Case Study - Apartment Complex Foundation Void Fill

The owners of a college apartment complex in Georgia reached out to a local engineering firm after discovering a large void underneath the northwest end of the building. The issue was caused by stormwater collecting and flowing beneath the building's foundation. Over time, the foundation became exposed and now the grade has lowered by a foot from the original design.

Site Investigation and Initial Attempts

A dynamic cone penetrometer test revealed the soil was fragile, with two to three blow counts for the tested areas. Underpinning piles had been installed by the previous owner, but did not did not adequately stabilize the building's foundation. The shifting foundations caused the ceiling and wall in one of the top-level rooms to begin to crack and showed early signs of separation.

Proposed Solution

The engineering team recommended using injectable polyurethane grout. AP Lift 430 was selected for the job because of its high expansion of 22x and compressive strength of 7,200 psf. This polymer is also hydrophobic, which prevents water from causing further erosion of the void.

Implementation

1. Rod Placement: Injection rods were inserted horizontally into the void’s back, spaced 4 feet apart.
2. Foam Injection: Foam was injected through the rods until emerging from the exposed void area. This step ensured that the foam reached the farthest areas of the cavity.
3. Free-Shooting: The remaining exposed void was filled by direct foam application, ensuring comprehensive coverage.

Results and Conclusion

The end result was 658 gallons of expanded AP Lift 430 injected underneath the void. The polyurethane injection, combined with regrading work done in collaboration with a local engineering firm, successfully solved the issue and prevented further void formation underneath the student housing.

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