SuperPile 2017 Online Proceedings, (DFI)
Load Transfer Mechanism of Micropiles in Weathered Rock
Ed Theinat, E.I.T.
Micropiles foundations are small size piles drilled and installed with cased and bond zones. They are more often used to support bridges at high capacities and in groups to resist static and dynamic loads. To transfer the structural load, geotechnical designers rely primarily on the strong bond of the grout to ground in the bond zone. Due to the drilling and construction nature of micropiles, it is assumed that the cased zone will have low friction interaction with the surrounding ground, and subsequently poor load transfer. In this study, numerical simulations of micropiles interacting with soil, weathered rock, and competent rock are presented. The micropile foundations instrumented are located under Bridge No.2 of the Foothills Parkway in the Great Smoky Mountains, TN. Several micropiles were installed through overburden soil and weathered/competent rock. For example, a typical foundation element consists of a 93-ft long micropile with a 70-ft cased zone (50-ft in the overburden soil and 20-ft in weathered rock). The bond zone is embedded in 23-ft of competent rock. The instrumentation program installed vibrating wire strain gages at five locations along the length of the micropile. A 3D finite element simulation was conducted to examine the load transfer behavior and compared with the instrumented data recorded during bridge construction. This paper presents the effect of the soil/rock mechanical properties to the load transfer mechanisms. The research findings show that the load transfer contribution in the cased zone is significant, primarily in the weathered rock; in some cases most of the load is absorbed before reaching the bond zone in competent rock. The increase in the load transfer adds a significant safety margin in the load carrying capacity of the micropiles, subsequently, the superstructure.
|article #2678; publication #1031 (SP-2017)|