A true triaxial apparatus for unsaturated soil has been developed from the true triaxial apparatus for sand by attaching a device supplying suction to the specimen. The true triaxial tests ( sigma _1> sigma _2>= sigma _3) on a unsaturated silty soil have been carried out under a constant suction using the negative pore water pressure method (S=-u_w>0, u_a=0) for applying suction S (S=u_a-u_w, u_a:pore air pressure, u_w:pore water pressure). It is shown that the true triaxial test results under different three principal stresses are uniquely arranged on the "Extended Spatially Mobilized Plane (Extended SMP)" for fricitional and cohesive materials, which is modified form the original SMP [1,2,3,4] for frictional materials by introducing "bonding stress sigma _0(c=c centre dot cot phi )". it is also shown that the shear strengths of the unsaturated silty clay obtained by the true triaxial apparatus agree well with the Extended SMP failure criterion ( tau _SMP/ tau _SMP = const.).
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