When Opus North Corporation set out to construct a four-story office building in a northern suburb of Chicago, it quickly became clear that a ground improvement solution would be necessary to support the 43,000-square-foot building with column loads ranging from 90 kips to 730 kips. The geotechnical investigation revealed subsurface conditions of a few feet of clayey topsoil or silty clay fill materials underlain by stiff silty clay soils extending through boring depths of 35 to 70 feet below existing grade, with groundwater at 34 feet below grade.
Bill Kwasny, principal engineer with American Engineering Testing, Inc. in St. Paul, Minn., considered caissons for foundation support, though ultimately recommended a Geopier Rammed Aggregate Pier (RAP) solution because of its cost-effective constructability.
“The Geopier RAP System has proven to be a reliable and cost-effective method of improving soils to permit the use of conventional shallow foundations, and avoiding deep foundations,” says Kwasny, who has worked with Geopier systems on numerous occasions.
Installation of RAP elements involved drilling a 30-inch diameter cavity to design depths ranging from 10 to 21 feet. Thin lifts of aggregate were then placed within the cavity and vertically rammed using high-energy patented beveled impact tamping devices. The high-frequency energy delivered by the modified hydraulic hammer, combined with the beveled shape of the tamper, not only densifies the aggregate vertically to create a stiff aggregate pier but also forces aggregate laterally into the sidewall of the hole, resulting in lateral stress increase in surrounding soil. The lateral stress increase reduces the compressibility of the surrounding soil and promotes positive coupling of the RAP element and the soil to create an improved composite, reinforced soil zone.
The replacement Geopier GP3 system provided an allowable bearing capacity of 6,000 pounds-per-square-foot while controlling total and differential settlements to less than one inch and one-half inch respectively, all while costing considerably less than cassions. Four hundred and one piers were installed in nine days to complete the ground improvement solution. The system performance was confirmed with a full-scale field modulus test performed during production that showed a deflection of less than 0.2 inches at 100 percent of the Geopier design stress of 15,790 psf.
The Geopier system that was chosen uses replacement RAP elements to reinforce good to poor soils, including soft to stiff clay and silt, loose to dense sand, organic silt and peat, and variable, uncontrolled fill. The system allows for visible inspection of the spoils, and the opportunity to address changing ground conditions as they happen.