The shear behaviour of soft joints containing infill materials was investigated in the laboratory under constant normal stiffness (CNS) conditions. Tests were conducted on joints with asperities having inclinations of 9·5°(type I) and 18·5°(type II), under a given range of initial normal stresses (σ(no)) varying from 0·30 to 1·10 MPa, and at a constant normal stiffness of 8·5 kN/mm. It was found that the shear strength of joints decreases considerably even with the addition of a thin layer of infill. Results also show that the effect of asperities on shear strength is significant up to an asperity height to infill thickness (t/a) ratio of 1·4-1·8, whereas the shear behaviour is controlled by the infill alone beyond this critical ratio. The shear displacement corresponding to the peak shear stress is considerably reduced once the infill starts to govern the shear behaviour of the joint. In this study, the drop in peak shear stress under CNS conditions has been modelled by a hyperbolic relationship. In relation to 'clean' joints, it is verified that the proposed equation can predict the drop in shear stress as a function of the infill thickness, with good agreement with the measured data. In order to predict the dilatancy behaviour of 'clean' joints, a Fourier transform method is introduced, which can be used to predict the shear strength of joints under CNS conditions.