Preloading with prefabricated vertical drains (PVDs) is a widely used ground improvement technique in geotechnical engineering to enhance the strength and stiffness of soft soils. PVDs facilitate a more effective route for the dissipation of pore water, accelerating consolidation. A smear zone forms due to the installation of the mandrel, in which in this zone the permeability reduces and volume compressibility increases toward the drain. Existing studies reported that the consolidation process slows down when the distance between PVDs falls below a threshold spacing value. However, analyzing this minimum spacing, with the realistic distribution of permeability and volume compressibility in the smear zone, has not been examined. This gap in research would result in an appropriate design procedure. This paper reviews the existing studies for different distributions of permeability and volume compressibility in the smear zone to assist geotechnical engineers in analyzing the minimum spacing between drains to attain the optimal consolidation of soft soils. A better understanding of this factor enhance the efficiency and sustainability of ground improvement procedures in soil consolidation using preloading with vertical drains.