Monitoring Micropile Foundations of Bridge during Construction

Information regarding the load transfer of micropiles during construction loading has not been reported in the literature. The instrumented micropile foundation system reported herein was for Bridge No. 2 of the Foothills Parkway in eastern Tennessee completed in December of 2012. Vibrating wire strain gages were installed at select locations along the micropiles within two of the pile groups. Two strain gages were installed in the bond zone and the remainder near the top and middle of the cased length. Subsurface conditions consisted of medium dense to very dense sand and gravel with varying amounts of silt and clay overlaying conglomerate bedrock. The bond zone of the micropiles was installed within the bedrock strata and the dramatic dipping of the top of rock impacted the length of the micropiles and load transfer. Data was collected from the array of strain gages, such that each set of data corresponded to a different bridge construction stage. Monitoring during construction loading indicated that the variable pile lengths that were necessary because of the geologic setting resulted in load concentration on the shorter piles within the pile groups. Based on the strain data measured along the entire length of the micropiles, load transfer occurred within the bond zone, but significant load was also transferred along the cased length. All the measured loads were well below the structural capacity for the piles.