Top 10 US Cities for Retaining Walls

10. Minneapolis, MN
Minneapolis hits the environmental trifecta of retaining wall requirement. Although the city proper and areas near the river tend to be flat, other parts of the city are sloping with relatively unstable soil that create a need for retaining walls. Climactic factors are also an issue as summer rainstorms can dump inches of rain in a short time span. Frost is also a hazard: As the topmost layer of soil freezes it can push against the top of your retaining wall, while the bottom of the wall doesn't receive this pressure. Without proper drainage, this force can threaten to damage or topple walls in cold climates like Minneapolis. There has also been a sharp increase in the number of residential wall projects in Minneapolis relative to other cities, moving from #17 in 2006 to #7 in 2008-to-date.

9. Phoenix, AZ
The sand and clay of the American Southwest may make Phoenix seem like a Mecca of retaining walls, but the city ranks only a modest #19 in terms of residential retaining walls. The relatively flat terrain of the city reduces the need for residential and commercial retaining walls. Interestingly, Phoenix's major contribution to the retaining walls is technology. Post-tension retaining wall technology uses the extra tension created by posts to circumvent the need for grout and rebar. The result is a stronger wall for a fraction of the time and cost to build a conventional retaining wall. Many of the new retaining walls throughout California and the American Southwest uses the post-tensioning technology developed by Harold Greenberg in Phoenix.

8. Dallas, TX
Although not as hilly as some cities on this list, the soil in Dallas and north Texas in general is notoriously unstable. If you do any regrading whatsoever, you need to protect these landscaping changes and/or soil erosion with a retaining wall. On its own Dallas ranks #5 on the list of cities for retaining wall installations, but if you were to include Ft. Worth, the combined cities jump to #2. Much like Minneapolis, depending on the size and design of the retaining wall, homeowners frequently need to include a drainage system to prevent damage to the wall.

7. Los Angeles, CA
L.A. is the city of the most recent and dramatic example of what happens when a retaining wall system fails to hold back the Earth and its submission to gravity and water. The deadly La Conchita landslide of 2005 damaged 31 homes and killed 10 people. Most people don't know that there was actually an 18-foot retaining wall built after the previous landslide of 1995. This wall wasn't necessarily designed to hold back the entire slope of land, but needless to say, the wall toppled when 7,000,000 cubic ft. of dirt fell during the landslide. Throughout L.A., the number of retaining walls projects has moved the city down to #9 this year, although over the past four years L.A. has ranked #4 overall.

6. Pittsburgh, PA
The extensive system of hills and rivers that wind through the city of Pittsburgh has given birth to many residential and commercial retaining walls and extensive tunnels that allow for travel back and forth from the city. Rough-faced cut stone connects the city with a type of retaining wall aesthetic unity from the numerous city parks to the famous Tunnel Walk. Pittsburgh has also made the most significant jump in the number of residential retaining wall installations, jumping from #13 in 2005 and 2006 all the way up to #3 in 2008-to-date.

5. Galveston, TX
The Galveston Seawall is a study in the difference between retaining walls and flood walls. Retaining walls prevent soil erosion and maintain a difference in elevation on each side of the wall. Flood walls or levees are designed, naturally, to prevent an area from being flooded. The Galveston Seawall is both. Not only does it prevent Galveston from the being completely wiped out by flooding, the city's elevation itself was raised to the level of the wall, in some cases, as much as 17 ft. Thus, the Galveston Seawall is both a retaining wall and a flood wall. Hurricane Ike was only the latest of a handful of hurricanes that have threatened to wipe out the city, as was done in 1900, during the worst natural disaster in the country's history. The Army Corps of Engineers credits the wall with sparing the city $100 million worth of damage in 1983 alone.

4. Washington, D.C.
Home to one of the famous walls of any kind in the U.S., the city's Vietnam Veteran's Memorial uses 74 granite panels, held in place by two 246-foot long retaining walls. These two retaining walls are constructed with 140 concrete pilings driven 35 feet into the underlying bedrock. Plus, Washington D.C. is the #2 city for the number of residential retaining wall projects. The hilly terrain and rapidly expanding D.C. suburbs have kept D.C. retaining walls at extremely high levels for the past several years.

3. Atlanta, GA
Atlanta may not have a famous commercial retaining wall, but it is the undisputed champion of residential retaining walls, outstripping even much larger metropolitan areas. Situated at the Appalachian foothills, not only is the city covered with rolling hills, but many older retaining walls were built with crossties that used coal tar creosote for pressure-treated lumber. When this wood rots, it leaks the creosote into the groundwater. Unfortunately, creosote is described by the International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA) as carcinogenic. Thus, replacing older retaining walls in Atlanta has become a public health and safety issue.

2. Cincinnati, OH
Named for the Roman General Cincinnatus and the Seven Hills of Rome, Cincinnati spends more money per capita on retaining walls and landslide damage repair than any other U.S. city. The loose clay that much of the city sits upon is dangerously susceptible to landslides and shifting. Often, homeowners repair damaged foundations, decks, and windows without realizing their home is experiencing a slow landslide that endangers the future of their homes. Between the University of Cincinnati, the U.S. Geological Survey, and federal funding, the city also generally spends more money per capita on retaining wall and landslide research. The cost and necessity of building massive retaining wall systems has left many of satellite hills around Cincinnati undeveloped.

1. Seattle, WA
In terms of both residential and commercial retaining walls, no city relies so heavily on these walls than Seattle. At the beginning of the 20th century, 16 million cubic feet of dirt were removed by blasting 20 million gallons of water into Denny Hill from Lake Union. Half of the dirt was used to fill in low areas of the city; the other half was used to create Harbor Island. To fully appreciate the history and importance of retaining walls, you should really check out the Seattle Underground, the walking tour that takes people under the city itself. The city is ranked #3, over the past four years, in terms of residential wall projects. Meanwhile, the commercial retaining walls of Seattle are like the levees of New Orleans. Without them, the city simply wouldn't exist.