Oxford Manor Apartment Complex - Gainesville, FL
Challenge
Eleven single-story, slab-on-grade buildings within the Oxford Manor Apartment Complex were experiencing settlement, evident by cracked and sloping floors, cracks in the drywall, and cracks within the brick and mortar veneer. The buildings, constructed in the early 1980’s, had a history of prior movement. The foundations for one of the buildings had been previously underpinned and the floors within that building lifted with expansive polyurethane grout. This building, however, continued to settle with recent, measured differential movements up to three inches. Differential settlements within the other ten buildings ranged from ¼-inch to eight inches. Cosmetic repairs were continuously being completed in order to make the units "presentable" for rent. Test borings encountered sand fill and medium dense clayey sand from the surface to depths of about five to eight feet. Plastic clays were sampled between the near-surface sands and the medium dense to very dense weathered limestone at 21 to 25 feet. Recent settlement of the buildings was likely prompted by drought-like conditions and drying and shrinking of the clay soils beneath the foundations and slabs. Large trees and bushes surrounding the buildings contributed to the movement by drawing moisture from the foundation soils.
Solution
Alpha Geotechnical and Testing Services, Inc. recommended that the exterior foundations be supported with retrofit helical piers and the interior floor slabs be supported with hydraulically-driven “push” piers. The piers were to penetrate the upper clay soils for bearing within weathered limestone. Four hundred eight (408) Model 288 (2.875-inch OD by 0.276-inch wall) round shaft helical piers were installed along the foundations of the eleven buildings. The 8”-10” double-helix lead section was selected to support the design working load of 18 kips. The helical piers were installed to an average depth of 30 feet and to torque values of at least 4,000 ft-lbs, correlating to ultimate pile capacities of at least 36 kips (FOS ≥ 2). Two hundred eighty (280) Model 288 (2.875-inch OD by 0.165-inch wall) slab piers were installed to an average depth of 25 feet and an average ultimate capacity of 12 kips. Design working loads for the slab piers were generally less than five kips. The slab piers were generally installed in a grid pattern with a spacing of five feet. Following lifting operations of the floor slabs, void space was filled with cement grout injected at low pressure. Due to previous cosmetic repairs that were completed, the foundations were generally lifted less than one inch. The foundations for one building were lifted as much as four inches to recover some of the measured eight inches of differential settlement.
Project Summary
Products Installed: (408) FSI Model 288 Helical Piles, 8”-10” Lead Section, Installed to an average Depth of 30 feet, Design Working Load of 18 kips; (280) FSI Model 288 Slab Piers, Installed to an Average Depth of 25 feet, Design Working Load of 5 kips
