The unexpected softening of the undrained shear strength of organic and silty clays in Rhine delta: a conceptual study

Abstract

The current Dutch norms for the macro-stability of ood protection embankments and more specifically the determination of SHANSEP parameters has been subject of debate since its implementation in 2017. The main concerns arise from the apparent over-conservative nature of the guidelines and the apparent underestimation of shear strength of organic and silty clays. In this Master thesis, an assessment of the normalised normally consolidated undrained shear strength is performed for a dike stability reinforcement project between Gorinchem and Waardenburg in the South west of the Netherlands on a problematic clay layer called the `Gorinchem Clay' throughout this thesis. The main goal is the conrmation of the current norms or towards a more optimised and less conservative assessment of the strength parameters and in doing so, discusses one of the most recurrent questions in geotechnical engineering: do some of the measurements from laboratory tests reveal true material behaviour or do the limitations of such laboratory tests produce this particular behaviour? This analysis was performed on Triaxial Compression, Direct Simple Shear and Triaxial Extension tests from which the SHANSEP parameters are derived as input for the available shear strength along a slip surface according to the methodology developed by Ladd (1991) and following the Critical State Soil Mechanics. The structure of this thesis is as follows: rst, a literature study is performed on the strength parameters assessment in chapter 1, the principles of the Critical State Soil Mechanics and the diculties encountered in the determination of the strength parameters from laboratory tests in chapter 2. The laboratory results are then exploited in chapters 4 to 5 according to the current guidelines and more extended methods in which the limitations of the Classical Critical State Soil Mechanics are shown. The next chapters focus on a more fundamental understanding of the considered material consisting of modelling the material behaviour in chapter 6 using a simplied academic constitutive model featuring non-associative elasto-plasticity with mixed volumetric and deviatoric hardening allowing for hardening or softening. The outcomes of this thesis show Critical State conditions as traditionally understood could not be reached reliably for undrained conditions in Triaxial Compression, Triaxial Extension and Direct Simple Shear. The tests showed to be particularly unreliable beyond 10% axial strain in Triaxial Compression and Extension, and beyond 15% shear strain in Direct Simple Shear. The diculties encountered in determining the Critical State friction angle and undrained shear strength were shown to be minimised by performing drained and undrained Triaxial compression tests on slightly over-consolidated soil samples. Additionally, the limitations of the Classical Critical State theory were highlighted and a more advanced constitutive model including a non-associative ow rule and deviatoric hardening was successfully used to predict the behaviour in Triaxial Compression for undrained conditions. The predictions for drained conditions and particularly undrained Triaxial Extension were however limited.