Practice Exam Question - Rock Mechanics

Good morning all,

I am about to write my last two professional practice exams in order to obtain my P.Eng. and I came across a question on a practice exam that I am having a hard time solving it is as follows:

A servo-controlled compression test has been conducted on a weak soapstone such that the specimen length remained unchanged throughout: as the axial stress σa was increased, so the confining pressure p was increased so that no net axial strain resulted. A plot of axial stress (vertical stress) against confining pressure (horizontal axis) gave an initial straight line passing through the origin. At a critical confining pressure of p = 85 MPa (when σa =39.1 MPz), the slope of the σa-p plot suddenly changed to 29 degrees and remained constant for the remainder of the test. This sudden change in slope may be taken to represent the onset of yield. As such:

A. Determine an elastic constant from the slope of the initial portion of the σa-p curve.

B. Assuming that the Mohr-Coulomb criterion is applicable, determine σa, c and Ψ for the rock.

Now I have seen in the examples I've worked through that the elastic constant is essentially Young's Modulus and it is normally calculated from the slope of the line of the initial part of the curve. Does that relationship hold true on a graph that does not incorporate strain?

Thanks for any help!