Interpretation of layer boundaries and shear strengths for stiff-soft-stiffclays using cone penetration test: LDFE analyses

This paper reports on a new framework for interpreting cone penetrometer data in stiff-soft-stiffdeposits, with the aim at identifying layer boundaries and interpreting accurately the undrained shear strength for each layer. An extensive parametric study was conducted using large deformation finite-element (LDFE) analyses. The LDFE results were used to establish the interpretation framework. Regardless of the strength and the rigidity index ratio between two successive layers, where the cone penetrates from stiffto soft clay layers and the reverse, the layer interface can be demarked at 1.3D (D is the cone diameter) above and 0.8D below the gradient discontinuity on the penetration resistance profile in the soft layer. The undrained shear strengths of the first and third (stiff) layers can be interpreted using a single-layer approach and the resistance profile without the influence of the adjacent soft layer. The interpretation for the interbedded second layer necessitates implementing a correction factor, which is shown to be a function of the relative thicknesses of the first and second layers, and the rigidity indexes and strength ratios of the adjacent layers. By combining the findings with the results from a previous study on an interbedded stiffclay layer, a complete design chart is proposed for a thin soft or stifflayer embedded in a thick clay layer. The proposed design framework is illustrated by a flowchart for practical use.