3305
Safety of a protection levee under rapid drawdown conditions. Coupled analysis of
transient seepage and stability
La sécurité d’une digue de protection en conditions de vidange rapide. Analyse couplée des
écoulements transitoires et de la stabilité
López-Acosta N.P., Fuente de la H.A., Auvinet G.
Instituto de Ingeniería, UNAM, D.F., México
ABSTRACT: The rapid drawdown condition arises when submerged slopes of protection levees experience a rapid decrease of the
external water level. In this paper the safety of a protection levee under rapid drawdown conditions is studied by numerically
modeling this phenomenon as a coupled problem of transient seepage-deformation in a saturated/unsaturated medium. Analyses are
performed based on finite element method by using the PLAXFLOW program for transient seepage analysis and the PLAXIS
program for deformation, consolidation and stability analyses. The details of the proposed methodology are presented in this work.
Also, recommendations for definition of material type (drained or undrained), type of soil constitutive model (Hardening Soil and
Mohr Coulomb), boundary conditions and mesh generation of finite elements are provided. In the main part of the paper, the effects
of multiple parameters such as position of phreatic surface, water drawdown ratio, drawdown rate and hydraulic conductivity are
evaluated by a 2D model of stress-strain. Special emphasis is given to the study of the safety factor variation as a function of time
obtained when assessing the stability of these earth structures. Finally, concluding comments about the results are exposed.
RÉSUMÉ: La condition de vidange rapide survient lorsque les pentes de digues de protection submergées expérimentent une
réduction rapide du niveau d'eau externe. Dans cet article, la sécurité d'une digue de protection dans des conditions de vidange rapide
est étudiée par modélisation numérique de ce phénomène comme un problème couplé de flux transitoire -déformation dans un milieu
saturé / non saturé. Les analyses sont effectuées sur la base de la méthode des éléments finis en utilisant le programme PLAXFLOW
pour l'analyse d'infiltration transitoire et le programme PLAXIS pour les analyses de déformation, de consolidation et de stabilité. Les
détails de la méthodologie proposée sont présentés dans cet écrit. Également, des recommandations pour la définition du type de
matériau (drainé ou non drainé), type de modèle constitutive de sol (Hardening Soil et Mohr Coulomb), conditions aux limites et de
génération de maillage d'éléments finis sont fournis. Dans la partie principale de l'article, les effets de plusieurs paramètres tels que
l'emplacement de la surface phréatique, le taux de vidange rapide, le rapport de vidange et la conductivité hydraulique sont évalués
par un modèle 2D de contrainte-déformation. Une attention particulière est accordée à l'étude de la variation en fonction du temps des
facteurs de sécurité obtenus lors de l'évaluation de la stabilité de ces structures en terre. Enfin, les observations finales sur les résultats
sont données.
KEYWORDS: Protection levee, rapid drawdown, coupled analysis, transient seepage, slope stability, 2D model of stress-strain.
1 THE RAPID DRAWDOWN CONDITION
1.1
The water drawdown phenomenon
The water drawdown phenomenon can be divided in three
modes (Fig. 1): a)
fully slow drawdown
, b)
fully rapid
drawdown
, c)
general (transient) drawdown
(Duncan
et al.
1990, Griffiths and Lane 1999, Lane and Griffiths 2000,
Berilgen 2007, Huang and Jia 2009, Nian
et al.
2011).
In the
fully slow drawdown
condition (Fig. 1a), the soil is
assumed to be drained; in every moment of the drawdown the
water level inside the levee (water table) equals the water level
on the outside (the reservoir level), generating a steady-state
flow condition, therefore the pore water pressure within the
levee is hydrostatic condition. In the
fully rapid drawdown
mode (Fig. 1c), the soil is considered to be undrained, the water
table is conserved at the initial level of the reservoir for every
moment of the drawdown, so the pore water pressure inside the
levee is the hydrostatic pressure. In both extreme cases, the
water surface is assumed to be horizontal, except on the face of
the slope for fully rapid drawdown mode, as shown in Figure
1c. For the
general (transient) drawdown
mode (Fig. 1b), a
curvilinear water surface is generated within the soil structure
whose position depends on the drawdown rate and the material
properties (such as hydraulic conductivity, porosity, etc.),
consequently the remaining pore water pressure within the levee
is transient type (it varies as a function of time but also with the
soil’s ability to retain water).
a)
b)
c)
H
H
H
L
L
L
Initial water level
Initial water level
Initial water level
Figure 1. Water drawdown modes: a) Fully slow drawdown, b)
Transient drawdown y, c) Fully rapid drawdown (Berilgen 2007).
