A 15-foot by 95-foot area of the existing single-story, slab-on-grade building was experiencing differential settlement of the interior floor slab and adjacent exterior walls. Existing patches in the floor slab and tuck-pointing of the exterior brick veneer indicated that the settlement was not only recent, but had been going on for some time. Sections of the floor slab had received periodic overlays of self-leveling grout to hide the settlement, to the extent that the slab had become 14 inches thick in places.
A soil investigation, consisting of one soil boring to a 34-foot depth, was performed to determine the cause of settlement. The boring identified uncontrolled fill to a depth of eight feet underlain by loose to medium dense sand to the maximum depth explored. A probable ancient log was encountered around a depth of 20 feet. Groundwater was observed at a depth of eight feet at the time of the exploration. The geotechnical report concluded that a trench or remnant drainage canal was apparently back-filled with debris prior to construction of the building. Compression and possible decay of this debris over time was determined as the cause of the observed settlement. Furthermore, it was anticipated that additional settlement would likely occur if the foundation and floor slabs were not stabilized.
Helical piers were selected to stabilize both the interior concrete slab and the exterior walls. Six (6) Model 287 (2.875-inch O.D. by 0.203-inch wall) hollow round shaft piers with a 10”-12” double-helix lead section were installed to stabilize the settled sections of the exterior wall footings. The piers were installed to depths of 20 to 30 feet and to a torque-correlated ultimate capacity of at least 27 kips (FOS ≥ 2). The piers were fitted with retrofit brackets positioned beneath the existing footing. Hydraulic cylinders were used to uniformly load the piers to the design working load.
The interior concrete slab was stabilized with sixty-two (62) Model 237 (2.375-inch O.D. by 0.154-inch wall) hollow round shaft piers with an 8”-8” double-helix lead section. An 8” diameter hole was cored through the concrete slab at each slab pier location. The piers were advanced to depths of 20 to 30 feet and to a torque-correlated ultimate capacity of 25 kips (FOS = 2). The piers were installed with handheld installation equipment due to limited access for machinery. After installing the slab pier brackets, a uniform seating load was applied to the piers to remove any slack out of the system and to mobilize the slab. A flowable fill slurry was used to void fill underneath the slab and the surface of the floor slab was repaired with a self-leveling overlay. A total of sixty-eight piers were installed in just twelve days.