Stresses In Hong Kong

[sonamoni]

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.

 

[dgillette]

I probably can't really help with this, having been out of rock mechanics many years, but how is the strength being characterized? Does 30-35 deg refer to joint sliding, or to intact but decomposed rock treated as a continuum? I know nothing of Hong Kong geology, beyond that there is a lot of granite and decomposed granite, and landslide problems.

By stress ratio, you are referring to sigma1/sigma3? If so, there seems to be a mismatch between K=4 and the strength properties, which I think you are correct about. How much faith do you have in the measurements of strength and in situ stresses? What is the orientation of the principal stresses in the ground?

 

[dgillette]

Now that I have my calculator out, K=4 requires a friction angle of 37 degrees (or a lower friction angle with significant cohesion), assuming I pushed the right buttons.

sin-1[(sig1-sig3)/(sig1+sig3)]

Can the rock mass really be that weak if it has a high def modulus? 37 degrees is like moderately dense clean sand.