# Help needed: Modeling an Excavation in Clay with Mohr-Coulomb Model **URL:** \ **Category:** FLAC\ **Created:** [October 8, 2022, 9:27pm UTC](https://forum.itascainternational.com/t/help-needed-modeling-an-excavation-in-clay-with-mohr-coulomb-model/774 "2022-10-08T21:27:10Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![stmicheljerome](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/stmicheljerome/32/127_2.png) [@stmicheljerome](https://forum.itascainternational.com/u/stmicheljerome)\ **Post date:** [October 8, 2022, 9:27pm UTC](https://forum.itascainternational.com/t/help-needed-modeling-an-excavation-in-clay-with-mohr-coulomb-model/774/1 "2022-10-08T21:27:11Z")
Hi there, I’m trying to model an excavation in clay using FLAC2D. The stratigraphy goes as follow, 27m of clay, purple zones, laying on the bedrock, green zone. Here is my mesh: ![MESH](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/1X/e63202cd7f65fa9d459ca1e39db7de2ced590cf3.jpeg) Gravity is taken to be 9.81m/s2, units are m, Mg, kPa and s and boundary conditions are: fix x (sides) fix x y (bottom) The strategy is to use the Mohr-Coulomb Model to evaluate displacement resulting from the unloading of the clay (excavation). The rock is modelled as an elastic medium. Material properties are : model mohr group ‘Clay’ prop density=1.3 bulk=8300 shear=3800 group ‘Clay’ prop cohesion=13.5 friction=31.0 group ‘Clay’ model elastic group ‘Rock’ prop density=2.3 bulk=6.7E4 shear=4E4 group ‘Rock’ I established effective stresses and pore pressures using a FISH function called Ininv.fis and solved mechanical only. set flow off water density 1 ; ; Call Fish functions call ‘Ininv.fis’ set cd back set wth=17 k0x=1 k0z=1 ininv ; ; Solve history 999 unbalanced solve The equilibrium is reached with minimal displacements. Vertical displacement reached 0.4 m total which seems normal for a 272m high model. I then initialize velocities and displacement and proceed to excavate the clay using model null on zones to be excavated. initial xdisp 0 ydisp 0 initial xvel 0 yvel 0 set flow off water bulk 0 model null i=215,250 j=133,148 history 999 unbal solve The solution converges but the zones atop of the slope start to strain very weirdly. Anybody can help? ![Capture d’écran, le 2022-10-08 à 14.21.29](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/1X/5634f908d637713ef59a852c8f6090d37fbcca64.jpeg) ---
**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\ **Post date:** [October 10, 2022, 3:05pm UTC](https://forum.itascainternational.com/t/help-needed-modeling-an-excavation-in-clay-with-mohr-coulomb-model/774/2 "2022-10-10T15:05:26Z")
Why the water bulk is set to zero for the excavation stage? if it is set to zero, the pore pressures will be kept unchanged during excavation. ---
**Author:** ![stmicheljerome](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/stmicheljerome/32/127_2.png) [@stmicheljerome](https://forum.itascainternational.com/u/stmicheljerome)\ **Post date:** [October 10, 2022, 4:29pm UTC](https://forum.itascainternational.com/t/help-needed-modeling-an-excavation-in-clay-with-mohr-coulomb-model/774/3 "2022-10-10T16:29:45Z")
I thought making the mechanical calculation first made sense. Should I run a coupled analysis right away? ---
**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\ **Post date:** [October 10, 2022, 5:55pm UTC](https://forum.itascainternational.com/t/help-needed-modeling-an-excavation-in-clay-with-mohr-coulomb-model/774/4 "2022-10-10T17:55:13Z")
It does make sense to do the mechanical calculation first to simulate undrained response. Flow is set off, water bulk is specified with a nonzero value. When excavation starts, zones expand and pp decrease (with a nonzero water bulk). check Section 1.9.5 of the fluid flow manual.