The foundation of a building on the campus of Western Iowa Tech Community College (WITCC) was in trouble. One end of the building seemed to be twisting itself apart due to advanced settlement, causing structural and cosmetic damage. The building housed classrooms, utility space and a vital 911 call center. The facility was constructed with poured foundation walls, a brick fascia and a flat roof.
Allowing this large structure to continue to settle was not an option as the damage to the building's interior and exterior portions was becoming significant.
Often the two-part solution for repairing foundation and slab settlement is "piers, then poly." The priority is to stabilize and potentially "lift the structure." Get it secure again. Then, go back and level up the slab with polyurethane foam.
Nothing is as simple as it seems. The first challenge was that the majority of retrofit piers were located inside the structure. That meant tight spaces, an intrusive installation process and the "noise factor." Thrasher Commercial Production Manager Brent Foreman discovered right away that jackhammers and an active 991 Call Center were not a good mix.
A second test was the 20 feet of stiff compacted fill beneath the building. The engineer of record determined that rather than taking the load-bearing value of that first 20 feet into account, they would pre-drill and advance a pier through to the secondary layer. This solution would eliminate the skin friction factor of this layer of soil in the equation.
Solution: Ho Chunk Inc (HCI) contracted Thrasher Commercial Group to underpin the foundation and use our PolyLevelingTM product to lift and level interior floor slabs affected by the settlement. The project consisted of 63, 3.5" diameter push piers to support exterior and interior load-bearing walls. Also, 800 pounds of poly foam lifted, leveled and stabilized interior floor slabs.
"There was significant make-ready work for this project," said Foreman. "Not only did we have to find another solution for cutting the footings near the 911 center to reduce the noise, but we also ended up hand digging around pipes and cables inside the building." Since the building foundation's settlement was determined to be caused by weak soil below the first 20 feet of compacted fill, the piers would have to be pre-drilled. Push piers typically do not have to be pre-drilled to advance the pier tube sections. Still, due to this highly compacted fill and the importance of eliminating skin friction in the fill area while installing, the piers were pre-drilled to ensure they could be advanced to sufficient load-bearing soil strata.
The footings' bottom were over 20 feet, so the exterior excavation was performed with heavy equipment. Interior excavations were in tight spaces with possible complications, so 2-5-foot-deep hand digs were necessary. Thrasher pre-drilled each of the 63 pier locations using a hydraulic rotary percussion rock drill. After pre-drill, the piers were then driven and advanced to an average bearing depth of 112ft below the bottom of the footing. The piers were driven in at a maximum drive force of 55,500 pounds and were pressurized and loaded to support 37,000 lbs. per pier.
Thrasher backfilled access holes and replaced floor slabs after the foundation system installation. Thrasher then lifted and leveled approximately 800 square feet of interior floor slab affected by the settlement. The total duration of the project was seven weeks.
"Execution is key for us," said Todd Royal, Thrasher Solutions Specialist & Project Lead. There are always wrinkles to iron out in any project. We are judged by how well we come up with solutions that get the job done on time."