URBANIZATION PRESSURE INCREASES POTENTIAL FOR SOILS-

RELATED HAZARDS, DENTON COUNTY, TEXAS.

Harry Williams

Department of GeographyUniversity of North Texas

Introduction

•1990–2000, Dallas–Fort Worth region had the greatest % population growth in the US

•Population grew by 29.3%

•Denton County population grew by 58.3%

•Accompanied by rapid urbanization

•Denton County urban areas grew by 25%

A 1999 air photo of the same area

1978

Much of the growth in population and urban areas in Denton County was concentrated near its southeastern border with Dallas County.

Objective

Hypothesis: urbanization pressures in Denton County in 1990–2000 pushed development into areas poorly suited for urban development.

"Poor suitability" for urban development is defined primarily in terms of unfavorable soil conditions.

Materials and Methods.

•detailed soil map obtained from SSURGO database

•Soils are divided into 85 map units

•Average area of 10,243 mapped soil units in the county is 0.24 km2

Soil surveys – a wealth of information.

The soils are mapped at 1:24,000 scale on air photos:

•suitability of soils for urbanization obtained from the Soil Survey of Denton County

•elements considered: dwellings, streets, excavations and uncoated steel pipe

•elements rated on a number of factors including shrink-swell potential and soil strength

•soil survey assigns overall urbanization-potential rating in terms of very low, low, medium and high

Table 1.  Examples of Soil Conservation Service urbanization-potential ratings for four soils in Denton County.

The urbanization-potential ratings were used to create a theme map of urbanization-potential by soil unit

Urban potential of soils in urban areas.

•performed overlay analysis with GIS

•compared soils in pre- and post-1990 urban areas

•found extent (km2) of soils for each of the four urban-potential rating categories (very low, low, medium, high)

Results.

•Pre-1990: 23% of urban areas on soils of either very low or low potential for urban development

•1990-2000: 53% of urban areas on soils of either very low or low potential for urban development

Why do soils have a low rating for urbanization?

•many soils with a low rating for urbanization are expansive soils containing montmorillonite clay

•expansive soils can damage structures such as foundations, pavements and pipelines, because swelling can generate pressures of up to 273 metric tons/m2

Eagle Ford Shale. Eagle Ford Shale is composed of clay particles. It is finely layered, fissile and subject to severe erosion, mainly because of slaking (swelling and shrinking of the clay).

The rock breaks down easily and forms very clay-rich soils.

The resulting soils are typically deep and clayey and support dense grasslands which add abundant organic matter to the soil, giving it a black color. These soils have high shrink-swell potential (often indicated by large cracks).

Expansive soil hazards

•slab-on-grade foundations are susceptible to damage

•slab-on-grade foundations are used extensively in new housing in the Dallas-Fort Worth area

•one study found that 26–36% of ten-year-old houses surveyed in Carrollton, had damage attributable to expansive soils

•another study reports that approximately 8,500 foundations are damaged annually in Dallas County

Conclusion

The increase in urban development on expansive soils in Denton County has increased the potential for soils-related hazards. Given the documented history of damage to structures due to differential swelling, this type of damage is likely to increase in the county, particularly in the southeast where there is a concentration of new urban development on soils with high to very high shrink-swell potentials.