Analysis of undrained soil behaviour of Dutch organic clay in K0-consolidated triaxial tests

Abstract

In 2017 new guidelines concerning macro stability calculations were implemented by the Dutch Ministry of Infrastructure and Environment. These guidelines are formulated in “Wettelijk Beoordelingsinstrumentarium” (WBI). The largest difference with the previous version of the guidelines concerns the material model that is prescribed to determine shear strength parameters. In triaxial tests shear strength parameters are to be determined at ultimate state (25% axial strain), which is assumed to be a good representation of critical state. Critical state is a concept from Critical State Soil Mechanics (CSSM) and assumed to be a good representation of the state reached after large deformations induced by macro instability. Another fundamental assumption in the WBI is the use of the SHANSEP method. This method encompasses a laboratory procedure and a normalisation method. CSSM was originally defined and elaborated under isotropic stress conditions while in engineering practice anisotropic conditions are mostly used.
The goal of this thesis is to investigate the undrained soil behaviour of organic clay in triaxial tests following the SHANSEP procedure and compare the results to the CSSM framework. In order to do so a series of eight K0-consolidated triaxial tests is executed using silty organic Oostvaarders plassen clay, which is assumed to be representative for a typical Dutch soil. A large range of over consolidation ratios is applied (1-20). Both compression and extension tests are executed. The triaxial tests are complemented by two K0-CRS tests and an isotropic compression test to determine relevant soil properties. The results are compared to the CSSM. The qualitative soil behaviour is analysed as well as the actual predicted undrained shear strength Su. Parameter relationships as described by CSSM are tried to be established from the data. The undrained shear strength is predicted by using numerical and analytical formulations of the Modified Cam Clay model, which is the most basic implementation
of CSSM. From the data a clear failure line could be determined in p'-q space. In the triaxial compression tests failure at ultimate state gave very consistent result, a failure line could precisely be determined. In extension failure at peak strength showed the most consistent results. Ultimate state could not be reached under representative stresses because the formation of large shear bands and necking during shearing. A unique p'-q-e relation was much harder to establish. The general trend as described by the CSSM was clearly visible but the uncertainty was rather large. Several factors contributed to the uncertainty. Among which the void ratio determination method that was used to determine the void ratio after the triaxial test, which is prone to errors. The MCC model was not able to model the stress path correctly and p'_0 at failure was not correctly predicted resulting in incorrect Su prediction. The MCC model overestimated Su in compression tests. In extension, however, Su is well predicted by the MCC model. The SHANSEP method turned out to be a very convenient way to normalise the undrained shear strength of the triaxial tests, both in compression and extension. Only at very large OCR Su/σ'_v0 is slightly overestimated.