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22 September 2003, 11:27 AM
A new University of North Texas study shows that, in recent years, more and more homes here have been built on soil that's less than ideal for urban development
Michael Whiteley - Tarrant/Denton Editor
http://dallas.bizjournals.com/dallas/stories/2003/09/22/story4.html?page=1
Many builders involved in the 1990s residential boom in Dallas-Fort Worth, whose population growth outpaced the nation's other big metro areas that decade, know that prosperity was built on feet of clay.
Montmorillonite clay, to be exact.
But the clay's inclination to take huge gulps of water and swell enough to crack foundations is underscored in a new study by a University of North Texas geographer.
The study focuses on an area in and around Southeastern Denton County including Frisco, Hebron, Little Elm and The Colony. It concludes that 90% of the soils built on there during the 1990s are rated "low" to "very low" in their suitability for urban development.
The designations are two of four assigned through a computer data-driven mapping system known as GIS by the U.S. Department of Agriculture's Natural Resources Conservation Service.
In a study to be published this fall in the 2003 issue of Environmental Geology, UNT associate geography professor Harry F. L. Williams found that the Metroplex added 448 square miles of urban area and Denton County added 39 square miles during the decade.
Computer-mapped data taken from the U.S.D.A. and the U.S. Census Bureau defines urban areas as, among other things, those with 500 or more people per square mile.
Before 1990, 77% of the urban areas in Denton County were built over soil considered of medium or high suitability for urban development. But between 1990 and 2000, Williams found, 53% of the new urban areas were built on soil deemed of very low or low suitability.
"It does affect much of the Metroplex, not just Denton County," Williams said. "You find it in Collin County, Dallas County and Tarrant County. Expansive soil is a big problem in the Dallas-Fort Worth area. It's considered one of the worst areas in the country."
The study, while it draws a vivid picture of the problem, doesn't surprise the homebuilders who've struggled in North Texas' shaky soil for years.
Some have avoided towns with especially expansive soil. Others have built despite concerns about the soil because the market demanded they follow Interstate 35E and other manmade growth corridors.
D.R. Horton Inc., for example, opted not to build in Little Elm because the soil there was too expansive.
"I can tell you this: If the guy somewhere in that report said we're building on soil that's not suitable, and someday we'll pay for it, he's right," said Rick Horton, president of the Dallas division for the Arlington-based builder. "As a group, and I'm not saying it's us, homebuilders occasionally make decisions based on money and not common sense."
Horton said the worst soil forms a vein running through Frisco, Little Elm and all the way to Flower Mound. He suspects the vein was a river in prehistoric times.
"At its widest, the vein is probably 10 miles wide," Horton said. "At its narrowest point, it's probably two miles wide. You can build a lot of houses in two square miles."
Bob Bush, an Arlington attorney who represents many of the area's homebuilders, said builders have made a series of technological advances and developed complex engineering plans to grapple with the problem.
What isn't apparent from the points raised in the study, he said, is that expansive soil is "extremely site specific," and doesn't apply throughout entire sections of the Metroplex. But builders build to the market.
"Most of the residential development is basically adjacent to I-35. That's probably some of the least desirable soil from a construction standpoint," he said. "Ideally, you want to build on limestone. But people don't want to live there."
Officials in The Colony, Little Elm and Hebron could not be reached comment.
New regs in Frisco
Williams' study identified expansive soil as particularly severe in Colorado, Texas and Wyoming, and focused on local foundation nightmares dating back to the 1980s.
A 1986 survey of 10-year-old houses built in Carrollton, for example, showed that 26% to 36% had structural damage attributed to expansive soils. The earlier study also concluded that 8,500 residential foundations in Dallas are damaged each year when the ground swells and contracts.
Williams' study concludes that concrete-slab foundations are particularly prone to crack or break when soil expands. But builders use them anyway to save money, he said.
1998 survey showed that slab foundations were used in more than 95% of new homes in the city of Denton, Williams said. His random Internet survey of Frisco real-estate listings, he said, showed 49 of every 50 houses built between 1990 and 2002 had slab foundations.
The study looked at population growth in the 12-county region known as the Dallas-Fort Worth Consolidated Metropolitan Statistical Area, which logged a population jump of 29.3% from 1990 to 2000, adding 1,184,519 people. Denton County's population jumped by 159,451 people, or 58.3%. And Frisco's population exploded by 449% during the period -- growing from 6,141 to 33,714.
Williams linked the soil data, which was classified based on propensity to flood, texture and soil corrosivity to uncoated steel and concrete.
"You have the clay, and you have the climate that contributes to expansive soils," Williams said. "You get a dry summer and a wet winter, and you have the ability of the soil to swell up."
Aware of its role as the vortex of Metroplex growth, Frisco adopted one set of new foundation requirements in 2001 and a second set on May 1.
"We kind of saw this coming, and we've been trying to get ahead of the curve," said Donnie Mayfield, Frisco's chief building official. "We had observed what happened to our sister cities, mainly in the Las Colinas area. We talked to engineers. We talked to developers and we tried to come up with the criteria that would appeal to everybody and ensure the safety of the houses."
Frisco now requires an engineer of record for all homes, and asks builders to submit structural design plans and conduct geo-technical investigations. The city has adopted the recommended practices developed for foundations by the Texas section of the American Society of Civil Engineers, Mayfield said.
The problem also has become a focus for change by the Home Builders Association of Greater Dallas.
The association has been active in trying alternatives to standard concrete slabs. In Frisco, those have included steel-frame foundations and a process of injecting water into a layer of soil beneath the surface and covering that with a tarp before the final fill.
"History is going to tell what happens," said Frisco's Mayfield. "But a person buying a house in the city of Frisco is a lot better off than buying somewhere else, because of the requirements."
Contact DBJ Tarrant/Denton editor Michael Whiteley at mwhiteley@bizjournals.com or (817) 693-0023.
Michael Whiteley - Tarrant/Denton Editor
http://dallas.bizjournals.com/dallas/stories/2003/09/22/story4.html?page=1
Many builders involved in the 1990s residential boom in Dallas-Fort Worth, whose population growth outpaced the nation's other big metro areas that decade, know that prosperity was built on feet of clay.
Montmorillonite clay, to be exact.
But the clay's inclination to take huge gulps of water and swell enough to crack foundations is underscored in a new study by a University of North Texas geographer.
The study focuses on an area in and around Southeastern Denton County including Frisco, Hebron, Little Elm and The Colony. It concludes that 90% of the soils built on there during the 1990s are rated "low" to "very low" in their suitability for urban development.
The designations are two of four assigned through a computer data-driven mapping system known as GIS by the U.S. Department of Agriculture's Natural Resources Conservation Service.
In a study to be published this fall in the 2003 issue of Environmental Geology, UNT associate geography professor Harry F. L. Williams found that the Metroplex added 448 square miles of urban area and Denton County added 39 square miles during the decade.
Computer-mapped data taken from the U.S.D.A. and the U.S. Census Bureau defines urban areas as, among other things, those with 500 or more people per square mile.
Before 1990, 77% of the urban areas in Denton County were built over soil considered of medium or high suitability for urban development. But between 1990 and 2000, Williams found, 53% of the new urban areas were built on soil deemed of very low or low suitability.
"It does affect much of the Metroplex, not just Denton County," Williams said. "You find it in Collin County, Dallas County and Tarrant County. Expansive soil is a big problem in the Dallas-Fort Worth area. It's considered one of the worst areas in the country."
The study, while it draws a vivid picture of the problem, doesn't surprise the homebuilders who've struggled in North Texas' shaky soil for years.
Some have avoided towns with especially expansive soil. Others have built despite concerns about the soil because the market demanded they follow Interstate 35E and other manmade growth corridors.
D.R. Horton Inc., for example, opted not to build in Little Elm because the soil there was too expansive.
"I can tell you this: If the guy somewhere in that report said we're building on soil that's not suitable, and someday we'll pay for it, he's right," said Rick Horton, president of the Dallas division for the Arlington-based builder. "As a group, and I'm not saying it's us, homebuilders occasionally make decisions based on money and not common sense."
Horton said the worst soil forms a vein running through Frisco, Little Elm and all the way to Flower Mound. He suspects the vein was a river in prehistoric times.
"At its widest, the vein is probably 10 miles wide," Horton said. "At its narrowest point, it's probably two miles wide. You can build a lot of houses in two square miles."
Bob Bush, an Arlington attorney who represents many of the area's homebuilders, said builders have made a series of technological advances and developed complex engineering plans to grapple with the problem.
What isn't apparent from the points raised in the study, he said, is that expansive soil is "extremely site specific," and doesn't apply throughout entire sections of the Metroplex. But builders build to the market.
"Most of the residential development is basically adjacent to I-35. That's probably some of the least desirable soil from a construction standpoint," he said. "Ideally, you want to build on limestone. But people don't want to live there."
Officials in The Colony, Little Elm and Hebron could not be reached comment.
New regs in Frisco
Williams' study identified expansive soil as particularly severe in Colorado, Texas and Wyoming, and focused on local foundation nightmares dating back to the 1980s.
A 1986 survey of 10-year-old houses built in Carrollton, for example, showed that 26% to 36% had structural damage attributed to expansive soils. The earlier study also concluded that 8,500 residential foundations in Dallas are damaged each year when the ground swells and contracts.
Williams' study concludes that concrete-slab foundations are particularly prone to crack or break when soil expands. But builders use them anyway to save money, he said.
1998 survey showed that slab foundations were used in more than 95% of new homes in the city of Denton, Williams said. His random Internet survey of Frisco real-estate listings, he said, showed 49 of every 50 houses built between 1990 and 2002 had slab foundations.
The study looked at population growth in the 12-county region known as the Dallas-Fort Worth Consolidated Metropolitan Statistical Area, which logged a population jump of 29.3% from 1990 to 2000, adding 1,184,519 people. Denton County's population jumped by 159,451 people, or 58.3%. And Frisco's population exploded by 449% during the period -- growing from 6,141 to 33,714.
Williams linked the soil data, which was classified based on propensity to flood, texture and soil corrosivity to uncoated steel and concrete.
"You have the clay, and you have the climate that contributes to expansive soils," Williams said. "You get a dry summer and a wet winter, and you have the ability of the soil to swell up."
Aware of its role as the vortex of Metroplex growth, Frisco adopted one set of new foundation requirements in 2001 and a second set on May 1.
"We kind of saw this coming, and we've been trying to get ahead of the curve," said Donnie Mayfield, Frisco's chief building official. "We had observed what happened to our sister cities, mainly in the Las Colinas area. We talked to engineers. We talked to developers and we tried to come up with the criteria that would appeal to everybody and ensure the safety of the houses."
Frisco now requires an engineer of record for all homes, and asks builders to submit structural design plans and conduct geo-technical investigations. The city has adopted the recommended practices developed for foundations by the Texas section of the American Society of Civil Engineers, Mayfield said.
The problem also has become a focus for change by the Home Builders Association of Greater Dallas.
The association has been active in trying alternatives to standard concrete slabs. In Frisco, those have included steel-frame foundations and a process of injecting water into a layer of soil beneath the surface and covering that with a tarp before the final fill.
"History is going to tell what happens," said Frisco's Mayfield. "But a person buying a house in the city of Frisco is a lot better off than buying somewhere else, because of the requirements."
Contact DBJ Tarrant/Denton editor Michael Whiteley at mwhiteley@bizjournals.com or (817) 693-0023.