WSP USA developed a method of pit hydrogeology and slope depressurization at Sierrita Copper Mine that uses a fiberglass rod instead of the more traditional steel or PVC pipe in order to install groundwater pressure monitoring sensors – known as vibrating wire piezometers (VWPs) – to depths greater than 1,000 feet, as well as help guide geophysical logging tools in horizontal drain holes.

“Data collected from the VWP provide the site workers with accurate hydrogeologic characterization and improved hydrogeologic knowledge,” said Jamie Kennealy, WSP project manager.

Since 2014, WSP has been providing field services, modelling services and overall direction for Freeport McMoRan, the client for the ongoing pit hydrogeology and slope depressurization project.

“Our client is benefitting from faster, safer, installations with reliability at depths not possible for traditional methods,” Kennealy said. “The fiberglass method allows for faster installations to occur, and allows for accurate set depths with less disruption to mine operations, resulting in cost savings for the client.”

One challenge with using PVC or steel pipe for installation is that they require assembly during lowering, as well as significant time to maintain safety, quality and security. Accurately estimating installation hours is a challenge and it is common for project miscalculations to result in costly change orders for the operator.

“The ability to collect high-quality and reliable data from the beginning provides more accurate depressurization and pit slope pore pressure models, which are valuable planning tools for the mine,” Kennealy said.

During the past four years, WSP has installed over 100 VWPs using the fiberglass rod method, offering clients a combination of innovation, high-quality work, and a commitment to safety.

“All field work requires a firm handle on the method, potential pitfalls and the flexibility,” Kennealy said. “All of our field personnel are trained hydrogeologists with the expertise to make timely, site-specific decisions to address issues that can arise while in the field.”

VWPs are installed using a 1/2-inch fiberglass rod after a pilot hole is fully grouted using a cement-bentonite mixture. Multi-level VWPs use grouted-in pressure transducers, set at different depths within the same hole, to measure pore pressures and gradients in, and across, geologic units and structures. The resulting data can be compared with geotechnical design targets to manage and track slope depressurization, and provide a basis for calibrating 2D and 3D pit-slope pore-pressure models.

“The most accurate pore pressure gradients are measured in vertical VWPs,” Kennealy said. “However, in areas where data is needed but access for vertical holes is not available, we have adapted the fiberglass rod assembly for a secondary use.”

Traditional geophysical tools rely on gravity to run logs, but WSP has worked with a geophysical sub-contractor to use the rigid – yet flexible – fiberglass rod to push the geophysical tools to the terminal point of a horizontal drain hole. The logging tool can then be pulled out of the hole at a constant rate, providing an accurate log. The VWP is then installed using the traditional pipe method.

The process requires strong, continuous collaboration with the client, on-site drillers, and other WSP project managers for both time-saving ideas and equipment improvements.

“Every installation seems to be unique,” Kennealy said. “On each installation something new or different arises, so the equipment and the installation process are continuously refined.”

The Freeport McMoRan–Sierrita project is ongoing, with more than 10 projects that have, will, or are currently using the fiberglass rod method. In 2017, more than 30 fiberglass rod installations are planned, and Kennealy expects that number will continue to increase with new clients.

“Having completed numerous installations and shared knowledge and experiences with other field hydrogeologists, we have been able to easily overcome challenges and refine new equipment and installation methods that have been recognized by current clients as well as potential clients,” she said.

Each completed project using the fiberglass rod aids the next project.

“Ideas, innovation, and equipment improvements can all be used and shared among managers to continue building the uses of the fiberglass rods, providing high-quality work and cost savings to the clients,” she said. “Use of the fiberglass rod in horizontal drains has shown that the possible uses of the fiberglass rod are very extensive. We continue to innovate to deliver the optimal outcomes, high-quality service, and cost-reductions that our clients expect.”

The project has led to the creation of a guide that explains the technique WSP is using – and its benefits – to potential clients. It has been an exciting experience for Kennealy and her team, which includes Camy Noguera, project hydrogeologist and field lead, who helped develop the fiberglass rod method for the Sierrita mine.

“We are grateful to have had the opportunity to work on this project and be able to contribute with the improvements being done with the fiberglass rod,” Kennealy said. “It’s also been rewarding to be able to share our knowledge and assist others with their challenges when needed.”