In the HADES underground research laboratory in Belgium, a large-scale PRACLAY Heater test and a small-scale ATLAS Heater test are performed to examine the thermo-hydro-mechanical (THM) responses of Boom Clay to heating in configurations mimicking disposal galleries for heat-emitting waste, and to confirm and/or refine the THM property values previously obtained from the laboratory characterization programme. The determination of cross-anisotropic thermal conductivity of Boom Clay is the focus of the present work. Because of the small heater borehole diameter and moderate thermal loading, the ATLAS Heater test produces thermal responses depending essentially on the undisturbed Boom Clay thermal properties. A dedicated gallery with a diameter of 2.5 m was excavated to host the PRACLAY Heater test, and the thermal responses are measured within the excavation damaged zone (EDZ) and beyond as well. A 3D anisotropic thermal conduction model excellently reproduces all measured temperatures of ATLAS, using a pair of thermal conductivity values (par, per) = (1.9, 1.2) W/(m K). The PRACLAY model upscaled from ATLAS model with the same thermal parameter values is found to provide results in close agreement with measured temperatures within the EDZ and in the far field as well. Hence, the observed temperatures in both tests can be well reproduced and interpreted using a single set of cross-anisotropic thermal conductivity values for the Boom Clay, which indicates the impacts of excavation and THM perturbations on the Boom Clay thermal conductivity are not significant. The presented conductivity values could be used for the thermal design of disposal galleries.