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Recent research conducted in the U.S (Jones, & Holtz, 1973) has shown that the country spends an estimated 7 billion dollars each year as a result of damage to all types of structures build on expansive soils. The research further shows that more than twice as much is spent on damage due to swelling soils as is spent on damage from floods, hurricanes, tornadoes and earthquakes.
Certainly, the problem is of enormous financial proportions. The issue of the effect of expansive soils has probably received more attention than any other analysis associated with difficult soils. Numerous analytical procedures have been proposed in various countries.
However, most methods have been used to a limited extent within a limited geographical area (Jones, & Holtz, 1973). This term paper examines the issue surrounding expansive soils in relation to foundational engineering.
In order to establish the effect of poor foundational design in expandable soils, this paper will briefly present a case history for emphasis. In 1961, the Division of Building Research under the National Research Council carried out a study to monitor the performance of an industrial building in North-Central Regina. In the study, instrumentation was installed to monitor ground movements at various depths below the slab.
Approximately one year after construction, the owner noticed considerable cracking of the floor slab. Precise level surveys conducted at that time showed the maximum total heave to be 106 mm. The owner had also noted a significant increase in water consumption (i.e. 35 000 litres).
It was later discovered that a leak had occurred in the hot water line beneath the floor slab, at the location of maximum heave. Although this is just but a simple case history, it speaks volumes on the need for thorough foundational planning in constructions involving expansive soils (Yoshida et al, 1983).
The challenge with planning building foundations on moderate to highly expansive clay soils are the probable damaging effects of differential movements of the foundation structural elements owing to volumetric changes of the underlying and adjoining soils. In nonprofessional’s language, expansive soils swell up and cause heave with increasing soil moisture, or can dehydrate and cause subsidence with diminishing soil moisture.
According to Muckel (2004), movement of expansive soils is brought about by fluctuations in the moisture level of soil particles. Because homogenous expansive soils have very low permeability, fluctuations in the moisture essence of the soils might usually be expected to happen over a prolonged period.
However, permeability is amplified with geotechnical phenomena such as land faults, surface fractures due to waterlessness of clays, and decomposition of tree roots, which bring about cracks that become widely spread over time.
Due to the continued wetting, expansion, drying, and shrinking of the soil as it weathers, the cracks often fill up with sediment and sand, and create pathways for water that can aggravate the permeation process.
Additionally, water can also move easily through naturally occurring sand strata, sand seams, and micro-fissures in clay soil caused by earlier shrinkage. High negative pressures, also referred to as suction, in expansive soils with low water content also raise the tendency for absorption of water into the soil (Muckel, 2004).
Apart from environmental factors, expansive soils can also be affected by climatic conditions. Water removal by trees and other plants, a process known in science as transpiration, can become a basis for soil shrinkage.
Swelling can be caused by water penetration into the soil from neighborhood irrigation systems, broken water tubes, inundated and leaking service dugouts, poor drainage system, or dripping swimming pools, or it can be caused by slow moisture replacement and equalization after the confiscation of a tree.
The combined effect and inconsistency of all these possibilities make it difficult to precisely predict expansive soil ground movements (Muckel, 2004).
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In his book titled Foundation Design: Principles and Practices, Donald Coduto (2005) notes that foundation movements are viewed as problematic only if they bring about negative occurrences that destructively affect the functionality or exterior of the building.
The negative occurrences are considered to be structural if the weight carrying capacity of the superstructure or foundation elements are affected, or are deemed to be cosmetic if only the appearance of the outside covering or interior wall, floor, or ceiling finishing are affected.
Negative phenomena can also affect the serviceability of the building, such as the opening or closing of the doors. In most cases, negative occurrences due to foundation movement naturally occur because of disparity movements between various parts of the building.
These disparity movements in most cases lead to high internal stresses in building components often ending as distress in the form of gaping cracks, splitting, twisting, collapsing, or separations in the exterior covering systems such as block, cement board panels, or in the internal finishes such as drywall finishes, wood paneling and carpeting (Coduto, 2005).
Apart from reinforcing the building weight, the aim of engineering foundation design in expansive soil areas should be to economically alleviate the negative effects of foundation movement. This can be done by isolating rudiments of the foundation system from possible soil movements or by using design methods and details that help to manage the consequences of the soil movement.
In most cases, movements of expansive soils are generally constrained to an upper zone of soils referred to as the active zone. The lower boundary of this zone is usually defined as the line of zero movement. The depth of the active zone differs from location to location.
The depth of the active zone is an imperative design parameter used in the engineering design of foundations on expansive soils, principally when planning to apply deep foundations. Another general design consideration is the effect of the extent of extra pressure on the scale of heave that can occur.
Lightly loaded foundation components, such as physical framework, pavements, and building slab-on-grade floors are affected more by expansive soil volumetric variations than are heavily loaded foundation components such as heavily loaded bearing walls.
This is because heavy loads significantly reduce the amount of expansion that can occur. Numerous foundation system design options to meet these goals to varying degrees are available. There are also many options in the design and selection of components that constitute these foundation systems. However, these choices should only be based upon an engineered geotechnical investigation (Jones, & Holtz, 1973).
There are various types of foundation systems that are usually used for residential and other low-rise buildings in areas where expansive soil is predominant. The foundation systems are subdivided into two groups namely the deep support system and shallow support systems.
It is important to note that each of these systems has an associated level of risk of damage that can happen to the building superstructure and architectural components due to differential foundation movements. Each of these systems has a connected relative cost of construction.
When contrasting the various foundation systems, the intensity of risk is characteristically found to be inversely proportional to the level of cost (Muckel, 2004). In most cases, higher risks are usually acknowledged due to financial considerations.
For example, shallow support systems usually have a moderately higher level of risk compared to deep support systems, but are usually adopted due to finances and affordability (Jones, & Holtz, 1973).
Deep Support Systems
Deep support systems are characterized as foundations having deep components such as drilled piers or piles that expand way below the moisture active zone of the soils. These components function as a boundary to the vertical movements of the building by providing vertical support in a soil section that is not prone to downward movement brought about by moisture fluctuations.
The deep support system is divided in to several sub-systems all of which are adopted depending on location to location. These subsystems include; isolated structural systems with deep foundations, stiffened structural slab with deep foundations, stiffened non-structural slab with deep foundations, and non-stiffened slab-on-grade with deep foundations.
Each of these support systems has its own advantages and disadvantages but one thing that is common among them is that they are superior to shallow support systems. The only advantage that is synonymous with each one of them is the higher construction cost but this is secondary when compared with the safety of the building (Muckel, 2004).
Shallow Support Systems
Shallow support foundation systems are defined as foundations having shallow foundations that do not go below the moisture active zone of the soils and are prone to vertical movements due to volumetric changes of the expansive soils. Like the deep support systems, the shallow support systems are divided in to subsystems.
These subsystems include the grade-supported stiffened structural slab, grade-supported stiffened non-structural, slab and grade-supported non-stiffened slab of uniform thickness. In a large part, this system is preferred due to the fast time in construction and reduced costs.
However, this system has a high risk of vertical movement as compared to deep support system and it is therefore not recommended for use in high-rise buildings (Coduto, 2005).
Mixed Depth Systems
Just as the name suggests, mixed depth systems are foundations that extend to diverse bearing depths. Although their use is disallowed for some applications, mixed depth systems are sometimes utilized. The systems can be employed for new structures on sites with large plan areas located on a site with broadly changeable soil conditions and for new structures on locations with a considerable amount of deep fill.
Additionally, the systems can be used for new structures on a sloping hillside, for new structures located adjacent to a waterway or slopes greater than 5%, for existing structures when adding a new building among other places. When a new addition is added on to an existing building, consideration must be given to the depths of the new and existing foundation systems.
When dealing with expansive soils, it is advisable to employ various mitigation options to reduce the damaging effects of soil movement due to improper drainage and transpiration of trees and bushes. Employing a moisture control system prevents damage by controlling the amount of water and moisture that enter into the location soils.
This includes methods to direct storm water runoff and methods of providing irrigation to lawn vegetation. Some recommended ways of controlling site drainage include site grading, French drains, and area drains. These systems just like the support systems reduce vertical movements of building foundations by moderating the effects of seasonal moisture changes.
Coduto, D. (2005). Foundation Design: Principles and Practices. London: Prentice-Hall.
Jones, E., & Holtz, W. (1973). Expansive Soils-The Hidden Disaster. ASCE, Civil Engineering, 43 (2), 87-89.
Muckel, G. (2004). Understanding Soil Risks and Hazards: Using Soil Survey to Identify Areas with Risks and Hazards to Human Life and Property. Retrieved from ftp://ftp fc.sc.egov.usda.gov/NSSC/Soil_Risks/risk_low_res.pdf
Yoshida, R., Fredlund, D., & Hamilton, J. (1983). The Prediction of Total Heave of a Slab-on-Grade Floor on Regina Clay. Canadian Geotechnical Journal, 20 (3), 69-81.