Slope Stability Basics

The single biggest thing to know about slope stability, the breadth PE, is the factor of safety numbers:

1.5 – Permanent conditions

2.0 – Slope is supporting a building foundation

1.3 – temporary slope conditions

These will probably be in a word problem.

What is the factor of safety?

The factor of safety is a ratio of how stable the slope is given soil characteristics and geometry of the slope (angle and height etc).

The methods of determining the factor of safety for slope stability are somewhat advanced. I think they will definitely appear on the depth exam but likely not on the breadth. I suggest knowing they exist and where they are (Ch 210 Goswami, 40-7 CERM).

What Causes Slope Failure?

All of these things cause failure, eventually:

  • Steepening of slopes
  • Increase in groundwater pressure
  • Weathering (which may be related to steepness)
  • Vibrations, especially repetitive vibrations (traffic), or extreme vibrations (earthquakes)
  • Extra Loading above the slope
  • Excavation at the toe of the slope

 What prevents Slope Failure?

  • Making it less steep through grading
  • Retaining structures
  • Weight at the tow
  • Less weight above the slope
  • Reduce or remove vibrations
  • basically reverse the causes….


About Conrad

I am a Civil Engineer. I work in San Diego and am preparing to take the PE Exam. I am interested in surfing, business, travelling, and spending time with my wife. Thanks!

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