In the heart of Kelowna, B.C., lies Knox Mountain, where city residents and visitors come to hike recreational trails, visit the 367-hectare park and see Okanagan Lake.
In 2018, a section of hillside at the base of the mountain below Royal View Drive began to slowly slide, prompting closures of a route leading to Knox Mountain Park and causing concern for area walking paths and the above roadway. While studies confirmed the property above the slope failure was safe with minimal risk to public safety, restoration work was required to prevent further ground movement.
GeoStabilization International was contracted by the City of Kelowna to stabilize the slope at the historic Knox Mountain landslide area. The project scope included slope regrading, installation of grout-injected soil nails, interconnection of high-strength steel mesh and restoration and naturalization of the site area.
Value-engineered approach
In May 2022, the GeoStabilization team conducted a site visit with the City of Kelowna, where visual inspections, site photogrammetry, field measurements and site observations were completed. Based on the data collected and prior area reports and observations, it was determined that the previous slope failure was caused by the composition of the underlying soils, the over-steepened slope gradients and elevated groundwater levels. The elements also made the area susceptible to future movement.
With site data in hand, the engineering team proposed a value-engineered approach using targeted remediation in multiple areas within the site, which were over-steepened and had a history of or potential for slope movement. The targeted approach allowed the engineering team to develop a solution for each area, as opposed to a traditional design based on the critical section of the entire site. The solution, which included earthworks regrading to reduce the slope angle and installation of hollow soil nails and steel mesh, would resist further slope movement and protect area infrastructure beyond the crest of the slope.
The design was also created to provide long-term stability to the area with a factor of safety (FOS) of 1.5 for the deep-seated potential of landslide movement. With groundwater levels being a major contributor to the initial movement of the slope, the team also checked an elevated groundwater level to determine confidence in the stabilization solution if levels rise – and ensure the FOS remains above 1.2, meeting industry standards for short-term elevated groundwater.
Innovation at work
To provide regrading along the over-steepened areas of the slope, an earthworks contractor performed all excavation work throughout the project. Reducing the slope angle of these areas allowed for an optimized soil nailing pattern and depth of embedment.
Working with the excavation team, GeoStabilization installed injection drilled hollow bar soil nails. Having analyzed several cross-sections and interpreting ground conditions using the photogrammetry data, the team used hollow bar nails since the slope below Royal View Drive was still considered an actively moving landslide. Hollow bar soil nails are especially effective for this type of ground stabilization due to their high installation rates, pullout capacity and bond capacities compared to solid bar soil nails.
Using a SuperNailer™ unit to install the soil nails, the crew was able to access all necessary areas safely and efficiently, while reducing the time to set up and drill each soil nail. Once soil nails were installed, the team used an injection grouting method to avoid having to case any of the holes due to collapsing soils. Soil nails were drilled to below the potential slip planes to intersect and resist mobilization of the slope mass, with anchors 18 meters deep in some areas. In total, 20,500 meters of hollow bar soil nails were installed along the slope – with the soil nail depth based on information procured from the team’s initial site exploration.
Safety was top of mind from set-up to construction to site cleanup.
With a half a metre of the soil nail bar exposed, the team installed a high-strength steel mesh over the nail heads. The mesh was then fastened to the nails at the specified depth from the soil slope, with a total of 6,350 metres of mesh installed in 11 targeted areas.
The high-strength steel mesh serves as a facing material and ties the soil nails together. To promote area regrowth and naturalization of the slope, hydro-seeding followed the stabilization work.
Safety at the forefront
Mitigating geohazards involves risk, but employing a highly trained crew, a robust safety program and constant communication throughout the project will significantly reduce that risk.
Prior to construction, GeoStabilization worked with the City of Kelowna to identify a suitable laydown area to facilitate project work. The identified area was positioned to maximize worker, asset and public safety. Additionally, installation of environmental controls, including erosion control devices, was conducted in conjunction with the earthworks contractor, access to the slope exaction areas was carefully maintained and existing monitoring wells were marked and protected to eliminate the risk of damage during construction.
Safety was top of mind from set-up to construction to site cleanup. The team ensured all practices and procedures followed local, provincial, federal and client regulations and guidelines. All personnel were up to date on necessary safety certifications and were trained to perform tasks, including working with equipment, safely and appropriately. Additionally, crews conducted daily tailgate meetings at the start of each shift to address project activities, hazards and concerns and to provide a look-ahead plan.
Knox Mountain Hillside restored
Through site analysis, innovative techniques and constant communication with the client and contracting partners, GeoStabilization provided a value-engineered solution that was completed on time and within budget – with work being completed within 70 total working days and area closures reopened in spring 2023. The use of hydroseed restored the slope to its natural aesthetic, while grout-injected hollow bar soil nails with high-tension steel mesh ensure long-term stability and provide peace of mind to the area public, property owners along Royal View Drive and visitors using the route to access Knox Mountain Park and take in its breathtaking views.