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Coupled THM mechanical model for porous materials under freezing condition
Couplé THM modèle mécanique pour les matériaux poreux dans des conditions de congélation
Shin H.
University of Ulsan, Republic of Korea,
Ahn J.-H.
Pusan National University, Republic of Korea
Kim Y.-T.
Pukyong National University, Republic of Korea
Lee S.-R.
Korea Advanced Institute of Science and Technology, Republic of Korea
ABSTRACT: Recent growing interests associated with frozen ground have required to advance fundamental theories and to precede
systematic researches of soil behavior under freezing conditions. Unlike the well-established soil mechanics’ theory, temperature
variation and phase change of pore-water cause water migration to cold side, ground heaving, sharp increase in earth pressure, etc., and
they bring about serious problems to freezing geotechnical structures. Elasto-plastic mechanical constitutive model for frozen/unfrozen
soil subjected to fully coupled THM phenomena is formulated based on a new stress variable that is continuous in frozen and unfrozen
regions. Numerical simulations are conducted to discuss numerical reliability and applicability of the developed constitutive model. The
numerical results show that developed model can efficiently describe complex THM phenomena of frozen soil, and it can be utilized to
analyze and design the geotechnical structures under freezing conditions, and predict long-term behavior of them.
RÉSUMÉ : Récentes intérêts croissants associés à un sol gelé ont besoin pour faisons avancent des théories fondamentales et de pour
précédent recherches systématiques du comportement du sol sous des conditions glaciales. Contrairement à la théorie de la mécanique
des sols bien établis , les variations de température et de changement de phase de l'eau interstitielle migration de l'eau à cause de côté
froid, soulèvement du sol, forte augmentation de la pression de la terre, etc, et ils produisent de sérieux problèmes à geler les structures
géotechniques. Élasto-plastique modèle mécanique constitutive d' frigorifié / soluble sol à charge des phénomènes parfaitement couplés
THM est formulé à partir d'une variable nouvelle contrainte qui est continue dans les régions frigorifizé et soluble. Des simulations
numériques sont accomplies pour discuter de fiabilité numérique et l'applicabilité du développé constitutif modèle. Les résultats
numériques montrent que le constitutif modèle peut décrit des phénomènes complexes de THM sol gelé, et cela utilisé pour analyse et
dessiné les structures géotechniques dans des glaciales conditions, et de prédire le comportement à long terme d'entre eux.
KEYWORDS: frozen soil, mechanical constitutive model, THM coupling, heaving pressure.
1 INTRODUCTION
Soil behavior during a freezing process has been intensively
studied mainly in Canada, the United States, Russia, and Japan
from the 1920s. In Korea, a recent Antarctic bases and
gas-pipeline negotiations to introduce Russia's natural-gas line
with 1100km long have attracted attention to understand
fundamental phenomena of natural or artificial frost ground.
Typically, freezing region within soil is divided into frozen zone,
frozen fringe, and unfrozen zone in Figure 1. New ice lens
formation occurs on the boundary between frozen zone and
frozen fringe, called a freezing front. The frozen fringe is the
very thin layer with 1mm ~ 10mm thick, and soil type and
freezing rate control its thickness. Cryogenic suction due to
temperature gradients within the frozen fringe absorbs water
from unfrozen zone, and form ice crystals onto the freezing front
to separate between the soil particles, called segregation freezing.
Konrad and Morgenstern (1981) defined the ratio of a thermal
gradient within the frozen fringe to liquid inflow rate as
segregation potential, and empirical correlations between
segregation potential and basic properties of the soil were
proposed to evaluate freezing susceptibility (Konrad, 1999).
Silty soil with relatively high hydraulic conductivity and
triggering high suction is prone to freeze and form an ice-lens
due to temperature drop. Slowing freezing and resulting low
temperature gradient forms thicker ice crystals in the soil
(Lawrence, et al., 2005). Konrad and Morgenstern (1982)
suggested segregation potential function to consider the effect of
an external load on the inflow rate into the freezing fringe due to
a thermal gradient. It was known that overburden pressure limits
the amount of heaving, but the presence of shut-off pressure
restraining further expansion is still debating (Arvidson &
Morgenster, 1977). Electrolyte in the soil pore water reduces
total suction, and plays a role to decrease the amount of frost
heaves of the soil.
Phase change between liquid water and solid ice in porous
material intimately affects deformation characteristics, as water
and heat flow do. Thus, fully coupled THM
(Thermo-Hydro-Mechanical) phenomena in the porous material
require well-established governing equations, and necessitate
solving nonlinear equations and complex numerical correlation
between constitutive models. Up to date, many numerical studies
have been conducted to imitate fundamental characteristics of
frozen soil. However, most cases performed combined TH
analysis without considering mechanical effects (Tan, et al.,
2011; Painter, 2011). Lately mechanical analyses based on the
total stress was attempted for frozen soil (Michalowski and Zhu,
2006), and frozen ground was assumed as a linear elastic material
and interpretations was carried out (Liu & Yu, 2011; Thomas, et
al., 2009), except for the case of Nishimura et al. (2009).
In this paper, THM elasto-plastic mechanical constitutive model
is presented to reproduce freezing action in porous material
consisted of soil particles, unfrozen water, and ice. The adopted
new stress variable represents the sum of ice and soil skeleton
stress to maintain continuity across the frozen-unfrozen
transition regions, and stress-strain relationship is derived as the
form of incremental formulation. After conjunct with
pre-developed THM finite element program, THM numerical
analyses for freezing process in porous materials were performed
to evaluate the performance of the mechanical model.
M dèle mécanique couplé THM
