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Introduction
A lack of appreciation of the behavior of expansive clay causes billions of dollars of damage to buildings, roads, pipelines, and other structure every year in the United States. Many of the most populated areas of the country are underlain by expansive soils. The expansive soils undergo large changes in volume due to increased or decreased moisture content. (Kariukiet al, 1; Agee) Problems occur across the construction field due to lack of understanding, under estimations, and low respect for the highly expansive clays.
Identification of Expansive Soils
Very fine weathered soils and minerals are the expansive clays that pose a problem for building construction. The greatest risk lies with the expansive clays that are created due to the weathering of mudrocks and have a high intensity of drying conditions.(Doornkamp, 197) These clays are classified by their extremely fine grain size. A generally accepted classification for the measurement for the grain size is no larger than 4 µm. Geologist, soil scientists, and engineers from time to time disagree on the correct term usage of clays and their properties. Another generally accepted classification for clay is its high plasticity (Agee), which is its ability to be molded into any shape. Clays are dominated by plastic fines that have low resistance to deformation when wet and form hard cohesive masses when dry. (FHA, 12) These clays that are typically very fine sediments are generally very well graded as to produce an overall latticework that creates the clay that is difficult to work with mechanically.
The clay minerals make the clay susceptible to absorbing water. Clays are difficult to compact when excessively wet and generally impervious and impossible to drain by ordinary means. (FHA, 12; Agee) Clay can expand 10 % or more as it becomes wet. The clay is capable of exerting pressures of 20,000 pounds-per-square-foot or greater on foundations, slabs, and other confining structures. (Polk County, 2) Identification of clays can be made with a microscope or a small hand lens. Clays generally are composed of one or many minerals. Most clay is phyllosilicates with exception to other minerals. (dolomite, mica, quartz, feldspars, organic phase elements, and many others) The most common clay with highly expansive properties usually contains montmorillonite.
When the clay absorbs water the volume increases, the more water the clay absorbs, the more the clay’s volume increases. When the particles lose water or water is removed from the clay particles, the clay can shrink significantly. The highly expansive clays will be dominated by the water absorbing clay minerals so therefore the clay material is subject to changed in volume with changing moisture conditions. This category of clay mineral, typically used as one of many construction materials for foundations, could pose a threat to building construction. The clay typically has a swelling potential that is dependent on the clay content. ( Kariuki et al, 7)
These clays could possibly be observed performing extremely poorly with even human foot traffic over a short period of time. The highly expansive clay is generally a soil that will react to moisture with great differential in volume. Imagine the size of a popcorn cornel before it is heated to make popcorn. The size of the popcorn cornel can be related to the initial size of the clay particle. Once the cornel is popped, the difference between the size of the cornel and the popped cornel is relative to the difference of swelling per clay particle after absorbing water. Now, if that clay particle is multiplied a couple thousand times to make up a small area of clay, the swell potential could be great. That small area of clay can then be imagined to be the foundation under a building, if that area of clay receives a significant amount of moisture, the clay will swell considerably and cause damage to the structure.
Potential Problems
Potentially, there are literally thousands of problems, if not more, with highly expansive clays. Expansive clays pose a problem for building construction. The expansive clays with a large margin of potential of expanding or contracting due to moisture gain or loss can be classified as having a large shrink-swell potential.
Expansive soils can be a problem for a house built on the expansive clay. The JCP Geologists describe that house will move if the foundation was not designed to take this soil type into account. They describe that movement occurs because the expansive clays expand so forcefully causing the foundation to actually move. At different parts rates and distances, the different parts of the house will move causing the foundation to crack. (1) The JCP Geologists explain that the foundation cracking will begin to cause doors and windows to become jammed and the looks of the home could be affected. Not only are houses affected by the swelling, other structures can be affected too. Basically anything that is built on these untreated expansive clays could have problems with settling and expansion of the surrounding soils. Structures most often damaged from swelling soil include building foundations and walls of residential and light (one or two story) buildings, highways, canals and reservoir linings, and retaining walls. (Polk County, 2)
The Oregon Natural Hazards Workgroup, in Polk County, presented a thorough report on structures built on expansive soils. The workgroup describes how buildings that are less able to suppress the differential heave such as lightly loaded one-or-two story buildings, warehouses, residences, and pavements are especially vulnerable. Heave over time is hardly predictable because it is not easily foreseen as to the location and time the water is available to the soil. (Polk County,1) “Most heaving and associated structural distress occurs within five to eight years following construction, but the effects of heave may also not be observed for many years until some change occurs in the foundation conditions to disrupt the moisture regime” (Polk County, 3) This means that the foundation of the structure built on the expansive soils, will over time retard either the absorption or dewatering of the expansive soils to a point that eventually the area will begin to react to the flux in moisture. If the foundation of the structure encompasses, let’s say, a half acre of expansive soil, then that area that underlies the foundation is protected against the elements of air or water infiltration. But over time, the surrounding soils will slowly provide moisture to the underlying expansive soils and causing it to slowly gain moisture and expand. In the adverse case the surrounding soils could be drier than the expansive soils beneath the foundation and slowly dry out the soils causing sustenance. Both the expansion the soil, also called heave, and the subsidence will cause extensive damage to anything founded on this expansive clay. If the swelling soils are not recognized, the result can be a costly problem. (Polk County, 2) Poor design and construction of structures can worsen the problem. (Polk County,5) Improper building or structural design, faulty construction, inappropriate landscape and long-term maintenance practices unsuited for the soil are just a few problems that contribute to the increasing cost. (Polk County, 5)
Brink and his associates presented in their book “Soil Survey of Engineering” that houses are lightly loaded structures that are particularly vulnerable to heaving. They describe that the central area of the foundation is where the maximum upheaval is usually where the greatest moisture content is located. The upheaval will result in a dome-shaped distortion beneath the house and will produce widening-out cracks on the house. (116)
The Polk County Workgroup presents a short list of potential losses from building on expansive soils that have a large shrink-swell potential. Losses included but are not limited to structural damage, cracked driveways, sidewalks, basement floors, heaving of road and highway structures, condemnation of buildings, and disruption of pipelines and sewer lines.(Polk County, 3)
Polk County Natural Hazards Workgroup Indicators of Expansive Soil Movements
Exterior Interior
Diagonal cracks in brick walls, sagging brick lines, bowed or non-vertical walls, separation of wood trim joints at crevasse, separation of concrete driveway/ patio/ or sidewalk, tilting of landscape/retaining walls Cracks in sheetrock or ceilings, bowed of non-vertical walls, bottom of wall separated from floor, cracks of wall corners, cracks above doors, sticking windows, sticking doors, sloping floor surface, cracks in ceramic or vinyl tile
The Polk County Workgroup describes that major movements originate from the fluctuation of moisture content. Major structural damage is the effect of differential rather than total movements on the foundation. (2) The probability of damage to the structure can be increased by differential movement by shrink-swell of the expansive soils. (Polk County 3) The shrink of the expansive soils due to drying out or dewater is a major settlement problem. “Clay soils change their volume on drying out, particularly these containing active minerals”. (Brink et al, 108) Under drought conditions when the water table is lowered, substantial shrinkage can occur and will produce differential settlement in buildings especially under external walls where there is the least amount of protection against evaporation. (Brink et al, 108) The reverse process, heaving, may occur due to the wetting of these clay soils.(Brink et al, 108) Water loving trees can draw water from the clays that act as the foundation to the structure. (Brink et al, 109) These trees with their extensive root system such as poplars, willows, eucalyptus, and elmo must be planted at a distance from the structure less than 1.5 times their ultimate height. (Brink et al, 109)
Doornkamp described that one possible problem of subsidence due to the dewatering of the expansive clay, causing shrinkage, would be the matter of management and financial policies. Doornkamp describes that it would be prudent for companies to not only to create an insurance policy for the subsidence but to invest their money into building up the value of the property by taking into account the predictions of the soils and climate and working to improve the property that holds their interest. (201-202)
Structural damages from shrink-swell can include: cracks in grade beams, walls, and drilled shafts; distortion and cracking of pavements and on-grade floor slabs, jammed and misaligned doors and windows, and failure of steel or concrete blocks supporting grade beams. (Polk County, 2) “The magnitude of damages to structures can be extensive, impair the usefulness of the structure, and detract aesthetically from the environment. Maintenance and repair requirements can be extensive, and the expenses can grossly exceed the cost of the original cost of the foundation”. (Polk County, 2)
John Agee, a geotechnical engineer in Nashville, described that many qualified firms have a strong and firm understanding in the science of soil engineering. He acknowledged that many of today’s firms have engineers on staff within construction and building firms. Agee described that these engineers help provide better productivity and are very skilled on how the politics and physics of soil engineering affect a job. Mr. Agee provided an example story about shrink-swell where a building in Texas that had piles for the foundation. As the piles were in the ground and held in place by friction, the surrounding soils went through a series of shrinking and swelling from the addition of moisture. Due to the shrink-swell process, the piles were pushed out of the ground.
Living With Expansive Soils Action Plan
TYPICAL SOURCES – POSSIBLE PROBLEMS – POSSIBLE ACTIONS
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- Rainfall — Non-uniform runoff from roof may result in localized heave. Maintain soil sloping away from all sides of the foundation for a distance of at least 5 feet, use gutters with downspouts that discharge at least 3 feet from the foundation.
- Gutter Down Spout– Concentrated sources of water may lead to non-uniform foundation movements. Extend discharge a minimum of 3 feet from the foundation and use splash blocks to avoid erosion or use flexible discharge tubes.
- Poor Drainage– Localized source of water from rainwater flowing or ponding next to the foundation may lead to localized heave of the foundation. Slope ground away from all sides of the foundation for a distance of at least 5 feet, create drainage swales to divert water away from the foundation, keep dirt line several inches below the brick line, use clay soil fill to create positive slope away from the foundation. Do not use SANDY SOILS for fill next to foundation, use CLAYS. Compact the fill to shed water, not absorb it.
- Flower Beds– Localized source of water not on all sides of foundation may result in non-uniform foundation movements. Do not flood or pond irrigation water, slope ground surface away from the foundation, do not trap water near the foundation with edging, use mulch to slow evaporation.
- Sprinkler Valves– Valves frequently leak and joints may leak with time, resulting in localized water sources which may cause non-uniform foundation movements. Locate at least 5 feet from foundation and inspect valves frequently.
- Over Watering– Provides excess source of soil water for suction to draw moisture under foundation which may cause a stable area to begin heaving and damaging your structure. Water just enough to keep plants and grass alive and growing, not thriving and lush through saturating the ground.
- A/C Unit Condensation– Concentrated source of water which can result in non-uniform foundation movements. Direct the discharge line to drip on a concrete pad or splash block which has been properly sloped away from the foundation.
- Hot and Dry Climate– Loss of soil moisture from under foundation edges may cause foundation settlement. Uniformly water landscape planting and area next to all sides of the foundation, instal1 automatic sprinkler systems, add sidewalks adjacent to the foundation.
- Excess Drying on the West Side / Non-uniform Moisture Loss– Non-uniform drying on all sides of foundation from the sun or failure to provide watering on all sides of the foundation may cause non-uniform foundation movements. Apply more landscape water on drier sides of the foundation, use mulch to slow evaporative drying, plant quality shade trees along with installation of a tree root/vertical moisture barrier.
- Trees– Tree roots grow under foundation and dry out soils causing non-uniform foundation settlements. Plant tree a distance greater than their mature height from the foundation. If existing trees are closer-instal1 an approximately 4- foot deep tree root/vertical moisture barrier system near the foundation and possibly prune trees (to limit moisture stress) if barrier system is under the drip line of the tree. Water tree roots away from the foundation.
- Landscape Planting– Drying from roots, transpiration and soil suction may cause non-uniform foundation movements. Plant bushes and shrubs away from the foundation, uniformly water plants, do not flood or pond water next to the foundation.
- Landscape / Retaining Walls — Non-uniform drying on all sides of foundation may result in non-uniform foundation settlements. Apply more landscape water than other sides of the foundation, use mulch to slow evaporation.
- Plumbing Line Leaks– Leaks in sewer or water lines provides localized source of water that may lead to localized foundation movements. Monitor water bills, get leak detection plumber to isolate and repair leaks, verify repairs with pressure tests.
- Shallow Subsurface Seepage / Moving Down Slope– Concentrated source of water to foundation soils may result in non-uniform heave of the foundation. Install interceptor trench drain up slope to collect and divert seepage water around foundation soils and discharge down slope or to a sump.
- Moisture Vapor Rising from Wetter Soil Beneath Foundation– Gradual and uniform rise in soil moisture under foundation may lead to gradual heave of structure. Normal occurrence, foundation stiffness should be designed and constructed for this long term condition.
