Building site presents retaining wall design and installation challenges.

The relocation site for The Metropolitan Plant Exchange in Fort Lee, N.J., provided a larger-sized area needed to operate its retail nursery, florist, and greenhouse services. However, the land needed to be leveled before the facility could be built. The proposed site solution of building a structural retaining wall, first thought to be routine, was complicated by rock outcroppings, the discovery of subsurface bedrock, and local construction codes.

Rock anchors were used in areas where bedrock in the wall’s reinforced zone prevented geogrid from being placed at its required length.

The final solution was to excavate some of the area’s bedrock and build a wall on and in vertical coordination with the remaining bedrock. Engineers selected a Keystone Victorian Stone wall because it met the site’s challenging demand for a design-flexible, structurally sound wall system that was both long-term and aesthetically-pleasing.

The Metropolitan Plant Exchange’s site is located about 10 miles from New York City, at an intersection of major highways in downtown Fort Lee, N.J. To level the land, a 36-foot retaining wall was to be built on the site’s south side, an area with a 45-degree slope, rock outcroppings, and related subsurface bedrock. In some areas, the bedrock reached a height of approximately 5 feet above finished grade. The related subsurface bedrock varied in depth. Along with the site conditions, the project was further complicated by a construction code that restricted blasting in the area. Also, all construction plans and building had to be approved by the New Jersey Turnpike Authority and local municipal authorities.

A Keystone Victorian Stone retaining wall allowed developers to level a rocky building site.

"Local codes did not allow for any blasting, and hammering out all the bedrock would have used more time and equipment. It was cheaper to just go over the bedrock, so you had to work with it," said George Kreis, general contractor, J. Fletcher Creamer & Son, Inc.

Since excavation of all bedrock was not an option, the wall was to be reinforced with a combination of geogrid and drilled-in-place rock anchors. Extensive site surveying and soil testing established what portion of the bedrock was competent to hold the anchors. The testing also classified some of the soils as SM or SP/SM soils and suitable for use as a controlled compacted fill for the retaining wall.

Weaker bedrock was removed in a non-benched excavation by drilling, splitting, and hammering. Excavation of non-competent bedrock produced gaps along the wall’s base, which were filled with crushed stone. Excavation efforts also included establishing the retaining wall’s leveling pad and drilling holes for rock dowel placement. After the dowels were placed, the concrete leveling was poured and pinned to the bedrock and the first course of the wall was pinned and grouted to the leveling pad.

"It was a restricted space, and we couldn’t work from the front at all," said Wall Contractor Joseph Pillari, Pillari, LLC. "Also, both contractors had to work simultaneously in the same area. At some points, we would build the wall to a certain elevation, and they would continue driving in the rock dowels as we got to a certain elevation. Overall, the installation went well; it was just a coordination effort."

The presence of bedrock in many sections of the wall’s reinforced zone prevented some geogrid from being placed at its required length. Rock anchors were used in these areas (see Figure 1). More than 200 rock anchors were used at various elevations at the base of the wall. Some anchors were placed as high as 8 feet on the wall. A 5-inch-diameter, galvanized schedule 40 steel spreader pipe was drilled and fastened to the rock anchors.

Figure 1: Typical retaining wall section on the Metropolitan Plant Exchange site.

Much of the remaining bedrock occurs at varying angles that produce additional forces exerted on the anchors and grid. The additional forces required extensive analysis that included a combination of hand and design software calculations.

Also engineered was a length extension for the wall. The Keystone Victorian Stone wall system’s flexibility allowed the wall to be extended to take advantage of additional parking space; however, the extension required execution of an acute angle. The engineered design solution combined the reinforcement grid and lean concrete to accommodate the 60-degree-angled corner.

The Metropolitan Plant Exchange presented unique design and installation challenges requiring a flexible wall system. The Keystone Victorian Stone provided a long-term and aesthetically-pleasing solution that produced the maximum structural integrity while maintaining a reasonable project cost.

Tanya Higdem, communications specialist, and Jon Hansberger, staff engineer, are with Keystone Retaining Wall Systems, Inc., in Minneapolis. They can be contacted at thigdem@keystonewalls.com and jhansberger@keystonewalls.com, respectively.