Interpretation of layer boundaries and shear strengths for soft-stiff-soft clays using CPT data: LDFE analyses

© 2015 American Society of Civil Engineers. This paper describes a new approach for interpreting cone penetrometer data in soft-stiff-soft clay deposits. The identification of layer boundaries and interpretation of shear strength profile were of particular interest. The proposed approach was based on an extensive parametric study using large deformation finite-element (LDFE) analyses, with a standard cone penetrometer penetrated continuously from the soil surface. The LDFE model has been validated against existing theoretical solutions and numerical results, with good agreement obtained. Regardless of strength ratio between two successive layers, the interface of soft-stiff layers can be identified at 0.8D (D is the cone diameter) below the kink in penetration resistance curve in the top soft layer. The interface of stiff-soft layers can be demarked at 1.3D above the kink in the penetration resistance profile in the bottom soft layer. The undrained shear strength of a soft clay layer can be interpreted using a single-layer approach and the resistance profile without the influence of the adjacent stiff layer. The interpretation for the interbedded stiff layer necessitates implementing a correction factor, which is shown to be a function of the thickness and rigidity index of the stiff layer and the strength ratio between that layer and the bottom layer. The proposed design framework is illustrated through a flowchart to be used in practice.