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Numerical modelling of desiccation crack induced permeability
Modélisation numérique de la perméabilité induite par la fissuration des sols
Stirling R.A., Davie C.T., Glendinning S.
Newcastle University, Newcastle-upon-Tyne, UK
ABSTRACT: The development of cracking as a result of desiccation and the apparent increase in permeability of cracked fill is
increasingly under investigation. Rainfall infiltration into soil surfaces that experience cracking increases due to the additional,
preferential transmission of water. This in turn results in cycles of rapidly elevated pore water pressure and is widely cited as a
significant mechanism for strength reduction that leads to embankment failure. A two-phase flow numerical model that allows the
partially saturated behaviour of the desiccated medium to be captured is presented based on the finite difference code FLAC 2D. The
material properties of the developed model, including soil stiffness and strength, are incorporated as a function of drying. The model
has allowed investigation into the factors influencing the incidence and scale of cracking.
RÉSUMÉ : L’infiltration des précipitations dans les sols sensibles à la dessiccation augmente comme résultat de la transmission
préférentielle, additionnelle d’eau. Ce phénomène se traduit par des cycles de pression interstitielle rapidement élevée, et est
largement cité comme un mécanisme important de la réduction de la résistance qui conduit à la rupture des remblais. Un modèle
numérique de l’écoulement diphasique, permettant la prise en compte du comportement partiellement saturé du milieu desséché, est
présenté. Ce modèle est basé sur un code de calcul de différences finies, FLAC 2D. Les propriétés du matériau du modèle, y compris
la rigidité et la résistance du sol, sont incorporées comme fonction du séchage dans la description de la courbe caractéristique sol-eau.
Le modèle a permis également l’évaluation des principaux facteurs qui influencent l'incidence et l'ampleur de la fissuration des sols.
KEYWORDS: Numerical modelling, Unsaturated soils, Soil behaviour
1 INTRODUCTION
Cracking within clay fills has been an accepted phenomenon
for many decades. The engineering study of desiccation
cracking has been motivated by its impact upon the
effectiveness of many earth structures including liners (Philip et
al 2002), foundations (Silvestri et al 1992), cuttings and
embankments (Smethurst et al 2006) due to an apparent
increase in water infiltration.
Desiccation cracking is the product of volumetric shrinking
of clays brought about by a reduction in soil-water content.
Cracking initiates when tensile stresses generated by increasing
suctions exceed the soil strength, which in itself, is controlled
by soil water content. Variability in soil-water content is
primarily
the
result
of
seasonal
fluctuation
in
precipitation/evaporation in addition to the transient demands of
vegetation and the infiltration potential of the soil surface and is
therefore largely governed by climate.
Predicted climate change scenarios are recognised to have
the capacity to more frequently bring about conditions
conducive to the increased occurrence of this behaviour because
of the increased occurrence of warmer and drier summers
experiencing rainfall events of shorter duration and higher
intensity (Hulme et al 2002, Jenkins et al 2010).
Progressive failure is thought to be largely governed by
permeability which is in turn controlled by the micro- and
macro-scale structure of the soil. Previous studies have
established that current permeability measurement techniques
produce discrepancies between both laboratory and field
established values and numerically simulated pore-water
pressure values (Smethurst et al 2006, Rouainia et al 2009).
These differences have been identified as being caused by
permeability values ranging by up to three orders of magnitude
(Nyambayo and Potts 2005, Rouainia et al 2009). Albrecht and
Benson (2001) identified the same increase in hydraulic
conductivity of three orders of magnitude in laboratory testing
of small cracked samples when compared to equivalent non-
cracked samples of the same material. This supports the notion
that it is the presence of pervasive cracks that results in the
elevated permeability. An empirically reasoned permeability
modification has been employed in the modelling of
embankment pore pressures (Nyambayo et al 2004).
Many researchers have attempted to model the mechanisms
involved in crack initiation and propagation, particularly with
respect to crack pattern. Kodikara and Choi (2006) present a
simplified analytical model for laboratory cracking which has
subsequently been implemented by Amarasiri et al. (2011) into
a distinct element code. Their work describes the modelling of
cracking behaviour in slurried clays under given laboratory
boundary conditions and incorporates material changes due to
drying. More recently, work has been carried out using the finite
element method to investigate the development of tensile
stresses associated with desiccation (Trabelsi et al 2011, Peron
et al 2012). In contrast, this work models partially saturated
flow throughout the medium induced by a simulated
evaporation boundary and combines this mechanism with the
ability to capture a fracturing geometry.
2 TWO-PHASE FLOW
Modelling has been carried out using the commercial finite
difference code, FLAC (Fast Lagrangian Analysis of Continua)
(ITASCA, 2002). The internal programming language, FISH,
has allowed material variables to be defined as a function of
water content. Given the fundamental influence of water content
in desiccation cracking, it is important to be able to capture the
partially saturated behaviour of the medium. To do this, the
Two-phase Flow (tp-flow) option available with FLAC was
used.
