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Stability improvement methods for soft clays in a railway environment
Méthodes d’amélioration de la stabilité des argiles moles sous remblai de chemin de fer
Mansikkamäki J., Länsivaara T.
Tampere University of Technology
ABSTRACT: In October of 2009 a full-scale railway embankment failure experiment was conducted in Finland. The data gathered
from the test established a good verification base for the soil models used in this study. In Finland, the most commonly used
improvement method for railway embankments with low stability is a counter weight berm, which is designed based on the undrained
shear strength of clay. Undrained shear strength is often underestimated and this inaccuracy is constantly leading to overdesigned
counter weight berms, which can be tens of meters wide. This paper introduces an evaluation of alternative methods to improve
embankment stability with wooden pile structures or with sheet pile walls. The study contains a comparison of different pile elements
and an evaluation of piles capability for stability improvements. The evaluation is based on 2D and 3D finite element analysis and to
the soil behavior calibrated in the failure test and existing, well investigated Finnish railway embankments with poor stability
conditions.
RÉSUMÉ : Une expérimentation grandeur réelle d’une défaillance de remblai de chemin de fer été menée en Finlande en octobre
2009. Les données recueillies à partir de ce test ont fourni une base pour le modèle de géométrie et de simulation du comportement
des sols exploité pour cette étude. En Finlande, la méthode d'amélioration la plus fréquente est une berme contre-poids conçue sur la
base de la résistance au cisaillement de l'argile. La résistance au cisaillement non drainé est souvent sous-estimée et cette imprécision
conduit à des bermes contre-poids surdimensionnées, qui peuvent avoir des dizaines de mètres de largeur. Cet article présente une
évaluation de méthodes alternatives pour améliorer la stabilité du remblai à l’aide de pieux en bois ou de murs de palplanches.
L’étude présente une comparaison des différents éléments de piliers et une évaluation de la capacité des piliers à améliorer la stabilité.
L’analyse s’effectue par éléments finis 2D et 3D, avec un comportement du sol calibré dans le test de défaillance et l’existence bien
documentée de remblais finlandais dans des conditions médiocres de stabilité.
KEYWORDS: FEM, 3D analysis, soft clay, embankment, wooden piles, sheet pile wall, stability improvement, railway.
1 INTRODUCTION
Stability of railway embankments on soft clays is commonly
calculated with limit equilibrium method (LEM) using
undrained strength parameters. In Finland the undrained
strength is defined with the Field Vane Test. However,
calculations with undrained strength might in some cases
underestimate the factor of safety. In some of the soft soil areas
the calculated total factor of safety is less than F=1.0 for
existing embankments. On the other hand, LEM calculations
with effective strength parameters
φ’
and
c’
tend to
overestimate the safety factor for undrained conditions, when
the excess pore pressure is not accurately taken into account.
A major problem in effective stress analyses is the
assumptions for stress and pore pressure distribution and the
difficulty in accounting for yield induced pore pressure. These
can be taken into account with finite element method (FEM), if
the analyses are conducted with advanced material models and
correctly defined parameters (Mansikkamäki et. al., 2011).
To clarify the real stability conditions of Finnish railway
embankments, a full scale failure test was conducted in October
of 2009 on a soft marine clay deposit in southern part of
Finland. Embankment was loaded to failure in 2 days as shown
in figure 1. The goals for the test were to gather data for the
purpose of improving stability calculation methods and testing
the suitability of different instruments for monitoring
embankment stability.
The extensive instrumentation is well documented in the
work by Lehtonen (2011). Data considering displacements and
excess pore pressure development has given good basis for the
evaluation of FE analysis and the material models.
Figure 1 Test site after the failure. Instrumented area is between the
containers and the ditch. Loading structure has overturned and the slip
surface is protruding from the ditch.
So far FEM stability analyses have been mostly done using
plane strain 2D analyses. Recent development of FE programs
and increase of the computational capacity have enabled an
increasing use of 3D analysis (e.g. Nian et.al., 2012). A stopped
freight train on embankment is relatively close to a plane strain
stability problem, even though 3D modeling provides
possibility to analyze effect of axles or concentrated bogie
loads. What comes to stability improvement methods, modeling
of three-dimensional structures, for example piles, can be much
more precise with a 3D analysis compared to a plane strain
approximation.
