ABSTRACT A dike for a reclaimed land was constructed on a highly plastic marine clay. Immediately after completion, it failed and sunk into the sea. Both field and laboratory tests were conducted in order to obtain the undrained strength of the clay and the actual failure plane. Undrained strength for the stability analysis was determined by 1) unconfined compression test, 2) in-situ vane test, 3) the modified Bjerrum’s method and 4) the SHANSEP method. Both strength anisotropy and the strain rate effect were considered for the strength obtained from the modified Bjerrum’s method and the SHANSEP method, while only the strain rate effect was considered for the in-situ vane strength. Results of stability analysis are summarized below: — 1) Factors of safety obtained using the strength values from the modified Bjerrum’s method showed good agreement with the field behaviour of the dike. 2) Factors of safety obtained using the strength values from unconfined compression and in-situ vane tests were slightly unconservative. 3) Factors of safety obtained using the strength values from the SHANSEP method were conservative.