Supportworks Case Studies: Chiller Equipment and Slab Stabilization - Naples, FL

Wednesday, December 3rd, 2014 by John Siwicki

Challenge

October 2012 - Chiller equipment was installed circa 2000 for a new middle school addition. The equipment was originally installed within a stand-alone, roofless structure. During a 2005 renovation and expansion project, the stand-alone chiller building was modified and incorporated into the southeast corner of a new addition. While the surrounding walls remained stable, soil consolidation and washout created voids beneath the slab on grade. Slab settlement in one corner of the room measured 5.5 inches. A large chiller unit supported on the slab settled differentially up to 2.5 inches, putting a bind on its plumbing. Eight hand auger borings with static cone penetrometer testing identified very loose to medium dense sands with minimal amounts of shell and gravel from the underside of the chiller room slab to the termination depth of eight feet. Support of the chiller equipment and new slab would require deep foundations to minimize risk of future settlement. The equipment also had to remain operational during the construction process.

Solution

Helical piles were considered, but then dismissed as a viable option, even with handheld installation equipment, due to the limited working space. Driven pin piles were ultimately selected to support the relatively light equipment and slab loads. Pin piles could be installed within the tight areas around the existing equipment and piping. Six (6) Model 288 (2.875-inch OD by 0.165-inch wall) pin piles were proposed with low-profile retrofit foundation brackets to re-support and re-level the chiller unit. Thirty-two (32) Model 288 pin piles were proposed to support a new eight-inch thick reinforced structural slab. In order to provide an ultimate pin pile capacity of two kips, project specifications required that the piles be driven to refusal with a 110-pound pneumatic hammer. Refusal was defined as one minute of continuous driving without pile advancement. Sections of the existing slab around the chiller unit were removed for the installation of the six pin piles and retrofit brackets. After the chiller unit was effectively stabilized, lifted and re-leveled, all remaining sections of slab within the chiller room were broken up and removed. The 32 slab support piles were installed in a grid pattern and fitted with new construction brackets to be cast into the structural concrete. Pin pile depths ranged from seven to 45 feet within the highly variable soil profile. Twenty-six of the 38 pin piles were driven to depths of seven to 11 feet, while the remaining 12 piles advanced to depths of 40 to 45 feet before refusal was encountered.

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