North Atlanta Windermere Project Surmounts Stormwater Management Issues

Staff | September 28, 2010

In many of Atlanta's northern suburbs, retail projects are the bread and butter of many commercial contractors. As populations grow, demand for such projects increases — and one that's currently under construction is the Shops of Windermere, a project being developed by Easlan Capital of Atlanta, Inc., near Cumming, Ga.

The project, being constructed on a 4-acre site, will feature 24,000 square feet of retail space. General contractor is J.K. Lockwood Construction Company, Alpharetta, Ga.

The building itself is a straightforward structural steel structure which will be finished with brick, stucco, stone, and structural masonry. But other project elements — including an underground stormwater retention pond and a challenging concrete block wall — presented unusual sets of challenges for the construction team.

The Wall

One challenge on this project came in the form of a modular concrete block retaining wall which was constructed along the back of the site. The wall shores up a major cut along the back of the building complex, a cut made necessary by the site's steeply sloping topography.

Originally, the wall was designed as a cast-in-place concrete wall with a stone veneer applied to areas that would be exposed to public view. However, the design was changed to utilize modular block instead for reasons of cost as well as feasibility.

The modular block wall, with an overall length of about 850 feet and a maximum height of about 20 feet, is being constructed by Vecco, Inc., Alpharetta, Ga., using precast wall blocks from Keystone.

The wall sits atop a gravel sill foundation. Excavation for the wall and for the sill was handled by Triton Industries, Conyers, Ga., and one of the first challenges faced during wall construction was the matter of what to do with the excavated material. Plans called for using that material as backfill once the wall was complete, but in the meantime the only place to put it was on the building pad. Thus, construction of the wall became an important part of the scheduling operation, since building work could not get under way until that excavated material was removed from the building pad and placed as backfill.

The construction of the wall, which is built at a slight batter, progressed smoothly. Crews constructed it by placing several rows of block at a time, with geogrid tied to the block at the start of construction of each set of rows. The geogrid extended behind the wall for about 20 feet, and as each set of rows was completed the area behind the wall was backfilled, anchoring the geogrid and securing the wall. This process was repeated until the wall had reached the desired height.

Two different block surfaces were used, with more decorative facing on architecturally significant portions of the wall.

The Underground Pond

A second challenge, and a major factor during the site work phase, was construction of an underground stormwater retention pond. Management of stormwater runoff is an important issue on many projects. Frequently, runoff is retained in surface ponds prior to controlled discharge. However, where space is at a premium, other approaches must be used.

At Windermere, for example, the design includes an underground stormwater retention pond. As originally designed, this would have been a 20-foot by 240-foot cast-in-place value. However, value engineering yielded an alternative utilizing precast concrete components — an alternative which saved money as well as construction time. As finally designed and constructed, the finished vault measures 42 feet wide and 121 feet long and has a height of 10 feet 9 inches. It will be totally buried (and totally out of sight) once the project is completed.

Work on the pond began with construction of concrete footings around the pond's perimeter — but only after groundwater was brought under control.

"The original water table was 5 feet higher than the finished floor of the underground pond," notes project superintendent Mike Crossan, "so we had to put in French drains to lower the water table." Triton Industries handled the necessary excavation for drain installation (as well as all other excavation on the project) using a Cat 330 and a Cat 963.

Art's Concrete of Lawrenceville, Ga., then began constructing the underground pond's perimeter footings, which measure about 6 feet wide and 1 foot deep and completely ring the perimeter. Concrete from Ernst Concrete was placed directly from ready mix trucks, which accessed the pond construction area via a ramp constructed at the west end of the site.

The area inside the footings — that is, the bottom of the underground pond — consists of a 1-foot lift of gravel sitting directly atop graded dirt.

Once footing work was finished, crews from precast concrete supplier Metromont began assembling the underground pond's precast wall sections. All precast components were set by Superior Rigging & Erecting using a 70-ton crane. The wall sections, which were installed first, had been manufactured with an inside lip at the top of each section. Once the walls were in place, traffic-rated precast double tees were set into position on the lips to form the "lid" atop the underground pond.

"The largest precast piece weighed about 32,000 pounds," notes Mike Crossan, superintendent on the project, adding that installation went quickly.

"Metromont started placing precast on a Thursday morning," he says, "and was finished by Friday night."

A rubber membrane and tar were used to seal the joints between wall panels inside and out, and edges were grouted by hand. The exterior of the underground pond was then backfilled, with fill compacted using a small roller which could work close to the concrete walls.

To finish the underground pond, the double-tees were subsequently overlaid with a 5-inch lift of concrete reinforced with #4 and #6 bars. This resulted in a traffic-rated roadway over the top of the underground pond, necessary since the project's parking lot would subsequently be constructed on top of it. Subsequent site work overlaid the underground pond's concrete top with a foot of dirt, 6 inches of crusher run stone, and 2 inches of asphalt to create the parking area.

Water will enter the underground chamber via one 36-inch pipe and two 18-inch pipes and will exit the chamber through an outlet control structure with 1-inch holes, pass through an oil and grit separator, then continue out through a 24-inch pipe to a manhole located adjacent to the nearby highway.