08-11-2012, 05:57 PM
Lime Stabilization of Soils: Reappraisal
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Introduction
Lime, or CaO or CaðOHÞ
2, the burned byproduct of lime stone
(CaCO3), is one of the oldest developed construction materials,
and humans have been using it for more than 2,000 years, when
the Romans used soil-lime mixtures to construct roads. However,
its utility in modern geotechnical engineering applications was
limited until 1945, mostly because of lack of proper understanding
of the subject (Herrin and Mitchell 1961). Today, lime stabilization
of soils is widely used in several structures such as highways, railways,
airports, embankments, foundation base, slope protection,
canal linings, and others. (Anon 1990; Wilkinson et al. 2010). This
prevalent use of lime is primarily because of its overall economy
and ease of construction, coupled with simplicity of this technology
that provides an added attraction for engineers. Several research
studies highlighted the beneficial effect of lime in improving soil
performance.
Materials Used
Primarily, an expansive soil (ES) and a non expansive residual soil
(RS), which represent the extreme soil type, are used in this study.
The expansive soil is a commercially available bentonite. Its liquid
limit and plastic limit are found to be 459.9% and 53.7%. As per
ASTM D2487 (2006b), the soil is classified as clay with high plasticity
(CH). Given its high expansiveness, particle size analysis using
a sedimentation process is extremely difficult. However, given
the very high plasticity characteristic, the soil can be assumed to
have 100% clay-size particles. The BET specific surface area and
cation exchange capacity of this soil are found to be 86:45 sq:m∕g
and 69:12 meq∕100 g, respectively.
Experimental Program and Test Details
To cover a wide range of plasticity, six different soil samples were
constituted by mixing the expansive soil and residual soils in different
proportions, the details of which are given in Table 1. All of
these soils were amended with lime (i.e., 1, 3, 5, 9, and 13% by
weight of dry soil) and cured for varying periods (i.e., immediate,
3, 7, 21, and 28 days), following which different tests were
conducted. Tests under series 1 were carried out to study the influence
of lime on the plasticity characteristics of soils. The swelling
behavior was studied in test series 2 using oedometer swell
tests. Strength improvement attributable to lime amendment was
evaluated through unconfined compressive strength (UCS) tests
in series 3. The microstructural characteristics were examined
through scanning electron micrograph (SEM) and X-ray diffraction
(XRD) under test series 4.
Results and Discussions
Plasticity
Plasticity of a soil is closely correlated with the liquid limit, the
plastic limit, and their derivative, the plasticity index. Therefore,
a detailed study of the variation of these parameters attributable
to lime amendment will assist in understanding the plasticity
behavior of the lime-stabilized soils.
Influence of lime and curing on liquid limit of the soils, 100%ES
and 100%RS, are depicted in Fig. 3, and that of a typical soil mix
(40%ES þ 60%RS) is shown in Fig. 4. The responses of the other
three soil mixes (i.e., 80%ES þ 20%RS, 60%ES þ 40%RS, and
20%ES þ 80%RS) are intermediate to these trends. Initially, all
of the specimens showed a decrease in the liquid limit with an increase
in lime content. This reduction is maximum for the expansive
soil (100%ES) and gradually declines with increased content
of residual soil. With the addition of lime, Caþ ions are released
into the pore fluid. As a result, the electrolyte concentration of
the pore water increases and decreases the thickness of the diffuse
double layer held on to the soil and leading to a lower liquid limit.
For expansive soil, the liquid limit continues to decline until approximately
3% of lime, beyond which it does not change much.
However, with very high lime content (i.e., 13%), when cured long
(i.e., 21 days, 28 days), the liquid limit showed an increasing
trend. This behavior is more prominently observed for specimens
having high percentage of the silica-rich residual soil.