This paper tackles the issues related to the excavation of a horizontal gallery carried out in Boom clay, a tertiary clay that hosts the Underground Laboratory of the Belgium Nuclear Agency (SCK-CEN). The gallery is 85 m long, 5 m wide and connects one of the laboratory access shafts to a horizontal drift drilled from the second access shaft. Displacement and pore water pressure sensors installed from both gallery ends allowed for a detailed monitoring of the hydro-mechanical response of the clay rock before, during and after gallery excavation. A striking feature of the response concerns the strong changes measured in pore water pressure at distances as large as 60m from the excavation front. To explore and discriminate the mechanisms controlling such pore pressure changes, 2D axisymmetric Finite Element hydro-mechanical calculations have been carried out. An elastoplastic constitutive law based on Mohr-Coulomb criterion has been considered for the material. Several types of analyses have been performed: a) material and stress state are isotropic; b) material is isotropic but stress state is orthotropic and, c) material and stress state are orthotropic. Results allow for explaining the field measurements and identifying the key variables that control the clay response around the drift.