Landscape Architect Articles

LASN PMBR March 200403-01-04 | News

The Benefits of Machine Placed SRWs:
Increased Speed, Decreased Costs

By Tyler Matys and Allison Uher

Larger retaining wall systems can incorporate such structural items as poured-in-place crash barriers, flexible steel beam guardrails, and wood fences; all are easy to combine with a machine-laid wall.

Retaining walls have become a mainstay in landscape construction. Segmental retaining walls (SRWs) in particular have been lucrative for landscape architects and contractors with their relative ease and speed of installation, as they do not require an expensive poured-concrete footing in most cases, and there is no need to apply mortar. Some manufacturers have made these walls even simpler to install by producing solid, self-aligning units (no need to waste time filling the cores) that don?EUR??,,????'???t require any additional pins or clips.

Segmental retaining wall systems can be broadly divided into two main categories: hand placed and machine installed.

Depending on the scope of the project and other situational factors that may come into play, you may find that it is more cost-effective to choose an SRW system that is machine placed. This article will discuss the benefits of machine-installed systems, and provide some installation tips.

Broadly defined, machine-placed systems are characterized mainly by their massive size and weight. Typical machine-installed systems may range between 400 and 1,700 pounds. In most cases, these units are placed using a simple clamp device. The clamp and block are lifted with common construction equipment such as an excavator. Despite their large size, some machine-placed systems have a highly aesthetic split-rock appearance, making them suitable for a wide range of applications, from residential to commercial.

A JCB 407 articulated wheel loader moves pieces of a segmental retaining wall. Many landscape professionals have found that specifying machine-placed SRWs is more cost efficient that hand-placed SRWs, especially for large projects.

One of the major benefits of machine-installed systems is increased efficiency and productivity. These larger systems should allow the contractor to place more square footage per day when compared to hand-placed installation, due in part to the larger unit size, but also because of the increased speed of the machines. This means less dependence on the energy and physical strength of those doing the installation, factors that often will slow a project down and waste valuable time, and in turn, money. Reliance on the energy level/capacity of the workers adds another level of variability to estimation, speed of installation, and scheduling. Remember that simplicity reduces variability, if some of the more difficult and time-consuming tasks are alleviated from the shoulders of the contractor, it is easier for you to estimate the speed and therefore the cost of construction. To give you an idea of how reducing labor costs can reduce the project cost, a recent article in an industry publication estimated labor as 55-65 percent of the cost of a hand-placed SRW installation.

An equally important reason to select a machine-assisted SRW is to protect the health and safety of your employees. The National Institute for Occupational Safety and Health (NIOSH) has recently lowered the recommended acceptable lifting weight of an industrial worker to well below the single unit weight of many hand-placed systems. Selecting a machine-placed system will let the equipment do the heavy lifting, protecting your workers from job-related injuries and reducing costly downtime.

Another consideration with regard to the workforce is the availability of manual laborers. Our conversations with contractors in various areas of the United States and Canada seem to indicate that the market has recently been showing a downward trend in the availability of manual labor. The consensus among landscape contractors is that a good man is hard to find, and when you do, he may be tough to keep. One possible solution to this potential shortage of workers is to move some of the brute lifting to the backs of machines.

This joint close-up shows how easily a landscape feature like this wooden fence can be added to a machine-laid segmental retaining wall.

Beyond the aforementioned effects on labor, a machine-laid SRW may have several other advantages that will save time and money, or will allow for a more seamless integration with other landscape features.

One structural advantage is the additional stability inherent with larger units. A greater mass will tend to better compensate for the potential variability of site conditions. Larger SRW systems will be less sensitive to overcompaction or the presence of heavy machinery behind the wall facing, or other minor contractor error. Larger retaining walls can also be helpful because they incorporate other structural items well. Poured-in-place crash barriers, flexible steel beam guardrails, and wood fences are all easy to combine with a machine-laid wall. These larger SRWs by sheer weight and size are more resistant to vandalism and damage, an unfortunate fact of life in public areas.

Another benefit of larger SRW systems is that they may minimize excavation costs. Because of their larger unit dimensions and weight, they can in many cases be constructed as conventional gravity walls (not requiring geogrid reinforcement). Because the wall units are better able to rely on their own gravity for stability, the need to lay geogrid is often eliminated, thereby reducing the amount of excavation to be done on-site. This decreased area of excavation will permit the wall to be built closer to existing obstructions (structures, trees, etc.), as well as solving difficult property line issues (as geogrid reinforcement cannot extend over property lines).

Any type of segmental retaining wall system will bring added dimension and beauty to your landscaping project; even the smaller, more ornamental garden wall will add charm and a finished look to your design. Next time you have a larger, more demanding site to contend with, consider a machine-placed SRW. You might be pleasantly surprised at how easy they are to install, and how striking they are to look at.

About the authors: Tyler Matys is the manager of engineering design at Risi Stone Systems. Allison Uher takes care of the marketing and communications for Risi Stone Systems.