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Simulation of Delayed Failure in Naturally Deposited Clay Ground by Soil-water
Coupled Finite Deformation Analysis Taking Inertial Forces into Consideration
Simulation de rupture différée d'un sol d'argile naturelle sédimentaire à l'aide de l'analyse des
déformations finies de squelette couplé eau-sol en tenant compte de la force d'inertie
Yamada S., Noda T.
Nagoya University, Japan
ABSTRACT: A bearing capacity analysis was carried out for a highly structured naturally deposited clay ground using the soil-water
coupled finite deformation analysis code
GEOASIA
, which takes inertial forces into consideration and employs the SYS Cam-clay
model, which is capable of describing the work of the soil skeleton structure. The following results and conclusions were obtained. 1)
When a ground that exhibited localization of deformation and formation of a circular slip failure accompanied by load reduction as a
result of loading by displacement control was loaded by load control, it failed dynamically in association with acceleration motions
after reaching the peak load obtained during displacement control. To date, the bearing capacity problem has only been dealt with
quasi-statically, but it is essential to take inertial forces into consideration in order to reproduce this type of failure behavior. 2) Using
the analysis code, it was possible to reproduce the behavior before, during, and after the delayed failure phenomenon, as well as
whether or not there is a load threshold for occurrence of delayed failure. To reproduce this type of phenomenon, a time-dependent
constitutive equation is not necessarily required.
RÉSUMÉ : Nous avons effectué l'analyse de capacité portante d'un sol d'argile naturelle sédimentaire ayant développé une structure à
l'aide du programme
GEOASIA
d'analyse des déformations finies de squelette couplé eau-sol en tenant compte de la force d'inertie, et
équipé du modèle SYS Cam-clay qui inclut la fonction de la structure du squelette du sol. Les résultats sont indiqués ci-dessous. 1) La
soumission d'une charge au sol par commande de déplacement produit une localisation des déformations et simultanément la création
d'une rupture coulissante en forme d'arc alors qu'en soumettant une charge par commande de charge, après avoir atteint le pic de
chargement de la commande de déplacement, le sol subit une rupture dynamiquement avec l'accélération de l'activité. Jusqu'à présent,
le problème de la capacité portante n'avait été traité que de manière quasi-statique mais afin de reproduire ces comportements de
rupture il est nécessaire de prendre en compte de la force d'inertie. 2) Il est possible de reproduire le comportement avant, pendant et
après la rupture du phénomène de rupture différée à l'aide du même programme d'analyse du seuil de charge s'il y a rupture différée.
Pour reproduire ce phénomène, une équation constitutive dépendante du temps n'est pas forcément nécessaire.
KEYWORDS: Inertial force, Soil-water coupled finite deformation analysis, Delayed failure.
1 INTRODUCTION
Starting in the 1990s, the Nagoya University geo-mechanics
group has been engaged in developing soil-water coupled finite
deformation analysis employing an elasto-plastic constitutive
equation (Asaoka et al. 1994). In 2002, with the goal of
developing a constitutive equation capable of handling the full
range of mechanical behavior of a wide range of soil textures
from clay to sand and intermediate soil, the group proposed the
SYS Cam-clay model as an elasto-plastic constitutive equation
based on the concept of the soil skeleton structure (Asaoka et al.
2002). More recently, the group developed a soil-water coupled
finite deformation analysis code
GEOASIA
that accounts for
inertial force (Noda et al. 2008), which enables the simulation
of ground deformation and failure behavior without having to
distinguish between static and dynamic problems.
While the importance of accounting for inertial force is
widely recognized in seismic response analysis, the same cannot
be said for phenomena that, up to this point, have been handled
as quasi-static bearing capacity problems. Thus, in this paper,
taking the bearing capacity of a highly structured naturally
deposited clay ground as an example, we demonstrate that there
are situations in which it is important to account for inertial
force, even in the case of phenomena that have traditionally
been treated as quasi-static. Furthermore, in order to show the
robustness of the soil-water coupled skeleton approach, we
again employ the
GEOASIA
code to demonstrate the possibility
of simulating delayed failure of ground, which previously was
explained as a rheological property of the soil skeleton, without
having to impose a time dependence on the constitutive
equation.
2 ANALITICAL CONDITIONS
The simulations were performed using the soil-water coupled
finite deformation analysis code
GEOASIA
,
which accounts for
inertial force, mounted with the SYS Cam-clay model to
represent the work of the soil skeleton structure. The finite
element mesh and boundary conditions used in the simulations
are presented in Figure 1. Computations were conducted under
two-dimensional plane strain conditions. We examined the
loading of a rigid frictional foundation, represented in the
simulations by imposing linear constraint conditions (distances
constant and angles constant; Asaoka et al. 1998) on the nodes
constituting the foundation. In order to prevent asymmetrical
motion of the foundation due to slight numerical errors, we
fixed horizontal displacement of the central node of the
foundation and imposed direction constant condition. The
material constants used in the simulation were adjusted to
reproduce the elasto-plastic behavior of a typical clay soil
(degradation rate of overconsolidation is greater than the
degradation rate of structure, and development of anisotropy is
slow). In the initial stage prior to analyzing the bearing capacity
problem, we simulated the consolidation following the removal
of a load (98.1 kPa) from the surface of a normally consolidated
clay ground with highly developed structure and anisotropy up
to the achievement of a steady state. The bearing capacity
