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Experimental investigations of the mechanics of gassy sands - testing methodology, shear tests, and imaging
Citation Link: https://doi.org/10.15480/882.13225
Publikationstyp
Doctoral Thesis
Date Issued
2024
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2024-08-24
Institute
TORE-DOI
First published in
Number in series
62
Citation
Technische Universität Hamburg (2024)
ISBN
978-3-936310-64-1
Most soils in the offshore area contain small amounts of gas; either originating from bacterial metabolistic processes or from thermogenic gas production in deeper layers and subsequent upward migration. For a variety of research questions, for example in the fields of submarine slope stability and carbon capture and storage, the interaction of soil grains, pore water, and gas becomes a relevant aspect. However, the impact of occluded gas bubbles on the soil's stress-strain behaviour has not been thoroughly investigated to date, with the existing studies focusing only on very specific boundary conditions. Particularly the characteristics of the gas phase in the pore space and how they influence the macroscopic stress-strain behaviour remain a matter of theoretical assumptions not validated to date. A holistic assessment of the implications of marine gas occurrence is therefore not possible with the existing knowledge and a requirement for further laboratory investigations can be derived. The overarching objective of this thesis is to forward the general understanding of gassy soil mechanics to allow for a holistic understanding of geological systems such as the continental slopes. A micro-to-macro approach was chosen for the experimental investigations. This approach allows for the analysis of microstructural controls for the macroscopic stress-strain behaviour of gassy soils and therefore offers the potential for a more fundamental understanding of the soil mechanical processes. To this end, a sample preparation methodology is developed which is subsequently employed in macroscopic CU triaxial tests as well as in microscopic microCT experiments. Finally, all experimental results are combined in a joint interpretation. The conclusions drawn from the investigations conducted within the scope of this thesis can be summarised as follows: The advancement of the axis-translation method for gassy soils, as introduced in this thesis, is suitable for a reliable preparation of gassy soil samples under different experimental boundary conditions. The application of the method on two poorly graded model sands was successful in the triaxial as well as microCT experiments. Within the test series two different gas morphologies can be identified in the two investigated gradations: gas clusters that grow by capillary invasion of a stationary grain skeleton in medium sand and macropores within a saturated soil matrix that grow by fracturing in fine sand. Therefore, the grain size is the governing factor for the pore habit of the gas phase. The basic soil mechanical assumptions regarding the distribution of the gas phase within the pore space differ from the observations in this thesis; e. g. in contrast to the literature assumptions capillary forces play a great role in gassy sands. Consequently, the mechanical implications are likewise diverging. The shearing behaviour is impacted differently in the two gradations. The friction angle is slightly elevated in the gassy medium sand compared to its saturated equivalent. In the gassy fine sand, a substantial capillary cohesion is induced by the gas phase. Both model sands fail at significantly lower stress levels than the saturated baseline tests. Thus, a general negative impact of the gas can be deduced.
Subjects
gassy sand
partial saturation
granular soils
triaxial tests
capillary effects
fracturing
imaging
computed tomography (CT)
DDC Class
550: Earth Sciences, Geology
624: Civil Engineering, Environmental Engineering
620.1: Engineering Mechanics and Materials Science
Publication version
publishedVersion
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Name
Dissertation-Kaminski-30082024.pdf
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88.39 MB
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Adobe PDF