| dc.description.abstract | Through a literature study, different aspects of frozen ground, physical properties, mechanical behavior of frozen ground, the relation between permafrost and climate, and erosional processes are described. The mechanical behavior of frozen ground is governed by the thermal properties, and changes in ground temperature can result in a significant reduction in soil strength. The ice in frozen ground acts like a strong bounding agent, increasing the soils strength. Through an increase in ground temperature, ice will melt, unfrozen water content will increase, and the strong bounding will disappear.
The investigation site, Vestpynten, is located at the coast at Svalbard, near Longyearbyen. Six thermistor strings are instrumented at the site to measure the ground temperatures, and a study of the erosional mechanisms started in 2012. The ground at Vestpynten is composed of medium to poorly sorted gravel with layers of pebbles and sand. These poorly bounded sediments are mainly glued together by the ice bounding, making the ground vulnerable to permafrost degradation.
Previously there has been proposed a modelling approach for the thawing stability of the coastal slope at Vestpynten. This modelling approach was implemented to the numerical program GeoStudio. Using a simplified thermal model, the thermal regime was established, before a sequential thermo-mechanical coupling was used to analyse the thawing slope stability. The modelling gave good results, but with some limitations. It was therefore stated that further numerical simulations were required to capture the stability evolution of the frozen slope. One proposal was that the modelling approach could be implemented in a fully thermo-hydro-mechanical coupling.
The modelling approach is simplified to capture the thawing slope stability at the site. Other factors affecting the coastal erosion, including wave action and snow melting, is therefore ignored. For this thesis the main objective is to capture the stability evolution through a fully coupled analysis with the newly developed thermal module in the finite element program Plaxis. However, the frozen soil material model in Plaxis is not ready and some simplifications is done to make the thermo-mechanical coupling. The modelling is conducted in two parts, first a thermal analysis and then a slope stability analysis.
The accuracy of parameters are central in a good model, but not necessarily easy to retrieve for frozen ground. The majority of the material parameters are determined by engineering judgement together with empirical data and trial and error. Through the model, a better understanding for the thermal, physical and mechanical parameters influencing the thawing slope stability is created. Identifying the field parameters, investigations and experiments needed, is an essential part of a model and provides a good basis for further studies. The main advantage of the thermal analysis is the great visualization of the evolution of the thermal regime.
The high strength of frozen ground is confirmed by the modelling results, and the slope stability analysis show that the failure does not necessarily follow the thawing depth. The model try to resemble the natural failure of the slope by removing the failed material. The results show that a point is reached where an increase in thawing depth does not have any influence and the reestablished geometry created by the failures give stability in the thawed ground. The volume of failed material is identified, and both the volume and failure mechanism resemble with observations in the field. | en |
