Segmental Retaining Walls
Offer Cost-Efficient Grade Solutions

By Dean Sandri, PE
Soil Retention Systems, Carlsbad, CA

 

Beautiful, plantable retaining walls like this one at a residential housing project are enjoying an increased popularity because of their low costs and environmental esthetics.

Efficient use of land is becoming more important in today’s competitive marketplace and nobody knows that better than those in the land development business. Sky-high land prices, regulatory mandates for land consuming public “perks” (parks, schools, access roads and other amenities) and consumers demand for large homes on big lots in lavishly landscaped, upscale communities, at reasonable prices are forcing developers to scrutinize every detail of the development process. To keep costs down, developers are looking to unique products and processes to facilitate efficient and innovative methods of construction while maximizing the desirability and use of property.

Segmental Retaining Walls

Developments constructed in sharply sloping terrain typically require retaining walls to maximize the use of available land and keep unit and lot costs under control. While retaining structures are typically one of the highest cost items associated with land development, segmental retaining walls are recognized at cost-effective retaining wall variant. Prudent use of retaining structures can also maximize lot premiums by optimizing view perspectives. On one recent project, incorporating segmental wall alternatives created half-million dollar lot premiums for the developer.

The segmental retaining wall, still under construction, is contoured and sculpted to blend with the natural topography of the site.

Given the cost advantage of segmental retaining walls and the aesthetic advantages of walls that can be vegetated, plantable retaining walls are enjoying an increased demand. Developers are attracted to the low cost, while public agencies readily accept these structures for their soft, natural, “green” appearance and public focus on environmental responsibility.

Flexibility

Cold, stark, masonry retaining structures used to be the norm in providing sharp grade transitions between terraces of graded tracts. Today, the flexibility of segmental walls has enabled developers to cost-effectively extend the use of “contour” grading to the retaining walls of a project. Segmental retaining walls that “follow the grade” or are contoured and sculpted to blend with the natural topography of the land provide aesthetic superiority to conventional structures while being easy and cost effective to construct. The freedom provided by this newfound flexibility enables civil engineers and landscape architects to create natural and interesting environments for the homeowner. Once constructed, plantable walls can be vegetated with plants selected for their natural appearance or aesthetically enhancing properties. The combination of contour grading and vegetation helps to create a natural ambiance in an otherwise sterile environment.

Timing

Installation of retaining walls is often the “critical path” item in releasing lots for construction and is sometimes required before creating construction entryways when site access is limited. On large projects, retaining wall choices can impact pad delivery by several months. A delay in getting critical walls completed can result in project delays and immeasurable costs to the developer. Anything that can be done to shorten the construction time of critical retaining structures is important.

Construction Integration

Significant time and money can be saved if wall construction can be integrated into the mass grading. Integration begins by including retaining walls in the grading permit, which can shave months off subsequent permitting phases. It also provides the added advantage of enabling the grading, including the walls, to be completed without interruption.

The cost savings of integrating wall construction with the grading operation will result in significant cost savings over alternative methods, but the placement of the geosynthetic reinforcement and wall facing must be integrated with the placement and compaction of the fill. If this can be facilitated, the cost and time of double dirt handling is avoided.

To effectively integrate the two processes, the contractor must be able to coordinate schedules and build fast enough to keep up with the grading operation. In most cases, automated fascia handling equipment and experienced crews are required. Specialized, state-of-the-art equipment is available to facilitate fast and efficient wall construction.

When building large commercial retaining walls, experienced crews and automated handling equipment like this “grapplegrab” (background) and side loader (foreground) are essential to keep up with tight construction schedules.

In addition to the advantages of being able to raise the walls concurrently with the site grading, investments in efficiencies in the wall construction typically reduce the bottom line. With a $5,000 per day high-efficiency installation crew, installing at the rate of 500 square-feet per day, the unit cost is $10 per square-foot. The same crews installing only 2,000 square-feet of wall per day produce a unit cost of a meager $2.50 per square-foot. In short, a contractor that doesn’t keep pace with the grading operation can be disastrous to the project, especially if the delays adversely affect the developer’s ability to timely show home models or gain building occupancy permits.

Experience

Integrating retaining walls into a development is best accomplished by engaging an experienced contractor early in the planning phase. Selecting a contractor by inquiring about past performance and wall problems (if any) is a good place to start. Securing the advice and assistance of an experienced plantable wall-building contractor will ensure that a “proven” retaining structure is acquired and that cost and time-efficient installation techniques are employed.

Keep in mind that the cheapest wall provider may not be the most economical. It is typical for the “cheapest” price to be absent of necessary functions that must be done by someone else, usually at significant additional expense. These added costs are transparent or invisible to all but those who have been previously stung by slick marketing documents and savvy salespeople. Be wary of choosing a contractor to build your plantable walls based on information provided in a spreadsheet.

Rosemary, ivy, or flowering vines make great retaining wall plantings. Soil and moisture is designed to be continuous in a good wall system so heat absorption in the concrete blocks won?EUR??,,????'???t desiccate the plants.

Partnering with an experienced wall-building contractor can streamline construction and improve the final layout and design of the walls and associated improvements. Utilization of plantable, visually soft retaining walls provides the development with an attractive, pleasant, nonsterile environment enhancing the aesthetics of the community. The combination of aesthetics, design flexibility and overall cost advantage has made segmental retaining walls the product of choice for grade transition features. Incorporation of plantable retaining walls into your next project will help to put additional green onto your bottom line.

Cost of SRWs

30-40 – Cost of concrete blocks & soil reinforcing (geogrid or geotextile fabric) as a percentage of total cost of a segmental retaining wall

5% – Wall designer’s engineering cost

55-65% – Labor cost

Source: Scott Miller, a Little Rock, Ark. & based civil engineering consultant.