Test Pile Curing and Strain Measurement at Millennium Tower

Researchers: Andrew Yeskoo, Ruonan Ou, Yaobin Yang, Peter Hubbard, Linqing Luo

Client/Owner: Applied Foundation Testing (AFT)

Technologies: Distributed Fiber Optic Sensing

The UC Berkeley team performed temperature and strain monitoring using point-based FBG and distributed fiber optic monitoring systems on a full-scale test pile for the Millennium Tower retrofit. The system involved two parallel monitoring systems – one consisting of multiple chains of FBG strain sensors and a second with distributed thermal and fiber optic cable. The thermal system was used to monitor the heat generation and dissipation of the concrete pile during curing to evaluate the integrity of the pile. Strain measurements were taken during the load test, performed using three bi-directional load cells installed at separate depths in the lower portion of the pile. The results of the fiber optic monitoring, as well as the parallel conventional instrumentation program, were used to refine the pile design prior to the retrofit moving into production. A key challenge of the installation included designing a specialized detail to allow the fiber optic cables to pass through the bi-directional cells, allowing the cable to lengthen during expansion without breaking and losing the ability to pass light to the sensing points below the cell.