969
A model study of strains under footings supported by floating and end-bearing
granular columns
Une étude sur modèle réduit des contraintes sous semelles isolées reposant sur des colonnes
granulaires flottantes et encastrées
Tekin M.
GEOMED, Geotechnical Consultancy, Investigation Supervision & Trd. Co.Inc. Ankara, TURKEY
Ergun M.U.
Civil Engineering Department, Middle East Technical University, Ankara, TURKEY
ABSTRACT: A model study was performed in order to examine settlements in the presence of floating and end-bearing granular
columns and without columns (untreated) under D=100 mm and D=200 mm circular loading plates (footings). It is aimed to find
effective length in floating type granular columns that provides significant settlement improvement under footings. In addition to total
footing settlements recorded by displacement transducers, subsurface displacements both along the column and in the untreated soil
below column were measured by miniature borehole settlement gages for different column lengths and loading plate diameters.
Settlements and strain distributions obtained from displacement measurements with depth show the role of column length in
settlement reduction.
RÉSUMÉ : Une étude sur modèle réduit a été réalisée pour examiner les tassements du sol en présence ou non de colonnes
granulaires flottantes et encastrées, sous des plaques de chargement circulaires de diamètre D=100 mm et D=200 mm (semelles
isolées). Le but est de trouver la longueur optimale des colonnes granulaires flottantes permettant d’obtenir une amélioration
significative des tassements sous semelles. Pour différentes longueurs des colonnes et différents diamètres de fondations, les
tassements des semelles ont été enregistrés à l’aide des transducteurs. En plus de ces enregistrements, les déplacements en profondeur,
le long des colonnes et en dessous de celles-ci ont été mesurés par des gauges miniatures. Les tassements et les distributions des
contraintes déduits des enregistrements de déplacements en profondeur ont montré le rôle de la longueur de la colonne dans la
réduction des tassements.
KEYWORDS: model test, granular column, floating stone column, settlement, subsurface displacement
1 INTRODUCTION
Granular columns as semi-rigid inclusions are used to reduce
settlements under foundation loads in compressible soils. The
column length is a significant design decision in settlement
reduction in deep compressible soils.
An experimental research was performed to find out an
effective length in floating type columns to get benefit of them
as settlement reducers.
2 LABORATORY TESTS
2.1 Material Properties
The model tests were carried out in loading tanks designed as
large oedometers (d=410 mm, h=390 mm). The clay
‘foundation’ soil loaded in model test was obtained by
consolidating commercially available remoulded kaolinite type
clay from paste (moisture content w=42 %) inside the loading
tank under a pressure of
v
=50 kPa. Kaolin clay has a plasticity
index of PI=22 % (liquid limit, LL=51% and plastic limit,
PL=29 %). Plasticity of kaolin eliminates swelling and
shrinkage problems.
Foundation soil has an average undrained shear strength of
c
u
=25 kPa. Coefficient of compressibility, m
v
values for the
pressure intervals in main tests are given in Table 1.
Granular columns with a relative density of 80 % were
formed by compacting sand material. Grain size distribution of
the sand has been arranged based on Dc/D (column
diameter/typical particle size) ratio in stone column
applications. The grain size distribution and physical properties
of sand are shown in Table 2 and Table 3 respectively.
Table 1. Laboratory consolidation test results of clay foundation soil
Test No
50-75 kPa
mv (m
2
/kN)
75-100 kPa
mv (m
2
/kN)
Test No:1
Test No:2
Test No:3
0.000409
0.000442
0.000324
0.000345
0.000363
Table 2. Grain size distribution of sand used in granular column
Sieve
D(mm)
Percent finer than D(%)
50
70
100
200
325
0.315
0.210
0.149
0.074
0.035
100
70
38
6
0
Table 3. Physical properties of sand with relative density of 80%
USCS
e
min
e
max
G
s
SP
0.961
0.581
2.683
42
2.2 Experiment Set-Up and Test Procedure
Following the completion of consolidation inside loading tank,
clay foundation height is fixed to H=290 mm by removing some
soil at the top. To form granular columns, soil was drilled by 20
mm auger to the desired depth with the aid of steel template
placed on soil and then the bored hole was filled with sand and
