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Author ORCID Identifier

https://orcid.org/0000-0003-4532-4618

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Civil and Environmental Engineering

Year Degree Awarded

2020

Month Degree Awarded

May

First Advisor

Don J. DeGroot

Second Advisor

Guoping Zhang

Third Advisor

Julie Brigham-Grette

Fourth Advisor

Jason T. DeJong

Subject Categories

Geotechnical Engineering

Abstract

Silts are considered a challenging material to deal with in geotechnical engineering design practice and there has been limited research on determining the engineering parameters of silts either by in situ or laboratory testing. This thesis presents results of an extensive research program that investigated the in situ and laboratory behavior of a low plasticity silt deposit at the Norwegian National Geotechnical Test Site at Halden, Norway. Results from multiple in situ tests including: piezocone, pore pressure dissipation, in situ pore pressure measurements, field vane, self-boring pressuremeter and screw plate load tests were synthesized to characterize the Halden silt. Soil sampling using a suite of different samplers of varying sampler geometry and sampling methods were conducted. Laboratory tests performed on the collected samples included: index and soil classification, oedometer, consolidated undrained and drained triaxial, bender element and constant volume direct simple shear. The laboratory tests provided data for interpretation of geological setting, depositional history, deformation, strength, stiffness and hydraulic flow properties of the different soil units at the site. Moreover, simulated tube sampling performed on block sample and reconstituted specimens of silt using the ideal sampling approach complemented data provided by the different soil samplers. These results advanced the understanding of the effects of tube sample disturbance on engineering parameters in this soil type. Results revealed two soil units of low plasticity clayey silt (ML) over silty clay (CL). Geology and the normally consolidated stress state of the underlying clay unit indicates that the silt is near normally consolidated as well. Interpretation of the undrained shear strength of the silt specimens was complex as the in situ tests were potentially influenced by partial drainage while conventional undrained triaxial tests displayed dilative type behavior with no unique (peak) undrained shear strength. Significant alteration of the intact or reconstituted soil state occurred during field sampling using a poor geometry sampler and likewise during laboratory simulation of poor geometry tube sampling. Yet, the clay-based sample quality assessment methods using recompression strains did not track sample quality well for the Halden silt nor did shear wave velocity. The effects of sample disturbance were very pronounced in undrained triaxial shear with generally increasing undrained shear strength with increasing disturbance but with little to no change in the effective stress friction angle. Based on a collective evaluation of the laboratory and in situ screw plate load tests practical recommendations on selection of undrained shear strength for design and associated foundation performance are provided.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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