# Homework Help: Rock Mechanics: Mohrs Stress Circle - Principal Stresses

1. Oct 10, 2015

### iloveit6969

1. The problem statement, all variables and given/known data
A series of shear tests were performed on a weak rock. Each test was carried out until the sample sheared, and the principal stresses for each test were:

S. No.--- σ3 (KN/m2) --- σ1 (KN/m2)
1 --- 200 --- 570
2 --- 300 --- 875
3 --- 400 --- 1162

Plot the Mohr’s stress circle and hence determine the angle of internal friction and
cohesion of the rock from Mohr’s envelope. If the confining pressure was 600KN/m2 extrapolate the failure stresses of the rock sample.

// I've done solid mechanics and have a good idea of Mohrs circle, but im just confused on how its written and dont know how to start and have a few questions:
- Is the element in plane strees? and is the 2nd principal stress 0?
- Do i evaluate each sample seperetly?

Thanks

2. Oct 10, 2015

### Staff: Mentor

Yes. Assume it is.
No. You evaluate them all together on the same graph.