Homework Writing
Expansive Soils
19 November 2020
Expansive
Soils
Abstract
Expansive soils are the type of
soils that when water is added in them, they expand and shrink when they dry
out. These types of soils may also be referred to as shrink-swell soil. The
expansion and shrinking of these types of soils leads to changes in volume of
the soils that in turn results to damages on the building foundations that may
further cause more damages to an entire building or structure. The main focus
in this paper is, therefore, to discuss different ways that should be adopted
to control soil expansive. This will include giving a definition of expansive
soils, describing the behavior of this type of soil and the damages it may
cause. In addition, giving solutions, methods of improvement and treatment of
expansive soils to control or eliminate the damages that may result from this
type of soil. Finally, the paper will also discuss design strategies for
expansive soils.
Keywords:
Expansive soils, Swelling, Shrinking.
Introduction
As
defined earlier, expansive soils are the type of soils with the ability to
expand or shrink depending on the content of water in them. The mineral
composition of this type of soil is responsible for the characteristics of
these soils in such a way that they determine the amount of moisture that can
be retained in the soils. Soils with clay mineral composition have great
ability to retain more moisture and hence show the swell-shrink characteristic.
Expansive
soils changes in volume depending on the amount of moisture in the ground. The
more the moisture the more they expand and shrinks when moisture is reduced.
Soil expansion of more than ten percent usually has consequences on building or
structures. It may cause cracking of foundations, floors or the basement wall
among other damages.
When
they dry up, expansive soils shrink. This shrinkage may cause damages on the
subsidence of a structure or building by removing support on the foundation.
The soils may also crack creating fissures that allows deep penetration of
water in the moisture conditions. This repetitive cycle of swelling and
shrinking of soil exerts stress on structures and buildings that weakens them,
hence causing serious damages.
Due
to the damages that may come from expansive soils, it is important to adopt
effective techniques that will prevent or reduce the risk of damages that may
result from the properties of expansive soils. It is therefore important to
take necessary control measures in construction before and after building as a
way of preventing damages on building. It is also important to adopt design
strategies for expansive soils to prevent their effects.
Behavior of Expansive Soils
Expansive
soils experiences changes in volume depending on the content of water into the
soil. The ability of the soils to swell or shrink depends on the mineral
composition. To be more specific, the expansive property depends on the
percentage of clay in the soil.
Different
types of soils are composed of different materials that give them different
abilities to expand in presence of moisture. Soils with clay minerals are
highly expansive. Some of the highly expansive minerals in clay include the
bentonite, smectite, beidellite, montmoeillonite, attapulgite, vermiculite,
nontronite, chlorite and illite. Soils with large amount of these soils will
have greater ability to expand in presence of moisture.
Expansive
soils absorb and retain water leading to expansion of the soils. During high
moisture, the soils heave. This may cause uplifting or unintended pressure on
the structures and building around the soil. In low moisture, the soils
collapse, releasing pressure on the building and structures.
When
expansive soils gain more moisture, they increase in volume and loss strength.
The soils, therefore collapses weakening building foundations or causing slope
failure. Fluctuation in the amount of moisture in expansive soils causes
repetitive swelling and shrinking of soils. During high moisture, the soils
gain more water and increase in volume. When moisture reduces, the soil shrinks
leading to development of fissures. These fissures take in more water in
another season of high moisture leading to more expansion. This cyclical
swelling and shrinking of soils causes tension on foundations and other
structure on the ground causing damages.
Foundation Requirement in Expansive
Soils
The
first and the most significant thing before commencing construction activities
in areas with expansive soils is to conduct and assessment on the degree of
expansiveness of the soil and the swelling pressure on the intended structure
so as to come up with effective preventive measures for any damages(Chen, 2012).
Due to the risk of damage that may arise from construction on areas with such
soils, there are certain standard requirement for building foundation in
expansive soils. The requirements include:
1. All
exterior wall foundations should not be less than 24 inches and the interior
wall foundation should extend not less than 18 inches in clear grounds with no
obstacles.
2. There
should be adequate support on both exterior and interior walls from continuous foundations.
3. Every
foundation should have a minimal of four continuous reinforcement bars. At
least two of the deformed reinforcing bars of half an inch should be placed
within 4 inches of the top of the foundation and another two deforming
reinforcement bars of the same size should be placed 3-4 inches of the bottom
of the foundation.
4. All
foundations of both exterior and interior bearing walls should be reinforced
through tying them to the floor slabs using reinforcing bars of not less than
half an inch diameter. The reinforcement bars should be placed at an interval
that does not go beyond sixteen inches. The reinforcing bars should up to at
least forty bar diameter into the foundation and the slab.
5. It
is also required that the concrete slab for the floor to be cast over two
layers of two inch sand fill. The concrete should also be enhanced with
moisture barrier membrane of a minimum of six mils that is sandwiched between
the two-two inch layers.
6. Finally,
t is required that the soil below the interior concrete slab of the foundation
to be saturated with clean water up to a depth of 18 inches before pouring the
concrete.
The above requirements help a great deal
to reinforce the foundation to withstand tension that may be exerted on them by
the action of swelling and shrinking of expanding soils.
Damages Caused by
Expansive Soils
Expansive
soils may lead to adverse effects on building and structures build on them.
This because the building and other structure on the ground are always
sensitive to the movement triggered by the swelling and the shrinking of
expansive soils (Krohn & Solson, 1980).
One
common way through which expansive soils damage foundations is through
uplifting caused by increased volume of soil due to increased moisture. This
may cause continuous distress on the floor that causes cracks on the slabs of
the floor. The resultant uplift usually depends on different ways through which
a structure’s foundation is constructed. The movement of soil also causes
distress to the framing of a building and other structures.
Expansive
soils may also cause damages on pipes buried under the ground. Moisture
fluctuation in the ground leading to swelling and shrinking of soil exposes the
pipes to enormous stress. This stress can bend the pipes or sometimes break
them. The swelling can be exacerbated by breakage of the pipes causing
spillage. Such damages on pipelines may cause big shortages of fuel or water
that is transported through the pipes.
Expansive
soils may also may have great damage on concrete devices such as the concrete
ditches. Swelling of the soil leads to cracking and lifting of the concrete
causing serious impairment on its ability to convey runoffs. Subsequently, the
shrinking of the soil leaves an empty space under the concrete. This may provide
channel for erosion since runoffs will flow through the space below the ditch.
Another
damage that may result from these type of soils is on site improvement like
walkways, patios and swimming pools. Such structures are extremely sensitive to
the movement of soil that causes serious cracking or dislocation on the
facilities.
The
soils can cause severe damages to swimming pools and its associate
improvements. They lead to rotation, heaving or cracking of the pools. Cracks
on a pool leads to leakages that may be very expensive to repair. Swelling and
shrinking of soils also affects pavements resting on expansive soils. The
pavements may be dislocated, uplifted or their directions changed.
Another
serious damage that may result from swelling and shrinking of soils is the soil
creep. The continuous swelling and shrinking of soils makes the soil less
cohesive. Due to the force of gravity, such soils in slopes creeps down. This
may lead to injuries, great losses of property or lives.
Expansive
soils can have adverse damages to property and people hence making life to be
uncomfortable. It is therefore important to come up with effective precaution
measures against the effects of expansive soils.
Improvement and Solution Before and
After Building a Construction
1.
Improvement
and Solution Before Building a Construction
When improvement
on grounds with expansive soils, there are ground movements that should be
anticipated. It is, therefore, very important to consult relevant design
professions like geotechnical engineers who will help in determining the
expected risk. This will help to design the construction taking into
consideration effective precautionary measures.
Keeping in mind
that ground movement causing soil creep are more likely to occur at the top or
edges of slopes, it is important to keep costly structures like swimming pools,
spas and other facilities that may be affected by soil movement way from tops
or edges of slopes. This will enable reduce the high cost that would be
required for design and construction in areas that are more likely to face the
effects of ground movement. There are always additional precautions that should
be taken for constructions around slope set-back zones. It is important to
consider strengthening and reinforcing of facilities like decks, pools and
patio slabs that are greatly affected by expansive soil forces. Tilting of
pools may be anticipated after some period of time. It is important that pools
should be designed to allow slight movement without cracking.
It is also very
important to consult experts when designing exterior landscape of a
construction. This is important since different landscape may have different
effect on the soil. Some vegetation and trees with large roots absorb more
moisture from the ground hence leading to shrinkage of soil. On the other hand,
a landscape may also favor retention of moisture by the soil leading to
swelling.
Expansive
soils should not be left to dry out during or before construction. In order to
maintain the moisture in the soil, it should be wetted or covered with plastic.
Sometimes, it is necessary to wet the soil for an extended period of time
before the construction. However, pre-wetting of the soil should only be done
after the consultation of a geotechnical professional.
2.
Improvement
and Solution After Building the Construction
Proper
maintenance of buildings and structures after construction is very important in
preventing the effects of expansive soils. First, it is important to maintain
the required concentration of moisture of the soils in the construction sites. This
may involve moderate and consistent watering of yards to ensure that the soil
does not dry out. More watering of the yard should be done during dry seasons
and reduced in wet seasons in order to maintain the concentration of moisture.
Excess moisture may lead to swelling up of the soil while reduced moisture will
result in shrinking of the soil.
It is also
important to repair any water leaks around constructions located in areas with
expansive soils. Water leakages when not stopped in the right time, they will
increase moisture content of the soil leading to swelling up. This may lead to
land movement that may cause damages to the construction.
Efforts of
preventing effects of expansive soils should not only be confined within the
construction structure but also to other adjacent lands especially slopes.
Adjacent slopes should be well maintained to prevent soil movement. They should
be irrigated during dry seasons to prevent the soil from drying up.
Proper
maintenance that involves repairs of building and structures in expansive soil
areas should be observed. For instance, cracks that may develop in pool decks
and patio slabs should be repaired in time to prevent water penetration that
will increase moisture concentration in the soil.
Proper drainage
should be maintained around the construction. Water should not collect in area
that will increase moisture in the expansive soils around the construction. The
modification of surface drainage pattern and the ground level should only be
done after the consultation of design or construction professionals.
A building or
any other structure in expansive soils regions should be regularly inspected.
This is important so as to ascertain whether there are any effects caused by
expansive soils in order to apply appropriate maintenance required. Special
attentions on the building and structures should be put on perimeter walls and
areas near slopes. Regular inspections will enable identification of areas of
weakness and application of the relevant maintenance.
Field and Lab Tests for
Expansive Soils
Expansive soils may be identified either
through field or laboratory testing.
1.
Laboratory
Testing
Expansive soils
can be tested using three methods namely; Mineralogical Identification, Direct
measurement and indirect methods.
Mineralogical
identification. It is an effective laboratory test
method for identification of the basic properties of clay soils. However, this
method is uneconomical and impractical for engineering practice.
Direct
measurement. In this method, the soil type and its
mineral composition can be identified using both laboratory tests and field
observation. This method is considered to be the most useful for engineering
(Jones & Jefferson 2012). The method is however, expensive and time
consuming. It requires expensive laboratory equipment and trained personnel to
perform the test.
Indirect
method. This method requires that one understand the type
of soil and its mineral composition in order to identify its properties.
Through this method, one can come up with appropriate methods that can be used
to alter the behavior of the soil.
2.
Field
testing of Expansive Soils
Expansive soils
can also be identified through field testing. Field testing involves direct
observation of the soil and testing its physical characteristics like the
texture of the soil, permeability and how it behaves with moisture and when it
dries up.
Design
Strategies
Contemporary,
expansive soil is a big concern in construction. It is important to come up
with effective control strategies for the effects of expansive soils. In order
to overcome the effect of expansive soils, pre-wetting of the soil before
construction may be necessary before placing the foundation. This will help to
minimize further expansion pressures on the foundation and the building or
structure to be constructed.
Another alternative strategy recommended
by soil engineers is that the upper level of expansive soil in a construction
site be removed and replaced with material that are non-expansive(William &
Donaldson, 1980). Such materials are usually compressed to create a stable
layer for construction. In some cases, expansive soils may be mixed with
non-expansive material depending on the potentiality of the soil to expand.
This helps in reduction of the expansive of the property up to a desirable
level.
There are also
design strategies on the effects of expansive soils on building and structures
that have already been built. In case of movement of the foundation of an
existing building or structure due to swelling or shrinking of soil repair
designs can be employed. These design include making thicker slabs, deepening
the footings and increased reinforcement in all concrete foundation. In most
cases, underpinning technique is applied on the building to transfer the load
of the building to deeper and more stable soils. There are different methods
used for underpinning including concrete piers, use of grade beams, screw
anchors, pipe piles among other systems used.
Conclusion
As it has been
discussed in this paper, expansive soils may have adverse effects that causes
much losses of property or even life. It is, therefore, very important to
adhere to the design strategies developed by qualified construction and soil
experts in controlling the expansive soils. Before commencing any construction
activity, one should make sure that they have considered all factors necessary
for controlling the expansive soils. Necessary precautions should not only be
focused on the construction period but also proper maintenance should be
observed on existing structures.
Reference
Williams, A. A.,
& Donaldson, G. W. (1980). Building on expansive soils in South Africa:
1973-1980. In Expansive Soils (pp. 834-844). ASCE.
Jones, L. D., &
Jefferson, I. (2012). Expansive soils.
Mitchell, P. W.
(1980). The structural analysis of footings on expansive soil. InExpansive
Soils (pp. 438-447). ASCE.
Chen, F. H. (2012). Foundations on expansive soils (Vol. 12). Elsevier.
Miller, D. J. (1997). Expansive soils: problems and
practice in foundation and pavement engineering. John Wiley & Sons.
Krohn, J. P., &
Slosson, J. E. (1980, June). Assessment of expansive soils in the United
States. In Expansive Soils (pp. 596-608). ASCE.