The use of prefabricated vertical drains with preloading is now common practice and is proving to be one of the most effective ground improvement techniques known. The factors affecting its performance, such as the smear zone, the drain influence zone, and drain unsaturation, are discussed in this paper. In order to evaluate these effects a large scale consolidation test was conducted and it was found that the proposed Cavity Expansion Theory could be used to predict the characteristics of the smear zone based on the soil properties available. Moreover, the procedure for converting an equivalent 2-D plane strain multi-drain analysis that considers the smear zone and vacuum pressure are also described. The conversion procedure was incorporated into finite element codes using a modified Cam-clay theory. Numerical analysis was conducted to predict excess pore pressure and lateral and vertical displacement. Three case histories are analysed and discussed, including the sites of Muar clay (Malaysia), the Second Bangkok International Airport (Thailand), and the Sandgate railway line (Australia). The predictions were then compared with the available field data, and they include settlement, excess pore pressure, and lateral displacement. The findings verified that smear and well resistance can significantly affect soil consolidation, which means that these aspects must be simulated appropriately to reliably predict consolidation using a selected numerical approach. Further findings verified that smear, drain unsaturation, and vacuum distribution can significantly influence consolidation so they must be modeled appropriately in any numerical analysis to obtain reliable predictions.