Sand/bentonite mixtures are considered as buffer material in low and medium radioactive waste disposal schemes. While the bentonite provides low permeability and swelling capacity, the sand is intended to improve mechanical stability and to reduce the amount of bentonite required. One of the requirements of the buffer is to ensure the isolation of the gas-emitting waste. The pressurized gas may migrate towards the host geological media through several mechanisms (dissolution, two-phase flow, preferential paths). This paper is a contribution to this topic and presents experimental results as well as some modelling work. Laboratory experiments were performed on statically compacted 80/20 sand/bentonite mixture recovered from the in situ large scale GMT (Gas Migration Test) buffer. The sample was tested in a specifically designed Perspex test cell which allows the visualization of the boundary of the sample. Two needles inserted inside the sample are used to inject and recover gas. The formation of preferential paths following the compaction layers and the sample-wall interface were detected in the test. The test is analyzed by hydro-mechanical numerical simulations considering randomly distributed properties (porosity, permeability and hardening parameter) and the possibility of opening discontinuities in the permeability law are considered. Also a computational experiment inspired in the conditions prevailing on the in situ large scale GMT test, is presented.