Project Profile: Optimizing Space
Finding land for a new development is usually not an easy task, and that is especially true for the United States in the densely populated areas of the Northeast. Developers must balance the attractiveness of the location with the cost of the parcel, and then again with the cost of developing that parcel. Easily developed parcels are usually no longer available, and thus developers are forced to work with sites that often provide significant challenges just to prepare for the building pad. Because the flat land with lots of elbow room is all spoken for, today’s developers are looking to develop sites that 20 years ago would have been ruled out as simply too challenging or expensive to prepare.
With the development of new sites, the project team is generally trying to make the most of the space available. Frequently, that requires cutting into existing hillsides to maximize usable real estate at the bottom of the cut, filling low elevations with newly imported soils to maximize the usable real estate at the top of the fill, or a combination of both. When a developer is cutting into the side of an existing embankment, the goal is to push the cut back into the hillside as far as possible without crossing a property line and without causing site disturbance to the adjacent property during the construction process.
This was the case when real estate developer Murray Hills Inc. began the process of planning for a new condominium complex in downtown Burlington, MA. The site selected had the great location that was essential, but also had its fair share of preparation challenges. The site was at the base of a substantial hill, and to create the footprint necessary to accommodate the proposed 37 condominiums, 70 parking spaces, and additional green space, excavation into the hillside was required. The cut would require construction of a retaining wall that would reach 24 feet in height, and to complicate things, utility lines were buried in the hillside just 10 feet behind the face of the proposed retaining wall.
The advent of large precast segmental retaining wall systems in the past 10 years has brought a new tool to the aid of the developer trying to stabilize a cut into the side of a hill. These systems have a size (from 5 to 8 square feet of face per block) and mass (ranging from 1,000 to 4,000 pounds per block) that can often stabilize the cut without the use of additional soil reinforcing geogrids, which would extend back into the slope behind the wall. These walls are called gravity walls, and because they do not require geogrid, they minimize the amount of excavation into the hillside that would otherwise have been required. In addition, without geogrids, a gravity wall can be placed closer to the property line or can eliminate excavation that might not be possible if utilities already exist in the hillside. These large precast segmental retaining wall systems have aided many developers in solving site development challenges in an efficient and affordable way.
“When trying to decide which wall system to utilize, we took several things into consideration,” says Michael Murray Jr. of Murray Hills Inc. “Appearance, cost, and engineering were all essential.” This led the company to select the ReCon Wall System, manufactured by Shea Concrete of Amesbury, MA. “The ReCon system met all of our needs. Their new Rustic texture provided an authentic stone appearance that was appropriate for the site. Also, being able to design the wall without geogrids was very helpful. It eliminated a significant portion of the excavation and associated replacement of fill,” says Murray. “In addition, it avoided the need to remove existing trees that provided a buffer between our project and the adjacent residential properties. All in all, the ReCon Rustic block look and the gravity wall engineering was a perfect fit for our site and project.”
Given the height of the wall-24 feet-as well as the need to complete the construction in the winter months and the limited area available for construction equipment and block storage onsite, it was important that the wall contractor be experienced and that the block supplier be able to consistently deliver materials in a “just in time” fashion. Murray Hills selected John E. Farmer & Sons of Billerica, MA, to construct the wall and Shea Concrete to supply the ReCon block.
Mark Farmer, P.E., project manager for John E. Farmer and Sons, notes the site posed a number of challenges for the construction of the retaining wall. “The site was tight, with not a lot of room for staging ReCon block. At the same time, when the cut was open, it was essential that the block be delivered on time to keep the crews productive and to minimize the chance for slope erosion during construction. Because of the height of the wall, construction was done in sections, 50 linear feet at a time. At times work was completed using a trench box to protect the workers. Not having to excavate further into the slope for geogrids was essential. In some locations, the face of the wall was within 10 feet of existing utilities buried into the slope. All of the construction was done from the front of the wall using a backhoe to place the ReCon block. As the wall got taller, soil was temporarily placed in front of the bottom 10 feet of the wall so that the backhoe could reach to the top of the 24-foot wall.”
Greg Stratis, manager of Shea Concrete’s Amesbury, MA, facility, worked with Civil Solutions Group LLC of Bloomington, MN, to complete the design for the project. The mass at the bottom of this gravity wall utilized both 60-inch- and 39-inch-deep ReCon blocks stacked directly behind each other. “We pride ourselves in working with our customers to identify solutions to their grade separation challenges,” says Stratis. “In this case, a unique ReCon gravity wall application delivered on the need to find a cost-effective, well-designed, and aesthetically pleasing solution for the Burlington Heights condominium project.”