The general stress condition underground is
therefore one in which the three mutually perpendicular
principal stresses are unequal. If
fluid pressure were applied locally within rocks
in this condition, and if the pressure were increased
until rupture or parting of the rocks resulted,
that plane along which fracture or parting
would first be possible would be the one
perpendicular to the least principal stress. It is
here postulated that this plane is also the one
along which parting is most likely to occur
(Fig. 1).