An Undrained Friction Angle for Embankment Design on Soft Clays

Abstract

The undrained strength analysis (USA) of soft clays behavior has been instrumental in the design and staged construction of embankments on such clays for a few decades. The USA accounts for the gain in strength with vertical loading and consolidation as the principle of stress history and normalized strength dictates. Employing USA in numerical analysis is not straightforward. Most finite elements softwares consider either Undrained ‘A’ conditions, which use effective stress parameters (Effective Stress Analysis), or Undrained ‘B’ conditions, which use total stress parameters (Total Stress Analysis). The total stress analysis does not accommodate the change in strength with increasing effective stresses. The effective stress analysis requires accurate formulation to calculate shear induced pore pressures; otherwise, the analysis leads to erroneous predictions. Hence, research uses sophisticated constitutive models such as the Modified Cam Clay (MCC) in Undrained ‘A’ analysis. This paper presents the derivation of an undrained friction angle () to improve simple Mohr-Coulomb (MC) predictions when used an input parameter. The evaluation of () effectiveness involves the case study of highway No. 34 (Banga-Trad) embankment on Bangkok clay. Finite element computation uses coupled analysis in the general purpose software ABAQUS to simulate the embankment construction in a single stage and two stages. The predictions of MC model are compared to that of MCC. Both models estimate the same maximum stable embankment heights in the single and two stage simulations when  is used.