# (Apparent?) tensile rupture in joint with a hydrostatic model

**URL:** <https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285>\
**Category:** 3DEC\
**Created:** [November 21, 2025, 3:55pm UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285 "2025-11-21T15:55:26Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![Theophile](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/theophile/32/299_2.png) [@Theophile](https://forum.itascainternational.com/u/Theophile)\
**Post date:** [November 21, 2025, 3:55pm UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/1 "2025-11-21T15:55:26Z")

</div>

Hi everyone!

Doing some tests on hydromechanics in 3DEC, I came up ith this situation I cannot explain: when considering pore pressure in joints (fluid **not** configured), I get tensile rupture even when specifying extreme tensile strength (10^10 Pa versus 10^7 Pa pore pressure).

If I consider a model with fluid configured and flow **off** , the tensile ruptures disappear.

[Documentation](https://docs.itascacg.com/3dec700/3dec/docproject/source/theory/fluid/fluid_introduction.html) mentions that when model is not configured for fluid, pore pressure are only used to estimate elastoplastic state:

> The user can simply specify water pressure everywhere in the model and these pressures are used to calculate effective stresses. Effective stresses are then used in the calculations to determine if there is failure of solid material or slip on joints.

However, I would not expect rupture to appear when pore pressure is way below normal stress and tensile strength.

Which brings my question: is the tensile rupture state apparent or actual (hence influencing mechanical equilibrium resolution)?

I provide a detailed exemple in PS of this post.

Thanks a lot!

Cheers

Theophile

* * *

PS :  
I considered a really simple scenario:

- 1x1x1 km^3 cube
- single joint
- elastic matrix and elastoplastic joint (Mohr-Coulomb)
- gravity
- oedometric boundary conditions (i.e., blocked normal velocity on all faces except top one which is free)
- 3-step balancing (with displacements and state resetting between each):
  - elastic joint + no pore pressure
  - elastoplastic joint + no pore pressure
  - elastoplastic joint + hydrostatic gradient

 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/c/ce4c753e8597d799851a3db0295c2020ad34217f.jpeg)

Normal stress seems to be affected (poorly, though).

 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/f/fb12baf880434b014f1db6b565755478a2f07dea.jpeg)

---

<div class="post-metadata">

**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [November 22, 2025, 2:42am UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/2 "2025-11-22T02:42:02Z")

</div>

My guess is that the joint failed in shear first. This would cause the tensile strength to drop to zero. If this is not the answer, please post your simple data file.

---

<div class="post-metadata">

**Author:** ![Theophile](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/theophile/32/299_2.png) [@Theophile](https://forum.itascainternational.com/u/Theophile)\
**Post date:** [November 24, 2025, 3:46pm UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/3 "2025-11-24T15:46:11Z")

</div>

Thank you, @jhazzard !

You guessed right: adding residual tensile strength preserves from having tensile rupture (while having only tensile strength alone did not).

We still get some differences in contours but these are acceptable.

 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/3/304f935f96bc8a8a7b0ead2a228ece3681898fc4.png)  
 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/7/788703554003ffbdc491d135766c0d3a995631f3.png)

I still do not understand why no tensile rupture appears when considering `config fluid`/`flow off`, even with no residual tensile strength?  
Aren’t we supposed to be in the same conditions as no config fluid?

Just in case, I provide below the scripts for the study

Thanks!

Theophile

PS :

> **Script WITHOUT configure fluid**
>
> ```auto
> ; Testing pore pressure effects on tensile rupture: no config fluid
> ; Model
> ; - elastic matrix
> ; - elastoplastic joint
> ; - gravity + oedometric BC
> ; - hydrostatic profile
> ; - 3-phase equilibrium
> ; o elastic, no pore pressure
> ; o plastic, no pore pressure
> ; o plastic, pore pressure
> ; Results
> ; --> tensile rupture appears except if one gives a residual tensile strength (suggestion by J. Hazzard)
> 
> project new
> model large-strain off
> block mechanical mass-scale on
> block mechanical damping global
> 
> ; GEOMETRY
> ; ;;;;;;;;
> 
> ; Parameters
> def geom_parameters
> global xmin=0
> global ymin=0
> global zmin=-1000
> global xmax=1000
> global ymax=1000
> global zmax=0
> global fault = map('origin', vector(500,500,-500), 'dip', 70, 'dip-dir', 270)
> end
> [geom_parameters]
> 
> ; Ranges
> ; - main fault
> model range create 'right' &
> plane origin [fault('origin')->x], [fault('origin')->y], [fault('origin')->z] &
> dip [fault('dip')] dip-direction [fault('dip-dir')] below
> model range create 'left' named-range 'right' not
> 
> ; Geometry+mesh
> block create brick [xmin],[xmax] [ymin],[ymax] [zmin],[zmax]
> block cut joint-set &
> origin [fault('origin')->x], [fault('origin')->y], [fault('origin')->z] &
> dip [fault('dip')] dip-direction [fault('dip-dir')]
> block zone gen edge 50
> 
> ; Groups
> block group 'side=right' range named-range 'right'
> block group 'side=left' range named-range 'left'
> block contact group 'contact=fault' range group-intersection 'side=right' 'side=left'
> 
> ; Boundaries
> block face group 'west' range pos-x [xmin]
> block face group 'east' range pos-x [xmax]
> block face group 'south' range pos-y [ymin]
> block face group 'north' range pos-y [ymax]
> block face group 'bottom' range pos-z [zmin]
> block gridpoint group 'east' range pos-x [xmax]
> 
> ; RUN: PHASE 1/3 ELASTIC, NO FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> def hm_parameters
> ; Gravity
> global grav_z = 9.81
>     
> ; Zones
> global young_ = 56.5e9
> global nu_ = 0.28
> global rho_ = 2680
> ; Contacts
> global kn_ = 10.e9
> global ks_ = 5.e9
> global cohe_ = 0.
> global phi_ = 15.
> global psi_ = 5.
> global usc_ = 1.
> ; NB: turn to non-zero fictitious value to avoid tensile rupture
> global rt_tres_ = 0
>     
> ; Fluid
> global density_water = 1000
> global bulk_water = 0 ;3.8e5
> global viscosity_water = 1e-3
> global pp_top = 0 ; Pore pressure at model top
> global pp_grad = -math.abs(density_water*grav_z) ; increase with depth (z<0)
> global pp_z0 = pp_top - zmax*pp_grad ; pore pressure at z=0
> end
> [hm_parameters]
> 
> ; - generic
> model gravity [grav_z]
> ; - zones
> block zone cmodel assign elastic
> block zone property young [young_] poisson [nu_] dens [rho_]
> ; - contacts
> block contact jmodel assign elastic
> block contact property &
> stiffness-normal [kn_] &
> stiffness-shear [ks_]
> 
> ; BC = oedometric
> def sigH_on_sigV(nu)
> ; SigmaH / SigmaV ratio considering an overburden in an elastic, homogeneous
> ; and isotropic material
> ; nu: float, Poisson ratio
> sigH_on_sigV = nu/(1-nu)
> end
> block insitu topography ratio-x [sigH_on_sigV(nu_)] ratio-y [sigH_on_sigV(nu_)] ratio-z 1
> block face apply velocity-normal 0.0 range group 'west'
> block face apply velocity-normal 0.0 range group 'east'
> block face apply velocity-normal 0.0 range group 'south'
> block face apply velocity-normal 0.0 range group 'north'
> block face apply velocity-normal 0.0 range group 'bottom'
> 
> model solve ratio 1.e-6
> model save 'no_config_fluid_1-on-3.3dsav'
> 
> ; RUN: PHASE 2/3 PLASTIC, NO FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> block gridpoint initialize displacement 0 0 0
> block contact reset displacement
> block contact jmodel mohr range group "contact=fault"
> block contact property &
> stiffness-normal [kn_] &
> stiffness-shear [ks_] &
> cohesion [cohe_] &
> friction [phi_] &
> dilation [psi_] &
> dilation-zero = [usc_] &
> tension-residual = [rt_tres_] &
> range group "contact=fault"
> model solve ratio 1.e-6
> model save 'no_config_fluid_2-on-3.3dsav'
> 
> ; RUN: PHASE 3/3 PLASTIC, FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> block gridpoint initialize displacement 0 0 0
> block contact reset displacement
> block contact reset state
> ; Hydrostatic
> block insitu &
> pore-pressure [pp_z0] &
> gradient (0,0,[pp_grad])
> model solve ratio 1.e-6
> model save 'no_config_fluid_3-on-3.3dsav'
> 
> ```

> **Script WITH configure fluid/flow off**
>
> ```auto
> ; Testing pore pressure effects on tensile rupture: with config fluid
> ; Model
> ; - elastic matrix
> ; - elastoplastic joint
> ; - gravity + oedometric BC
> ; - hydrostatic profile
> ; - 3-phase equilibrium
> ; o elastic, no pore pressure
> ; o plastic, no pore pressure
> ; o plastic, pore pressure
> ; Results
> ; --> no tensile rupture
> 
> project new
> model large-strain off
> model configure fluid
> block mechanical mass-scale on
> block mechanical damping global
> 
> ; GEOMETRY
> ; ;;;;;;;;
> 
> ; Parameters
> def geom_parameters
> global xmin=0
> global ymin=0
> global zmin=-1000
> global xmax=1000
> global ymax=1000
> global zmax=0
> global fault = map('origin', vector(500,500,-500), 'dip', 70, 'dip-dir', 270)
> end
> [geom_parameters]
> 
> ; Ranges
> ; - main fault
> model range create 'right' &
> plane origin [fault('origin')->x], [fault('origin')->y], [fault('origin')->z] &
> dip [fault('dip')] dip-direction [fault('dip-dir')] below
> model range create 'left' named-range 'right' not
> 
> ; Geometry+mesh
> block create brick [xmin],[xmax] [ymin],[ymax] [zmin],[zmax]
> block cut joint-set &
> origin [fault('origin')->x], [fault('origin')->y], [fault('origin')->z] &
> dip [fault('dip')] dip-direction [fault('dip-dir')]
> block zone gen edge 50
> 
> ; Groups
> block group 'side=right' range named-range 'right'
> block group 'side=left' range named-range 'left'
> block contact group 'contact=fault' range group-intersection 'side=right' 'side=left'
> 
> ; Boundaries
> block face group 'west' range pos-x [xmin]
> block face group 'east' range pos-x [xmax]
> block face group 'south' range pos-y [ymin]
> block face group 'north' range pos-y [ymax]
> block face group 'bottom' range pos-z [zmin]
> block gridpoint group 'east' range pos-x [xmax]
> 
> ; RUN: PHASE 1/3 ELASTIC, NO FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> def hm_parameters
> ; Gravity
> global grav_z = 9.81
>     
> ; Zones
> global young_ = 56.5e9
> global nu_ = 0.28
> global rho_ = 2680
> ; Contacts
> global kn_ = 10.e9
> global ks_ = 5.e9
> global cohe_ = 0.
> global phi_ = 15.
> global psi_ = 5.
> global usc_ = 1.
>     
> ; Fluid
> global density_water = 1000
> global bulk_water = 0 ;3.8e5
> global viscosity_water = 1e-3
> global pp_top = 0 ; Pore pressure at model top
> global pp_grad = -math.abs(density_water*grav_z) ; increase with depth (z<0)
> global pp_z0 = pp_top - zmax*pp_grad ; pore pressure at z=0
> end
> [hm_parameters]
> 
> ; - generic
> model gravity [grav_z]
> block fluid property bulk [bulk_water]
> block fluid property density [density_water]
> block fluid property viscosity [viscosity_water]
> ; - zones
> block zone cmodel assign elastic
> block zone property young [young_] poisson [nu_] dens [rho_]
> ; - contacts
> block contact jmodel assign elastic
> block contact property &
> stiffness-normal [kn_] &
> stiffness-shear [ks_]
> flowplane vertex property &
> aperture-initial 1e-5 &
> aperture-minimum 1e-5 &
> aperture-maximum 1e-5
>     
> model fluid off mech on
> model cycle 0
> 
> ; BC = oedometric
> def sigH_on_sigV(nu)
> ; SigmaH / SigmaV ratio considering an overburden in an elastic, homogeneous
> ; and isotropic material
> ; nu: float, Poisson ratio
> sigH_on_sigV = nu/(1-nu)
> end
> block insitu topography ratio-x [sigH_on_sigV(nu_)] ratio-y [sigH_on_sigV(nu_)] ratio-z 1
> block face apply velocity-normal 0.0 range group 'west'
> block face apply velocity-normal 0.0 range group 'east'
> block face apply velocity-normal 0.0 range group 'south'
> block face apply velocity-normal 0.0 range group 'north'
> block face apply velocity-normal 0.0 range group 'bottom'
> 
> model solve ratio 1.e-6
> model save 'config_fluid_flow_off_1-on-3.3dsav'
> 
> ; RUN: PHASE 2/3 PLASTIC, NO FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> block gridpoint initialize displacement 0 0 0
> block contact reset displacement
> block contact jmodel mohr range group "contact=fault"
> block contact property &
> stiffness-normal [kn_] &
> stiffness-shear [ks_] &
> cohesion [cohe_] &
> friction [phi_] &
> dilation [psi_] &
> dilation-zero = [usc_] &
> range group "contact=fault"
> model solve ratio 1.e-6
> model save 'config_fluid_flow_off_2-on-3.3dsav'
> 
> ; RUN: PHASE 3/3 PLASTIC, FLUID
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> block gridpoint initialize displacement 0 0 0
> block contact reset displacement
> block contact reset state
> ; Hydrostatic
> block insitu &
> pore-pressure [pp_z0] &
> gradient (0,0,[pp_grad])
> model solve ratio 1.e-6
> model save 'config_fluid_flow_off_3-on-3.3dsav'
> 
> ```

---

<div class="post-metadata">

**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [November 24, 2025, 5:46pm UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/4 "2025-11-24T17:46:29Z")

</div>

OK - it seems I was not correct. Your tensile strength is 0, so the fact that the joint fails in shear is irrelevant. The real problem is the global damping. This should never be used - it is only still in the code for people who want to try to reproduce model results from the old days. In the model with no fluid and global damping, there is a bit of instability when the pore pressure changes, which causes a bunch of tensile failure. Just comment out line 17, and the results are much more similar.

---

<div class="post-metadata">

**Author:** ![Theophile](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/theophile/32/299_2.png) [@Theophile](https://forum.itascainternational.com/u/Theophile)\
**Post date:** [November 25, 2025, 10:38am UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/5 "2025-11-25T10:38:08Z")

</div>

It seems to me you were correct!  
Turning **residual** tensile strength to non-zero does prevent from having tension rupture, even with a zero tensile strength.  
It makes sense because, as you presumed, slip rupture happens first and 3DEC directly switches to residual strength.

Turning global damping off indeed brings very similar results to the model with config flow.  
However, when turning residual tensile strength to 0 for the no config fluid model, there is still tension rupture.

 ![image](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/e/ed2ea203c7d19b6efb3b9a6c351023c7a2ac0434.png)

To sum up - and taking the model with config fluid, no global damping and zero residual tensile strength as reference (this model has no tensile rupture) - we get for the no config fluid model:

- with global damping and with zero residual tensile strength: tensile rupture; more or less comparable shear/normal results to reference
- with global damping and with **non-zero** residual tensile strength: no tensile rupture; more or less comparable shear/normal results to reference
- **without** global damping and with zero residual tensile strength: tensile rupture; very comparable shear/normal results to reference
- **without** global damping and with **non-zero** residual tensile strength: no tensile rupture; very comparable shear/normal results to reference

The 4th model is the best candidate.

Theophile

---

<div class="post-metadata">

**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [November 25, 2025, 2:32pm UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/6 "2025-11-25T14:32:17Z")

</div>

Probably the timestep/mass density scaling is a bit different when you turn on fluid, so the abrupt application of pore pressure causes tensile failure in one case and not the other. I was running this in version 9, where the default damping is “combined” and the results were more similar.

---

<div class="post-metadata">

**Author:** ![Theophile](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/theophile/32/299_2.png) [@Theophile](https://forum.itascainternational.com/u/Theophile)\
**Post date:** [November 26, 2025, 7:54am UTC](https://forum.itascainternational.com/t/apparent-tensile-rupture-in-joint-with-a-hydrostatic-model/2285/7 "2025-11-26T07:54:24Z")

</div>

Thanks a lot for your clarification, I’ll dig in this direction!

Cheers

Theophile