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.
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.
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.
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.
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