FAQs and Glossary

Why is the City of Whitehorse concerned with mitigating the escarpment geohazards? 

In 2002, the City completed an Escarpment Study which examined the stability of the escarpment and identified high, medium and low hazard zones for consideration of potential slope instability. As a result of the 2002 study, several properties were purchased in the high hazard zones and any new developments in the medium hazard zones to be assessed by a geotechnical engineer to determine what mitigation measures are required to protect the development.

While the escarpment has been relatively stable for a long time, over the past two years the frequency and severity of landslides have become more pronounced. As a result, the City is continuing to take steps to reduce the risk to people and infrastructure by mitigating the geohazards.  

Why is there an issue?

That is a great question and extremely tough to answer. Failures of the escarpment are typically driven by groundwater seepage. Climate change predictions show that the north is expected to get warmer and wetter, and 2021 and 2022 saw some of the highest snowfall levels on record. This is likely the key reason driving the failures along the escarpment.  

What steps is the City of Whitehorse taking? 

Long-term geohazard mitigation and risk reduction take time. The City of Whitehorse is taking a number of steps, including working with geotechnical consultants to develop long-term strategies to address various areas along the escarpment. Investigation, analysis, design, and construction could take a few years to complete. 

The City is also continuing to investigate this matter. Once more information is obtained, analysis and design for further solutions can begin.

The City of Whitehorse is already exploring long-term mitigation options for Robert Service Way at a high level. Those options will need to be carefully considered and presented to Council to identify the option that best suits the City’s needs in all the circumstances. The City will continue with long-term planning in 2023. 

How well does the City of Whitehorse understand the groundwater that causes seepage?

Extensive groundwater monitoring has been performed along the airport bench over the years; however, the City is continuing to investigate how groundwater moves about the sand/silt interface. As these investigations are completed the City be in a better position to further assess how groundwater flow affects seepage. 

What is the City of Whitehorse doing to understand the groundwater that causes seepage?

The City of Whitehorse is engaging local groundwater experts to determine the investigation program required to understand the regional groundwater flow. In the interim, we are working with local consultants to install groundwater data loggers and remote telemetry to monitor groundwater levels through freshet. 

Why were there so many landslides in 2022?

The City of Whitehorse and its experts cannot say with absolute certainty why so many landslides happened in 2022. It is believed that the landslides were contributed to by multiple factors, including:

• Two years of above-average snowfall;
• Groundwater levels not subsiding during the winter of 2021/2022;
• Instabilities had developed in the subsurface over time that were not detectable from the ground surface; and/or
• Slow progressive deformation had globally weakened the escarpment soils, requiring a small push to cause landslides.

We may never know with certainty why 2022 was an extreme year, but the City is planning further escarpment studies to help understand why so that we can be better prepared in the future. 

What has the City of Whitehorse been doing since the Robert Service Way landslide?

The City of Whitehorse has retained various hydrological and geotechnical consulting firms to provide:

• A high-level options analysis for long-term geohazard mitigation strategies along Robert Service Way;
• 2023 spring monitoring plan; and 
• Updated geohazard report for the entire escarpment from Robert Service Way to Takhini (ending at the intersection of Range Road and Mountainview Drive).

More specifically, future work may include:

• Working to better understand the groundwater conditions in the escarpment area;
• Identifying long-term mitigation strategy options;
• Exploring the completion of a risk analysis to help inform future decisions; and

The City takes the escarpment seriously and will continue working towards reducing the hazards for Whitehorse residents.

Will the escarpment ever be stable?

Over long periods (thousands of years), any over-steepened slope may naturally stabilize through a series of landslides. Without stabilization measures, the escarpment may not stabilize in a timeframe that does not require intervention. Therefore, the City of Whitehorse will remain proactive and work towards long-term hazard reduction and risk mitigation strategies.

What is the 2023 monitoring plan for Robert Service Way?

The City of Whitehorse has retained a geotechnical engineering team to monitor Robert Service Way through 2023 freshet. Inspections will begin in mid-April and is expected to continue until experts determine that freshet is over (likely end of June).

Multiple methods for monitoring the slope are being used, and they include:

• Visual inspections;
• Targeted survey monitoring of critical slope areas;
• Drone surveys; and 
• A radar-based slope scanner.

The monitoring frequency is determined by slope conditions and conducted as per recommendations from geotechnical engineers.

What is the 2023 monitoring plan for the rest of the escarpment?

The monitoring plan for the rest of the escarpment is similar to the monitoring plan for Robert Service Way, but a slope scanner will not be implemented. Geotechnical engineers will continue to perform regular visual inspections, and the monitoring plan will remain flexible to adapt to changing slope conditions.

Can future landslides be predicted before they occur, and what is the City of Whitehorse doing to identify future failures?

Predicting landslides including the magnitude (or size) and timing is extremely difficult to do. For example, an engineering consulting firm identified the Jeckell Street area as having the potential for a large landslide to occur in 2021, which prompted the closure of Cliffside Park and area; however, that slope did not fail until 2022. On the other hand, Robert Service Way did not show any signs of developing instability in 2021, and it failed in 2022.

The City of Whitehorse retains geotechnical engineers to provide recommendations for future work and monitoring to predict failures better. Regional monitoring will be performed to inform yearly susceptibility mapping. During freshet, monitoring will continue until the City of Whitehorse is advised otherwise.

What could cause the escarpment to fail?

Many geologic factors could contribute to slope failures of the escarpment, but the primary reason in this case is believed to be groundwater seepage. Seepage is when groundwater flows readily through sand and slowly through the silt, meaning groundwater can flow through the sand on top of the silt and out the escarpment face.

At a high level, the process is:

• The escarpment face freezes during winter and traps seepage within the escarpment soils.
• Groundwater can build up behind the escarpment face throughout the winter, which saturates the soils.
• Water in the soil plus freezing can lead to ice lenses and frost fracturing of the soils.
• As the soils thaw in the spring, they are saturated and lose their strength depending on the degree of saturation. Strength is reduced by ice lenses and frost fracturing as well.
• On steep slopes, these factors can allow the escarpment soils to fail with failure planes up to 5 m below ground surface. Sometimes failures occur because the near surface soils are thawed, and the subsurface soils are frozen.

Why did the Robert Service Way landslide on April 30, 2022, travel further than the landslide at Jeckell Street?

It is believed that the soils in the landslide at Robert Service Way were more saturated than the soils in the landslide at Jeckell Street. Saturated fine-grained soils can flow like a liquid when disturbed (i.e., movement down a slope), causing them to travel faster and further.

What experts have the City of Whitehorse retained to provide escarpment recommendations?

The City of Whitehorse has retained local geotechnical and hydrogeological consulting firms and a third-party engineering consultant specializing in geohazards to review work conducted by the local firms. The team comprises:

• Tetra Tech Canada Inc: escarpment monitoring, geohazard zone updates, long-term options analysis, and various other escarpment tasks requiring local expertise.
• BGC Engineering: third-party review and recommendations.
• SLR Consulting: groundwater monitoring at the airport and hydrogeological input.

The Yukon Geological Survey has also engaged with the City of Whitehorse. It has provided its expertise and observations as required.

What is freshet, and when does it occur?

Freshet is the period every year when seasonal snow and ice melt and influence river and groundwater systems. The duration of freshet varies year by year, but it generally occurs between end of April and June 30 for the Whitehorse escarpment.

Why was the landslide that occurred on April 30, 2022, not identified before it failed?

No significant signs of slope instability had been detected in that area in previous years.  

What was the purpose of the sheet pile wall installed in spring of 2022?

Inspections of the area conducted after April 30, 2022 noted signs of additional unstable soil masses above the failed slope, with volume estimates up to 4,000 m³, or about 400 dump trucks worth of soil. Based on the advice of the geotechnical engineers, the area was too hazardous to allow vehicle traffic without additional mitigation in place.

The sheet pile wall installed in the spring of 2022 served a few purposes, including: to reduce landslide risk and allow Robert Service Way to be reopened before the end of freshet; to protect the highest risk area in 2023 because of the soil masses that did not fail; and to provide additional protection of the area until the City can implement a long-term solution.

How does the sheet pile wall work?

The sheet pile wall creates a barrier that blocks landslide debris from reaching the roadway. To fully stop a landslide, sufficient volume must be behind the wall to capture the debris before the area is filled up. If there is insufficient volume, the landslide debris may overtop the wall. Still, its energy will dissipate, significantly reducing the chances of serious risk or damage.

The area behind the wall is expected to be enough to capture landslides of a similar magnitude to the one that occurred on April 30, 2022.

What is the long-term solution for Robert Service Way?

The City is exploring options for long-term solutions to protect all of Robert Service Way. The options for long-term solutions will need to be carefully considered and presented to Council to identify the option that best suits the City’s needs in all the circumstances. At this time, it is too early to identify the optimal solution.

What is the long-term plan for the rest of the escarpment?

The City of Whitehorse’s updated geohazard assessment for the escarpment is the first step in long-term planning for the rest of the escarpment. The geohazard assessment allows the City to begin long-term planning for the entire escarpment. The current conditions of the escarpment do not indicate any destabilized areas that affect residences downtown, but the City is mindful that conditions could change and will continue to monitor the escarpment and act accordingly.

Can the City install a sheet pile wall the entire length of Robert Service Way?

The simple answer is no because of how the wall works. A sheet pile wall, soil/concrete berm, or other means of stopping a landslide rely on a catchment area behind the mitigation structure to trap landslide debris. The volume of the catchment is controlled by the height of the mitigation measure and the area behind it before the toe of the slope.

North of the sheet pile wall, the toe of the slope is too close to the road to install a wall that has sufficient catchment for a landslide that could impact the roadway. South of the sheet pile wall, there is so much catchment area and runout length that a soil berm can be implemented if additional protection is required.

The sheet pile wall was the most effective mitigation measure that could be installed in a timely manner to mitigate risks.

Who do I contact if I see something concerning around the escarpment?

The City welcomes citizens contacting them if they see anything concerning along the escarpment. For:

• Emergencies (imminent slope failure or public safety risk): contact the Whitehorse Fire Department using 9-1-1. 
• After hours (non-emergency): contact the City reporting line at 667-2111
• General slope observations: contact City of Whitehorse staff using the following ways:
  • Engage Whitehorse
  • City reporting line:

What is the City doing to understand the geohazards along the escarpment in light of changing climate?

The City is regularly engaging with geotechnical consultants to continue planning investigation work to fully understand the conditions of the escarpment and how they may change in the future. 2021 and 2022 highlighted the need for updated groundwater information that is key to understanding its influence on slope stability.

Further investigation work along the escarpment is being planned to:

• Better understand soil conditions;
• Inform slope stability modelling using software;
• Inform mitigation requirements at various locations for existing and future infrastructure using acquired data;

With this ongoing investigating, the City will be better equipped to conduct a full, escarpment-wide analysis to predict how a changing climate will affect slope stability in the future.

How will the City communicate geohazard risks to the public?

The City will endeavor to take a multi-faceted approach to communicate the most recent information to the public as resources permit:

• For 2023, we will have an Engage Whitehorse page that will be updated as frequently as possible.
• The City’s communications team will use social media to release important information. Information will also be provided to local media to be released to the public if necessary.
• Depending on the condition of the escarpment, the City may also provide information to residences backed along the escarpment and give those residents the ability to speak one-on-one with City representatives to help them understand any implication if instabilities are detected. 

If I live near the escarpment, should I be concerned, and who can I contact to discuss in more detail?

The City expropriated properties in the high-hazard zone and some in the moderate-hazard zone after the 2002 geohazard study to mitigate risks. If you are concerned about the escarpment slope, you can contact the City of Whitehorse Engineering Department at: 867-667-6401.

Will Robert Service Way  be closed in 2023?

Robert Service Way may be closed this year if geotechnical engineers recommend closure based on slope conditions. There are three levels that will be planned for:

• Level 0 (no trail or road closures)
• Level 1 (modified/restricted access only)
• Level 2 – full closures

Closing or reducing traffic through Robert Service Way is a decision not taken lightly and is done to protect the public. The City of Whitehorse understands the inconvenience this may cause however, sometimes avoidance is the best method to mitigate a hazard and sometimes closing Robert Service Way is the best method to protect the public when slope movements suggest imminent failure.

What is the City of Whitehorse doing to keep Robert Service Way open this year?

Like last year, the City of Whitehorse’s geotechnical experts will continue to monitor the area. Among other things, the City of Whitehorse will be using a radar slope scanner for 2023 that will help gather data and assist in the assessment of whether it is safe to keep Robert Service Way open for 2023. If the slope scanner is successful, it may be recommended each year until a long-term slope stabilization method is designed and constructed.

How did the City of Whitehorse manage hazards and risks in 2022?

In order to manage hazards and risks, the City of Whitehorse, supported by its geological engineers, took a number of steps during the 2022 freshet. These steps included: keeping Robert Service Way closed until the sheet pile wall was installed, continuing to monitor slope movement data to ensure safety, installing the sheet pile wall, closing trails along the escarpment, informing the public to avoid the escarpment, and increasing inspections by engineers.

After April 30, 2022, slope movement data indicated that the risk of another landslide was too significant to open the road to the public as well as allow the cleanup of RSW. It took two weeks for slope movements to begin settling down so that City of Whitehorse staff could work safely unprotected below the slope. During the cleanup, continuous visual monitoring, both at the bottom and top of the slope, was required to ensure the safety of City staff clearing the area.

The City will continue to employ methods and lessons learned from 2021 and 2022 to reduce the geohazards during freshet and reduce the risk to life by closing areas, trails, and roadways as required.

Will the City of Whitehorse be closing trails and areas below the escarpment again in 2023?

The City of Whitehorse will likely be closing the following areas at the beginning of freshet:

• West side of 6^th Avenue at Jeckell Street;
• East airport access road between Hanson Street and Rogers Street;
• The dog park at the end of Main Street;
• The area at the end of Wood Street; and
• The south side of Puckett’s Gulch access to Black Street stairs, but the primary access and stairs will remain open.

The City is prepared to close the entire escarpment area if 2023 proves to be as active as 2022. We ask residents to respect closure areas for their safety and only to use designated trails open to the public to traverse the escarpment slopes.

How do geohazards impact development within the area of the escarpment?

The Whitehorse escarpment has three geohazard zones: low, moderate, and high. No development is allowed in the high-hazard zone, and unrestricted development is permitted in the low-hazard zone. Any new development in the moderate zone must have a site-specific geohazard analysis by a geotechnical engineer licensed to practice in the Yukon.

A site-specific geohazard analysis may not recommend any geohazard mitigation measures, or could recommend measures such as a berm or increased structural integrity of walls facing the escarpment and restrictions on where bedrooms are permitted in the building. 

The City expects to continue to require these site assessments to allow development in the moderate zone. The City retained Tetra Tech to update the previous geohazard report using modern methods and has recently received the report. The report provides updates to the hazard zones, and the City will update the maps accordingly once the report is finalized.

What is geohazard mitigation?

Geohazard mitigation methods reduce the hazard, which can be a combination of:

• Public education;
• Avoidance;
• Monitoring;
• Berms, walls, barriers, etc;
• Slope netting;
• Soil anchors and shotcrete; 
• Slope angle relaxation; and/or
• Other methods that are not listed.

Debris Flow: like a debris slide, but sediment is water-laden, fast-moving, and forms thick muddy deposits. Typically, they are channelled in gullies where their speed and energy increase. These flows can readily move boulders and uproot trees.

Debris Slide: a relatively dry landslide of unconsolidated soil or rock that has slid or rolled rapidly down a steep slope to form an irregular hummocky deposit. These movements are somewhat slower than others.

Deep-Seated Failures: failures that initiate back from the crest of the slope with a failure plane that extends beyond the toe. This slope process has not been observed along the Whitehorse escarpment to date.

Earthflow: a viscous flow of fine-grained materials downslope saturated with water and moving under the force of gravity. This is an intermediate flow between downhill creep and mudflow.

Escarpment: a long, steep slope, especially at the edge of a plateau or separating land areas at different heights.

Extensometer: a device used to measure the displacement between two points using a string, weight, and ruler affixed to two posts anchored in the ground. Typically, extensometers are placed in a manner where one pole is anchored in the stationary soil mass and the other in the mass that is moving. These are usually installed perpendicular to tension cracks on a slope.

Failure Plane/Surface: a plane drawn through the soil that becomes the sliding surface for a landslide. The failure plane may be relatively uniform or it may undulate depending on soil conditions.

Freshet: a term used to describe a period of snow melt, an annual high water event on rivers from snow melt, and river ice melting. For the Whitehorse escarpment, freshet season is considered from mid-April to the end of June each year. During freshet, increased surface water and seepage flows are observed.

Frost Fracturing: the result of frost penetration into fine-grained soils that causes expansion and ice lenses. When the ground thaws, these fractures remain and can increase in size the following winter. Frost fracturing contributes to shallow mass movements on the escarpment.

Ground Based Interferometric Radar: radar technology used for continuous slope scanning to help find and track developing instabilities. 

Headscarp: the head is the upslope portion of a landslide, and the scarp is a steeply inclined failure surface with exposed soil and/or rock that marks the top of a landslide. Headscarps are found throughout the escarpment and are easily identified as “large bowls” above gullies or where near vertical faces of soil are exposed.

Ice Lenses: Massive ice, millimetres to centimetres thick, forms as fine-grained soil freezes with excess pore water. Due to many years of growth, ice lenses can grow to be metres thick in permafrost areas.

Liquefaction: a process that occurs in saturated soil where disturbance causes excess pore water pressure and changes the behaviour of the soil from a solid to a liquid. The April 30, 2022 landslide’s reach was primarily due to saturated silt and clay liquefaction.

Mass Movement (landslide): a catch-all term for the bulk movement of soil and rock debris downslope in response to the pull of gravity; debris slide, debris flow, earthflow, and mudflow are all examples of mass movements.

Mudflow: similar to an earthflow, but sediment has liquefied and moves quicker.

Pore Water: water trapped in the interstitial space between soil grains. 

Rilling Erosion: shallow channels formed in soils from the flow of water down a slope.

Runout: a term used to describe the reach of a landslide once it reaches the toe of a slope. Runout is controlled by many factors; topography, soil composition, velocity, etc.

Seasonal Frost: the seasonal freezing of near-surface soils. The expected frost penetration depth in Whitehorse is 2.4 m in undisturbed areas. It can reach up to 5 m depth in disturbed areas.

Seepage: the flow of fluids through soil. Groundwater is the most common form of seepage, but it can be other fluids such as air, hydrocarbons, etc. Seepage occurs throughout the Whitehorse escarpment, most notably in Baxter’s and Puckett’s Gulch. Generally, the number of seepage points from the escarpment face increases moving south.

Shallow Failures: the typical slope processes of the escarpment where failures occur within the slope between the crest and toe. 

Tension Crack: soil has little to no tensile (pull apart) strength. Tension cracks are formed when a mass of soil moves differentially from another. These cracks are an indication of ground movement and can be measured using instrumentation.