Building a Waffle on a Peat Bog
This apartment project won't get bogged down by the neighborhood's geology
Rafn is pleased to be partnering once again with a local family on the development of a 5.5 acre urban village in Seattle's Greenwood neighborhood. The fourth project in this partnership is called Greenwood Town Center North and will consist of 141 units of market-rate apartments over retail spaces and parking.
Getting this project off on the right foot took extra care due to a geological oddity of the Greenwood neighborhood: It's built on a peat bog.
It's interesting geology, says Rafn Senior Superintendent Phil Wheeler. Bogs are made up of partially decayed organic material that has filled in a marshy area over hundreds of years. Chances are you've seen a bog, or one developing, and not realized it, Wheeler says.
While the soil of a mature bog might appear stable enough to build upon, it requires a continuous supply of water to keep the soil moist. Without that moisture the ground will sink.
"Think of it as a waterbed," Wheeler said. "If you placed a box on a waterbed and I let the water begin to leak out, the box would begin to sink."
As the total area of impervious surfaces, such as roads and parking lots, has grown over the years, the bog has begun to be deprived of water, resulting in some areas of noticeable differential settlement. Sidewalks, streets and even homes have been affected.
Geotechnical Engineers Provide Solution
Several years ago, the City of Seattle took action to address settlement due to development on bogs around the city by adding Environmentally Critical Area (ECA) regulations to its municipal code. Among those regulations is the requirement for a geotechnical evaluation of major construction sites.
Having worked on projects in the Greenwood area for more than a decade, Rafn is well-versed in working with the City for permitting and geotechnical engineers for consulting, Wheeler said.
As part of the pre-construction phase, the geotechnical engineers took core samples from the site to determine the soil composition and recommended the best course of action for site preparation and construction of the foundation.
The solution required drilling down past the peat and the lake silt and into the hard, glacial till, Wheeler said, to anchor 260 auger-cast piers into stable soil. The length of each pier which was determined on site during the drilling process by the geotechnical engineer and ranged from 21 feet to 41 feet.
After the drill reached the appropriate depth, high-strength concrete grout was pumped into the 18-inch diameter hole. The concrete displaces the water and silty sludge, pushing it out the top of the hole. Steel supports are then sunk into the concrete to serve as the underpinning of the foundation.
An additional nine pilings were drilled to support the tower crane that will be used for construction of the building.
Once the concrete has cured, the foundation is formed and poured. Again, due to the nature of the soils, special consideration is given to this step of the build.
This recipe calls for a foundation system nicknamed “the waffle”. This style of foundation gets its name from the Belgium-waffle appearance the foundation has on its underside. The deep pockets of the waffle are filled with drainage rock that allows for the natural groundwater to ebb and flow without causing the foundation to crack and settle.
Steel reinforcements are placed in a grid pattern over the waffle forms before the concrete pad is poured. Then reinforcing steel (rebar) is carefully placed inside the concrete. The concrete resists the compression stresses and the steel inside the concrete resists the tension stresses. When combined it's called a composite but more commonly referred to as reinforced concrete.
"The waffle is a series of beams that run in two directions that tie it all together," Wheeler explains. "These structural beams transfer the load into the 260 piers below, which transfer the concentrated loads into the load bearing glacial till. Structural engineers call this the 'load path.'"
Methane barriers are then placed around the edge of the foundation. It is part of a methane collection system that mitigates the natural methane produced by the decomposition of the organic material in the peat bog.
Once the foundation is poured and set, the construction of the apartment building is business as usual. The geotechnical engineers will continue to help monitor the groundwater on site during construction and be available to provide advice throughout the project.
To celebrate the successful completion of these first phases of construction, Wheeler hosted a "Waffle Day" for his crew, cooking waffles and sausage on site. And Wheeler didn't forget the real Vermont maple syrup, a tribute to his home state.