Ground Improvement Using Rammed Aggregate Piers

• Phil Wheeler and John Kellberg
• Senior Superintendents
• Rafn Company

You walk out onto the construction site where a large excavator is beginning to dig down for a new building foundation. You’ve worn tall rubber boots, but a nervous feeling sets in as you begin to sink in the mud. If this site can’t even support your weight, how will it ever support the weight of the new building? Luckily you stop sinking before the mud gets to the top of your boots, but you are not going any farther out on site. After climbing back to the sidewalk, you begin to appreciate the strength, solidity, and simplicity of the rammed aggregate pier method of soil reinforcement that is about to turn this site from soupy mess to solid base.

Many individual aggregate piers can be installed in the soil and together they bring otherwise unsuitable soil to a condition able to be built upon. The installation process for each rammed aggregate pier begins with a special mandrel attachment on a trackhoe or similar machine. The mandrel is placed at a predetermined location on the construction site and creates a shaft down to bearing soil by pressure and vibration (step 1). A small amount of stone aggregate is then placed at the bottom of the shaft (step 2). It is then compacted down and out to form the bulb shaped element of the pier (step 3). More stone aggregate is placed on top and compacted to form successive bulbs building up to nearly the top of the shaft (step 4). In softer soils or soils with a lot of organic materials, grout can be added in the shaft as well as mixed with each bulb of compacted aggregate stone to create an even stronger pier. This process is repeated throughout the unsuitable soil areas on site. The new building’s foundation can then be constructed on a thin layer of soil on top of the piers.

Northgate Eighth Apartments

At our new market-rate apartment building in the Northgate neighborhood of Seattle, the beginning soil conditions were so poor due to the peat bog below that we needed to find a way to stabilize the soil and support the building. The method selected by the project’s engineer was the installation of 600 grouted aggregate piers with Haywood Baker contracted for the installation. This project is the first for Rafn to use the rammed aggregate pier method of soil stabilization.

The sheer number of piers on the relatively small one acre site created such a density that instead of potholing the site to determine what underground cleanup would be required, it turned out to be easier just to clean up the entire site. And due to the weight and use of the tower crane, we put in 13 additional piers under its footing.

AMLI Spring District Apartments

Starting construction in late 2017 in the new Spring District neighborhood in Bellevue, our new market-rate apartment building is located on a site with very soft soil. To stabilize it and provide strong support for the building’s foundation, engineers selected a rammed aggregate pier system. 698 piers are being installed by Geopier Northwest, Inc. on half of the 62,811 square foot site.

The installation is going smoothly even though the current soil conditions were extremely muddy and required us to bring in over 1,500 tons of rock to stabilize the soil enough to support the aggregate pier machine itself. The small area where ground improvement is needed also plays a role in the ease of installation with the machine needing space around it in all directions to both press the shaft and place the aggregate material and grout.

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