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The results are presented of a site investigation for the Heimifeng, China, pumped storage power station project which showed the presence of swelling behaviour in the granite that resulted in slaking and disintegration of the rock blocks. It was shown that hydrothermal alteration of the granite was primarily due to the intrusion of multi-phase igneous magma and that clay minerals such as montmorillonite, chlorite and kaolinite were the main causes of the swelling behaviour. Alteration occurred mainly along faults or fractures in the rock masses, resulting in roof caving if water was present. Free and confined swelling tests were carried out on samples to determine the times to completion of the swelling process. Contraction of the samples occurred after swelling was completed and the strength of the granite was reduced. On the basis of the results, it is recommended that a support or lining should be put in place immediately after excavation, the shotcrete should be designed to take into account swelling of the rock when encountering water, water should be drained at an appropriate time during the construction of access tunnels and caverns, and drainage directly from surface should be avoided to prevent floor heave., The results are presented of a site investigation for the Heimifeng, China, pumped storage power station project which showed the presence of swelling behaviour in the granite that resulted in slaking and disintegration of the rock blocks. It was shown that hydrothermal alteration of the granite was primarily due to the intrusion of multi-phase igneous magma and that clay minerals such as montmorillonite, chlorite and kaolinite were the main causes of the swelling behaviour. Alteration occurred mainly along faults or fractures in the rock masses, resulting in roof caving if water was present. Free and confined swelling tests were carried out on samples to determine the times to completion of the swelling process. Contraction of the samples occurred after swelling was completed and the strength of the granite was reduced. On the basis of the results, it is recommended that a support or lining should be put in place immediately after excavation, the shotcrete should be designed to take into account swelling of the rock when encountering water, water should be drained at an appropriate time during the construction of access tunnels and caverns, and drainage directly from surface should be avoided to prevent floor heave.