Boom clay formation is a potential host rock for geological disposal of high-level nuclear waste in Belgium. Heating pulse tests with controlled power supply and controlled hydraulic boundary conditions were performed under constant volume conditions to study the hydraulic impact of thermal loading on the clay. Selected test result s of intact borehole samples retrieved in horizontal direction are presented a nd discussed. The study focuses on the time evolution of temperature and pore water pressure changes along heating and cooling paths, i.e. pore pressure build-up during quasi-undrained heating and later dissipation at constant temperature towards the applied hydraulic boundary condition.