In this thesis the techniques used for determining the shear strength of rockfill, both in the field and in the laboratory, are reviewed. Very little research has been carried out to date to predict the shear strength of rockfill from laboratory investigations. It was thought that if geometrically similar gradings of a material were tested under similar conditions a similar behaviour might be expected. A large scale triaxial apparatus was designed and constructed and a series of experiments were then carried out on geometrically similar gradings of silurian cleaved mudstone, granitic gneiss and chert. From this investigation a number of conclusions have been reached on the behaviour of rockfill materials.