857
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Development of excess pore-water pressure in thawing process of frozen subgrade
soils: Based on analytical solutions and finite element method.
Dégel des sols et variation de la pression d'eau interstitielle: application de méthodes analytiques et
des éléments finis.
G.Y.Yesuf & I. Hoff
Norwegian University of Science and Technology, Trondheim, Norway
J. Vaslestad
Norwegian Public Roads Administration, Oslo, Norway
ABSTRACT: The spring thaw of a frozen soil is controlled by boundary conditions and soil thermal properties. Frozen soils have
substantially reduced permeability and the melting water in the thaw front cannot drain through the still-frozen soil. Consequently,
temporary excess pore-pressure is generated in the process which degrades the shear strength of the soil. This will ultimately reduce
the bearing capacity in roads. In this paper, analytical solutions and a finite element method are used to estimate the thawing rate of
frozen soils, in which a very good agreement is obtained for one-dimensional thawing. Axisymmertic geometry was used in Abaqus
FEA to model the pavement layers with a sinusoidal surface temperature. From the numerical simulation, it was obtained that a
constant rate of thawing can be assumed for frozen subgrade layers for one directional top-bottom thawing. The excess pore-water
pressure largely depends on the initial ground temperature as well as on the magnitude of surface temperature.
RÉSUMÉ : Le mécanisme de dégel des sols est déterminé par les conditions limites et les propriétés thermiques des matériaux. Les
sols gelés ont une perméabilité sensiblement réduite. De plus, lors du dégel, l'eau ne se draine pas toujours au même rythme que la
fonte. Une fonte rapide entraine un excès de pression interstitielle, ce qui diminue la résistance au cisaillement et entraîne une
diminution considérable de la portance des sols et des chaussées. Ce papier présente les résultats de l'estimation du taux de dégel des
sols par des méthodes de résolution analytique et des éléments finis. Une très bonne corrélation est obtenue dans le cas de la
simulation du dégel en une dimension. Les couches de chaussées ont été modélisées dans Abaqus FEA par géométrie asymétrique, en
appliquant une courbe de température de surface sinusoïdale. Une simulation numérique a permis d'établir l'hypothèse d'un dégel
unidirectionnel depuis la surface, à taux constant. L'excès de pression interstitielle dépend grandement de la température initiale du sol
et de la température de surface.
KEYWORDS: FEM, pore-water pressure, temperature, thawing, thawing rate
1 INTRODUCTION
Climate condition is one of the factors that affect design and
performance of pavements. Epecially in cold regions, seasonal
freezing and thawing process may occur in subgrade soils. The
extent of damage on the pavement surface due to freezing and
subsequent thawing of subgrade soils depends on many factors
such as the thermal gradient, availability of water in the sub-soil
layers, frost susceptibility of the soil, consolidation coefficient,
permeability and drainage conditions. If the rate of generation
of water exceeds the discharge capacity of the soil, excess pore
pressure will develop, which can lead to failure of foundations
and slopes (Morgenstern and Nixon 1971). A pavement
structure will be most susceptible to breakup during the period
when excess water cannot drain downward through still-frozen
soil. A major practical aspect of predicting the thawing
mechanism can be for effective road management (especially
for countries that imposed load restriction during spring
thawing) and maintenance programs. When the bound layer of
a road is thinner, the anticipated traffic load in the subgrade is
high. Consequently, the excess pore water pressure(in the short
term) during thawing increases, partly due to the phase change
from the ice state, and partly due to the additional load from the
traffic. The cumulative effect can be severe and this has been
true in many cases especially for low-traffic volume roads since
maintenance budgets are relatively low and appropriate
drainage is missing. Full scale tests conducted at the Vormsund
test road (Nordal and Hansen 1987) showed that the excess
pore-water pressure developed during the spring thaw was the
primary reason for the reduced bearing capacity. Pore water
pressures of up to 0.90m above the drainage level was
registered during thawing.
The problem of spring thawing has no exact solution.
Analytical solutions for heat conduction are well known and are
obtained from the Newmann’s solution (Carslaw and Jaeger
1959). Nixon(1973) formulated an approximated analytical
solution from the theory of consolidation and principle of heat
conduction for the development of excess pore water pressure
following the thawing process. This analytical solution is valid
for thawing of soils over thick ice layers. The impact of
seasonal frost penetration on pavement has been widely studied,
with considerably less focus on thaw weakening from thawing
(Simonsen and Isacsson 1999).This paper discusses on the rate
of thawing (thaw advancement) in the frozen soil layers in
pavements and the subsequent excess pore water pressure. The
study is based on the existing analytical solutions and finite
element method (FEM). The general FEM program, Abaqus has
been used to model the thawing process. The thawing process is
widely understood qualitatively. For example, the type of
subgrade soils that are frost susceptible are well known
(Johnson et al. 1986; NPRA 2011) and some empirical
correlations exist relating the depth of frost penetration to the
Freezing Index (Andersland and Ladanyi 2004). The study
presented here focuses on the quantitative explanation of the
thawing process based on the thermal properties of pavement
materials and thermal boundary conditions. With a better
understanding of the thawing process, optimization process can
be carried out during the design phase, operation and
maintenance of roads.
