C+S March 2021 Vol. 7 Issue 3 (web)

Installing Micropiles in Two Phases UMA ended up securing two of the three phases of the addition in November 2019, including an addition to the fabrication building (Building F) and an expansion to accommodate the massive turntables used by the manufacturing facility (Building J). Among the complexities of the project was the sheer volume of mi- cropiles to be installed – 304 to be exact. The first phase, Building F, contained 290 piles, and Building J contained 14. Team UMAhit the site during Christmas of 2019. The first test pile was installed in late December and load tested in January. Two load tests were conducted at 480 kips. UMA designed piles to achieve loads of 240 kips for Building J, and 200 and 240 kips in Building F. UMA started production shortly thereafter, installing (or drilling) 20-plus piles per day and working two shifts. The job was finished before the February deadline. Once the micropiles in Building F were completed, UMA left for a month before returning to finish up the 14 piles in Building J. True Design-Build UMAwas initially supplied with a geotechnical report fromCharleston, South Carolina-based Terracon. Although it was thorough, geotechni- cal borings are only guaranteed accurate at the boring. The distance between borings can vary and so can the soil characteristics. “They basically said, here's the geotechnical report and the spacing we want to try to utilize," says DeSpain. “Our job was to come up with the diameter, depth, and means and methods used to install the piles. UMA worked with general contractor Fessler & Bowman, Inc. and the owner’s engineer to select a design-build deep foundation sys- tem that would meet their needs. The selection was a 7-inch Outside Diameter (O.D.) casing. The casing was installed the full length of the micropile for structural capacity, and the bond zone was pressure UMA worked two shifts to install more than 300 micropiles to support a fabrication facility addition for a global manufacturer in Goose Creek, South Carolina.

Drilling in proximity to the existing structure was required for Building F.

grouted through the casing. A 14" x 14" x 2" plate was attached to the top of each pile with a non-structural bar and nut and embedded in the structural concrete slab. Team UMA laid out the micropiles 20 feet apart and in a perfectly aligned grid pattern. Fessler & Bowman followed up by pouring a multi-layer reinforcement slab for the foundation. “With a caisson or a drilled pier, you would assume an end bearing pressure at the bottom and then you could also use frictional strength between the grout and the ground,” says DeSpain. “With micropiles, the design standard for end bearing capacity is zero, so you rely strictly on the friction between the grout and the ground.” UMA’s design-build skills were unexpectedly tested when its work was more than half-way completed. The contractor was informed that an additional four columns in the middle of the building would require support. “They had us add four micropiles for each of the four columns,” DeSpain recalls. “We continued construction, did a design and got it approved, and then implemented the new design while we were on-site without delay.”



march 2021

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