2517
Interaction of stone column and surrounding soil during its construction: 3D
numerical analysis
Interaction d’une colonne ballastée et du sol environnant pendant sa construction : analyse
numérique 3D
Klimis N.S.
Civil Engineering Department, Democritus University of Thrace (DUTh), Xanthi, Greece
Sarigiannis D.D.
AUTh, MSc DIC
ABSTRACT: This work deals with a simulation of a construction sequence of a stone column in two distinct stages: a) a one stage
excavation and b) a multi-stage backfilling of the column stone excavation with crushed gravel at ascending steps of 1m. Simulation
of this procedure is attempted using a 3D model which represents the stone column and the surrounding soil. Analysis is carried out
using a numerical code, called FLAC3D, based on finite differences. The mathematical model incorporates geometry and boundary
conditions of the problem, profile of soil layers with their physical, deformational and mechanical properties and their constitutive
laws, as well as, initial conditions of stresses and deformations of subsoil stratums of the examined area. Special emphasis is given to
simulation of an harmonically imposed vertical loading of the vibrating column, into an equivalent static vertical loading and
subsequently into an equivalent radial pressure against internal wall of the cylindrical excavation of the constructed stone column.
Results clearly denote that there is a strong interaction of the complex system in the kinematical and stress field, which satisfactorily
justifies modification of the final diameter of the constructed stone column compared to the theoretical proposed diameter.
RÉSUMÉ : Ce travail se réfère à une simulation numérique de la séquence de construction d’une colonne ballastée, en deux étapes
séparées : a) une étape unique d’excavation, et b) plusieurs pas successifs de remblayage de l’excavation cylindrique de la colonne
ballastée, avec du matériau granulaire écrasé, à des pas montants de 1m. La simulation est effectuée à l’aide d’un modèle 3D qui
représente la colonne ballastée et le sol environnant. Le code numérique utilisé est FLAC3D et il est basé sur le modèle des
différences finies. Le modèle mathématique intègre la géometrie et les conditions limites du problème, le profil du sol avec leurs
propriétés physiques, mécaniques et de déformation, ainsi que leurs lois de comportement et les conditions initiales de la région
examinée. Une attention particulière est donnée à la simulation d’un chargement harmonique vertical imposé à la colonne vibrante, à
un chargement équivalent vertical statique, et par la suite, à une pression équivalente radiale exercée sur l’intérieur de l’excavation
cylindrique de la colonne ballastée construite. Les résultats démontrent clairement l’interaction prononcée du système complexe, qui
justifie aisément le grossissement du diamètre construit par rapport au diamètre théorique conçu lors du dimensionnement du projet.
KEYWORDS: stone column, excavation, multi-stage backfilling, Flac3D, interaction, complex system, diameter.
5 INTRODUCTION – SCOPE OF THE WORK
The present work focuses on the investigation of kinematic and
strain interaction of a complex system consisting of a single
column stone and the surrounding soil, during the excavation
stage and the backfilling stage with crushed gravel.
The scope of this work is the investigation and a possible
explanation of the problem concerning modification of the
constructed stone column diameter, versus the theoretical
(design) one, taking into account the procedure of the stone
column construction, its geometrical characteristics and the
geotechnical model representing the surrounding soil and its
physical, deformational and mechanical properties.
In the framework of this work, a summary of geological,
geophysical, geotechnical and seismological data are presented
in a succinct way in the following chapters, for the examined
area, based on a number of corresponding projects performed in
the recent past. After a short technical description of the stone
column constructing procedure adopted for this project, the
numerical model is determined and numerical analyses results
are presented, in an attempt to explain the deduced discrepancy
between “constructed” and “designed” stone column diameter.
The examined area is located in the wide bed of a river in
northern Greece, prone to liquefy, where a bridge is founded.
2.
GEOLOGICAL AND SEISMOLOGICAL
DESCRIPTION OF THE SITE
According to geological and geotechnical data, resulting from
preceding investigation projects on this area, the surface is
covered by deposits that belong to the Quaternary and is
subdivided into: a) river deposits (RD) consisting of silty sands,
clay-silty sands, gravels and locally cobbles of gneiss or marble,
and b) alluvial deposits (AL), consisting mainly of sands with a
largely fluctuating percentage of clays, silts and gravels, of a
thickness ranging from 12 to almost 55m.
The geological bedrock of the examined site consists of
rocks of the alpic age and belongs to the Rodopic Mass,
consisting mainly of biotitic gneisses (gn) interpolated by
amphibolites and marbles green-gray coloured. The upper part
of the gneissic rockmass appears intensively weathered to
totally weathered, consisting thus the weathering zone of 2 to
4m of thickness. The permeability of different geological
formations is quite heterogeneous: the riverbed deposits, mainly
gravel consisting (RDg) are a rather permeable soil formation
(k
), whilst alluvial deposits present a rather low
permeability (
sec /
10
3
m
10
sec).
/
10
5
7
m k
As for the seismological data, the examined site belongs to
zone I of low seismic hazard, with a horizontal free-field peak
ground acceleration value: a
max
=0.16g, according to the most
recent Hellenic map of seismic zones, valid from 1/1/2004.
3. GEOTECHNICAL CHARACTERIZATION
According to the entity of the geotechnical and geophysical
investigation programs performed on the broad area
(geotechnical boreholes, CPTs and Cross-Hole tests), it results
that the prevailing soil formation are alluvial deposits consisting
2. Geologicaland seismiological
Description of the ste
