Abstract: This paper presents the implementation of the Barcelona Basic Model (BBM) into the TOUGH–FLAC simulator analyzing the geomechanical behavior of unsaturated soils. We implemented the BBM into TOUGH–FLAC by (1) extending an existing FLAC3D module for the Modified Cam-Clay (MCC) model in FLAC3D and (2) adding computational routines for suction-dependent strain and net stress (i.e., total stress minus gas pressure) for unsaturated soils. We implemented a thermo-elasto-plastic version of the BBM, wherein the soil strength depends on both suction and temperature. The implementation of the BBM into TOUGH–FLAC was verified and tested against several published numerical model simulations and laboratory experiments involving the coupled thermal–hydrological–mechanical (THM) behavior of unsaturated soils. The simulation tests included modeling the mechanical behavior of bentonite-sand mixtures, which are being considered as back-fill and buffer materials for geological disposal of spent nuclear fuel. We also tested and demonstrated the use of the BBM and TOUGH–FLAC for a problem involving the coupled THM processes within a bentonite-backfilled nuclear waste emplacement tunnel. The simulation results indicated complex geomechanical behavior of the bentonite backfill, including a nonuniform distribution of buffer porosity and density that could not be captured in an alternative, simplified, linear-elastic swelling model. As a result of the work presented in this paper, TOUGH–FLAC with BBM is now fully operational and ready to be applied to problems associated with nuclear waste disposal in bentonite-backfilled tunnels, as well as other scientific and engineering problems related to the mechanical behavior of unsaturated soils. [Copyright &y& Elsevier]