Expansive Soil Stabilization
Expansive Soil Stabilization
Expansive Soil Stabilization
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ABSTRACT:
Keywords: Soil stabilization, clay soils, expansive soils, lime, hydraulic binders,
bearing capacity, improvement, and treatment
INTRODUCTION
Active clay soils present a problematic challenge for civil and geotechnical
engineers all over the world. Costs associated with the phenomena of swelling and
shrinkage of active clay soils run into several millions of euros annually. Key aspects
that need identification when dealing with expansive soils include: soil properties,
suction/water conditions, water content variations temporal and spatial, (generated by
trees and the seasonal change), and the geometry/stiffness of foundations and associated
structures built on active shrink/swell soils. [13]
The current paper reviews the phenomena of active clays from a mineralogical,
mechanical and especially a geotechnical point of view.
Clay soils exhibit, sometimes, a significant volume change due to the variation
of water content in the mass of the soil, in response to climatic conditions and the action
of vegetation.
These volume changes affect the function of the constructions and foundations
in contact with the soil and they represent the causes of damage, especially intense,
during periods of drought. [4]
17th International Multidisciplinary Scientific GeoConference SGEM 2017
The definition of clay is diverse. Variations exist among different fields even
though fundamentally there is much in common among the visualized materials. The
definitions so far used may be grouped into three kinds as follows (summary from Sudo
et. al, 1981) [11]
Clay comes predominantly by alteration of rocks and it is in perpetual evolution
on a geological time scale, passing from one to another clay mineral (from smectite to
illite, for example) and changing its initial properties depending on the environmental
factors.
Clays have been defined on the basis of an assembly of certain specific
characters such as plasticity, small particle size, hardening on firing, and chemical
constitution (i.e. as consisting largely of silica, alumina and water). This definition is
applied in a broad sense.
The definition of clays that has been given in terms of the relative proportion of
the clay fraction in rocks or soils (Clay-area designated on the triangular diagram with
the component proportions of sand and silt) to the fractions of sand and silt is, however,
inconsistent from Sudo et. al point of view.
The most important grain property of fine-grained soils is the mineralogical
composition. If the soils particles are smaller than 0.002mm, the influence of the
gravitational force on the particles is insignificant compared to the electrical force
acting on the surface of the particle. (Terzaghi et al.) [14]
All the clay minerals are crystalline hydrous aluminosilicates having a lattice
structure, similar to the pages of a book, in which the atoms are arranged in several
layers. The colloidal particles of soil consist primarily of clay minerals that were
Section Exploration and Mining
derived from rock minerals by erosion, but that have differing mineralogical structure
from those of the initial minerals. [12]
Three important structural groups of clay minerals are described for engineering
purposes as follows:
• Kaolinite group – generally non-expansive
• Mica-like group – includes illites,vermiculites and chlorites, which can be
expansive, but generally do not pose significant problems.
• Smectite group – includes montmorillonites, which are highly expansive and are
the most troublesome clay minerals. (Nelson et al. 1992) [6]
It is well known that if the moisture content of the clay remains the same, its
volume won’t change, hence the structures built on clays with constant moisture content
will not be subjected to heave stress.
Compared to the typical three-phase (solid particle, water and air) structure of
the general soil groups, clay soil has a thin layer of water bounded to its clay mineral
surfaces.
The bound water is comprised of water molecules that are oriented due to the
charged mineral surface. These molecules cannot rotate as freely as the bulk pore water
under alternating electric field, resulting in less polarization compared with that of “free
water”, and a lower measured dielectric constant. The volume fraction of bound water is
determined by the soil specific surface and soil dry density (Stanciu 2006) [1].
This property is linked to their chemical activity and common cation substitutions. The
ions or molecular species are attracted by a charged surface of clay particles (process of
adsorption) or to the internal surface of clay particles (process of absorption) (Velde,
1995).
Clay mineralogy is one of the properties that influence the volume change in
clay soils. Montmorillonites, Vermiculites and some mixed layer minerals typically give
shrink-swell potential to clayey soils, in opposition to Illites and Kaolinites that are
infrequently expansive, though, they can cause volume changes when particle sizes are
extremely fine (less than a few tenths of a micron). (Mitchell, 1976) [15]
Soil water chemistry – swelling is repressed by increased cation concentration
and increased cation valence. Mg2+ cations in the soil water would result in less
swelling than Na+ cations. (Mitchell, 1976) [15]
Soil suction – Soil suction is an independent effective stress variable, repressed
by the negative pore pressure in unsaturated soils. Soil suction is related to saturation,
gravity, pore size and shape, surface tension and electrical and chemical characteristics
of the soil particles and water. (Miller, 1992) [6]
Plasticity – In general, soils that exhibit plastic behavior over wide ranges of
moisture content and that have high liquid limits have greater potential for swelling and
shrinking. Plasticity is an indicator of swell potential.
Soil structure – Flocculated clays tend to be more expansive than dispersed
clays. Cemented particles reduce swell.
Dry density – Higher densities usually indicate closer particle spacing, which
may mean greater repulsive forces between particles and larger swelling potential.
(Chen, 1975) [2]
Initial moisture condition – A desiccated expansive soil will have a higher
affinity for water, or higher suction, than the same soil at higher water content, lower
suction. Conversely, a wet soil profile will lose water more easily on exposure to drying
conditions, and shrink more than an initially dry profile.
Moisture variations – Changes in moisture in the active zone near the upper part
of the profile primarily define heave. It is in those layers that the widest variation in
moisture and volume change will occur. (Miller, 1992) [6]
Stress history – an over consolidated soil is more expansive than the same soil at
the same void ratio but normally consolidated. Swell pressure can increase on aging of
compacted clays, but amount of swell under light loading has been shown to be
unaffected by aging. Repeated wetting and drying tend to reduce swell in laboratory
samples, but after a certain number of wetting – drying cycles, swell is unaffected.
(Mitchell, 1976) [15]
increases the strength of the soil, reduces swelling potential without adverse effects.
With modern construction techniques, it is possible to scarify, pulverize, and recompact
the natural soil effectively without substantially increasing the construction cost.
Subgrade stabilization is one of the most common uses of geotextiles. The
process requires extent knowledge of the characteristics of the geotextiles, thus they
must have specific mechanical and hydraulic properties in order to perform properly.
Stone columns are extensively used to improve the bearing capacity of low grade soils.
A stone column is one of the soil stabilization methods that is used to increase strength,
decrease the compressibility of soft and loose fine graded soils, accelerate a
consolidation effect. They are mainly used for stabilization soft soil such as soft clays,
silts and silty-sands. It is believed that this method was used first in France in 1830s.
The columns consist of compacted gravel or crushed stone arranged by a vibrator.
Chemical stabilization practice includes the use of lime and/or other chemicals,
both organic and inorganic to stabilize expansive soils. These products can be cement,
fly ash, and combinations between them in different percentages depending on the
stabilized soil desired characteristics. Other chemicals are sodium or calcium based
compounds (calcium hydroxide, sodium chloride etc) (Chen, 2003) [3]. In general soil
stabilization is a practice used in the field of highway construction and road pavements,
but has been used successfully in the stabilization of the subgrade soil for individual
buildings.
Hydraulic binders:
• Portland Cement
• Lime (Hydrated Lime, Quicklime, Slurry)
• Fly Ash
• Pozzolanic materials
Organic binders:
• Tars
• Bitumen
• Organic resin
• Polymers
The National Lime Association in the USA gives a general context about lime treatment
procedure. Lime in the form of quicklime (calcium oxide – CaO), hydrated lime
(calcium hydroxide – Ca[OH]2), or lime slurry can be used to treat soils. Quicklime is
manufactured by chemically transforming calcium carbonate (limestone – CaCO3) into
calcium oxide. Hydrated lime is created when quicklime chemically reacts with water.
It is hydrated lime that reacts with clay particles and permanently transforms
them into a strong cementitious matrix. (LIME, 2004) [9]
Hydrated lime is the most used in chemical stabilization application, whilst
quicklime stabilization represents only 10 percent of the lime used in stabilization
procedures.
Quicklime is more difficult to use in day to day practice due to its corrosive
properties, increased security measures are required, although in some cases it has been
proven to be more effective than hydrated lime. Quicklime can be effectively used for
the procedure of drying of the soil, due to its strong exothermic reaction in the presence
of water.
Most lime used for soil treatment is “high calcium” lime, which contains no
more than 5 percent magnesium oxide or hydroxide. On some occasions, however,
"dolomitic" lime is used. Dolomitic lime contains 35 to 46 percent magnesium oxide or
hydroxide. Dolomitic lime can perform well in soil stabilization, although the
magnesium fraction reacts more slowly than the calcium fraction.
Nelson and Miller [6] recommend a percentage of 3 to 8% of the weight of the
soil mass of hydrated lime to be added to the top several centimeters of the soil. Ismaiel
(2006) recommends a percentage between 3-5%, American Association of State
Highway and Transportation Officials (AASHTO) percentages of 2, 4 and 6%. Thus,
we conclude that the percentage of lime depends on the properties of the treated soil, the
final destination of the site and therefore the desired change in geotechnical parameters
of the treated soil compound.
The improvement of the geotechnical characteristics of clay soils by lime
stabilization occur in two general steps, from a chemical point of view. On a short term
reaction cation exchange and flocculation are included. Lime interacts with the clay by a
strong alkaline reaction, causing a base exchange, where calcium ions displace the
potassium, sodium and hydrogen cations. This causes a decrease in plasticity of the clay
soils by the appearance of flocculation and aggregation.
On a long term basis the pozzolanic reaction is defined by the appearance of the
silicate rich hydrates groups by the interaction of the calcium from the lime or
quicklime with the clay minerals.
Advantages and disadvantages of different types of lime include: for the dry
quicklime, the first advantage is the economical one. Being more concentrated a smaller
quantity is required in practice. The National Lime Association defines the report of dry
quicklime to dry hydrated lime to be ¾. [9] Disadvantages include special precaution
measures and special attention to ensure adequate water conditions, mellowing and
mixing. On the other side, for the dry hydrated lime, the advantages is that it can be
applied more rapidly than slurry lime, and that it can also dry the soil, but not as
effectively as quicklime. The major disadvantage of hydrated lime stabilization in
practice is the environmental factor. The hydrated lime consist in very fine particles,
17th International Multidisciplinary Scientific GeoConference SGEM 2017
dust can be a problem and renders this type of application in dense populate areas
almost impossible.
The effectiveness of the lime treatment on clay soils is time dependent.
The outcomes of lime stabilization of the soil include:
• Dewatering of the soil as a reaction with Quicklime.
• Decreasing the plasticity index (Ip) and Augmentation of the plasticity limit
(Wp)
• Flocculation – change in grainsize distribution
• Reduction of the maximal Proctor density - Flattening of the Proctor Curve
• Augmentation of the CBR
CONCLUSIONS
We conclude that in general, all lime treated fine-grained soils exhibit decreased
plasticity, improve workability and reduced volume change characteristics. We need to
take into consideration that final aim improving also the strength characteristics of the
soil. It should be emphasized that the properties of soil-lime mixtures are dependent on
many factors such as soil type, lime type, lime percentage and curing conditions.
REFERENCES
[1] A. Stanciu and I. Lungu., Fundatii – Fizica si Mecanica Pamanturilor,
Editura Tehnica, Bucuresti, 2006;
[2] Chen, F. H., Foundations on expansive soils, No. Report Number 12
Monograph, 1975.
[3] Chen, W. F., & Liew, J. R. (Eds.), The civil engineering handbook, pp. 51-1.
Boca Raton: CRC Press, 2003;
[4] Felt, E. J. Influence of Soil Volume Change and Vegetation on Highway
Engineering, Highway Conference of the University of Colorado, 1953, January.
[5] Ismaiel, H. A. H., Treatment and improvement of the geotechnical properties
of different soft fine-grained soils using chemical stabilization, Shaker, 2006;
[6] John D. Nelson, Debora J. Miller, Expansive Soils – Problems and Practice
in Foundation And Pavement Engineering, Department of Civil Engineering Colorado
State University, 1992;
[7] M. Ray Tucker, Clay Minerals: Their Importance and Function in Soils,
1999;
[8] Manoliu I. and Radulescu N., Geotehnica I, Conspress (U.T.C.B.) Bucuresti,
Romania, 2010;
[9] National Lime Association, Lime-Treated Soil Construction Manual - Lime
Stabilization & Lime Modification, Bulletin 326, 2004;
[10] NORMATIV NP 126-2012 “Fundarea Constructiilor pe Pamanturi cu
Uumflari si Contractii Mari.”, INCERC Romania;
[11] Sudo T., Shimoda S., Yotsumoto H., Aita S., Electron Micrographs of Clay
Minerals, 1981;
[12] Velde B., ed., Origin and Mineralogy of Clays, 1995;
[13] Chindris L.C. Analiza fenomenului de contracție-umflare a pământurilor
argiloase: impactul pierderii umidității solului asupra clădirilor ușoare, 2013.
[14] Karl Terzaghi,Ralph B. Peck,Gholamreza - Soil Mechanics in Engineering
Practice
[15] Mitchell, J.K – Fundamentals of soil Behaviour, 1976 – New York