Expansive or swelling soil is a highly plastic soil that normally contains montmorillonite and other active clay minerals. Expansive soil is a commonly identified problem which has made scientists concern about the design, protection, and operating of highway and structural systems. Expansive soils can be found in arid/semi-arid areas, where even moderate expansive soils can cause major damages to the structure or in humid environments where just expansive soils with high plasticity index (Ip) can lead the structure to be damaged. The behaviors of an expansive soil can be affected by many factors, among which the principal ones are the availability of moisture, and the amount and type of the clay-size particles in the soil. It is worth mentioning that when the water changes in expansive soil, the volume would be changed as well. These volume changes can lead to either swelling or shrinkage and that is why expansive soils are also known as swell/shrink soils (Ardani, 1992; Day, 2000; Jones & Jefferson, 2012; Zemenu et al., 2009; Ito and Azam, 2010; Liu et al., 2015). Although expansion can be the result of the chemically induced changes, most of the times soils which have swelling and shrinkage behavior contain expansive clay minerals. It can be resulted in the fact that, the more the clay exists in the soil, the higher the soil swells and the more water the soil can absorb. In addition, the more water they absorb, the more their volume increases. In other words, when the water is attracted by these type of soils, they would get increased in volume and hence they would swell. In contrast, the shrinkage happens when the soil gets dry. Researchers have reported the safe expansion percentage equal or smaller than 10% for most of the expansive clays (Jones & Jefferson, 2012).

In order to determine the soil shrinkage and swelling amount, the water content in the near-surface zone can be measured. Normally, most of the significant action happens in the depth not more than 3 meters, although if the tree roots exist in the area, this amount can be extended. The characteristics of fine-grained clay-rich soils leads them to be capable enough to absorb large amount of water which makes them to be heavy and sticky. On the other hand, these types of soils can become very hard when they dry, which leads them to shrink and have cracks on their body. This procedure is known as "shrinkageswell" behavior (Jones & Jefferson, 2012).

Swelling and shrinkage are not wholly reversible procedure (Holtz & Kovacs, 1981). Shrinkage results in cracks on the body of the soil which cannot close-up completely by re-wetting the soil. Therefore, it makes the soil to bulk-out somehow. In addition, it can help the water to penetrate into the soil more easily during the swelling process. It is worth mentioning that when substances like sediment enters the existing cracks in the soil, the soil is unable to get rid of them and go back to its previous situation, hence it will result in the increase in the swelling pressure. Sometimes, the shrinkage cracks may be filled with sediment which leads to the incompatibility of the soil.

One of the primary problems with the expansive soil is that deformations are considerably greater than the predicted ones which is obtained from the classical elastic and plastic theory (Jones & Jefferson, 2012).

The two major factors in treating expansive soils are: (1) Identifying, and (2) estimating the anticipated potential volume change of the subgrade soils (Ardani, 1992) .

In this paper, firstly the expansive soil is identified comprehensively. Second, the causes and required treatments for expansive soil is discussed.

The success, safety, and economy of the structure is strongly dependent on the situation of the rock and soil at the place as well as the interaction of the ground materials during and after the construction. Large amount of ground deformations is one of the most significant problems in expansive soils which is the result of swelling and shrinkage of the soil. The excessive movements can lead to both damage and negative impact on the structural performance in terms of cost and time (Reddy et al. 2009). Therefore, identifying the expansive soils can help researchers reduce the imposed damages resulted from the expansive soil to the structure.

Identification of the expansive soil can be expressed in the two following categorizations; (1) Those used for mineralogical identification, and (2) those used for direct physical properties (Ardani, 1992). Figure 1 lists the methods used in each group.

Figure 1. Listings of the Methods Used in Each Categorization for Identification of the Expansive Soil

It is worth mentioning that mineral identification methods are not only too time-consuming, but also requiring special expertise and equipment. Therefore, most experts prefer simple identification tests based on physical properties of the soil.

Expansive soils are the ones which generally exhibit a large amount of volume changes due to environmental changes and water absorption of the soil. These types of soils contain clay particles of one or more minerals, which are eager to absorb a large amount of water. When the water is absorbed, these particles grow and hence, the materials containing the clays would have expansion (Ardani, 1992).

The soil mineralogy can be explained as the chemical organization of molecules into sheets that results in having the clay smaller than 0.002 millimetre, which have the ability to swell and shrink easily. The clay particles should be made up of many layers, either 1:1 or 2:1. If they are made up of 1:1 clay layers, they would be slightly expansive. On the other hand, if they are made up of 2:1 clay layers, they would be expansive soils. It is important to mention that the non-expansive clay minerals does not exist. Clay minerals can be defined as hydrous aluminium phyllosilicates minerals that are fine grained with sheet like structures and very high surface areas. The clay minerals are the combination of silcon-oxygen tetrahedral, aluminium or magnesium, and the brucite or gibbsite sheet in the octahedral layer (Lucian, 2008). Figure 2 shows the Basic layer structure of a natural clay mineral (Daulton, 2005).

Figure 2. Clay Mineral Layers (Daulton, 2005)

Investigation have shown that MONTMORILLONITE minerals have the greatest volume change in comparison to all the other clay minerals. This fact can be expressed as the penetration of the water into the interlayer molecular spaces in MONTMORILLONITE minerals. It is worth mentioning that if the sodium exists in these minerals, it can result in the swelling of the clay multiple times its real volume. The effect of MONTMORILLONITE minerals on expansive soils can be presented by the vertical in situ suction profile (Ardani, 1992).

The basic parameters which affect the potential expansiveness of the given soil are: (1) The type and swelling potential of the clay mineral, (2) the moisture content of the soil, and (3) the density of the soil (Ardani, 1992).

In order to classify the shrinkage and swelling, the scientists have done many researches to obtain a universally applicable system. Some have even tried to create a unified swelling potential index using commonly used indices (Kariuki and Van Dur Meer, 2004; Yilmaz, 2006) or from specific surface areas (Yukselen-Aksoy and Kaya, 2010), but these are as yet to be adopted. Figure 3 shows the examples of different schemes which are generally used all over the world.

Figure 3. Commonly used Criteria for Determining Swell Potential from Across the World (Jones & Jefferson, 2012)

Scientists believe that there is still the lack of standard definitions of the swell potential due to the fact that both testing factors and sample conditions vary extensively (Jones & Jefferson, 2012).

The seasonal volumetric change of desiccated soil is complex which enhances with severity of the existing shrinkage (Jones & Jefferson, 2012). The vertical in situ suction profile, water content profile and the degree of saturation reflect this fact as it is demonstrated in Figure 4.

Figure 4. Examples of Total Suction Profile (Fityus et al., 2004)

Here pF is the base 10 logarithm of the water potential in cm.

The relative values of suction are dependent on the composition of the soil, specially its particle size and clay mineral content. The soil hydraulic conductivity may fluctuate both seasonally and over longer timescales. It is worth mentioning that fabric changes, tension cracking, and shallow shear failure during the swelling and shrinkage process which may affect the subsequent moisture movements can lead to the secondary permeabilities (Jones & Jefferson, 2012). Expansive soil problems usually happen because of the changes in the water content in the upper few meters, with deep seated heave being rare. The climate and environmental parameters considerably affect the water content in these upper layers. The water content is generally termed the zone of seasonal fluctuations or active zones as shown in Figure 5 (Jones & Jefferson, 2012).

Figure 5. Water Content Profiles in the Active Zone (Jones & Jefferson, 2012)

In the active zone, negative pore water pressures exist, however, if the surface absorbs extra water or if evapotranspiration is eliminated then water content will enhance and heave will happen. In addition, temperature can influence the migration of water through the zone as it is shown in Figure 5. Therefore, when the site investigation is performed, it is significant to verify the depth of the active zone. The depth of this zone will increase if the drying is greater than rehydration.

"Active Zone" have been explained with different meanings which are explained as the followings (Jones & Jefferson 2012):

• Active Zone

The zone of soil that contributes to soil expansion at any particular time.

• Zone of Seasonal Moisture Fluctuation

The zone in which water content change due to climate changes at the ground surface.

• Depth of Wetting

The depth to which water contents have enhanced because of the introduction of water from external sources.

• Depth of Potential Heave

This is the maximum depth of the active zone. The depth at which the overburden vertical stress equals or is greater the swelling pressure of the soil.

There are many towns, transport routes and buildings which are located on clay-rich soil and rocks. The clay within these materials is so sensitive to shrinkage and swelling which is due to changes in the water content. This can lead to a significant hazard to engineering construction. Change in water content may be due to seasonal effects which are based on rainfall and evapotranspiration of vegetation, or it might be due site changes such as leakage from water supplier or drains, changes to drainage of the surface and landscaping including paving, or following the planting, removal or severe pruning of trees or hedges, as man is unable to supply water to desiccated soil as efficiently as a tree originally extracted it through its root system (Jones & Jefferson, 2012).

The permanent water deficit may grow during a long dry period which leads to the soil higher dry depth than normal and hence, long-term subsidence of the ground. This fact can explain why expansive problems are usually found in arid areas. As this water deficit dissipates, it is possible that long-term heave may occur (Jones & Jefferson, 2012).

Pressure resulted from swelling can cause heaving or lifting, while shrinkage can cause differential settlement. In general, when the soil volume changes are unevenly distributed beneath the foundation, failure occurs. For instance, if the water content changes in the soil around the perimeter of the building, it can lead to the swelling pressure beneath it, although the water content of the soil beneath the center of the building stays the same. This results in failure known as end lift. On the other hand, the opposite situation of the end lift is known as center lift, which swelling happens beneath the center of the structure or which shrinkage occurs under the edges (Jones, 2011) . Both of the cases are shown in Figure 6.

Figure 6. Foundation Movements Which Cause Building Distress

In addition, an example for results in failure known as end lift is shown in Figure 7.

Figure 7. Structural Damage to House Caused by 'End Lift' (Mokhtari & Dehghani, 2012)

Normally, damage foundation due to expansive soil results from tree growth which happens in two general ways,

• Physical disturbance of the ground, and
• Shrinkage of the ground by removal of water.

Physical disturbance of the ground caused by root growth is most of the times seen as damage to pavements and Broken walls. Figure 8 shows an example of vegetation induced shrinkage causing differential settlement of building foundation. Vegetation induced changes to water profiles can also have an important effect on other underground feature, including utilities. Clayton et al. (2010) reported monitoring data for a two-year duration of a pipe in London Clay, which led to understanding the significant activities of the ground. These movements are in both vertical and horizontal direction which generate significantly tensile stresses when in the neighbourhood of trees. Such trees induced movement has the ability to be an important contributor to failure of old pipes placed in clay soils near deciduous trees (Clayton et al., 2010) .

Figure 8. Example of Differential Settlement due to Influence of Trees (Jones & Jefferson, 2012)

Another case is shown in Figure 9 which the damage is caused by upward soil expansion.

Figure 9. Damage to Residential Building Caused by Highly Plastic Clays Shrinking and Swelling with Moisture Changes in the Foundation (Mokhtari & Dehghani, 2012)

In addition, Figure 10 is showing an example of expansive soil problems on the structure of the building.

Figure 10. Major Cracks in Exterior Walls at Doors and Windows (Mokhtari & Dehghani, 2012)

Figure 11 shows another example of the damage caused by expansive soil.

Figure 11. Residual Driveway Damaged by Expansive Soil (Mokhtari & Dehghani, 2012)

5.1 Foundation Options in Expansive Soils

The most common way which foundations can be damaged by is uplift which is caused by expansive soils. Swelling soils lift up and crack lightly-loaded, continuous strip footings, and frequently cause distress in floor slabs. The resultant uplift varies in different areas under the building due to the various building loads on different portions of a structure’s foundation. According to Figure 12, the outer corners of a uniformly-loaded rectangular slab foundation will only exert about one-fourth of the usual pressure on a swelling soil located at the central portion of the slab. Accordingly, the corners tend to be lifted up relative to the central portion. Moisture differentials within soils located at the edges of the slab intensify this incident. Such foundation differential movements can result in the distress to the framing of the structure (Rogers et al., 1993).

Figure 12. A Uniformly Loaded Rectangular Slab which Tend to Lift Up in the Corners Due to Less Confinement (Rogers et al. 1993)

Foundation types and design methods can be affected by a lot of parameters which include climate, financial, legal, and technical issues. It is worth mentioning that swell/shrink behaviour usually does not manifest itself for a long time and so design alternatives must take account of this. Regularly higher initial costs are offset many times over by a reduction in post construction maintenance costs when dealing with expansive soils (Jones & Jefferson, 2012).

Foundation alternatives when encountering the potentially expansive soils follow three choices of:

• Using the structural alternatives, e.g. stiffened raft,
• Using the ground improvement techniques, and
• Combination of 1 and 2 (Jones & Jefferson, 2012).

Two factors are important to be paid attention while dealing with expansive soil:

• Isolate structure from soil movement, and
• Design a foundation stiff enough to resist movements (Jones & Jefferson, 2012).

Scientists have reported major types of foundations used in expansive soils from around the world are pier and beam or pile and beam systems, reinforced raft sand modified continuous perimeter spread footings as are summarized in Figure 13 (Jones & Jefferson, 2012).

Figure 13. Foundation Types Used in Expansive Soils (Jones & Jefferson, 2012)

In addition, a brief description of each is given below.

5.1.1 Pier and Beam/ Pile and Beam Foundations

In this case, for the purpose of supporting the structural loads and transferring the loads to the piers and piles, the foundation should consist of a ground beam. In order to isolate the structure and prevent uplift from swelling between the pier/pile and ground beam, a void is supplied. Floors are then constructed as floating slabs. The piers/piles can be explained as reinforced concrete shafts with or without belled bottoms, steel piles driven or pushed, or helical pile which is used to transfer loads to stable strata. Under-reamed bottoms and helical piers/piles can perform successfully in soils with a high swell potential overcoming the impractical length. Otherwise straight shaft piers/piles would be required in this situation. In addition, under-reamed bottoms and helical piers/piles can be used in the regions where the loss of skin friction due to increasing groundwater levels is possible. Figure 14 illustrates a typical pier and beam foundation from US practice (Jones & Jefferson, 2012).

Figure 14. Illustration of a Pier and Beam Foundations (Jones & Jefferson, 2012)

It is worth mentioning that there should be enough anchorage below the active zone. Pier/pile diameters are kept small, which is typically 300 to 450 mm diameters. Any smaller size will result in poor concrete placement and related defects, e.g. void spaces. Other problems that can occur include 'mushrooming' next to the top of the pier/pile, which if occurs presents additional vicinity for uplift forces to act. Cylindrical cardboard forms are a solution to avoid this in many countries. They are usually employed and removed after the beam is cast to prevent a means to transmit swell pressures. It is important to mention that all the approaches are strong enough to prevent possible pathways to let water to enter to deeper layers as this will cause deep seated swelling (Jones & Jefferson, 2012).

5.1.2. Stiffened Rafts

Usually stiffened slabs are reinforced concrete, although in countries like the United States post tensioned systems are used. Design procedures are explained as determining bending moments, shear and deflections associated with structural and swell pressure loads. Figure 15 shows the general layout used commonly in the United States (Jones & Jefferson 2012).

Figure 15. Typical Detail of a Stiffened Raft (Jones & Jefferson, 2012)

For designing the Stiffened Rafts, the slab should be considered as a beam resting on an elastic medium or loaded plate. This design is modelled on soil-structure interaction at the base of the slab. There are two strict modes of movement:

• Where a ground profile develops assuming a weightless slab, and
• Where a slab of infinite stiffness is placed on the Delling soil.

The actual movement of the slab exhibits some flexibility and hence, is something between these two movements. All the three modes of movements are illustrated in Figure 16 (Jones & Jefferson, 2012).

Figure 16. Profiles After Construction for Various Stiffness of Raft (a) Profile with No Load Applied, (b) Profile with Infinitely Stiff Slab, (c) Profile with Flexible Slab (Jones & Jefferson, 2012)

The required geotechnical information include size, shape, and properties of the distorted soils surface that developed beneath the slab. This is dependent on many parameters such as soil stiffness, heave, initial water content, climate, water distributions, time post construction, loading and slab rigidity. Usually, center of the slab is subjected to severe long term distortion. In this method, the slab through its elimination of evapotranspiration promote the largest increase in water content near to the center of the slab and hence, it can solve the distortion problem of the center of the slab. On the other hand, it is found that the maximum differential heave varies between 33 to 100 % of total maximum heave. When the exterior of a structure experiences water content growth before the interior areas heave can occur on occasion edges (Jones & Jefferson, 2012).

5.1.3 Modified Continuous Perimeter Footing

Whenever expansive soils exists in a region, shallow footing should be prevented. However, if they are used in an area, in order to reduce the effect of swelling/shrinkage; some modifications would be required which are:

• Narrowing footing width,
• Providing void spaces within support beam/wall to concentrate loads at isolated points, and
• Increasing perimeter reinforcement taking this into the floor slab stiffening foundations (Jones & Jefferson, 2012).

Whenever the narrow spread footing is used in expansive soils, the use of it should be limited to soils exhibiting 1% swell potential and very low swell pressures (Jones & Jefferson, 2012).

5.2 Pavement and Expansive Soils

Pavements are vulnerable to expansive soil damage because of their relative lightweight nature extended over a relatively large area. Damage to pavements on expansive soil comes in four major forms:

• Severe unevenness along significant lengths- cracks may or may not be visible (particularly important for airport runways),
• Longitudinal cracking,
• Lateral cracking developed from significant localized deformations, and
• Localized pavement failure associated with disintegration of the surface (Jones & Jefferson, 2012).

Pavement designs are considered based on either flexible or rigid pavement systems (Nelson & Miller, 1992) . However, whenever we want to assume the effect of the expansive soil, a number of issues should be considered:

• Choose an alternative route and avoid path on expansive soil,
• Remove and replace expansive soil with a nonexpansive alternative,
• Design for a low strength and allow regular maintenance,
• Physically modify expansive soils through disturbance and re-compaction,
• Stabilization through chemical additives, such a lime treatment, and
• Control water content changes although very difficult over the life of a pavement (Jones & Jefferson, 2012).

In order to select the most suitable treatment method, it is essential to predict the potential volume change of expansive soil accurately. It is worth mentioning that there is no definite dividing line between the methods of predicting the volume change and some identification methods. To overall, three different techniques are used to predict the soil volume change which are:

• Soil suction test,
• Odometer swell test (consolidometer testing), and
• Potential Vertical Rise (PVR) (Ardani, 1992) .

Once an expansive soil has been known and distinguished using the stated methods, the volume change can be anticipated. These methods are explained by detail in the following:

6.1. Soil Suction Test

As the volume change is considerably dependent on the water content of the soil, soil suction test can be used to estimate this change. In order to apply the theories of the engineering behaviour of unsaturated soils, it is mandatory to measure the soil suction. The mechanism of the soil suction measurement can be explained as evaluating the moisture condition of the unsaturated soils. The filter paper method is both inexpensive and simple laboratory test method which can measure both total and matric suction. If the soil suction test is performed accurate, the initial and final soil suction profiles can be obtained from samples taken at convenient depth intervals. The change in suction with seasonal moisture movement is a significant information for many scientists to predict the potential volume change of the soil (Bulut et al., 2001).

6.2. Odometer Swell Test (Consolidometer Testing)

The one-dimensional consolidometer has been used widely for testing the swelling tests. This test is one of the most popular tests in geotechnical engineering for measuring the consolidation properties of the soil. In order to evaluate the deformation response in this test, different loads should be applied to the specimen. The results of odometer swell test illustrates the deformation of the soil with respect to its change in effective stress.

Although this test is used broadly for measuring the swelling clays, the procedures which are utilized for this test are quite diverse. Fredlund (1969) has mentioned two types of swelling test as the most popular ones;

• Free Swell test (FS), and
• The Constant Volume test (CV) (Fredlund, 1969) .

In the free swell test, the soil volume is permitted to be changed until equilibrium is achieved. The specimen should be both loaded and unloaded in conventional manner. In order to reduce the volume of the specimen to its original volume, the specific pressure is required which is called the swelling pressure of the soil. In the constant volume test, the total stress on the soil specimen is increased after submerging in order to prevent it from volume changing. In this method, the pressure at which there is neither a tendency for enhancing or reducing the volume is termed the soil swelling pressure (Fredlund, 1969).

6.3 Potential Vertical Rise (PVR)

The Potential Vertical Rise method design procedure was initially created by Chester McDowell in 1956. In this method, the anticipated vertical rise is related to both the plasticity index of the soil and the field loading. It can be also identified as the soil potential to swell at a specific loading condition, moisture, and density. Comparison of potential vertical rise with field results illustrates that in most of the cases, the PVR method over predicts the vertical rise that would happen (Zornberg et al. 2009).

The following methods have been presented for the treatment of expansive soil (Ardani, 1992):

• Sub-excavation and removal of expansive soil and replacement with non-expansive soil.
• Application of heavy applied load to balance the swelling pressure.
• Preventing access of water to the soil by encapsulation.
• Stabilization by means of chemical admixtures.
• Mechanical stabilization.
• Pre-wetting the soil.
• Avoiding the expansive soil.

7.1 Sub-excavation and Removal of Expansive Soil and Replacement with Non-Expansive Soil

Sub-excavation and replacement is the result of removing and replacing the expansive subgrade soils. The replaced material should not make problems with respect to the in situ material. For instance, granular soils should never be utilized as backfill for sub-excavation and replacement plans. If the granular material is used in the process, the surface of the underlying in situ materials would absorb the water. It has been reported in a case on Interstate 70 east, between Watkins and byers in Colorado that the performance of this treatment which is the replacement containing 18 to 30 inches of sand has been unsatisfactory. It is important to mention that the backfill materials should always be waterproof and preferably nonswelling. In addition, backfill substances particularly remolded in situ soil, should be replaced and compacted with desirable density and moisture management (Ardani, 1992).

7.2 Application of Heavy Applied Load to Balance the Swelling Pressure

In order to prevent the swelling, method of loading the expansive soil with pressure higher than the swelling pressure can be used. Although pavement loads are not big enough to prevent the expansion, buildings and structures carrying high loads can response to this method effectively. However, this method can be used in highway construction as well which is limited to the swelling soils with low expansive pressures (Ardani, 1992).

7.3 Preventing Access of Water to the Soil by Encapsulation (Catalytically Blown Asphalt Membranes)

Ardani has reported Catalytically Blown Asphalt Membranes as a successful method for minimizing the subgrade moisture variations and the associated volume change of expansive soil in Colorado. Using the asphalt membrane have been admired during the late 1960’s and early 1970’s (Ardani, 1992).

As a direct result of some of the early investigations on asphalt membrane on the experimental project north of Grand Junction, a full size, non-experimental project was constructed in northwestern Colorado during the summer and fall of 1967 (Brakey, 1968).

The catalytically blown asphalt membrane has been used in a project located just west of the village of Elk Springs on US 40 and it was placed on all the bases of the expansive soil cuts. In order to cover two miles of 2-lane roadway, four hundred and forty five tons of catalytically blown asphalt membrane were used in this project. The rate of application has been 1.3 gallons per square yard which is approximately 3/16 of an inch (Brakey, 1968).

Ardani has also reported the placement of the catalytically blown asphalt membrane on a cut base on the Agate- North project, located 65 miles east of Denver on Interstate 70. In order to record the moisture variation under the asphalt membrane and under the control sections, moisture cells were positioned. The results have shown that soil moisture directly under the asphalt membrane has been uniform and staying at optimum, while in the control section it has been seven to eight percent more than the optimum point. It should be mentioned that required care should be taken to provide a uniform and smooth surface prior to the placement of the membrane (Ardani, 1992) .

7.4 Stabilization by Means of Chemical Admixtures

In order to change the characteristics of clay mineral and decrease its potential for swelling chemical admixtures can be utilized. Probable materials for the stabilization could include lime, pozzolana, lime-pozzolana mixture, chemical grouting, cement, resins or fly ash or bituminous material. The choice of a material or a combination of materials depend on the size and importance of the building and economic consideration of the client. However, the need to strike a proper balance between quality and cost should not be overlooked (Lucian 2008) . One of the most effective and economical added materials in stabilizing the expansive soil is lime. The depth of treatment is limited to about 8 to 12 inches by conventional techniques (Ardani, 1992; Calik and Sadoglu, 2014).

There are different methods for stabilizing the expansive soil using lime. Ardani has reported two methods of lime shaft and lime-tilled stabilization as the most practical ones used by Colorado DOT. As lime cannot dissolve easily in water, distributing lime through natural soils with water in drill holes have not been successful. On the other hand, lime till stabilization will seal and decrease the swelling potential successfully, if it is combined with the soil to a desirable depth. There have been a numbers of projects which used this method and stabilized the soil by mixing the lime with soil (1 percent to 5 percent hydrated lime) to depth varying from 1 ft. to 3 ft. (Ardani, 1992).

Scientists believe that more investigation should perform on understanding of lime-soil interaction due to lacking of enough information. There is still not enough knowledge in estimating the depth of treatment for various expansive soils and appropriate quantity of lime and more research should be done in this area. There are many significant variables which affect this issue which are lime type, soil type, lime percentage, and curing conditions including temperature, time, and moisture. Preferably, the investigation should be based on tests that afford fundamental engineering properties rather than empirical test results (Ardani, 1992).

7.5 Mechanical Stabilization

Mechanical stabilization which is also called compaction, is the compression of soil using the application of mechanical energy. Although there would not be much change in water content in this procedure, the densification happens when air is expelled from soil voids. It is important to mention that if significant moisture fluctuations are imposed to these soils, this method may not be valuable. The efficacy of compaction may also reduce with an increase of the fine content, fraction smaller than about 75 μm, of the soil. The reason is that during compaction cohesion and inter particle bonding interferes with particle rearrangement (Little & Nair, 2009) .

7.6 Pre-Wetting the Soil

Expansive soils can be wetted before the construction which leads them to expand prior to anything happens. This method has been used a lot in many parts of the USA for different types of structures. Ponding has been suggested as one of the most effective techniques for accelerating swelling. There is still not enough approaches about how long the material should be ponded and what depth the moisture should penetrate to be effective. It has been suggested that the dry season is the best time for ponding when the natural cracks are open because of the desiccation (Ardani, 1992).

Sometimes the water penetration could be assisted in the problem cuts by drilling a numerous number of small holes into the swelling materials, which would allow irrigation water free access to them. Although, pre-wetting the soil is one of the most economical treatment methods, it is not completely a reliable method because of complexity of distributing the uniform moisture penetration in a reasonable time (Ardani, 1992).

7.7 Avoiding the Expansive Soil

Avoiding the expansive soil is a practical solution instead of having the more favorable subgrade conditions. It is worth mentioning that this can be used only in specific situations as it is strongly related to the local social, environmental, or economic considerations (Snethen, 1979).

Expansive soils are one of the most major ground related hazards found worldwide, contributing billions of pounds annually. Expansive soils are found all over the world and are most probable in arid/semi-arid regions, where their high suctions and potential for large water content changes on exposure/deficient which water can cause extensive changes in soil volume. This paper has reviewed the expansive soil and its causes and treatments comprehensively. Based on other researcher’s work, the following conclusions has been taken out from their investigations.

Removing the expansive soil and replacing with nonexpansive soil can be a reliable cure, however sometimes it is time-consuming and not economical. This method can be valuable in the projects with small amount of expansive soil, but it might cause both problem and a lot of expenses for huge projects.

In addition, Granular soils should not be used alone for subexcavation and replacement of the soil as they lead to collection of water at the surface of the underlying in situ materials.

Due to higher loading pressure of buildings and structures in comparison to pavements, application of heavy applied load to balance the swelling pressure is more effective for buildings and structures. Hence, it is better this method be used mostly for the structures and buildings.

Lime shaft stabilization has not reported a high-quality method for controlling the explosive soil as the lime is only slightly soluble in water and cannot be dispersed enough. On the other hand, Lime till stabilization can be an effective method on sealing and reducing the swelling behaviour only if it is combined with the soil appropriately to a suitable depth. As this method is a very complex one, it needs more research to be done on to recognize the desirable depth of treatment for different expansive soils and pleasant amount of lime.

Mechanical stabilization which is the result of the mechanical energy rarely is used and the technique may not be effective if the soils are subjected to significant moisture fluctuations.

Ponding has been reported as the most effective applied method for pre-wetting of soil. More research should be done in order to gain the satisfying depth to moisture and also the length of time the material should be ponded. Dry seasons has been suggested as the best time for ponding because the natural cracks are open due to desiccation.

Preventing access of water to the soil by encapsulation method can be a satisfactory method if the careful attention is paid to the material which are supposed to be used and also the situation of the expansive soil. The most probable material in this method is Catalytically Asphalt Membranes which mostly is used in pavements and transportation paths.

Finally, avoiding the expansive soil is a practical solution, although it can be used only in particular situations.