Foundations On Collapsible and Expansive Soils An Overview
Foundations On Collapsible and Expansive Soils An Overview
Foundations On Collapsible and Expansive Soils An Overview
Abstract: This study discusses various types of problems associated for constructing engineering projects
when the soil consists of collapsible and expansive soils. Collapsible soils and expansive soils causes heavy
damages to civil engineering structures like buildings, roads, runways, pipe lines etc., These soils are subject
to changes in volume and settlement in response to wetting and drying and resulting in severe damage to
structures. Expansive soils in India are popularly known as Black cotton soils. Black cotton soils accounts
about 20% of land area in India and are predominantly located in the Deccan trap covering the states. Most
Indian Black cotton soils are rich in Montmorillonite, this mineral is responsible for swell-shrink behaviour
of the soil. Collapsible soils make the construction of foundations extremely difficult in its natural state. It
may cause high differential movements in structures through collapse settlement. Therefore, to study soil
replacement technique taking into consideration geotechnical requirements and cost to achieve the optimum
replacement layer thickness corresponding to minimum cost of foundation works and reduce the collapse
settlement under foundation as well.
Keywords: Expansive soils, collapsible soils, problems, soil improvement methods, re-compaction, soil
replacement techniques.
I.INTRODUCTION
Soils are unconsolidated materials, composed of solid particles, produced by the disintegration of
rocks. Soils are formed by weathering of rocks due to mechanical disintegration or chemical decomposition.
Physical disintegration or mechanical weathering of rocks occurs due to the physical processes like temperature
changes, wedging of ice, spreading of roots of plants, abrasion. In all the processes of physical disintegration,
there is no change in the chemical composition. The soil formed has the properties of the parent rock. Coarse
grained soils, such as gravel and sand, are formed by the process of physical disintegration. When chemical
decomposition or chemical weathering of rocks takes place, original rock minerals are transformed into new
minerals by chemical reactions. The soils formed do not have the properties of the parent rock. The chemical
processes generally occur in nature are hydration, carbonation, oxidation, solution, hydrolysis. Chemical
composition of rocks results in formation of clay minerals. These clay minerals impart plastic properties to soils.
Clayey soils are formed by chemical decomposition.
Major soil deposits in India are (1) Alluvial deposits:A large part of north India is covered with alluvial
deposits. The thickness of alluvium in the Indo-Genetic and Brahmaputra flood plains varies from a few metres
to more than one hundred metres. Even in the peninsular India, alluvial deposits occur at some places. (2) Black
cotton soils: A large part of central India and a portion of South India is covered with black cotton soils. These
soils are residual deposits formed from basalt or trap rocks. The soils are quite suitable for growing cotton. (3)
Lateritic soils: Lateritic soils are formed by decomposition of rock, remov4al of bases and silica, and
accumulation of iron oxide and aluminium oxide. The presence of iron oxide gives these soils the characteristic
red or pink colour. These are residual soils, formed from basalt. Lateritic soils exist in the central, southern and
eastern India. (4) Desert soils: A large part of Rajasthan and adjoining states is covered with sand dunes. In this
area, arid conditions exist, with practically little rainfall. (5) Marine Deposits: Marine deposits are mainly
confined along a narrow belt near the coast. In the south-west coast of India, there are thick layers of sand above
deep deposits of soft marine clays.
Geology and climate play significant roles in the distribution of these problematic soils. Soils capable
of expansion can occur in both tropical and arid climates; however, those located in arid and semi-arid regions
are subject to more extreme cycles of expansion and contraction than those located in more consistently moist
areas. Collapsible soils are most often encountered in arid climates, where wind and intermittent streams deposit
loose sediment (Mulvey, 1992, Rollins et al., 1992).
Expansive and collapsible soils are some of the most widely distributed and costly of geologic hazards.
These soils are subject to changes in volume and settlement in response to wetting and drying, often resulting in
severe damage to structures.
II.EXPANSIVE SOILS
Some soils undergo slow volume changes when change water content that occur independently of
loading and are attributable to swelling or shrinkage. These volume changes can give rise to ground movement
which can cause damage to low-rise buildings that they don’t have sufficient weight to resist. These soils also
represent a problem when they are encountered in road construction, and shrinkage settlement of embankments
composed of such clays can lead to cracking and breakup of the roads they support. Construction damage is
notable, especially where expansive clay forms the surface cover in regions which experience alternating wet
and dry seasons leading to swelling and shrinkage of these soils. The principle cause of expansive soils is the
presence of swelling clay minerals such as Montmorillonite. The potential for volume change in soil is governed
by its initial moisture content; void ratio and vertical stress as well as the amount and type of clay minerals.
Cemented or undisturbed expansive soils have a high resistance to deformation. Therefore, remoulded expansive
soils tent to swell more than undisturbed ones. Expansive or swelling soils usually are those types of soil that
swell when they are subjected to moistures or partially saturated. Also, these types of soils shrink due to losing
moisture contents. Generally, their plasticity indices range high and their bearing capacities differ from when
wetted with when dried. These soils are mostly found in arid and semiarid areas and contain large amount of
clay minerals. Expansive soils are found in large areas of southwest and western United States including
Oklahoma, Texas, Colorado, Nevada, California, Utah, and others. These soils are also found in large areas of
India, and Australia (sometimes called black cotton soils), South America, Africa, and the Middle East (Bowels,
1988; Kalantari, 1991; Murphy, 2010).
III.COLLAPSIBLE SOILS
A collapsible soil is also known as loess. It is basically a clayey silt with a metastable structure.
Collapsible soil shows high strength and stiffness at normal water content conditions. On the other hand, upon
wetting it suffers a sudden plastic deformation, i.e., collapse, which could be severe in some cases
(wesigeotechnica).
Collapsible soils consist of loose, dry, low-density materials that collapse and compact under the
addition of water or excessive loading. These soils are distributed throughout the southwestern United States,
specifically in areas of young alluvial fans, debris flow sediments, and loess (wind-blown sediment) deposits.
Soil collapse occurs when the land surface is saturated at depths greater than those reached by typical rain
events. This saturation eliminates the clay bonds holding the soil grains together (Mulvey, 1992). Like
expansive soils, collapsible soils result in structural damage such as cracking of the foundation, floors, and walls
in response to settlement. In one case of soil collapse, 14 houses in a Cedar City, Utah neighbourhood had to be
jacked off their foundations and relocated due to severe settlement (Rollins et al., 1992). Human activities that
facilitate soil collapse include: Irrigation;Water impoundment; Watering the lawn; Changing the natural
drainage; andDisposal of wastewater.
A large part of the central India a part of the south India is covered with expansive soils. Although
these soils are good for growing cotton, they are treacherous for foundations of structures. Heavy damages may
occur to buildings, roads, runways, pipe lines and other structures built on such soils if proper preventive
measures are not adopted. The damages can be prevented to a large extent if the characteristics of the expansive
soil are properly assessed and suitable measures are taken in the design, construction and maintenance of
structures built on expansive soils.
Problems often associated with expansive soils include: Foundation cracks; Heaving and cracking of
floor slabs and walls; Jammed doors and windows; Ruptured pipelines; and Heaving and cracking of sidewalks
and roads.
Geologists work with geotechnical engineers to evaluate soil and rock prone to shrinking and swelling.
These areas are mapped and denoted for their expansion potential. Expansive soil and rock be removed and
replaced with non-expansive materials to provide a suitable foundation for new structures. Expansive materials
can also be chemically treated, preloaded, or prewetted to decrease swell potential (Engineering and
Environmental Geology of Southwestern Utah, Utah Geological Association Publication 21, K.M. Harty, Editor,
1992.)
Fig 3: Photograph of expansive soils under very dry conditions. Severe soil shrinkage has resulted in desiccation
cracks
A collapsible soil suddenly decreases in volume when it becomes saturated. Collapsible soils are
generally aeolian (wind-deposited) soils which have low water content and high void ratio in natural state. Such
soils usually have a honey-comb structure in which porous structure is maintained by a water-soluble
interparticle bond. When the water content of the soil is increased, the interparticle bond is broken and the soil
mass suddenly decreases in volume causing its collapse. Buildings and other structures constructed on a
collapsible soil have large settlements causing damage. Roads, highways, pipelines and other utilities
constructed on such soils have maintenance problems (Dr K R Arora).
Human activities that facilitate soil collapse include: Irrigation; Water impoundment; Watering the
lawn; Changing the natural drainage; and Disposal of wastewater.
Geologists work with geotechnical engineers to identify soils prone to collapse and evaluate their
potential to fail under loading and/or saturation. Collapsible soil be removed and replaced with approved and
properly compacted materials. Collapsible materials can also be saturated (hydro compaction) to force the soils
to collapse prior to construction.
Conditions in arid and semi-arid climates favour the formation of the most problematic collapsible
soils. The mechanisms that account for almost all naturally occurring collapsible soil deposits are debris flows,
rapid alluvial depositions, and wind-blown deposits(loess). Collapsible soils are moisture sensitive in that
increase in moisture content is the primary triggering mechanism for the volume reduction of these soils. One
result of urbanization in arid regions is an increase in soil moisture content. Therefore, the impact of
development-induced changes in surface and groundwater regimes on the engineering performance of moisture
sensitive arid soils, including collapsible soils, becomes a critical issue for continued sustainable population
expansion into arid regions. In practicing collapsible soils engineering, geotechnical engineers are faced with (1)
identification and characterization of collapsible soil sites, (2) estimation of the extent and degree of wetting, (3)
estimation of collapse strains and collapse settlements, and (4) selection of design/mitigation alternatives
(SANDRA L. HOUSTON).
For the compressible soils susceptible to wetting to a depth of greater than 1.5m, the following methods are
quite effective.
1. Ponding: In this method, low dikes are constructed around the construction site and the whole site is
flooded with water to cause collapsible of the soil before the construction is started. However, this
method is effective only when there is an impervious stratum beneath the collapsible soil deposit to
prevent seepage.
2. Vibro-flotation: Vibro-flotation is used to compact the collapsible soil before construction. This
method is effective only for free-draining soils.
Soil replacement is one of the most familiar techniques in dealing with collapsible soils. It is
implemented by removing the weak soil and replacing it with a better compacted soil. Unfortunately, the
determination of replacement layer thickness is questionable because it is based on experience (Morsy,Osama,
Bassioni,Hesham, Mostafa,Tareq).
Rollins and Rogers reported that this method offers several advantages, the first that it decreases the
amount of collapsible material in the zone of significant stress, the second that it increases the depth to which
water must percolate before reaching collapsible material, and the third that it decreases the induced stress to
which the collapsible soil is subjected. This reduction in the induced stress many keep the stress below the
critical value necessary to produce significant collapse settlement. In addition, this technique minimizes the
differential settlement under the footing.
Experimental tests had been also conducted on circular footing resting on collapsible soil and water
could infiltrate into soil from the bottom. Using compacted sand cushion with thickness equals twice the footing
diameter resulted in a significant reduction in collapse settlement.
Naema, Ali proved that the improvement of collapsible soils by sand/crushed stone replacement is
possible to control/mitigate their risk potentials against sudden settlement when exposed to water. She also
found that the soil replacement with compacted cohesion less soil reduces the foundation settlement by about
50% and increases bearing capacity by about 100%. The subgrade should be improved with compaction and
pre-wetting before placing the top compacted sand replacement to obtain good results of higher bearing
capacity, and low and uniform settlement. The most effective thickness for the compacted sand layer, within the
tested range, was found to be equal to the plate width.
IX.CONCLUSION
There are many available improvement techniques that can be used for the purposes of increasing
bearing capacity and decreasing settlement of collapsible soil such as soil replacement, prewetting, stone
columns, stabilization with additives and dynamic compaction. Most of researches investigated the effect of
using different soil improvement techniques on increasing soil bearing capacity and /or decreasing the expected
settlement while, there is a lack of researches which consider the cost of foundation works as one of the
governing factors when selecting between different soil improvement techniques. Using compacted sand
cushion with thickness equals twice the footing diameter resulted in a significant reduction in collapse
settlement.Determining of physical properties such as dry density and liquid limit of soils can be helpful to
identify collapse potential of soils. High Expansive Black cotton soil can be effectively utilized as a Geo
technical material by addition of 30-40% fly ash and 6-10% cement. At these dosage of admixtures, the Black
cotton soil can be behaving non-plastic and non-swelling can reduce the problems of volume change (Jaya
Prakash Babu.V). According to Miss Kapilani S. Gaikwad, Lime-stabilization of geo-materials by producing
cohesive materials in the soil increases the strength and decreases materials plastic properties. This is why these
materials can be used for projects where high strength and high-performance materials are desirable. The
increase in strength of lime stabilized materials in compression as well as in tension is attributed to the reactions
between clay particles and lime. The clay content of lime-stabilized materials can affect the strength of the
materials. The clay–lime compound provides the cemented material in soil.
X.ACKNOWLEDGEMENT
Greatly thanks to Dr G Sivanatha Reddy MSc, PhD and M V Ravi Kishore Reddy, Ph.D. Student,
Geotechnical Engineering Laboratory, Earthquake Engineering Research Centre, IIIT Hyderabad for their
contribution.
XI.REFERENCES
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Abbas, Iran
[2] Soil Improvement Techniques of Collapsible Soil, Morsy, Osama, Bassioni,Hesham, Mostafa,Tareq
[3] Diversity of Characteristics of Sandy Soils in Relation to Foundation Engineering, Josef Musílek, Petr
Hrubý, OndrejStopka
[6] Geotechnical Properties of Problematic Soils Emphasis on Collapsible Cases, Mohsen Rezaei, Rasoul
Ajalloeian, Mohammad Ghafoori
[7] Effects of Polypropylene Fibers on the Shear Strength of Sandy Soil, Mousa F. Attom, Adil K. Al-
Tamimi
[8] Engineering Properties of Black cotton soil Modified with Fly ash and Cement Jaya Prakash Babu.V,
Satyanarayana.P.V.V, Surya Manikantha, Abdul Moin
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Mehta, Rutvij J. Sonecha, Parth D. Daxini, Parth B. Ratanpara, Miss Kapilani S. Gaikwad
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HOUSTON, CLAUDIA E. ZAPATA and CHRIS LAWRENCE
[11] Study of black cotton soil characteristics with cement waste dust and lime,J.B.Ozaa