When Hydroseeding Isn’t Enough: Stabilizing Steep Slopes & Landslide Prone Terrain

Lake County Slide

Although hydroseeding can often be an effective treatment for surficial slope instabilities, steep slopes and landslide prone terrain call for an altogether different approach.

The first step in analyzing a particular site is to determine the primary source of instability. Most frequently this can be attributed to high soil saturation and poor drainage. When combined with weak soils, joints or fissures in the slope material, contrasting material permeability, and steep slopes, the result is often catastrophic.

California Landslide Susceptibility map

Estimated landslide susceptibility based on rock strength and slope steepness. Areas along the coast are among the most susceptible to deep landsliding. [California Geological Survey/USGS, 2011]

Trigger Events

In many cases, landslides occur immediately following a trigger event. Common triggers include prolonged, intense rainfall, accelerated snow melt, or seismic activity. In addition to the trigger event(s), a combination of natural and human contributions are almost always involved…

Natural contributions may include:

  • Joints and fissures within slope material
  • Rainfall
  • Contrasts in materials including permeability and stiffness
  • Earthquakes
  • Erosion (glacial, fluvial, wave)
  • Freeze-thaw cycles
  • Tectonic and volcanic activity

Human contributions may include:

  • Excavation
  • Vibration
  • Deforestation
  • Water leakage
  • Mining

Developing a Solution

Once the primary sources of instability have been identified and the soil properties analyzed, a mitigation plan can be developed for each area of concern:

  • Surface and ground water drainage
  • Failure planes/zones
  • Soil mass support
  • Rockfall hazard
  • Vegetative establishment

Common mitigation techniques include percussion driven earth anchors, structural repair with compacted soil lifts, surface and subsurface drains, soil nails, gabions, and gravity systems. Effectively integrating these methods is the key to long-term slope stability.

georgetown_gabions7c5f15 hwy_1_soil_nails hwy_26_earth_anchors trinity_co_slide45e67d

Logo_FINAL_2012_smallThunder Mountain Enterprises offers design-build services for slope stabilization and landslide repair. For more information, please call 916.381.3400