Article No: 26

2006-04-28 10:15:02
Using ICFs to build retaining walls

Insulating concrete forms (ICFs) are often touted as superior to wood and other materials for the construction of above grade walls. But some builders are also finding ICFs to have certain advantages for the construction of retaining walls.

Arkady Horak, owner and president of Austin Insulating Concrete Forms in Austin, Texas, says one of the chief advantages of ICFs for retaining walls is increased productivity. The company distributes forms in Austin and the Central Texas area.

"We recently used pre-assembly techniques to prestack wall form sections 4 feet tall by 8 feet long," Horak says. "The old fashioned way of stacking blocks is very time consuming. For example, a retaining wall that would typically take four to five days, we completed with Lite-form in half a day. So basically, in four hours, I was doing what took a contractor about 30 hours."

Photo courtesy of Austin ICF

A retaining wall is generally defined as a vertical wall that holds back earth, but this definition covers a host of uses. Some of these include embankments running along the side of roads and walls to separate older roads from expressways (these are often precast concrete); cutbacks to squeeze in parking areas; driveway walls leading to under-house garages; walls for septic tanks, swimming pools and outdoor fish tanks; and barriers for gasoline and volatile chemical storage tanks.

Terraced lawns, planter boxes and support around trees on slopes are some of the landscaping uses of retaining walls. Retaining walls are also used to keep river banks from eroding — San Antonio's famous River Walk depends on 30 or 40 miles of retaining walls.

Most of Austin ICF's retaining wall construction in Central Texas involves walls for three-sided basements. "Here a lot of people choose to live on the side of a hill or river, and that lends itself to walk-outs," Horak says. "Traditionally, the sloping site is excavated, and then the contractor pours the slab and footing. The 12-inch thick retaining walls, which may range in height from 8 feet to 12 feet or 14 feet, are considered part of the foundation structure of the home.

"Ninety percent or more of residential retaining walls are currently being built using conventional plywood forming techniques," Horak adds. "There are also a lot of metal form systems for retaining walls. There are some precast retaining walls, but precast is not a popular choice. Landscapers traditionally use stacked, non-mortared concrete block or brick, as their crews generally do not have retaining wall experience. Most landscape walls are not very high and therefore do not require much bracing. Taller walls develop more pressure on the forms due to the weight of the concrete, which is about 144 pounds per cubic foot."

Horak has found ICFs to be very efficient for retaining walls, although he has had to overcome challenges presented by both construction crews and engineers who are not accustomed to ICFs. "There are not a lot of people experienced with ICFs in the area," Horak says. "A large percentage of residential concrete construction crews in Texas depend on untrained workers who are not familiar with retaining wall construction techniques."

However, training unskilled laborers to work with ICFs is not difficult, Horak says. "Typically, I train crews onsite," he says. "I can train them in less than a day, and they are fully trained after one project. It is not rocket science. I can train most construction workers to stack ICF walls in 20 minutes."

Photo courtesy of Austin ICF

Perhaps one of the most important differences between ICFs and other methods for building retaining walls involves meeting code. Inspectors keep a close eye on retaining wall projects, watching for products and methods that are not suited for retaining walls. The first thing engineers check is whether the steel is properly placed and that the correct wall width is constructed.

One advantage of using Lite-Form is that it will create a flat finished wall, Horak says. Unlike in above grade housing, retaining walls must use ICFs that will create a flat wall of concrete. If the polystyrene foam is removed, one would not be able to tell whether the builder had built the wall using wood, steel or ICFs because the end result is the flat wall. Only that is acceptable to the engineers. Typically, no modification to the foundation is required when using an ICF system for a retaining wall.

Most above grade ICF walls in residential applications are not given the same attention as below grade retaining walls. Almost every municipality that inspects building projects will do a thorough pre-pour inspection of a retaining wall that must be designed by a state registered structural engineer.

Sometimes situations arise that call for an ICF. Horak described his experience with a subdivision project where a decorative perimeter block wall needed to have a retaining wall placed in front of it due to a new grade height introduced by a nearby road project. The subdivision developers would either have to tear down the non loadbearing block wall and replace it with a retaining wall or place a retaining wall in front of the existing block wall. This, however, presented a problem in that one face of the wall form would be left between the original block wall and the new retaining wall. The project engineers wanted a separation between the two walls, and the ICF system allowed for that with the 2-inch thick foam panels used to form the inside of the wall. ICFs were the only cost-efficient alternative because the city would not allow them to leave plywood forms against the wall, and leaving steel forms would be too expensive.

Because of their simplicity, many landscape retaining walls can be classified as do-it-yourself projects. However, Horak says many ICF manufacturers are looking to hardware and lumber stores as potential national or regional distributors due to the increase in the number of homeowners willing to take on this kind of project.

One of the main obstacles to such projects is the placement or pumping of concrete. "Pumper trucks can be rented for about $400 for an afternoon, but you still have to know what you are doing," Horak says. "When you fill ICFs made with polystyrene, it is very different than pouring concrete into wood or steel forms. ICF forms can blow out if the pump rate is too high. They can be less forgiving than wood or steel forms."

Horak considers 14 to 16 feet in wall height for a residential project as nearing the threshold of pain for single pour ICF projects. If a project requires a higher wall, the wall can be formed and poured in two levels.

"When we sell forms to a retaining wall customer, we remind them of the need for an engineered plan which must be approved by the local building authority," Horak says. "If this requirement is met, there is no excuse for not having good retaining walls."