Recently, the Institute for Radiological protection and Nuclear Safety (IRSN) has launched VSEAL (Vertical SEALing) project to investigate the impact of gas migration on the long-term performance of bentonite based vertical sealing systems (VSS). The first VSEAL in situ test was emplaced in IRSN’s Underground Research Laboratory (URL) in Tournemire (France) in 2019 and was equipped with 76 wired and wireless sensors. The test is still in progress, but thecollected set of data provides already valuableinformation ofthe hydro-mechanical behavior of VSS during hydration. The swelling coreconsists of a mixture of high-density pellets and powder of MX80 bentonite in a ratio of 80/20 (in dry mass). An innovative method was adopted to drill a 1-m diameter and ~10-m deep shaft in order to minimize the rock perturbation at the sidewalls. Because a specific protocol was adopted to install the bentonite mixture together with a careful characterization of the core during construction, VSEAL 1 constitutes the unique in situ sealing test with a well-known initial structural distribution of the pellets and the powder. Some heterogeneities occurred within the experiment during the installation process: a damaged zone developed around the shaft walls due to the interruption of the installation operations caused by COVID19 lockdown in France; a technological gap with a variable thickness between the last pellets layer and the top confining lid and a heterogeneous distribution of the bentonite powder at some layers inducing large internal pellets voids close to the bentonite-rock interface. Artificially injected water volume, relative humidity, water content and swelling pressure in both radial and axial directions were monitored. Comparison of the results showed that the presence of installation-induced heterogeneities led to the generation of preferential flow paths that influenced the swelling pressure evolution at radial and axial directions.