Fairfax County Public Safety Headquarters
This project consists of new construction of a 274,000 sf, 8-story headquarters building to house police officers and firefighters, and a 5-level, 850-space, secured parking garage for public safety vehicles.
The project has been designed to meet LEED Silver certification and will be the county’s first to use energy-efficient LED lighting throughout the entire building. Other environmentally friendly features include:
- Green roofs
- Low-flow plumbing fixtures
- Eco-smart environmental controls
- Permeable pavements
- Rain water harvesting
The use of local aggregates in the Geopier elements helped contribute to the LEED certification for the project.
Project Challenges included:
- An aggressive schedule – out of the ground before winter
- Heavy column loads – 1680 to 2000 kips
- Overlapping footing stress from closely spaced adjacent footings
- Uplift loads in shear wall footings ranging from 100 to 300 kips
- Poor foundations soils – 2.5 to 18 ft of soft fill soils ranging from silts and clays to silty sands, underlain by relatively low blow count (N= 6 bpf to 15 bpf) residual soils up to 45 ft deep underlain by weathered rock
- Deep Drill Depths – in order to install aggregate pier elements deep enough to tag weathered rock when needed
Due to the presence of a thick loose upper soil stratum, high column loads and design issues causing overlapping adjacent footing stress, shallow spread and continuous footings were not an economically feasible options.
After considering various options, 36-inch Geopier elements were selected to provide support in both the parking garage and operations building. Due to the heavy loads, adjacent footing design requirements and challenging subsurface conditions, utilizing a high capacity 36-inch diameter Geopier element proved to be the most economical solution. To handle the uplift requirements in the office building, 25 kip uplift anchors, consisting of a steel plate and two threaded rods, were installed within the Geopier elements supporting the shear wall footings.
Pier design lengths for the office and parking garage ranged up to 35 ft deep, due to the need to improve the ground underneath adjacent footings as large as 20 ft square. Due to changes in grades on the site and relatively thick footings, drill depths of up to 50 ft were required to install piers from the existing ground surface. This required the use of the Geopier X1™ mandrel which has been used to install piers up to 60 feet long. The Geopier X1 mandrel is attached to a vibratory pile driving hammer (same equipment used to install Geopier Impact Pier elements) which can provide up to 30,000 pounds of downward crowd force, in addition to the force imposed by the vibratory hammer.
The benefit of the 36-inch Geopier X1 mandrel is that allows for large diameter, high capacity piers to be built quickly. The mandrel stays in the drilled hole and stone is continuously poured into mandrel which results in quick compaction and minimizes the potential for caving. The Geopier X1 mandrel was the right tool for the challenges of the Fairfax County Public Safety Project!
County of Fairfax - Fairfax, VA
Manhattan Construction Company - Falls Church, VA
Hellmuth, Obata & Kassabaum, PC (HOK) - Washington, DC
William H. Gordon Associates, Inc - Chantilly, VA
ReStl Designers - Washington, DC
ECS Mid-Atlantic, LLC - Chantilly, VA