Two-dimensional finite element modeling of long-term frost heave beneath chilled gas pipelines

Abstract

Proper estimation of frost heave over the lifetime of a buried chilled gas pipeline is a key design concern. Previously conducted full-scale and centrifuge tests were continued for a limited period of time. Similarly, the numerical analyses with modeling of the coupled thermo-hydro-mechanical process of frost heave were limited to less than half of the design life of typical pipelines. Few studies estimated the long-term frost heave based on decoupled analyses where the heat transfer and frost heave were calculated separately, although both are interrelated processes. This study presents a two-dimensional coupled finite element (FE) modeling of frost heave under chilled gas pipelines buried in frost susceptible soil. The Konrad–Morgenstern segregation potential (SP) model is implemented in commercially available software using user subroutines. Simplified elastic-plastic models that recognize the key influencing factors, including temperature and volumetric ice fraction of frozen soil, are used for modeling the mechanical behaviour of soil. The present numerical approach can properly calculate the temperature gradient in the frozen fringe, which is essential in the SP model to define the volumetric expansion due to water migration to the frozen fringe. The FE calculated heave and frost front penetration are compared with the Calgary full-scale test results. The moisture content profiles obtained from FE simulation show a similar pattern as reported from a full-scale test. The challenges in modeling long-term heave resulting from continued ice accumulation in front of the final ice lens and potential warming of the leading part of the frozen bulb when the pipe moves further up are discussed.