Plotting the Hoek-Brown failure criterion used in Plaxis

Hi everyone,

for a study project I wanted to plot stress points along with the failure criterion I used in Plaxis.

Therefore I plotted the points in a sig1-sig3 plane and used the formula from the materials manual to plot the criterion:

def hoek_brown (sig_3,sig_ci,m_b,s,a):
    sig_1 = -(sig_3 - (abs(sig_ci)*(((m_b*sig_3/sig_ci)+s)**a)))
    return sig_1

def get_hoek_brown (sig_3_values,sig_ci=310,m_b=6.097,s=1,a=0.5):
    sig_1_values = [hoek_brown(sig_3,sig_ci,m_b,s,a) for sig_3 in sig_3_values]
    return sig_1_values

If I then plot everything together it looks like plaxis is using some different failure criterion:

The question now is what am I doing wrong, or am I just using the wrong equation?

Any help is appreciated, thanks a lot in advance!

Greetings

Lukas