SEALEX is a research project aiming at identifying the key factors that affect the long-term performance of bentonite-based sealing systems with an initial technological void. In this context, a series of in situ experiments have been performed in field conditions. Meanwhile, a small-scale test (1/10) was carried out in controlled conditions in the laboratory, aiming at providing useful information for analyzing the in situ tests in terms of saturation time and sealing effectiveness. In this paper, the results of the small-scale test are presented along with the results from the first in situ test (PT-N1). It was observed that during the saturation process, the evolution of the injected water volume followed a hyperbolic relationship with time in both the laboratory and field conditions. In the laboratory conditions, a decrease in axial swelling pressure occurred due to filling of the technological void. By contrast, this decrease has not been observed in the field conditions. Comparison of the injected water and the axial swelling pressure between the two different scales enabled the definition of a same time upscaling ratio of 2.5 (in situ experiment/small-scale test). Accordingly, the saturation duration of the in situ experiment was estimated to be equal to two years. For the small-scale test, a swelling strain evolution rate of 0.588 mm/day was identified in the case of infiltration from two sides of the sample. This is useful when predicting the evolution of swelling strain in the case of failure of the sealing plug. After filling of an additional 20% void, a swelling pressure of 0.18 MPa was obtained, indicating the favorable sealing capacity of the material after filling the technological void.