Field experience with sand drains indicates the need for a proper assessment of the horizontal coefficient of consolidation of laminated soils. The model sand drain test is thought to best simulate field conditions. A new oedometer is described wherein vertical drainage is prevented and radial drainage only is allowed to an axial sand drain. The influence of drain diameter and sand density on the compressibility and consolidation rate of a clay are investigated. It is shown that drains of small relative diameter, i.e., less than one-twentieth of the oedometer diameter, have negligible influence on the soil parameters measures in a test.