Site C Hydroelectric Dam – High Precision Monitoring

Project Snapshot

Project Overview

Introduction

Following completion of BC Hydro’s Site C Earth Filled Dam near Fort St. John, BC, Underhill was engaged to deliver a high-precision monitoring program to track structural behaviour during and after reservoir filling.

The project involved monitoring 73 monitoring points across the dam with millimetre-level accuracy, requiring a robust and carefully designed control network capable of maintaining precision over time and under changing conditions.

Project Challenges

Delivering consistent, high-accuracy monitoring on this project required overcoming a range of environmental and operational challenges:

• Extreme seasonal temperatures ranging from +30°C to -40°C
• Dust, heat shimmer, strong winds, and ice fog affecting visibility and instrumentation
• Frosted prisms and variable GNSS reliability due to solar activity
• Ongoing construction and evolving site conditions
• Seismic events, including two earthquakes during the monitoring period

Underhill’s Approach

Underhill designed a monitoring solution anchored by a rigorously tested and adaptable control network. A pre-analysis using least squares adjustment initially proposed three primary control points, but this was expanded in response to field conditions—resulting in a network of 13 densified control stations for improved reliability.

The approach integrated complementary technologies and verification methods:

• GNSS observations for primary positioning control
• Trimble SX10 scanning total station for conventional measurements
• Precise levelling between all of the primary control to act as a independent verification of GNSS measurements
• Monitoring frequency was strategically adjusted to align with project phases, increasing from monthly to weekly and up to twice-weekly observations during critical reservoir filling periods.

Noteable Outcomes

• Successful monitoring of 73 monitoring points with consistent millimetre-level precision
• A resilient and adaptable control network that performed reliably in extreme conditions
• High-confidence data supporting dam performance evaluation during and after reservoir filling
• Improved field efficiency and data processing through optimized monitoring workflows
• This project was the recipient of a 2026 David Thompson Award for Challenging Non-Cadastral Project

Technical Highlights

• Pre-analysis and Least Squares Adjustment used to optimize network design and performance
• Control network expanded from 3 planned stations to 13 densified stations
• Independent verification through precise levelling across all primary control points
• The GNSS, robotic total station and levelling measurements were all processed with MicroSurvey’s StarNet Least Squares Adjustment software.
• GNSS accuracy achieved within 5–10 mm, with conventional measurements within 3–5 mm of control
• To strengthen the network of monitoring stations prisms, we included 20 Leica 360 prisms to allow for conventional ties from multiple primary control stations.
• Field productivity and office processing efficiency was substantially increased by introducing Trimble Access’s Monitoring module

Project Impact

Underhill’s monitoring program provided critical, high-precision data to support the safe filling and ongoing performance assessment of a major hydroelectric dam.

By delivering reliable results under extreme environmental and operational conditions, the project demonstrates Underhill’s ability to design and implement resilient geomatics solutions for complex infrastructure, supporting long-term asset performance and risk management.