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Undrained shear strength of Finnish clays for stability analyses of embankments

Research output: Book/ReportDoctoral thesisMonograph

Details

Original languageEnglish
PublisherTampere University of Technology
Number of pages178
ISBN (Electronic)978-952-15-3806-3
ISBN (Print)978-952-15-3804-9
Publication statusPublished - 21 Sep 2016
Publication typeG4 Doctoral dissertation (monograph)

Publication series

NameTampere University of Technology. Publication
Volume1412
ISSN (Print)1459-2045

Abstract

The thesis deals with the undrained shear strength (su) of Finnish clays. The research study focuses on the evaluation and modelling of undrained shear strength for total stress stability analyses of embankments and it studies some special features of sensitive clays.

Firstly, a multivariate database of Finnish clay data points is compiled in order to derive correlations for undrained shear strength specific to Finnish clays. For each data point, information on su from field vane, consolidation stresses and other physical properties is available. The dependency of su on overconsolidation ratio (OCR) and index parameters is studied. The new correlations are derived through regression analyses. Results show that the dependency of su on index parameters is more marked when the uncorrected field vane measurements are considered. On the other hand, when measured su is corrected for strain rate and converted into mobilized su, such dependency becomes negligible. The new correlations are validated through comparison with existing correlations from the literature. Bias and uncertainties of the new transformation models are evaluated through an independent database consisting of clay data points from Sweden and Norway. The main result is that the new correlations are characterized by lower uncertainty than the other commonly used correlation models.

In order to study some of the special characteristics of soft sensitive clays, the Perniö failure test is analyzed through the finite element method (FEM) using the advanced NGIADPSoft model, which includes anisotropy and strain-softening behavior of sensitive clays. A series of laboratory and in-situ tests are used to determine the anisotropic shear strength of Perniö clay and Perniö dry crust. Stability analyses are performed using the software PLAXIS 2D and the influence of stress path dependency and post-peak strength reduction on the failure load is evaluated. Calculated displacements are compared to field measurements from the experiment. A good agreement is observed between field observations and calculation results. The study indicates that both su anisotropy and strain-softening have a notable impact on the undrained behavior of the Perniö embankment. Furthermore, it was shown how the modelling of post-peak properties influences the computed failure mechanism.

The issue of undrained shear strength increase in clayey layers under old embankments due to consolidation is studied through CPTU and field vane test results from Murro test embankment. Previous test results suggested a decrease of shear strength under the embankment after a few years of consolidation. The new test results show that the strength has increased and the soil has reached its normally consolidated state. Undrained shear strength and preconsolidation pressure are assessed using existing as well as calibrated transformation models. Data from the Murro test site shows that su has increased by over 50% in the uppermost part of the deposit.

Engineering aspects related to the topics object of study are discussed and some complex issues are addressed from a practical point of view. Firstly, some indications on the use of the new correlations for su of Finnish clays are provided. Secondly, suggestions about how to derive soil parameters for FE total stress soil models are given. Finally, a simplified methodology is proposed to model strength increase in total stress analyses.

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