The coefficient of consolidation is a foundamental parameter to estimate the rate of settlement of clayey ground. When clay is improved by vertical drains such as sand drains, the horizontal coefficient of consolidation ch is needed as well as the vertical coefficient of consolidation cυ. However the rate of settlement tends to be overestimated if we use ch that was obtained by conventional laboratory tests.
　In this report, a horizontal consolidation test is developed by using an improved consolidation device and a series of horizontal and vertical consolidation test is carried out. The main conclusions are as follows:
(1) The horizontal coefficient of consolidation ch of uniform Japanese marine clay is two times of the vertical one cυ.
(2) The disturvance by installing sand piles reducesch to 1/2～2/3 of undisturbed value.