Making productive use of the most square feet of a development site is a challenge for civil engineers everywhere. This challenge is even more acute in markets with limited available land and expensive real estate, such as New England. A particular challenge is constructing retaining walls to achieve necessary grade changes with limited setback from the right of way or from existing utility easements.
This CVS Pharmacy project in Plymouth, Mass., presented a common challenge. To make room for parking areas, grade changes of 10 feet or more were required adjacent to a state highway. However, to accommodate the site layout, the wall would be installed as close as 7 feet from the property boundary and as close as 13-1/2 feet from the highway. A geogrid-reinforced segmental retaining wall would not fit within the available space, and a cast-in-place cantilever retaining wall would be substantially more costly. The sitework contractor, SLT Construction of Carver, Mass., turned to a precast concrete modular retaining wall system that could be installed in a gravity configuration without encroaching into the right of way.
Gravity retaining walls are a category of structure that relies primarily on the self weight of the structure — instead of external reinforcements, tiebacks, or restraints — to resist sliding and overturning forces. Gravity retaining walls are not a new technology, but precast modular products have made this a much more economical earth retaining solution. These precast units also provide a significant decrease in the time required for installation and provide a more aesthetic solution.
The retaining walls for this project were constructed using Stone Strong retaining wall blocks produced locally by MBO Precast of Carver, Mass. The Stone Strong product consists of a large block with face dimensions of 96 inches long by 36 inches tall. Each block covers 24 square feet of wall face. The typical block is 44 inches wide (measured face to tail), creating a wide footprint for gravity wall applications. The Stone Strong units are precast with internal voids that are filled with drainage aggregate to provide interlock between courses and adjacent blocks, and to provide internal drainage within the completed wall. The precast concrete blocks weigh 6,000 pounds each and have a total weight of 10,000 pounds when filled with aggregate.
This project’s site design called for a retaining wall with exposed heights as high as 10 feet. The contract drawings required the contractor to provide a design-build solution to safely support the state highway right of way. Thiele Geotech prepared the shop drawings for the project, performing the analysis using proprietary spreadsheet solutions that were developed for Stone Strong Systems following American Association of State Highway and Transportation Officials (AASHTO) design standards. This is the only product that has undergone the rigorous review process of the Highway Innovative Technology Review Center (HITEC) to demonstrate conformance with AASHTO specifications.
The wall system provided an aesthetic solution that fit well within its historic New England context. The precast blocks had a chiseled granite face reminiscent of traditional stone masonry used throughout the region. The finished retaining wall provided a fitting appearance opposite an old church located across the state highway from the development site.
The completed wall was as tall as 12 feet including embedment. Mass Extender blocks were used for the bottom course in areas where the total wall height exceeded 9 feet. The Mass Extender block is 56 inches wide and weighs 10,000 pounds (14,000 pounds when filled with aggregate). Regular blocks were used for the remaining courses (see Figure 1). The wall units were set on a compacted dense-graded aggregate base and backfilled with native soils. By installing the units in a gravity configuration, the blocks were placed entirely within the property boundary and were installed without disruption to the highway.
In addition to the technical advantages of a gravity retaining wall system, the versatility of these precast units was an important factor in the success of the project. The precast modular system includes various accessories that allow the wall to adapt readily to the necessary site geometry. Accessories include end blocks, corner blocks, and smaller 6-square-foot and 3-square-foot blocks that match the 44-inch width of the larger units. By using these accessories, the facing was installed on a tight radius curve and included a 45-degree bend. The wall wrapped around the corner of the site and a tiered configuration was used to reduce the visual scale in an area where a secondary street angled away from the parking lot.
The contract drawings also required a decorative steel railing for the safety of pedestrians on a walkway installed between the wall and the highway. Utilizing the internal voids in the Stone Strong blocks, the posts for the railing could be installed inside of the block close to the wall face. The top course of the wall consisted of a coped “top unit” that allows landscaping right up to the face shell, eliminating the need for a separate cap block.
In addition to the other advantages of a precast modular gravity retaining wall system, SLT Construction chose the Stone Strong system for its speed of installation. The contractor was able to install three walls in seven working days with a four-person crew. The large precast blocks are set with an excavator, minimizing the labor associated with lifting and setting small segmental blocks.
The retaining walls were completed in 2008. With landscaping in place, the walls blend in with the historic character of this New England setting.
Daniel Thiele, P.E., is president of Thiele Geotech, Inc. Kelli James, P.E., is manager – Design Engineering at Thiele Geotech, Inc. Thiele Geotech is a 50-person firm with offices in Omaha, Neb., and Des Moines, Iowa.