Laboratory experiments show that preferential paths develop through saturated impervious clay bodies. A procedure to integrate gas transmission discontinuities into a general THM formulation is described. The technique has been incorporated into a general purpose FE THM code (CODE_BRIGHT) and it has been used to reproduce gas transmission experiments in specimens. The experimentally observed peaks in gas pressure and flow rates, when breakthrough conditions are reached, are reproduced. The paper discusses also the role of local heterogeneity in gas migration. A computational experiment, inspired in the conditions prevailing on the “in situ” large scale GMT test, has been preformed. All the elements of the discretization include an embedded discontinuity. The analysis shows that small variability in buffer properties enhances the development of preferential paths.