Prefabricted vertical drains: a simplified design procedure

The available design procedures for prefabricated vertical drain are discussed and a simple-to-use design equation is derived by which the drain spacing ratio, n, is expressed as an explicit function of the degree of consolidation required, time available and geotechnical properties of the compressible layer. Both smear and well resistance effects are included in the analysis. The proposed approach does not require the aid of a computer and the drain spacing is calculated avoiding any iterative process or interpolations. The design equation derived in this note can be considered an improvement over the design chart-based procedures as it provides a very accurate value of the drain spacing ratio instead of a graphical estimate. For smear and well resistance parameters varying in a range of practical interest the results of a validation phase demonstrate that the proposed approach can be effectively used for practical applications. A numerical example is given to illustrate the design procedure.