Learning Forum

[Konstantinos Chatzis]

Hello everybody, I am a PhD student and I am new at using LS-DYNA. My field is geotechnical engineering. I am trying to simulate simple geotechnical examples in order to get familiar with the software. However, I encountered some problems while simulating a saturated material.

I am modelling 1 3D cube, where the side boundaries are restricted in movement and a load is applied on top. Initially, I checked this model without saturation and the results where the expected (displacement at the top of the cube, horizontal and vertical stresses). However, when I included keywords for saturating the model the vertical stress is equal to zero. This is the case even when the ATYPE is 0 in the keyword *CONTROL_PORE_FLUID.

I am attaching one input file. I am looking forward to any recomendation. Thank you in advance.

 

$# LS-DYNA Keyword file created by LS-PrePost(R) V4.7.17 - 08Jul2020
$# Created on Jun-28-2023 (18:33:31)
*KEYWORD
*TITLE
$# title
LS-DYNA keyword deck by LS-PrePost
*CONTROL_PORE_FLUID
$# atype wtable pf_rho grav pf_bulk output tmf
0 0.0 1000.01.00000E-9 9810.0 2200.0 0 1.0
$# targ fmin fmax conv conmax eterm
0.0 0.0 0.0 1.01.00000E-41.00000E20 1.0
*CONTROL_TERMINATION
$# endtim endcyc dtmin endeng endmas nosol
1.0 0 0.0 0.01.000000E8 0
*DATABASE_BINARY_D3PLOT
$# dt lcdt beam npltc psetid
0.1 0 0 0 0
$# ioopt rate cutoff window type pset
0 0.0 0.0 0.0 0 0
*BOUNDARY_SPC_SET
$# nsid cid dofx dofy dofz dofrx dofry dofrz
1 0 0 0 1 1 1 0
*SET_NODE_LIST_TITLE
NODESET(SPC) 1
$# sid da1 da2 da3 da4 solver
1 0.0 0.0 0.0 0.0MECH
$# nid1 nid2 nid3 nid4 nid5 nid6 nid7 nid8
1 2 3 4 5 6 7 8
9 0 0 0 0 0 0 0
*BOUNDARY_SPC_SET
$# nsid cid dofx dofy dofz dofrx dofry dofrz
2 0 1 0 0 0 0 0
*SET_NODE_LIST_TITLE
NODESET(SPC) 2
$# sid da1 da2 da3 da4 solver
2 0.0 0.0 0.0 0.0MECH
$# nid1 nid2 nid3 nid4 nid5 nid6 nid7 nid8
3 6 9 12 15 18 21 24
27 1 4 7 10 13 16 19
22 25 0 0 0 0 0 0
*BOUNDARY_SPC_SET
$# nsid cid dofx dofy dofz dofrx dofry dofrz
3 0 0 1 0 0 0 0
*SET_NODE_LIST_TITLE
NODESET(SPC) 3
$# sid da1 da2 da3 da4 solver
3 0.0 0.0 0.0 0.0MECH
$# nid1 nid2 nid3 nid4 nid5 nid6 nid7 nid8
1 2 3 10 11 12 19 20
21 7 8 9 16 17 18 25
26 27 0 0 0 0 0 0
*BOUNDARY_SPC_SET
$# nsid cid dofx dofy dofz dofrx dofry dofrz
4 0 0 0 0 0 0 0
*SET_NODE_LIST_TITLE
NODESET(SPC) 4
$# sid da1 da2 da3 da4 solver
4 0.0 0.0 0.0 0.0MECH
$# nid1 nid2 nid3 nid4 nid5 nid6 nid7 nid8
19 20 21 22 23 24 25 26
27 0 0 0 0 0 0 0
*BOUNDARY_PORE_FLUID_PART
$# pid wtable pf_rho atype pf_bulk acurve wtcur suclim
1 1000.01.00000E-9 0 2200.0 0 0 0.0
*BOUNDARY_PWP_SET
$# sid lc cmult lcdr tbirth tdeath
4 0 0.0 0 0.01.00000E20
$# iphre itotex idrflag table
0 0 0 0
*LOAD_SEGMENT_SET_ID
$# id heading
1Pressure
$# ssid lcid sf at
1 1 1.0 0.0
*PART
$# title
Cube
$# pid secid mid eosid hgid grav adpopt tmid
1 1 1 0 0 0 0 0
*SECTION_SOLID_TITLE
Cube
$# secid elform aet
1 1 0
*MAT_ELASTIC_TITLE
Soil
$# mid ro e pr da db not used
12.00000E-9 90.0 0.2 0.0 0.0 0.0
*MAT_ADD_PERMEABILITY_TITLE
Soil permeability
$# mid perm
1 1.0
*DEFINE_CURVE_TITLE
Curve_Pressure
$# lcid sidr sfa sfo offa offo dattyp lcint
1 0 1.0 1.0 0.0 0.0 0 0
$# a1 o1
0.0 0.0
1.0 1.0
1.1 1.0
*SET_SEGMENT
$# sid da1 da2 da3 da4 solver
1 0.0 0.0 0.0 0.0MECH
$# n1 n2 n3 n4 a1 a2 a3 a4
24 23 20 21 0.0 0.0 0.0 0.0
23 22 19 20 0.0 0.0 0.0 0.0
26 25 22 23 0.0 0.0 0.0 0.0
27 26 23 24 0.0 0.0 0.0 0.0
*ELEMENT_SOLID
$# eid pid n1 n2 n3 n4 n5 n6 n7 n8
1 1 1 2 5 4 10 11 14 13
2 1 2 3 6 5 11 12 15 14
3 1 4 5 8 7 13 14 17 16
4 1 5 6 9 8 14 15 18 17
5 1 10 11 14 13 19 20 23 22
6 1 11 12 15 14 20 21 24 23
7 1 13 14 17 16 22 23 26 25
8 1 14 15 18 17 23 24 27 26
*NODE
$# nid x y z tc rc
1 0.0 0.0 0.0 0 0
2 500.0 0.0 0.0 0 0
3 1000.0 0.0 0.0 0 0
4 0.0 500.0 0.0 0 0
5 500.0 500.0 0.0 0 0
6 1000.0 500.0 0.0 0 0
7 0.0 1000.0 0.0 0 0
8 500.0 1000.0 0.0 0 0
9 1000.0 1000.0 0.0 0 0
10 0.0 0.0 500.0 0 0
11 500.0 0.0 500.0 0 0
12 1000.0 0.0 500.0 0 0
13 0.0 500.0 500.0 0 0
14 500.0 500.0 500.0 0 0
15 1000.0 500.0 500.0 0 0
16 0.0 1000.0 500.0 0 0
17 500.0 1000.0 500.0 0 0
18 1000.0 1000.0 500.0 0 0
19 0.0 0.0 1000.0 0 0
20 500.0 0.0 1000.0 0 0
21 1000.0 0.0 1000.0 0 0
22 0.0 500.0 1000.0 0 0
23 500.0 500.0 1000.0 0 0
24 1000.0 500.0 1000.0 0 0
25 0.0 1000.0 1000.0 0 0
26 500.0 1000.0 1000.0 0 0
27 1000.0 1000.0 1000.0 0 0
*END

[Ushnish Basu]

Konstantine, 

I will give the following general  tip that typically, pore-pressure problems use LOAD_BODY and INITIAL_STRESS_DEPTH to include the effect of gravity. 

You can search for the following paper online "Modelling liquefaction of soils with LS-DYNA using a SANISAND-based material model", as it has some discussion of modeling pore-water-pressure problems.

Also, please be sure to read through the Remarks under CONTROL_PORE_FLUID and INITIAL_STRESS_DEPTH in the latest versions of the manual

 

 

[Konstantinos Chatzis]

Hi Usnish,

Thanks for your reply. 

I looked at the paper that you proposed, but I couldn't find an explanation why I should use the Load_Body. I understand the Initial_Stress_Depth, but not the load_Body. 

Another question, if I want to have zero initial effective stresses, how can I achieve that since the water pressures due to gravity are always calculated in LSDYNA?

Finally, I cannot get the output of the pore water pressures. I read the manual, but I cannot find the right way to get this output. Could you help me also with this?

Thank you very much.

[Ushnish Basu]

The INITIAL_STRESS_DEPTH sets the initial stress due to gravity, LOAD_BODY maintains that stress throughout the simulation

A zero initial effective stress is inconsistent with hydrostatic pore pressures

The pore pressures are available as a history variables in d3plot and elout - see the section on "Output" in the Remarks under CONTROL_PORE_FLUID

Unfortunately, I am not able to provide more detailed help in this forum, hopefully other community members can help

[Konstantinos Chatzis]

Thank you  very much for your reply. It was already helpful.