Recently there has been increasing public interest in the disposal of low-level radioactive nuclide waste and energy storage. Underground rock caverns have been considered as possible repository sites. This paper presents a new grouting method which uses a suspension of liquefied ultrafine clay in fractured rock masses. In order to demonstrate the effect to block open cracks, two experiments were carried out on large-sized granite samples with open cracks. The experiments proved the method to be highly effective. Furthermore, experimental results were compared with those from a mathematical model based on successive blocking in parallel walls.