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Slope Stability 101 Basic Concepts and NOT for Final Design Purposes!

Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

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Page 1: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Slope Stability 101Basic Concepts and NOT for Final Design Purposes!

Page 2: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Slope Stability Analysis Basics

Shear Strength of Soils

Ability of soil to resist sliding on itself on the slope

Page 3: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Angle of Repose

definition

n1. the maximum angle to the horizontal at which rocks, soil, etc, will remain without sliding

Page 4: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 5: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Shear Strength Parameters and Soils Info

Φ angle of internal friction

C cohesion (clays are cohesive and sands are non-cohesive)

Θ slope angle

γ unit weight of soil

Page 6: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Internal Angles of Friction Estimatesfor our use in example

Silty sand Φ = 25 degreesLoose sand Φ = 30 degreesMedium to Dense sand Φ = 35 degreesRock Riprap Φ = 40 degrees

Page 7: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Slope Stability Analysis Basics

Explore Site Geology

Characterize soil shear strength

Construct slope stability model

Establish seepage and groundwater conditions

Select loading condition

Locate critical failure surface

Iterate until minimum Factor of Safety (FS) is achieved

Page 8: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Rules of Thumb and “Easy” Method of Estimating Slope Stability

Geology and Soils Information Needed (from site or soils database)

Check appropriate loading conditions (seeps, rapid drawdown, fluctuating water levels, flows)

Select values to input for Φ and C

Locate water table in slope (critical for evaluation!)

2:1 slopes are typically stable for less than 15 foot heights

Note whether or not existing slopes are vegetated and stable

Plan for a factor of safety (hazards evaluation)

FS between 1.4 and 1.5 is typically adequate for our purposes

Page 9: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

No Flow Slope Stability Analysis

FS = tan Φ / tan Θ

Where Φ is the effective angle of internal friction of soil

Where Θ is the slope angle

Page 10: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Flow Parallel to Slope

FS = (γ’ / γsat) (tan Φ / tan Θ

Where γ’ = buoyant unit weight of soil γsat -γwater

Where γsat = saturated unit weight of soilWhere γwater = unit weight of water = 62.4 pcf

(pounds per cubic foot)

Page 11: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

Horizontal Flow

FS = (γ’ – γw tan2 Θ) / γsat (tan Φ / tan Θ)

Where γ’ = buoyant unit weight of soil γsat - γwater

Where γsat = saturated unit weight of soil

Where γwater = unit weight of water = 62.4 pcf(pounds per cubic foot)

Where Φ is the effective angle of internal friction of soil

Where Θ is the slope angle

Page 12: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 13: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

FS > 1.5 so STABLE!!!

Page 14: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 15: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

FS < 1.4 so NOT STABLE!!!

Page 16: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 17: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

FS < 1.4 so NOT STABLE!!!

Page 18: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 19: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 20: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 21: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 22: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 23: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 24: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 25: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 26: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 27: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 28: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 29: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 30: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish
Page 31: Slope Stability 101 - University of Wisconsin–Stevens … Stability Analysis Basics Explore Site Geology Characterize soil shear strength Construct slope stability model Establish

QUESTIONS?!?!