The Zhanjiang clay is a typical structured soil with high strength and sensitivity

and its mechanical properties are easily affected by sampling disturbance. Based on the self-boring pressuremeter tests（SBPT） and seismic dilatometer tests（SDMT） in the typical structured clay stratum

the in-situ stiffness decay curves are obtained. Furthermore

the Stokoe model is used to describe the normalized stiffness decay behaviors. Results show that the in-situ stiffness decay curve can be obtained by combining the tangential shear modulus Gt from SBPT with the small-strain shear modulus G0 from seismic wave velocity Vs calculated by SDMT. The in-situ stiffness decay curves of structured clays at different depths have a declined trend with a hyperbolic type with the increasing strain levels

which proves that the mechanical properties of structured clays exhibit nonlinear characteristics. With the increase of depth

the in-situ stiffness parameters tend to increase under the condition of shear strain less than 10-3%

while they almost remain unchanged when the shear strain is larger than 1%. The normalized G-γ decay curves obtained from the in-situ tests are in good agreement with the fitting curves from the Stokoe model

indicating that the numerical model could accurately describe the in-situ stiffness decay characteristics of structured clays. This study provides the design parameters for geotechnical engineering constructions of structured clays in Zhanjiang and an important reference for similar formations.