|M.Sc Student||Nadav Dvir|
|Subject||Analytical-Empirical Approach for the Prediction of|
Kinematic-Response Relationships between
Hydrate-Bearing Soils and Standard
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Avi Ostfeld|
|Dr. Shmulik Pinkert|
|Full Thesis text|
Methane hydrate-bearing sediments (MHBS) are extensively explored in past decades, mainly for the vast potential as an alternative energy source and abundance in nature.
Current studies are focusing on resolving significant geotechnical challenges, which prevents commercial gas production.
Methane-hydrate within a soil pore-space may significantly modify the mechanical behavior of the composed sediment.
Preliminary experimental results have shown that the mechanical behavior of MHBS is sensitive to three key factors; amount and morphology of hydrate in the sediment, host soil properties, and the applied stress.
Furthermore, recent publications have proved that the effect of hydrate in the sediment does not involve internal cementation forces, but is related to a kinematic constraint of the solid hydrate in the pore space.
This thesis presents a new approach for predicting the kinematic response of MHBS by explicitly utilizing the measured host-soil experimental kinematic response.
The approach is demonstrated by developing an analytic-empiric expression, which relates the kinematic response of MHBS to that of standard soil.
The expression developed in this work is based on algebraic manipulation of well-established stress-dilatancy theory. It was found that the relationship between the kinematic response of MHBS and standard soil is strongly associated with the stress ratio response; MHBS to host soil.
Based on an extensive study of experimental results from the available literature, the current research analyses the stress ratio response and its major behavioral features are discussed; two clear stages (initial rise followed by steady decay), host-soil influence, all results exhibit proportional trends. In light of the trends observed, a generic shape function is suggested to describe the stress ratio response in the decay stage.
It is shown that three key factors govern the observed shape; soil-skeleton behavior, hydrate saturation, and stress state.
In order to comply with this observation, the suggested shape function includes three coefficients, where each of them is related to a different aspect; proportionality factor between stress ratios, decay rate, and interaction with soil skeleton.
A unique optimization process is used for each of the evaluated coefficients, and a sensitivity analysis is shown for the use of all of them in one expression. The model boundaries are defined according to the assumptions made and examined with all experimental test results.
MHBS calculated kinematic response is compared with measured results from various sources, showing good agreement.
In addition, good agreement was also found with independent undisturbed natural soil samples. Additional applications of the suggested expression are also shown, such as first-order evaluation of volumetric strain and implementation into a general plastic-flow rule.
The research in this thesis reveals two critical factors in understanding MHBS behavior by; attributing hydrate effect only to the kinematic response, integrating the host-soil experimental results.
The suggested approach offers a new line of thought regarding behavior analysis of MHBS, which may help reduce the number of model parameters and provide practical efficiency.