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The Drained Strength of Soft Clays with Partially Penetrating Sand Columns
at Different Area Replacement Ratios
La résistance drainée des argiles molles avec des colonnes de sable pénétrant partiellement
à différents taux de remplacement
Najjar S., Sadek S., Bou Lattouf H.
Department of Civil and Environmental Engineering, American University of Beirut
ABSTRACT: Granular columnar inclusions are generally used to improve the mechanical properties of soft clays. The objective of
this paper is to investigate the long term behavior of clay/sand column composites as represented by the fully drained loading
condition, for cases where the soft clay is reinforced by floating or partially penetrating sand columns. For this purpose, consolidated
drained triaxial tests (CD) were performed on back-pressure saturated normally consolidated Kaolin specimens that were consolidated
and tested at confining pressures of 100 kPa, 150 kPa, and 200 kPa. The sand columns were penetrated to 75% of the depth of the clay
sample to represent a partially penetrating condition, while the main parameter that was varied in the study was the area replacement
ratio which was varied from 7.9% to 17.8% to 31.2%. Results indicated that the positive effects of sand columns on strength are
minimal for small area replacement ratios and increase gradually as the area replacement ratio increases. The average percent
improvements observed for area ratios of 17.8% and 31.2% were 20% and 32%, respectively. These results indicate that partially
penetrating columns may provide effective strengthening for soft clays, provided that a suitable area replacement ratio is adopted in
design.
RÉSUMÉ: L’inclusion de colonnes granulaires est généralement adoptée pour améliorer les propriétés mécaniques des argiles molles.
L'objectif de cet article est d'étudier le comportement à long terme des composites d'argile / sable, représentée par la condition de
chargement complètement drainée, pour les cas où l'argile molle est renforcée par des colonnes de sable partiellement pénétrantes. A
cet effet, des essais triaxiaux drainés consolidés (CD) ont été réalisées sur des spécimens de Kaolin saturés et normalement
consolidés. Des échantillons consolidés à des pressions de confinement de 100 kPa, 150 kPa et 200 kPa ont été testés. Les colonnes de
sable ont été établis jusqu’à à 75% de la profondeur de l'échantillon d'argile pour représenter un état partiellement pénétrant, tandis
que le principal paramètre qui a été modifié dans l'étude était le taux de remplacement qui a varié de 7,9% à 17,8% à 31,2% de la
surface de section du spécimen. Les résultats indiquent que les effets positifs de colonnes de sable sur la résistance sont minimes pour
les petits taux de remplacement et augmentent progressivement avec l'augmentation du taux de remplacement. Les améliorations
observées en moyenne pour des rapports de surface de 17,8% et 31,2% étaient de 20% et 32%, respectivement. Ces résultats indiquent
que les colonnes partiellement pénétrantes pourraient être utilisées pour renforcer les argiles molles, à condition que le rapport de
remplaçant approprié soit choisi dans le design.
KEYWORDS: soft clay, sand columns, stone columns, consoldiated drained triaxial tests, soil improvement, floating columns
1 INTRODUCTION
Granular columnar inclusions in the form of sand
drains/columns or vibrated stone columns are commonly used
to improve the mechanical properties of soft clays. Historically,
experimental research studies have been designed to investigate
the behavior of sand/stone column-reinforced clay systems in
the laboratory using 1-g tests that are conducted in one
dimensional loading chambers (Hughes and Withers 1974, Muir
Wood et al. 2000, Malarvizhi & Ilamparuthi 2004, McKelvey et
al. 2004, Ayadat and Hanna 2005, Ambily & Gandhi 2007,
Gniel & Bouazza 2009, Murugeson & Rajagopal 2010, and
Fattah et al. 2011).
The limitations of 1-g model tests were recognized by many
researchers who resorted to testing soft clay specimens that
were reinforced with sand/stone columns under triaxial
conditions where the stress state, the drainage conditions, and
the loading rate could be controlled. Examples of such studies
include the work reported in Juran and Guermazi (1988),
Sivakumar et al. (2004), Black et al. (2006), Black et al. (2007),
Andreou et al. (2008), Najjar et al. (2010), Black et al. (2011),
and Sivakumar et al. (2011).
For cases involving sites with deep deposits of soft clay, the
use of sand/stone columns that fully penetrate the soft clay layer
is prohibitive and may not be practically achievable. As a result,
the use of partially penetrating columns is common as a
practical soil improvement scheme.
Current design methods for stone columns do not reflect the
effect of the degree of column penetration in the soft clay on the
response of clay/stone column system. As a result, there is a
need for investigating the behavior of clays with partially
penetrating columns using an experimental framework in which
the stress state and the drainage conditions could be controlled.
The objective of this paper is to investigate the load response
of soft clay that is reinforced with partially penetrating sand
columns in a triaxial framework. The parameters that were
varied in the experimental program are the area replacement
ratio which was varied from 7.9% to 17.8% to 31.2% and the
effective confining pressure which was varied from 100 to 150
to 200 kPa. All tests were conducted using columns that
penetrated the soft clay to a depth that is equal to 75% of the
height of the clay sample. Since the sand columns are expected
to act as drains that will facilitate radial drainage, fully drained
tests were conducted to represent the long term behavior of the
clay/stone column system and to provide an upper bound of the
response for practical loading conditions in the field where the
clay surrounding the columns is expected to be partially
drained.
