Vertical drains are widely used to accelerate the consolidation of soft clay deposits when preloading is used as a ground-improvement technique. Barron's theory is generally used for the design of vertical drains. One of the important input parameters required in Barron's theory is the coefficient of horizontal consolidation, ch. A reliable estimate of ch is, therefore, necessary for proper design. The values of ch can be determined by the radial-consolidation test, using either a central sand drain or a porous plastic peripheral drain. This paper compares the results of the radial-consolidation tests conducted using a 100 mm dia. Rowe consolidation cell under radially inward or radially outward flow conditions. The influence of the diameter of the sand drain (namely, 10, 15 or 50 mm) was also studied. It was found that the radially outward-consolidation test gave lower values of ch compared to the radially inward test. Also, it was observed that the measured values of ch decreased as the perimeter of the drainage boundary increased. © 2018 ICE Publishing: All rights reserved.