A yield stress of soft muds is an important rheologial property which represents the critical stress for the initiation of mud movement. It has been mainly measured by a coaxial-type rotary visco-meter. Since measurement with this visco-meter is so complicated, more general and simple methods are required.
　Vane tests and slump tests, which are commonly used in the field, were adapted in this study. Three kinds of soft muds, kaolinite, bentonite and kumamoto mud, were used in this experimemnt. Flow curves and yield stresses of these soft muds were investigated with the rotary visco-meter. Vane tests were conducted to obtain the relationship between the shear strength and the yield stress. The realationship between the shear strength and the yield stress. The relashinship between the slump value and the yield stress was obtained by slump tests. Main results are as bollows.
(1)Variations of flow curves with water content are classified in three types. Two kinds of the yield stress, τy1 and τy2, are defined.
(2)The stress-strain curves in vane tests are classified in two types. The first type has the maximum shear strengh τp in a small strain range and the constant sttrength τr in a large strain range, whereas the second type has only τr. The yield stress is estimated from τr by deducting the viscous resistance in it.
(3)A single relationship between the non-dimensional slump value and yield stress is obtained for the different kinds of soft muds. A slump-stress curve which is obtained by a simple assumption agrees well with this relationship.