When a rockfall on Highway 97 near Summerland, BC indicated the initiation of a large rockslide, Thurber supported MoTT with a quick response, timely recommendations, and a safety-first approach that opened the highway to traffic within two weeks.

Highway 97 sees heavy traffic. A rock fall event deposited approximately 3,000 m3 of materials onto the highway in August 2023 which closed the highway to traffic. Following the 2023 rockfall, MoTT observed a large tension crack extending behind the scarp from a 2008 slide. This indicated that the rockfall event was part of a larger event requiring immediate mitigation to protect the highway. MoTT engaged Thurber as part of the As-and-When emergency response contract. Thurber was onsite within 24 hours to help assess the situation and provide advice for a staged reopening of the highway.

The Okanagan Valley is prone to slides of which are highly influenced by a complex array of low and high angle faults. This is the second time this area has slid: in 2008, a slide occurred during the highway’s construction. Thurber carried out investigation and stability analysis to advise MoTT at that time and was retained again to assist in assessing and monitoring the most recent slide and designing temporary stabilization measures to safely reopen the highway.

Following our initial assessment, Thurber designed a 22-metre-high temporary soil and rock berm at the toe of the slide to resist slope movement. In parallel with the stabilization works, Thurber specified and sourced monitoring systems to assess slide activity and verify that the slide had stopped moving. The toe buttress successfully arrested slide movement and the continuous monitoring allowed for the staged re-opening of the highway for up to two travel lanes (of four) within 25 days.

With the slide now stopped and traffic moving, Thurber worked with MoTT on additional stabilization works including slide unloading, high strength anchoring, and structural walers.

Emergency response: Thurber’s team was onsite within 24 hours to conduct site reconnaissance and install cable and weight extensometers to assist in tracking slide movement . Stabilization: A 22m high temporary soil and rock berm was constructed at the slide toe to resist further movement, with a lock-block catchment wall along the berm toe to retain debris. The berm serves as a buttress until long-term stabilization measures are implemented. A key block at the slide’s south end was stabilized using high-strength anchors and reinforced concrete walers to achieve temporary stability. Slide unloading: The upper slide was unloaded by controlled rock blasting, using tight vibration controls, hand-operated equipment, and regular highway closures. Safety: During construction, 24/7 slide monitoring occurred using visual assessments, automated surveys, terrestrial LiDAR, extensometers, rainfall gauges, slope inclinometers, and tiltmeters. Regular LiDAR change detection analyzed aerial data, and emergency plans ensured prompt alerts and highway closures if needed. The toe-berm installation has prevented further large-scale movement.

Investigation and Design: Thurber conducted a geotechnical investigation alongside slide unloading. Geological mapping and 3D modelling helped develop a geological model and interpret the slide mechanism. The slide was subdivided into several interrelated blocks and analysed using 2D and 3D limit equilibrium methods using anisotropic strength parameters.

The temporary rockfill toe buttress, high-strength anchors and unloading of the upper slide stopped slide movement by increasing resistance and reducing driving forces. Stabilization allowed the highway’s staged reopening, with Thurber overseeing the analysis and design of permanent stabilization measures in parallel with construction activities. The monitoring systems verify that the highway can remain open with continual surveillance. Permanent stabilization works are planned for spring 2025