Recent studies on the structure systematics of dense oxides have shown that the garnet structure has extremely high creep strength. These results suggest that the transition zone between 400 and 670 km depth may be stronger than the lower mantle (perovskite), which, in turn, is sturdier than the upper mantle above 400 km. We have compared two models, (1) one with two layers separated at 670 km, (2) the other with three layers having thicknesses of 400 km (olivine layer), 270 km (hard layer) and 2270 km (perovskite layer). The viscosity contrast between the olivine and garnet layers varies between O(10) and O(102); the viscosity contrast between the olivine and perovskite layer varies similarly. We have employed two modelling approaches: (1) cartesian model for calculating geoid anomalies and surface topographies due to internal loading; (2) a spherical-shell model, based on analytical functions, for analysing postglacial rebound. The existence of the garnet layer induces distinct effects in both the geoid and postglacial responses. Geoid responses, as a consequence of the hard layer, become much stronger due to excitation of long-wavelength density anomalies in the interior of the three-layer model. To obtain similar magnitudes of geoid signatures, a larger viscosity contrast between the lower and upper mantle is required. For postglacial rebound and shallow-seated density heterogeneities a smaller viscosity variation between the upper and lower mantle is needed in the presence of the garnet layer. These findings may reconcile some of the discrepancies in lower mantle viscosity inferred from the two data sets.