A pressure-dependent cap model that is capable of describing the britt
le-ductile transition in porous rocks is proposed. The model consists
of a hardening Drucker-Prager yield cone with a nonassociative how rul
e, an ellipsoidal cap with an associative flow rule, and a failure or
limiting surface. This surface designates the stress states at the inc
eption of failure below brittle-ductile transition, or the onset of id
eally plastic response above brittle-ductile transition. The appropria
tely combined structures of the associative and nonassociative flow ru
les eliminate the difficulties associated with a singular corner regio
n at the intersection of the cap and failure surface. A cross hardenin
g between the cap and cone is discussed. The model is capable of a goo
d qualitative agreement with experimental data, as demonstrated by the
results of the triaxial compression tests under various amounts of th
e confining pressure. A stress-strain response with inelastic volume d
ilatancy at lower confinements, and an increased ductility and hardeni
ng rates at higher confinements, are well predicted.