Finite element and centrifuge modeling of frost heave and thaw consolidation settlement of pipelines in cold regions

Dayarathne, Rajith (2021) Finite element and centrifuge modeling of frost heave and thaw consolidation settlement of pipelines in cold regions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Frost heave and thaw settlement are two main issues that need to be considered in the design of pipelines in cold regions. Operating a chilled gas pipeline in unfrozen ground or a warm oil pipeline in frozen ground could create a frost or thaw bulb in the soils around the pipeline, which could cause significant ground movement that may impose unacceptable loads on the pipeline, especially in the proximity of thermal interfaces. Modelling of such ground movement and corresponding pipeline–soil interaction may become more complex due to seasonal variation of air temperatures and operating conditions of the pipeline (e.g., pressure and temperature at the compressor stations), which may induce freeze-thaw cycles in the soils downstream areas. The frost heave and thaw settlement around the pipeline under constant and cyclic temperatures at the pipeline and ground surfaces are the focus of the present study. An experimental investigation of displacement of pipelines buried in frost susceptible soils subjected to freeze-thaw cycles is presented first. The effectiveness of cyclic variation of pipeline operating temperatures as a frost heave mitigation measure is evaluated by analyzing 14 model pipes’ tests in a geotechnical centrifuge. Based on the experimental results, five types of possible freeze-thaw induced vertical displacement responses of the pipeline during operation have been identified. The cyclic pipeline operation (sub-zero in the winter and above-zero in the summer months) could reduce the heave rate and total heave compared to those observed in the tests operated under continuous sub-zero pipe temperatures. Secondly, a two-dimensional fully coupled thermo-mechanical finite element (FE) model is developed using Abaqus FE software for simulating the frost heave around chilled gas pipelines buried in frost susceptible soil. The mechanical behaviour of frozen and unfrozen soils is defined using elastic-plastic models that recognize the key influencing factors, including temperature and volumetric ice content in the frozen soil. The Konrad–Morgenstern segregation potential model and the mechanical behaviour of soil are implemented in Abaqus using user subroutines. The FE calculated results are compared with the Calgary full-scale experimental results of two pipe sections buried at different depths, namely control and deep-burial sections. The FE calculated frost front penetration, frost heave, and moisture growth agree well with the experimental results, which indicates that the present FE model can successfully simulate the frost heave around buried pipelines. The long-term frost heave (up to 20 years) is simulated. The decrease of heave rate after the formation of the final ice lens and associated warming at its leading edge is highlighted. The effects of key factors on frost heave and challenges in FE modelling of such large displacements are evaluated. The factors include the water migration modelling approach, soil properties, seasonal ground surface temperatures and operating conditions. Finally, a large-strain coupled thermo-hydro-mechanical FE model is developed using Abaqus FE software to simulate thaw consolidation. The variations of hydraulic conductivity, compressibility, and thermal properties of thawed soils during consolidation are implemented. One-dimensional FE simulations are performed first to verify the FE modelling approach and to show the limitations of the existing small-strain linear thaw consolidation model. Nonlinear variation of void ratio–effective stress–hydraulic conductivity is then considered for improved modelling of thaw consolidation. Finally, a two-dimensional FE modelling of thaw consolidation around a warm pipeline buried in permafrost is presented. The highly nonlinear void ratio–effective stress and void ratio–hydraulic conductivity relationships, specifically the high hydraulic conductivity at large void ratios and low effective stresses after thawing, cause pore water flow along the thaw front, instead of vertical flow in simplified one-dimensional thaw consolidation models, as assumed in previous studies.

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