Water-Weakening and Time-Dependent Deformation of Organic-Rich Chalks

The Ghareb Formation is a shallowly buried porous chalk in southern Israel that is being considered as a host rock for a geologic nuclear waste repository. Setup and operation of a repository will induce significant mechanical, hydrological and chemical perturbations in the Ghareb. Developing a secu...

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Bibliographic Details
Published in:Rock mechanics and rock engineering 2023-11, Vol.56 (11), p.8041-8059
Main Authors: Kibikas, William M., Choens, R. Charles, Bauer, Stephen J., Shalev, Eyal, Lyakhovsky, Vladimir
Format: Article
Language:English
Subjects:
Anisotropy
Axial strain
Chalk
Civil Engineering
Compaction
Deformation
Deformation effects
Earth and Environmental Science
Earth Sciences
Elastic properties
Elasticity
Escape behavior
Geophysics/Geodesy
Hydrostatic pressure
Hydrostatic tests
Mechanical unloading
Original Paper
Permeability
Perturbation
Pressure
Radioactive wastes
Repositories
Rock
Rock properties
Rocks
Saturation
Strain
Time dependence
Triaxial tests
Unloading
Water
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Summary:The Ghareb Formation is a shallowly buried porous chalk in southern Israel that is being considered as a host rock for a geologic nuclear waste repository. Setup and operation of a repository will induce significant mechanical, hydrological and chemical perturbations in the Ghareb. Developing a secure repository requires careful characterization of the rock behavior to different loads. To characterize hydromechanical behavior of the Ghareb, several short- and long-term deformation experiments were conducted. Hydrostatic loading tests were conducted both dry and water-saturated, using different setups to measure elastic properties, time-dependent behavior, and permeability. A set of triaxial tests were conducted to measure the elastic properties and rock strength under differential loading at dry and water-saturated conditions. The hydrostatic tests showed the Ghareb began to deform inelastically around 12–15 MPa, a relatively low effective pressure. Long-term permeability measurements demonstrated that permeability declined with increasing effective pressure and was permanently reduced by ~ 1 order of magnitude after unloading pressure. Triaxial tests showed that water saturation significantly degrades the rock properties of the Ghareb, indicating water-weakening is a significant risk during repository operation. Time-dependent deformation is observed during hold periods of both the hydrostatic and triaxial tests, with deformation being primarily visco-plastic. The rate of deformation and permeability loss is strongly controlled by the effective pressure as well. Additionally, during holds of both hydrostatic and triaxial tests, it is observed that when water-saturated, radial strain surpassed axial strain when above effective pressures of 13–20 MPa. Thus, deformation anisotropy may occur in situ during operations even if the stress conditions are hydrostatic when above this pressure range. Highlights Porous chalk was experimentally deformed over short- and long-time scales. Water saturation was shown to significantly degrade chalk elasticity and strength. Hydrostatic pressure above certain conditions can produce significant deformation anisotropy. Chalk samples exhibit strongly visco-plastic and compaction dominated behavior. Compaction dominant behavior could inhibit escape of nuclear waste in repository.
ISSN:0723-2632
1434-453X
DOI:10.1007/s00603-023-03453-6