Shear strength of foundation rock is one of the primary properties required for the design and safety analysis of dams. Direct shear tests are used to determine the peak and residual shear strength of the foundation rock mass. Literature study shows that spatial variations are common in shear strength behaviour, i.e., stress–strain curve, shear strength parameter ‘c’ and ‘ϕ’, shear stiffness, etc., due to variation in intact rock, discontinuities, rock mass condition, weathering, confinement, precision during site preparation, environmental, physical effects, etc. Present paper deals with results of in-situ direct shear tests conducted for rock over rock and concrete over rock interfaces on Quartzite rock mass at left and right bank of dam axis in three different drifts at EL 935.36 m, EL 931.92 and EL 877.9 m of Song Dam drinking water Project, Dehradun, Uttarakhand, with emphasis on the spatial variability in shear strength behaviour. The analysis and interpretation of test data with respect to spatial variability in shear strength behaviour of Quartzite rock mass at different locations is a challenging task, and an attempt has been made to explain the same in this paper. Further, the influence of geological features has been considered to explain the variation in shear strength behaviour with respect to different test locations in detail. However, present case studies are evident that the typical formation of shear key and its behaviour have played a very important role in the spatial variation of shear strength and shear strength parameters.