The paper introduces an original approach to face seismic hazard on existing buildings by treating with a soft grout a thin layer of soil at a certain depth. The paper presents a parametric analysis with reference to the case of a typical clayey soil, considering the ideal cases of an horizontal or vertical layer. The information gained from these two extreme cases are essential to understand the true behavior of a real soft caisson, whatever its shape. Surprisingly, the seismic isolation capacity of the vertical layer depends not only on shear stiffness but also on bulk stiffness, and this has to be taken into account in the design of a seismic barrier composed of both vertical and horizontal layers.