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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Engineering properties of an expansive soil
Propriétés mécaniques d'un sol gonflant
Azam S., Ito M., Chowdhury R.
Environmental Systems Engineering, University of Regina, Regina, SK, Canada
ABSTRACT: The engineering properties of an expansive soil were investigated using in situ and compacted samples. The soil water
characteristic curve was found to have a bimodal shape composing of two air entry values: an initial low value corresponding to
macroporous drainage followed by a high value related to microporous flow. Likewise, the shrinkage curve was found to be S-shaped
and included a low structural shrinkage followed by a sharp decline during normal shrinkage and then by a low decrease during
residual shrinkage.
RÉSUMÉ: Les propriétés mécaniques d'un sol gonflant ont été étudiées à l'aide d'échantillons compactés obtenues sur le terrain. La
courbe caractéristique en eau du sol s'est révélé avoir une forme bimodale avec deux valeurs d'entrée d'air: une valeur initiale
correspondant à un faible drainage macroporeux suivi d'une valeur élevée par rapport à l'écoulement microporeux. De même, la
courbe de rétrécissement est en forme de S et inclus un faible rétrécissement structurelle suivie d'une forte baisse pendant le
rétrécissement normal, suivie d’une une diminution faible durant le rétrécissement résiduel.
KEYWORDS: expansive soil, soil water characteristic curve, shrinkage curve.
1 INTRODUCTION
The capital of Saskatchewan, Canada, is founded on a glacio-
lacustrine clay deposit that exhibits significant volume changes
due to seasonal weather variations. Alternate swelling and
shrinkage in the expansive Regina clay has impaired civil
infrastructure such as transportation networks (Kelly et al.
1995), residential, industrial, and commercial facilities (Ito and
Azam 2010), and water supply and sewage collection systems
(Hu and Hubble 2005). Damages to engineered facilities are
clearly manifested in the form of differential heave in roadways
and sidewalks, inclined cracking in slab-on-grade basements
and masonry walls, and fatigue and breakage in underground
storage tanks and buried pipelines. The associated repair cost is
usually quite enormous. For example, the breakage rate in the
850 km long water supply network in the city has now reached a
30-year maximum of 0.27 breaks/km/year, costing more than $2
million in annual maintenance. Furthermore, the city is
currently going through a period of infrastructure development
including mega-projects such as the Global Transportation Hub
and the Downtown Covered Stadium with a 55000-seat
capacity. Clearly, there is a need to study site-specific soil
properties for the continuous maintenenace and improved
design of civil infrastructure systems in Regina.
Generally, volume changes in expansive soils are derived
from clay minerals that undergo hydration due to rainfall and
dehydration due to evaporation. This process is governed by the
attraction of bipolar water molecules to the negatively charged
clay particles possessing high specific surface areas (Mitchell
and Soga 2005). However, water access to individual clay
particles primarily depends on the following two factors: (i) soil
structure (mircopores within soil peds and macropores between
the soil peds) and (ii) soil state (void ratio and degree of
saturation). These parameters are respectively governed by
parent geology and construction practices prevalent in an area.
Recent research on local expansive clays has focused on the
determination of unsaturated soil properties using “undisturbed
samples” from the geological deposit (Azam and Ito 2011) and
on the correlation of these properties with morphological
observations using “cryogenic specimens” in a scanning
electron microscope (Ito and Azam 2013). These studies
concluded that the geologically-induced soil structure governs
the water migration and the swell-shrink patterns through the
expansive clay. The present study extends the current body of
knowledge to compacted soils thereby capturing the effect of
soil state on the the properties of local clays. Overall, a
generalized theoretical framework is developed to understand
the behavior of expansive soils.
The main objective of this paper is to understand the
engineering properties of Regina clay using in situ and
compacted specimens. Geotechnical index properties were
determined for preliminary soil assessment. The soil water
characteristic curve (SWCC) was determined to investigate the
water retention capacity of the soil. Likewise, the shrinkage
curve was determined to correlate volume changes with soil
saturation and desaturation.
2 RESEARCH METHODOLOGY
The expansive clay was retrieved from a local soil deposit that
was found to be desiccated in early Fall and exhibited extensive
fissuring oriented in all directions. High quality undisturbed
samples were obtained using the ASTM Standard Practice for
Thin-Walled Tube Sampling of Soils for Geotechnical Purposes
(D1587-08) from a depth of 0.6 m to 1.2 m. Likewise, disturbed
samples were obtained from bore cuttings according to the
ASTM Standard Practice for Soil Investigation and Sampling
by Auger Borings (D1452-09). All of the specimens were
plastic-wrapped and wax-coated and the entire collection was
transported and stored at the University of Regina as per the
ASTM Standard Practice for Preserving and Transporting Rock
Core Samples (D5079-08). The latter samples were compacted
in acordance with the ASTM Standard Test Methods for
Laboratory Compaction Characteristics of Soil Using Modified
Effort (D1557-12).
