Monitoring micropile foundations of bridge during construction

Information regarding the load transfer of micropiles during in-service construction has virtually never been monitored or documented. The primary objective of this research project was to monitor the load transfer versus the depth of production micropiles during construction. The micropiles being studied were used to support Bridge No. 2 of the Foothills Parkway in eastern Tennessee during construction. Two of the piers (1 & 2) were instrumented for the purpose of this study. Vibrating wire strain gages (VWSG) were installed on select micropiles within pile groups of twenty (20) micropiles. Seven vibrating wire strain gages were installed on each micropile at varying depths. Two strain gages were installed in the bond zone on each micropile. One gage, along with two pairs of strain gages, was installed in the upper cased portion of the micropiles to obtain both bending moment and axial strain readings. Subsurface conditions consisted of medium dense to very dense sand and gravel with varying amounts of silt and clay overlying conglomerate bedrock. Data was collected from the instrumentation such that each set of data corresponded with a different loading stage of the bridge construction process. A load test using fully loaded trucks was also conducted on the completed bridge near the end of this study. The results of both the construction loading and the load test indicated that the geologic setting with steep dipping angles caused significant uneven distribution of load on the pile groups. It was also shown that load was transferred to the ground in the bond zone, but significant load was also transferred along the cased lengths. All the measured loads were well below the structural capacity for the piles. Also, the moments measured by the pairs of twin gages were well below the calculated design bending moments for the piles --Abstract, page iii