I am analysing drainage tunnels in Hong Kong within zones of Granodiorite and Tuff rock mass.The rock mass has very low cohesion values and friction angle are 30-35 degrees. Deformation modulus are too high (65-70 GPa), so we are expecting very low deformations. Hydrofracture test data indicates average stress ratio of 4.0 at depth of 40m and 2.0 at depth of 120m.
In my Numerical analysis if I look at yield, every element has yielded in the first stage. Basically the material is failing insitu.I'm sure if I calculate by hand the stress state of an element under gravitational loading with a K=4, I'll find that the stress state always plots above the failure envelope.
Based on this, I wouldn't put any faith in the values calculated in the liner. However I can see how the liner would pick up a lot of load since you are supporting a large weight of failed material above the excavation.
My question is how to assign the correct stress conditions in my model so I can get no failure in in-situ conditions.