range position-y 10
model solve time-total 7200

zone face apply stress-normal 40e3 range position-y 10
model solve time-total 21600

zone face apply stress-normal 60e3 range position-y 10
model solve time-total 36000

zone face apply stress-normal 80e3 range position-y 10
model solve time-total 43200

Is this how you meant?

9

Is there any examples available in FLAC3D were the incremental load is applied similar to the one I mentioned above?

10

@sowmyaramanis,
The zone face apply commands you posted earlier are fine. Here’s a good example that does sequential loading.
Pile-Supported Highway Embankment — Itasca Software 9.3 documentation.

11

I am working on a consolidation analysis of soft soil improved by deep mixing columns. The load is incremented step wise i.e, I have applied 20 kPa for 1 day, 40 kPa for the next 1 day, 60kPa for the next 3 days and finally 80 kPa for the next 6 days.

The problem is the time it takes to run the code. Its been 2 days but even the first load solving itself haven’t completed yet. I have attached the code as well here. Kindly let me know what modification can be done in the code or how can the code be improvised further to speed up the simulation.
Thanks in advance.

Dat file:

model new
model large-strain off
model title ‘horpibuksul2012’
model configure fluid

zone create cylinder point 0 (0,0,0) point 1 (0.15,0,0) point 2 (0,0.0375,0) point 3 (0,0,0.15) size 4 1 4 group ‘sand_bottom’
zone create cylinder point 0 (0,0.0375,0) point 1 (0.15,0.0375,0) point 2 (0,0.2375,0) point 3 (0,0.0375,0.15) size 4 4 4 group ‘soil’
zone create cylinder point 0 (0,0.2375,0) point 1 (0.15,0.2375,0) point 2 (0,0.275,0) point 3 (0,0.2375,0.15) size 4 1 4 group ‘sand_top’
zone group ‘column’ range cylinder end-1 (0 0.0375 0) end-2 (0 0.2375 0) radius 0.025

zone property density 17000 range group ‘soil’
zone property density 14000 range group ‘column’
zone property density 18500 range group ‘sand_bottom’
zone property density 18500 range group ‘sand_top’
; — mechanical model —
zone cmodel assign m-c range group ‘column’
zone property young 120e6 poisson 0.3 cohesion 600e3 friction 25 range group ‘column’

zone cmodel assign s-s range group ‘soil’
zone property cohesion 1e3 friction 21 k-m 0.0095 l-m 0.095 poisson 0.3 range group ‘soil’
zone property stress-1-eff -20e3 stress-2-eff -10e3 stress-3-eff -10e3

zone cmodel assign m-c range group ‘sand_bottom’
zone property young 52e6 poisson 0.3 cohesion 1e3 friction 37 range group ‘sand_bottom’

zone cmodel assign m-c range group ‘sand_top’
zone property young 13e6 poisson 0.3 cohesion 1e3 friction 37 range group ‘sand_top’

zone face skin
zone face apply velocity-z 0 range group ‘bottom’
zone face apply velocity-x 0 range group ‘west’
zone face apply velocity-x 0 range group ‘east’
zone face apply velocity-z 0 range group ‘east’
zone face apply velocity (0,0,0) range group “south”

zone initialize stress-yy -20e3
zone initialize stress-xx -10e3
zone initialize stress-zz -10e3

; — fluid flow model —
zone fluid cmodel assign anisotropic
zone fluid property perm-xx 3e-11 perm-yy 1e-11 perm-zz 3e-11 range group ‘soil’
zone fluid property perm-xx 3e-9 perm-yy 1e-9 perm-zz 3e-9 range group ‘column’
zone fluid property perm-xx 5.83e-10 perm-yy 5.83e-10 perm-zz 5.83e-10 range group ‘sand_bottom’
zone fluid property perm-xx 5.83e-10 perm-yy 5.83e-10 perm-zz 5.83e-10 range group ‘sand_top’

zone gridpoint initialize fluid-modulus 9e8

; — pore pressure fixed at zero at the surface —
zone gridpoint fix pore-pressure 0 range position-y 0.275

model fluid active off
zone fluid fastflow on
model solve convergence 1

; — turn on fluid flow model —
model fluid active on
zone gridpoint initialize velocity (0,0,0)
zone gridpoint initialize displacement (0,0,0)

; — set mechanical limits —
model mechanical substep 1
model mechanical follower on
model fluid substep 1

; — histories —
model history name ‘time’ fluid time-total
zone history name ‘disp1’ displacement-y position (0.04,0.275,0.08)
zone history name ‘disp2’ displacement-y position (0.08,0.275,0.02)

zone history name ‘pp1’ pore-pressure position (0.05,0.057,0.05)
zone history name ‘pp2’ pore-pressure position (0.1,0.057,0.1)
zone history name ‘pp3’ pore-pressure position (0.05,0.087,0.05)
zone history name ‘pp4’ pore-pressure position (0.1,0.087,0.1)
zone history name ‘pp5’ pore-pressure position (0.05,0.210,0.05)
zone history name ‘pp6’ pore-pressure position (0.1,0.210,0.1)

zone history name ‘stress1’ stress-yy position (0,0.2375,0)
zone history name ‘stress2’ stress-yy position (0.092,0.2375,0.092)

model fluid timestep fix 4e-3
model fluid list

;incremental loading
zone face apply stress-normal -20e3 range position-y 0.275
model solve fluid time 86400 ; Run for 1 day (86400 seconds)

zone face apply stress-normal -40e3 range position-y 0.275
model solve fluid time 86400 ; Run for 1 more day

zone face apply stress-normal -60e3 range position-y 0.275
model solve fluid time 259200 ; Run for 3 days

zone face apply stress-normal -80e3 range position-y 0.275
model solve fluid time 518400 ; Run for 8 days

13

@cheng My model should be placed under 10 m of the sea water level. In other words, I am going to model the seabed and pondering about fluid boundary condition on the top (Z=-10), on the bottom (Z=-50) and sides X=30 and X=-30 and Y=-30 and Y=30. I want to analyse my problem using the couple mode. Please guide me about fluid boundaries. Thanks so much.

14

You need to fix or apply the pore pressure at the seabed surface

zone gridpoint fix pore-pressure …

or

zone face apply pore-pressure …

AND the static water pressure needs to be applied:

zone face apply stress-normal [value] gradient (0,0,-g) [range of seabed surface]

15

@cheng
Thanks. And for other boundaries? Side boundaries and the lower horizontal boundary? Do they need to be fixed ?

About fixing pp on the top boundary, even after that, the pore pressure change due to the condolidation. Just, the surface pore pressure remains without change becuse we have fixed the pore pressure for the face and not zone .