Many homeowners choose to place their home on lots with steep slopes. Generally, a contractor removes the “toe” by cutting a shelf into the side of the hill. The “toe” plays a critical role in keeping the upper portion stable. Removing the “toe” without reinforcement should be avoided. An unreinforced slope can erode and collapse, which can result in the home being pushed off its foundation.
Rainwater and runoff adds tremendous weight and pressure within the soil and acts like a lubricant. This combination reduces the frictional forces that hold the hill in place, causing the slope to become unstable resulting in potential failure (landslide).
When the home is located adjacent to a slope steeper than 1:3 (1’ vertical and 3’ horizontal), special setbacks (D) are required between the building and the base of the slope. This setback is to protect the home from damage caused by drainage, erosion, and failures of the slope (landslide). You should consult with the local building official or the state residential building code to determine these minimum setback distances (D).
If a building official or state building regulation is not available, then contacting an engineer to determine proper setbacks is also an option. The tables below can be used as a general reference for the minimum recommended setback for slopes from 1:3(18.5°) to 1:1(45°). Building sites with slopes greater than 45° or where the minimum setbacks can’t be met should be referred to a professional engineer or registered architect.
The Makeup of Soil
Soil is made up of soil particles, air, and water. Soils like clay are weaker (unless properly compacted and drained) because they contain more air. Excess water saturates the soil particles replacing the air, increasing the weight, and pushing the particles apart which reduces the friction and weakens the soil.
How to Prevent Landslides
- Keep vegetation in place to reinforce soil.
- Address the steepness by re-grading the slope.
- Divert roof water and slope runoff away from home.
- Properly compact fill to avoid pooling, uneven settling, and landslides.
- Stabilize slope with retaining walls. A retaining wall is recommended for slopes that exceed 1:1.5 (1’ vertical to 1.5’ horizontal or 34-degrees).