We give here a model for the pressure dependent, biaxial mechanical behavior of glassy polymers based on the thermally activated growth of deformation zones (Somigliana dislocation loops). The Coulomb criterion of plasticity, sigma(c) = S - m sigma(n), is found as the critical threshold needed to propagate Somigliana loops, in the same way as yield in crystals is found as the stress to move Volterra dislocation loops. While S is the shear strength, it is proposed that m follows basically from chain spacing fluctuations in the polymer glass; the temperature dependences of both parameters are derived. Application to tensile and compressive tests under a confinement pressure P is developed, with the aim to derive the pressure dependent (biaxial) strain-rate law. In particular, the pressure effect on dislocation density, that is, on plasticity defect nucleation, is shown to have a definite role in the plasticity of these solids. It introduces in the strain-rate law a normal stress dependent term (exp D sigma(n)), which may have a decisive importance in a number of situations like multiaxial solicitations, solid state polymer shaping, second phase effects in polymer blends, and so on. Finally, a set of constant strain rate experiments is presented on an unsaturated polyester resin crosslinked with styrene. Measurements fit reasonably well with the predictions of the above model up to similar to 50 K below the glass transition, at which collective molecular motions invalidate its basic assumptions. The fit includes : (i) the Coulomb Criterion and its temperature dependence; and (ii) the dilative and shear apparent activation volumes at yield at all pressures.