Help needed: Modeling an Excavation in Clay with Mohr-Coulomb Model

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:

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
; Solve
history 999 unbalanced

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

The solution converges but the zones atop of the slope start to strain very weirdly. Anybody can help?

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.

I thought making the mechanical calculation first made sense. Should I run a coupled analysis right away?

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.