Smithsonian National Zoo

A Design-Build Partnership at Smithsonian National Zoological Park

With an original design in hand for a cast-in-place wall at the Smithsonian National Zoological Park in Washington, D.C., the general contractor took a fresh look and settled on precast concrete. GC Hensel Phelps considered a few of the benefits of precast versus on-site construction – speed, less disruption at the site, less on-site labor needed, and, of course, aesthetics – and proposed a design-build partnership for the 850-foot long x 30-foot high simulated stone retaining wall. The wall would replace existing structures along an extensive drop off along the entrance road above parking lot C.

Enter Smith-Midland, partnering with contractors Hensel Phelps and Athena Construction Group to manufacture the precast concrete panels, and M & J Welding to complete the welding and wall erection. The new game plan involved the use of a shotcrete wall, with placement of precast concrete panels as a facing.

The panels were formed and hand-stained to mimic the look and color variations of a natural stone wall, based on the stone wall nearby on the Rock Creek Parkway. SMC prepared a mock-up of a wall section and delivered it to the National Zoo prior to production to serve as a proof of design for what was to be accomplished.

“This wall has all the color variations of a natural wall and is a permanent penetrating stain that will last indefinitely,” said Ashley Smith, President of Smith-Midland Corporation. “We are proud of this project that exemplifies Smith-Midland’s ability to innovate, improve and initiate products that are current and pleasing to the eye. We worked out the logistics with multiple contractors on this project and delivered a quality product for our customer.”

Panels were erected using a crane and were connected to the shotcrete wall with stainless steel plates and anchors. The erectors set seven panels per shift, a rate of about one panel per hour. Installation went quickly. Within a few weeks, a majority of the 30-foot high wall was completed. Hensel Phelps’ site manager, Kim Slusher, said the benefits of completing the project through design-build with precast were profound, compared to the original cast-in-place concept. “You have a better-looking wall this way,” Slusher said. “The joints all lined up well and the shoulder joints are set consistently. Speed of construction was good. The owner is pretty happy.”