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Effect of wetting- drying cycles on CBR values of silty subgrade soil of Karaj railway
Effet des cycles d’humidification et séchage sur les valeurs CBR des sols de limoneux de fondation
de la voie ferrée Karaj
Moayed R.Z., Lahiji B.P.
Imam Khomeini International University, Qazvin
Daghigh Y.
Azad University of Karaj, Karaj
ABSTRACT: In this research we have investigated the effect of lime-microsilica additive as a modern additive stabilizer on a silty
soil and have evaluated the wetting- drying cycles on it. Thus, for this purpose and also to observe their usage on a practical project,
we have taken some samples from bed soil of a region of Karaj railway in Iran, to improve its strength and use it as a railway
subgrade. Lime and microsilica in different percentage of dry soil weight were mixed with the soil at the soil optimum moisture. Then
after 28 days curing time, to create saturated condition, they were put in water for 96 hours under a surcharge load of 10 pound (4.5
kilogram). Then California Bearing Ratio (CBR) tests were conducted in order to find the best additive that have the maximum effect
on soil strength. In the next step, to observe the effect of wetting- drying cycles on the stabilized soil, several specimens which shows
the desired CBR value (from an economic and resistance viewpoint) were rebuilt and were exposed to wetting- drying cycles. Results
showed that the CBR values were greatly increased as the soil was stabilized with lime- microsilica additive. In addition, an increase
on the CBR values of the stabilized soil by wetting- drying cycles was observed. Results showed that lime- microsilica additive can
successfully be considered as a suitable option to stabilize silty soils.
RÉSUMÉ : Dans cette recherche, nous avons étudié l'effet d’un additif de chaux et microsilice en tant que stabilisateur moderne sur
un sol limoneux et avons évalué l’effet des cycles de humidification-séchage. Ainsi, dans ce but, et afin d'observer aussi leur
utilisation sur un projet concret, nous avons pris des échantillons de sol de la région du chemin de fer de Karaj en Iran, pour en
améliorer la résistance et pouvoir l'utiliser comme une plate-forme ferroviaire. La chaux avec microsilice a été mélangée avec le sol à
sa teneur en eau sol optimale à différentes teneurs en pourcentage du poids du sols sec. Puis, après 28 jours de temps de prise, les
échantillons ont été mis dans de l'eau pendant 96 heures sous une surcharge supplémentaire de 10 livres (4,5 kg), afin de créer des
conditions saturées. Des tests CBR tests ont été ensuite effectués afin de trouver le meilleur additif vis-à-vis de la résistance du sol.
Dans l'étape suivante, afin d’observer l'effet des cycles d’humidification séchage sur le sol stabilisé, plusieurs spécimens ayant la
valeur souhaitée de CBR (d'un point de vue économique et mécanique) ont été reconstitués et exposés à des cycles d’humification et
séchage. Les résultats ont montré que les valeurs de CBR ont été considérablement augmentées pour les sols stabilisés avec l’additif
de chaux et microsilice. En outre, une augmentation des valeurs de CBR du sol stabilisé par les cycles d’humidification séchage a été
observée. Ces résultats ont donc montré que l’additif de chaux et microsilice peut avec succès être considéré comme une option
appropriée pour stabiliser les sols limoneux.
KEYWORDS: Stabilization, Lime, Microsilica, CBR, Wetting - Drying Cycles.
1 INTRODUCTION
Increasing the bearing capacity of weak soils is always one of
the most important issues in civil engineering projects
especially in road construction. Silts are one of the problematic
soils which are needed to be replaced with suitable material or
improved by various improvement methods like compaction
and stabilization. Silt is a kind of sedimentary geomaterial
consisting primarily of very fine particles, including fine sand
particles, silt particles, and some clay particles which are often
less than 10% by weight. Silt is a type of transitional soil
between sand and clay. A soil is defined as silt if its plasticity
index is no greater than 10 and the amount of particles greater
than 0.075 mm is no greater than 50% of the total.
Silty soils aren’t considered as suitable materials in civil
engineering projects due to their low cohesion and friction
angel. Using the soils as a road or railway subgrade is generally
not possible without stabilization as their characteristics fall
below the minimum required. Consequently, stabilization is
needed for this kind of soil. Application of stabilizing agents on
soils has a long history. Cement was first used as stabilizing
agent at the beginning of the twentieth century to mix with soils
and form road materials in the United States. Since then, many
other kinds of materials, such as lime (Bell 1996) and special
additives such as Pozzolanic materials like Fly Ash (Dermatas
and Meng 2003), Microsilica (Abd El Aziz 2003), and Rice
Husk Ash (Choobbasti et al 2010), which are as waste material,
may be used for soil improvement. Most of the existing
stabilizers like lime and cement are not much useful for silts, so
the stabilized silts with such kind of stabilizing agents usually
cannot satisfy the requirements of road construction. The
encountered problems mainly are lower early strength, greater
shrinkage, easy cracking, and bad water stability (Bell 1996),
(Sheng and Ma 2001).
Indeed, a successful stabilization method depends on many
factors such as:
(1) Soil type and properties; (2) stabilizing agent; (3)
Stabilizer content; (4) Potential use of the stabilized soil; (5)
Field mixing method; and (6) Economical considerations
(Mohamedzein et al, 2003).
Therefore, new methods are still being researched to increase
the strength properties of silty soils. In this study we evaluate
the feasibility of using stabilized silt with microsilica and lime
for Karaj railway subgrade in Iran.
Microsilica (or silica fume) is one of the by- product
materials which is obtained from silicon material or silicon
alloy metal factories. It was discharged into the atmosphere by
the factories smoke before the mid-1970s. Nowadays each year
nearly 100,000 tons of microsilica is produced on purpose word
wide (Karimi et al, 2011). Iran also has a large amount of
microsilica production. Although the microsilica is a waste
