Vertical Machining Center Micropiles The MillAC-761 VII from Okuma was to be located in a factory area underlain by gravel and cobbles. This presented a challenge because the vertical machining center included a large worktable that undergoes rapid repositioning during operation. Because of this, a foundation was needed that prevented machine vibration and its movement along the floor, which would have affected the accuracy of the machining. The manufacturer specified pins to be epoxy set to hold the roughly 20-ton machine in place and required a concrete pad thickness of 450 mm (17.7 in). Meeting those specifications would have been time consuming; so, micropiles were incorporated instead in 2016 to support the machine's lateral load. The foundation was designed with six micropiles to support the machining center and the moment of its moving parts. The micropiles were constructed using a 38 mm (1.5 in) hollow bar with an 89 mm (3.5 in) carbide cross bit. Each pile was designed to hold 50 kips (222 kN). The larger pile size was designed to support the lateral loads generated by the rapidly accelerating table. Excessive grout takes and a lack of grout return during drilling prompted the team to identify a different system for the foundation. The revised process included: * Coring a uniform hole through the concrete pad and measuring its thickness. Contained drilling of five CMM micropiles * Determining if a short piece of casing was required to resist lateral loads. This step was completed for all six holes before the next step. * Drilling the holes by conducting an air flush, which allowed drilling to occur to a depth of 3 m to 6 m (9.8 ft to 19.7 ft). Air flushing was also successful in not plugging the rod or washing out gravel from around the piles. * Grouting using rotating drill tools, which served to mix the grout and cobbles around each pile. Coordinate Measuring Machine Micropiles Installation footprint for six MillAC micropiles The Hexagon Global coordinate measuring machine (CMM) posed the greatest challenge to the factory foundation construction - not because of its weight, but the precision needed for its functionality and its susceptibility to vibration. This foundation needed not only independence from the existing pad, but also complete isolation from any surrounding DEEP FOUNDATIONS * MAR/APR 2020 * 97