When a soft clay foundation under embankment loading is considered, settlement behavior is affected by the mass permeability and by the magnitude of applied load intensity, which is normalized by the initial undrained bearing capacity. Using these two parameters, the characteristic consolidation settlement behavior of a clay foundation can be represented by a family of S shaped curves. The range of mass permeability affecting residual settlements is confined to a narrow limit. For an embankment foundation, when these two parameters are known, settlement behavior can be determined uniquely. The effects of improvement of mass permeability by sand drains on a clay foundation are investigated using the macro element method (Sekiguchi et al., 1986). The sand drains improve the mass permeability to 30-300 times that of the non-treated ground. The above findings are applied to two case studies. In the first, where the ground is treated with sand drains under a high applied load it is found that only settlement observations can be used to assess the usefulness of sand drains in this ground. The other case is on coral soils which exhibit high permeability and a high initial undrained bearing capacity compared with common clay deposits. The effectiveness of sand drains in this ground is evidenced by a significant decrease in residual settlement.