The Curious Case of No. 57 Stone
The American Society for Testing and Materials, or ASTM, No. 57 stone is often used as sub base fill material below road surfaces and buildings. It is a fragmented stone with angular edges and is regularly utilized as a drainage layer when used with geotextile fabric. Although the material is touted by many as "self-compacting," excess voids left from zero compactive effort in locations with little confinement may not eliminate the possibility of future settlement. What happens when the environment above the No. 57 stone causes it to settle?
UNC Cogeneration Plant
Originally built in 1991, the Cameron Cogeneration Plant on the Main Campus of the University of North Carolina at Chapel Hill has been providing the campus with an on-site source of energy for 20 years. When cracks appeared around the base of a mill fan in the facility, management grew concerned about the settlement of the fan.
Falcon Engineering was contacted regarding observed signs of settlement of the fan foundation and the surrounding floor slab within the building. After geotechnical data was gathered and analyzed, it was discovered that the No. 57 stone below the fan had in fact settled. Engineers believed this was caused by the vibrations emanating from the mill fan, constantly disturbing the underlying stone. No. 57 stone should be placed with light vibratory compaction to “lock-in” internal friction and be visually verified by a geotechnical engineer. The presence of a large amount of stone and the knowledge that the building was supported on drilled shaft foundations, implied that subsurface soils at the site were unsuitable for foundation support. This led engineers to believe that normal soil consolidation settlement also contributed to settlement.
Uretek Mid-Atlantic offered the perfect solution to the settling facility. Uretek was contracted to inject its High Density Polyurethane Resin below the slab to improve the No. 57 stone by replacing air voids. The polymer acted as a kind of glue that permanently bonded the No. 57 stone in place. Not only was the slab lifted back into place, the vibrations of the fan would no longer affect the fill stone below.
Uretek’s patented deep injection process resulted in minimal downtime.
About the Author
Katherine Witt is a Marketing Assistant at URETEK, Mid-Atlantic, a licensed affiliate of GeoPrac sponsor URETEK ICR. URETEK Mid-Atlantic is a design build geotechnical contractor that serves North Carolina, South Carolina, Maryland, Virginia, and Washington, D.C. They have their own staff of experienced geotechnical engineers and and provide a range of geotechnical services including: soil nail walls, excavation support systems, micropiles, minipiles, High Density Polyurethane Resin injection, slab leveling, rock anchors, underpinning, and light weight cement fills. More information can be found at the URETEK Mid-Atlantic website.