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North Texas Soil Technical Data

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From 1990 to 2000, the Dallas-Fort Worth area experienced the greatest percentage population growth of the 10 largest metropolitan areas in the United States, according to the U.S. Census Bureau. To accommodate this population growth, the Dallas-Fort Worth region added 448 square miles of urban areas. In Denton County, urban areas grew from 158 square miles in 1990 to 197 square miles in 2000.



Dr. Harry Williams, associate professor of geography at the University of North Texas, is exploring how urbanization pressures have resulted in housing construction on soils poorly suited for development. Williams' research is described in his article "Urbanization Pressure Increases Potential for Soils-Related Hazards, Denton County, Texas." The article is slated for publication in the international journal Environmental Geology in the fall 2003 issue.



"There's a lot of housing being developed on poorly-suited soils," Williams said. "Unfortunately, many people will probably have foundation problems in the future." Williams says 90 percent of soils underlying Frisco, Hebron, Little Elm and The Colony, located in the southeast corner of Denton County, are rated low to very low for urban suitability, because of expansive soils -- soils that swell.



"Expansive soils can damage foundations, pavements and pipelines, because hundreds of tons of pressure can develop in these soils as they absorb moisture." Williams said.



According to a 1982 Federal Emergency Management Agency report, expansive soils have caused billions of dollars of damage in the United States. Today, damage from expansive soils is more costly than damage caused by earthquakes, floods, tornadoes and hurricanes combined.



"Slab foundations, commonly found in single-family dwellings, are particularly vulnerable to expansive soil damage," Williams said. "These foundations are used extensively in new housing in the Dallas-Fort Worth area, because of lower construction costs." 


Williams' research is based on several variables related to soil suitability and urbanization. He mapped these variables, layer by layer, into a geographical information system (GIS). The GIS map shows suitability of soils for urban development based on United States Department of Agriculture soil survey reports and pre- and post-1990 urban areas obtained from the U.S. Census Bureau. The USDA soil survey reports rate soil suitability for urban development based on propensity to flood, high water table, wetness, shrink-swell potential, soil strength, soil texture and soil corrosivity to uncoated steel and concrete. 


The results of Williams' analysis show a dramatic shift in the nature of soils in pre-1990 and post-1990 urban areas. Before 1990, approximately 77 percent of the 158 square miles of urban areas in Denton County was constructed on soils deemed either medium or high suitability for urban development. In 1990-2000, approximately 53 percent of the 39 square miles of additional urban areas was constructed on soils deemed either very low or low suitability for urban development.


Source: University of North Texas News Release (September 3, 2003) available at http://web2.unt.edu/news/print.cfm?story=8636



HAZARD IDENTIFICATION

Soils and soft rock that tend to swell or shrink due to changes in moisture content are commonly known as expansive soils. In the United States, two major groups of rocks serve as parent materials of expansive soils, and occur more commonly in the West than in the East. The first group consists of ash, glass, and rocks of volcanic origin. The aluminum silicate minerals in these volcanic materials often decompose to form expansive clay minerals of the smectite group, the best known of which is montmorillonite. The second group consists of sedimentary rock containing clay minerals, examples of which are the shales of the semiarid West-Central States. Because clay materials are most susceptible to swelling and shrinking, expansive soils are often referred to as swelling clays. Changes in soil volume present a hazard primarily to structures built on top of expansive soils. Most engineering problems caused by volume changes in swelling clays result from human activities that modify the local environment. 



They commonly involve swelling clays beneath areas covered by buildings and slabs or layers of concrete and asphalt, such as those used in construction of highways, canal linings, walkways, and airport runways.



The most extensive damage occurs to highways and streets. Damage to the built environment results from differential vertical movement that occurs as clay moisture content adjusts to the changed environment. In a highway pavement, differential movement of 0.4 inches within a horizontal distance of 20 ft is enough to pose an engineering problem if high standards for fast travel are to be maintained.



Buildings are capable of withstanding even less differential movement before structural damage occurs. Generally, a differential movement of 0.25 in between adjacent columns will cause cracking in loadbearing walls of a 20-ft wide bay. With differential movement of 1.5 in over a span of 20 ft, beams are likely to be structurally damaged. 


Houses and one-story commercial buildings are more apt to be damaged by the expansion of swelling clays than are multi-story buildings, which usually are heavy enough to counter swelling pressures. However, if constructed on wet clay, multi-story buildings may be damaged by shrinkage of the clay if moisture levels are substantially reduced, such as by evapotranspiration or by evaporation from beneath heated buildings.

The most obvious manifestations of damage to buildings are sticking doors, uneven floors, and cracked foundations, floors, walls, ceilings, and windows. If damage is severe, the cost of repair may exceed the value of the building.



Probably the greatest amount of small building damage has impacted those constructed when clays were dry, such as during a drought, followed by soaking rains that prompt swelling of clays. Other reported cases of damage involve volume increases due to moisture from broken or leaking water and sewer lines, watering of lawns and shrubbery, and modifications of the surface that produce ponding.



RISK ASSESSMENT

Many of the soils found in the central and eastern portions of the NCTCOG region are clay-rich, fine-grained soils.  These soils contain a class of clay minerals called “smectites”, which have the property of exaggerated bulk volume changes in the presence or absence of water. These smectitic soils in the NCTCOG region originate primarily from the calcareous Cretaceous–aged marls and clay rock formations, notably of the Austin and Taylor Groups. Extreme wetting and drying cycles on this soil accentuate the shrinking and swelling effects, and, as a result, these soils are commonly termed Vertisols due to their distinctive vertical shrink-swell features, clayey texture and common large, vertical cracks when dry. One of the best known and classic of these vertisols is the Houston Black, a Blackland Prairie soil that stretches over 2 million acres of land between Dallas and Houston. This soil has been nominated by the Professional Soil Scientists of Texas to be named the State Soil of Texas due to its unique and common influence on the lives of Texans.



Shrinking and swelling of these vertisols can come at a high economic price. Some of the most expansive of soils may gain or lose up to 75% of its original soil volume, causing radical gain and loss of a structure foundation’s continuity.  In addition, soil expansion and loss is rarely uniform across large areas; some areas with locally higher clay content may expand much more than a nearby siltier or sandier soil unit.



Other problems are caused by high plasticity soils. When soil has dried and cracked, water can travel along the cracks for several feet in all directions. If the soil around your foundation is dried and cracked, then water placed next to the foundation will run through the cracks and accumulate at the bottom of the grade beam (the thick portion of the foundation that is under the exterior walls). In some cases, an accumulation of water in the soil at the base of a foundation can cause the soil to lose some of its load bearing capacity. If the soil loses enough load-bearing capacity, the house will sink into the ground.



In addition, water that collects under the foundation, regardless of origin, is a major problem. “Upheaval” relates to the situation in which the internal and, on rare occasion, external areas of the foundation raises above the “as-built” position. In high-plasticity soils, this phenomenon results, almost without exception, from the introduction of moisture underneath the foundation. Once the slab heaves, the reinforcing steel is stressed into what amounts to a permanent elongation.



Expansive soils are one of the nation’s most prevalent causes of damage to buildings and construction. Annual losses are estimated in the range of $2 billion to $7 billion. However, because the hazard develops gradually and seldom presents a threat to life, expansive soils hazards have received limited attention, despite their costly effects. The losses include severe structural damage, cracked driveways, sidewalks and basement floors, heaving of roads and highway structures, condemnation of buildings, and disruption of pipelines and sewer lines. The destructive forces may be upward, horizontal, or both.



Design and construction of structures without attention paid to the existence and behavior of expansive soils can worsen a readily manageable situation. Where expansive soils are not recognized, improper building or structure design, faulty construction, inappropriate landscaping and long term maintenance practices unsuited to the specific soil conditions can become a continuing and costly problem. Design problems might include improper foundation loading, improper depth or diameter of drilled pier, insufficient reinforcing steel, and insufficient attention to surface and underground water.



Construction problems related to expansive soils include lack of reinforcing steel, insufficient or improperly placed reinforcing steel, mushroom-topped drilled piers, and inadequate void space between soils and grade beams. Allowing clays to dry excessively before pouring concrete and permitting the ponding of water near a foundation during and after construction also are contributing factors in expansive soil- related construction problems. Building without allowance for basement or ground floor movement in known expansive soils areas is a very common source of property damage. Improper landscaping problems include inadequately managing surface drainage and planting vegetation next to the foundation so irrigation water enters the soil.



Expansive soils are a profound nationwide problem, as shown by Jones and Holtz (1973): “ Each year, shrinking or swelling inflict at least $2.3 billion in damages to houses, buildings, roads, and pipelines –

more than twice the damage from floods, hurricanes, tornadoes, and earthquakes…Over 250,000 new homes are built on expansive soils each year. 60 percent will experience only minor damage during their useful lives, but 10 percent will experience significant damage--some beyond repair. One person in 10 is affected by floods, but one in five is affected by expansive soils. Swelling clays are one of the most significant, widespread, costly, and least publicized geologic hazards.” 


Although several visual methods for identification of potentially expansive clays exist, only a competent, professional soil engineer and engineering geologist should be relied upon to identify this potential hazard. Some warning signs for swell might include: a) soft, puff, "popcorn" appearance of the surface soil when dry; b) surface soil that is very sticky when wet; c) open cracks (desiccation polygons) in dry surface soils; d) lack of vegetation due to heavy clay soils; e) soils that are very plastic and weak when wet but are "rock-hard" when dry.



Engineering soil tests include index tests and design tests. Rapid, simple index tests are used to determine whether more complex design tests are necessary. Some index properties that may aid in the identification of probable areas of expansive clay include Atterberg limits, plasticity index, grain size determination, activity ratio, dry unit weight, and moisture content (Asphalt Institute, 1964). The primary design tests for expansive soils are the consolidation swell* test for buildings, and the California Bearing Ratio* swell test for roads (Asphalt Institute, 1964). 


Damage from expansive clays can affect, to some extent, virtually every type of structure in Texas. Some structures, such as skyscrapers in downtown Dallas, generally have well engineered foundations that are too heavily loaded for swelling damage to occur. At the opposite extreme are public schools and single family homes, which are generally constructed on a minimal budget and which may have under-designed lightly-loaded foundations that are particularly subject to damage from soil movements.



Homeowners and public agencies that assume they cannot afford more costly foundations and floor systems often incur the largest percentage of damage and costly repairs from expansive soil.  


No figures are available for the total damage to homes in Texas from expansive clays. However, several examples are known where the cost of repairs has exceeded the value of the house. Additionally, highways in some areas of Texas have required frequent and very expensive reconstruction or maintenance due to damage from expansive clay. 



SUMMARY

As depicted on HazMAP map 7-1, the central and eastern portions of the NCTCOG region are especially susceptible to expansive soil hazards. Soils that contain large percentages of swelling clays may experience volume changes of up to 40% in the absence or presence of water. This type of plastic deformation is common in the Blackland Prairie areas of the NCTCOG region, including Dallas, Ellis, Collin, Kaufman, Rockwall Counties, and portions of Johnson, Tarrant, and Hunt Counties. Over time, expansive soil hazards can have detrimental impacts on a variety of structures and facilities across many jurisdictions in the NCTCOG region. As a result, expansive soil hazards pose sufficient enough risk to receive mitigation consideration.


References:

Federal Emergency Management Agency (FEMA). (1997) Multi Hazard Identification and Risk Assessment.

Expansive Soils, Chapter 11.

This article can be found at:

http://www.hazmap.nctcog.org/risk_assessment/Chapter7.asp