Rammed Aggregate Piers To Support D.C. Condo

Staff | September 28, 2010

In a town where dirty laundry can have multiple meanings, the Yale Steam Laundry in Washington, D.C., makes a great backdrop for intrigue. In its day it was the destination for used linens from the White House, Congress and many hotels in the city, so the plant likely holds many secrets.

Today the mystery is why the early 20th century building at 4th Street and New York Avenue — in the heart of the prestigious Mount Vernon Square Historic District — has been vacant for 30 years. Whatever the reason, general contractor Clark Multi-Family Builders — MidAtlantic, LLC is making it a favored destination once again, but this time as a residence with more than 133 condominiums in a high-rise tower. Marrying the old industrial charm of the original 120-foot smokestack and three-story building with a contemporary 12-story structure and amenities presents a host of challenges beyond building codes and historic preservation issues.

The real "dirt" on the site was the dirt itself, a sandy clay that took as few as three blows in the standard penetration test to determine that it was too soft and prone to settlement. Even though a range of solutions could stiffen the soil, a main consideration faced by Clark was selecting the right one to avoid affecting the structural integrity of the original and adjacent structures.

Use of driven piles would have created too much vibration and noise in the tight urban space. Similarly, the depth of the underground parking garage meant a traditional deep foundation support would have required drilled shafts 80 feet deep — a costly choice. Also uneconomical would have been the deployment of spread footings by themselves due to the extra excavation required and the settlement between adjacent footings. Another pick could have been a mat foundation, but this would mandate a significant, expensive amount of concrete and reinforcement across the whole bottom of the 169,000-square-foot site.

With the limitations of these alternatives and the complications associated with the site, the option that quickly moved to the top of the list was geopier rammed aggregate piers, a system for ramming well-graded aggregate into compacted, 1-foot layers in drilled, 30-inch-diameter holes. Installed by design-build contractor GeoStructures, the system consists of a beveled tamper that forces the aggregate into the sidewall of the cavity, stiffening the surrounding soil enough to control settlement to one inch or less.

For Yale Steam Laundry, the piers range from only 8 feet long to 26 feet long and provide significant support such that spread footings could be designed for an allowable bearing pressure of 8,000 psf. The geopier-supported footings in turn support the dead load of up to 1,300 kips per column and up to the 2,900-kip transient load of a shear wall.

Helping to keep the construction schedule on track was the pre-construction work of the project team. Early coordination of the geotechnical and structural engineers, the contractors and architect allowed enough time to evaluate the ground-improvement options and address any foundation design changes. Optimizing the foundation system with smaller footings, for example, led to a cost-effective design that used less concrete. This coordination among team members kept the pre-construction efforts within the typical one-year timeframe, and it carried through the entire project so that ground-improvement work and quality control took only five weeks.

While the site was conducive to use of geopier elements for foundation support, it was less accommodating for installation. Because of the close proximity to other structures, there were many factors requiring coordination, according to Kris Manning, the project executive at Clark.

"We placed steel piles vertically in the ground to support the shoring around the perimeter of the excavation," he says, "so we needed to take extra caution regarding placement of the geopier elements because the drilling and compaction of the geopier system could affect the structural integrity of the support of the excavation system."

A ramp allowed equipment to be driven down in the hole, but as excavation continued this had to be eliminated. Stone was unloaded from the street level and moved via Bobcat to the 343 holes designated for geopier elements, which left little room to maneuver. When the time came to extract the equipment, including the 60,000-pound Lo-drill mounted on the CAT 320 excavator used in the installation, Clark had to get a public space permit and crane to hoist everything out of the 30-foot hole.

Another complication of working in such a site was the record rainfall at the time. As the project progressed and the soil became denser, there were few places for water to infiltrate so it created enough puddles to become a pool at times. Constant de-watering through a series of drilled holes with casings, transient well points and pumps kept the water at a manageable level and minimized project delays.

With the new structure Yale Steam Laundry will continue its reputation as a place where dirty laundry is washed, but now it will happen inside the condo units, and whatever secrets were contained in the old building are now buried for many years along with the sturdy foundation and support system.