Can The Problem Be Solved?

There are two conditions that must be satisfied before expansive soils become a problem: expansive soils must be present and the soil moisture condition must change.

Obviously, if expansive soils are not present, the extreme soil shrink and heave normally associated with expansive soils will not occur.

If the coil water content can be kept from changing, or at least the change kept to a minimum, lesser shrink or heave will occur and the problem created by expansive soils will be minimized. However, it must be recognized that constructing a house or a building interrupts an established energy gradient (principally due to surface evaporation and plant transpiration) that is causing soil water to move from depth to the surface or vice versa. This induced water flow will ultimately result in some shrink or heave, even in the volume of soil beneath the interior of the house or building that is not being influenced by outside factors such as climate. However, once the interrupted energy gradient has reached equilibrium, no further shrink or heave likely will occur unless something external happens to upset the soil moisture equilibrium. Three things that often can cause the soil water content to change are climate, site vegetation, and irrigation.

Climate. Expansive soils occurring in predominantly wet climates or predominantly arid climates typically do not produce the extent of damage that expansive soils occurring in semi-arid climates cause. Expansive soils in predominantly wet climates have, for the most part, already acquired nearly all of the soil moisture needed to produce soil heave. The damage to structures built over expansive soils in wet climates most often occurs during periods of drought. The United Kingdom is in a predominantly wet climate and although many of the clays occurring in that country are some of the potentially most active expansive clays in the world, little damage occurs to structures in the U.K. except during periods of drought or periods of less than usual rainfall. This is likely the reason that expansive soils are termed "shrinkable soils" in the U.K. In the U.S., Houston, Texas and Tulsa, Oklahoma are located in relatively wet climates. Droughts occurring during the 1980's produced millions of dollars of total damage to residential and other lightly loaded structures in those two cities because the normally wet soils dried and shrank in volume. 

Conversely, expansive soils that occur in arid climates typically do not cause much damage to structures constructed over them unless the clay experiences a major wetting period of episode. For example, houses in Amarillo, Texas experienced considerable damage in the early 1980's when a part of the city constructed in an expansive soil region was flooded in August, the hottest and driest month of a long, hot, and dry summer that year.

Most damage to structures from expansive soil movement occurs in locations that have a semi-arid climate. A "semi-arid" climate can be described as a climate that has periods of rainfall followed by long periods of no rainfall. This type of a climate typically exhibits rainfall over a period of several weeks which results in the soil becoming wetter and swelling. However, the rainy season is then followed by a longer period when little or no rainfall occurs, and the soil gives up the moisture that it acquired during the preceding rainy period, dries out, and shrinks. Houses supported on shallow foundations in a semi-arid climate experience and annual cyclic rise and fall of the structure as the soil heaves, shrinks, and heaves again.

Thus, the solution to controlling soil movement due to climate is to ensure that the soil beneath and around your home does not dry out if your home is in a predominantly wet climate. Similarly, you should take steps to ensure that the soil water content of the soil beneath and adjacent to your home does not appreciably increase if you live in a predominantly dry climate. The task of ensuring that the soil water content remains essentially constant becomes more challenging if you live in a semi-arid climate, but this task is no more challenging to any conscientious homeowner than other tasks related to good lawn or home maintenance. 

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Vegetation. Vegetation has been shown to be an important factor in causing changed soil moisture conditions. Vegetation can affect the soil conditions around and under a house or other structure in several ways. A common practice for watering shrubs, bushes, and flowers planted adjacent to a house or other building in the Southwestern U.S. is to excavate the vegetation bed a few inches below ground level and then let a garden hose run water into the depression until the bed is filled with water. The frequent result of this flooding practice is to induce water flow beneath the house or structure with a subsequent heaving around the outside of the building. Obviously, this is not the fault of the vegetation, but rather the fault of the owner for engaging in a poor watering practice.

Often, the vegetation itself can produce damage to structures. The damage is most frequently the result of plants withdrawing water from the soil and causing the soil to dry out and shrink. Large bushes or shrubs planted immediately along the outside of a building can withdraw water from under the edge of the building if the plants are not watered regularly. Trees are more frequently the cause of plant-induced soil shrinkage. Numerous instances of severe damage to buildings have been documented where roots from nearby trees have penetrated beneath a building's foundation and removed water from the soil beneath the building during periods of drought. The recommended practice for planting trees is to plant them far enough away from the building so that roots will not grow back underneath the building. This recommendation is usually hard for owners to follow because owners like to have large, shady trees around their houses or buildings. Roots generally grow out a little beyond the edge of the tree's limbs so that the roots can extract water entering the soil from the ground surface during rains or irrigation. This edge of the tree's limbs and leaves is sometimes referred to as the "drip line." Thus, one rule of thumb that has been shown to be successful is to plant trees no closer to the building than where the tree's drip line will be when the tree is mature. Often, it is difficult to tell where a mature tree's drip line will be 20 years after it is planted as a sapling. Another rule of thumb that is also used, but is a little more conservative, is to plant the tree a distance away from the building equal to the mature height of the tree. Arborists and nurserypersons can usually tell about how tall a tree will grow at maturity. The U.K. Royal Botanic Gardens suggests that the trees most likely to cause damage, in descending order of threat, are those shown in Table 1. 

Another instance of when vegetation can result in structural damage to buildings concerns constructing new buildings on sites where vegetation was removed shortly before construction. In constructing new homes or buildings that cover large areas, it is often the practice to remove all trees and large shrubs at the time the site is being leveled and graded. If trees or large shrubs are removed at the end of the dry season or at the end of a drought during this construction operation, the ground beneath and around the trees and shrubs will most likely be very dry and even desiccated. If the building is subsequently built over the desiccated site, the soil will subsequently wet up once the ground surface is covered. If artificial irrigation is also employed at the site, the resulting post-construction heaving is exacerbated and the heaving that occurs will likely be more than would have occurred without the irrigation. 

Thus, shrinking or heaving resulting from vegetation-related problems can also be controlled by simply understanding the effect vegetation has on soil water content and the impact of the location of the vegetation on the performance of the building. Shrinking and heaving can also be controlled by understanding how watering of vegetation affects soil water content and the subsequent soil movement. 

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Irrigation. Most people like to have lush, green lawns around their houses and often like to have similar attractive entrances to their businesses or other buildings. Often an automatic or semi-automatic sprinkling or irrigation system is installed to ensure that the lawn and bushes, shrubs, and flowers receive regular watering. Too many times, owners, thinking that if, for example, 1 in. of irrigation water twice a week is good for their lawn, then greater amounts or more frequent watering of the same amount is even better. Although the lawn and plants may not object to the additional watering, all too often the result is an increase in soil water content around the edge of the house or building. The increased soil water content, in turn, causes the soil around the edge of the building to heave and damage the building superstructure. Thus, when automatic sprinkling systems are involved, it is best to water only enough to satisfy the water demands of the lawn and the plants on the property, including the trees. 

In some parts of the U.S., "drip" systems are used instead of sprinkling systems to irrigate plants. This system allows water to "drip" or seep into the soil at the root level. A drip system is often preferable to a conventional sprinkling system because it provides water at the point where it is needed without saturating the surface. Drip systems are commonly used in California and other parts of the Southwest to water vegetation planted adjacent to the house.

Soil shrink and swell problems associated with underwatering as well as overwatering lawns and other vegetation can be mitigated by controlled irrigation practices. It is important to understand the watering needs of your lawn and the plants on your property, and to address those needs in a timely and conscientious fashion.

Thus, the answer to the question posed as the title of this section is that, yet, the problem of soil shrink and heave can certainly be minimized and, in many instances, be controlled very well.

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