CPT Installation of Fiber Optic Strain Extensometer

Members of the Soga Research Group distributed fiber optic team performed a field installation of a vertical fiber optic strain sensing cable using a Cone Penetration Test (CPT) truck on Treasure Island in the San Francisco Bay. UC Berkeley PhD students Andrew Yeskoo and Peter Hubbard partnered with John Rogie of Conetec and Chris Stouffer of ENGEO to test a new method of installing fiber optic cable for strain monitoring of vertical ground displacement developed at UC Berkeley. The Berkeley team has previously deployed vertical strain sensing fiber optic cable in drilled geotechnical boreholes, however this project represents the first deployment of strain cable using CPT. Distributed fiber optic sensing of subsurface strains offers a distinct benefit over conventional single- and multi-point borehole extensometer deployments; allowing measurement and localization of subsurface movements over the entire installed length rather than at discrete reference points. The resulting data can be used to estimate ground settlement and heave, as well as correlate observed movements with nearby construction and loading activities.

The ConeTec CPT truck used to install the fiber optic strain cable

The team was able to install a length of armored fiber optic strain sensing cable to a depth of 80 feet below the ground surface. The entire installation was completed in less than two hours. A series of baseline measurements were taken after the initial installation and weekly readings are ongoing as surcharge load is being progressively added to the site as part of a preloading program to reduce later settlements associated with consolidation in the Young Bay Mud. The team believes that this new installation method provides a fast and cost-effective way to monitor subsurface ground displacements as opposed to conventional extensometer installations. Future trials are planned to further optimize the installation technique and make the install even faster and easier to deploy.

The setup for weekly fiber optic readings using an Omnisens analyzer and portable generator