Slope Stability

Heavy rainfalls and high-volume snow melt can result in slope failures from time to time. Landslides have occurred in the District of Mission in the past, and property owners should be aware of the warning signs and preventative measures they can take.

Living and Working Near Gullies and Steep Slopes in the District of Mission

October 2011

Landslides have occurred within the District of Mission and have resulted in considerable loss of property and impacts to public infrastructure and usually occur as a result of heavy rains or snowmelt. Although not always the case, signs of instability usually show up in advance of slope stability problems occurring. Property owners should monitor their property for signs of instability before, during and after heavy rains. These signs can include:

• Cracks in the foundation walls or concrete patios.
• Leaning deck posts or excessively sloping decks.
• Slumps or slides in the stream banks.
• Erosion and soil exposure on the slopes.
• Fence posts or trees that are leaning downslope or upslope.
• Cracks or soft spots in the ground near the top of and on the slope.
• Trees with a strongly curved (“J”) shape at their base.
• Outbreaks of springs where there were none before.
• Undercutting of stream banks by creeks at the base of gullies.
• Presence of water loving plants on slopes – skunk cabbage, horse tail, devil’s club etc.
• Old wood retaining walls which have deteriorated due to age.
• Other retaining walls which exhibit signs of movement.

Property owners can take several steps to reduce the likelihood of slope stability problems affecting their property including, but not limited to:

• Hire a geotechnical engineer to complete regular stability assessments of your property and implement any recommendations. Ensure water from downspouts, driveways, lawns, decks etc. is directed away from steep slopes or conveyed to the base of the slopes in solid walled pipe. Refrain from dumping brush, leaves, grass clippings etc onto steep slopes. Ensure that retaining walls, swimming pools, ponds etc. on or above steep slopes are designed by an engineer.
• Ensure that any fill materials to be placed to develop yards are designed by a geotechnical engineer and placed under his or her supervision. Maintain appropriate setbacks from the top of steep slopes as directed by the geotechnical engineer.
• Have a geotechnical engineer assess the steep slopes for appropriate vegetation cover and adequate slope stability measures. Removal or topping of trees on the slope should only be undertaken with approval by a qualified professional arborist in consultation with a geotechnical engineer.

Geotechnical engineering firms can be found in the yellow pages. District engineering staff is available to answer any questions, please contact Mike Younie at 604- 820-3798.