3 Seismic Stability of Slopes
Page 1: Course Introduction
Course Title: CEE 541 Seismic Stability of Slopes
Instructor: Dr. Shriful Islam, PhD (UQ), MESc (UWO), BSc (SUST)
Contact Information:
Email: sharif_sust_cee@yahoo.com, sharif-cee@sust.edu
Emergency Call: 01716687869
Page 2: Overview of Earthquake Effects on Slopes
Impact of Earthquakes:
Slopes, embankments, and dams may suffer damage or failure from earthquake-induced shaking.
Historical examples illustrate severe landslides, including the 1964 Alaska Earthquake and the Lower Van Norman Dam failure in 1971.
Earthquakes pose a major risk to slope stability through slides, slumping, cracks, and permanent deformations.
Basic Mechanisms of Seismic Effects:
Failure mechanisms arise from:
Earthquake-induced forces and stresses.
Structural changes within the soil caused by seismic stresses.
Page 3: Mechanisms of Failure
Total Stress and Material Response:
Stiff materials (clay, gravel, dense coarse sand) fail when total stress exceeds material strength without structural change.
Fine, loose, saturated sands may liquefy, causing rapid dissipation of excess pore water pressure, e.g., during the 1964 Alaska Earthquake.
Similar abrupt changes may occur in sensitive clays, such as Canadian Leda clay and Norwegian quick clay.
Page 4: Factors Influencing Stability
Stability Considerations:
Essential factors: geological, hydrological, geometrical, and material characteristics are critical for stability evaluations.
Investigation Methods:
Subsurface investigations: excavation, boring, sampling, in situ testing, and geophysical testing.
Laboratory tests to determine soil properties: density, strength, stress-strain behavior, permeability.
Page 5: Static Slope Stability Analysis
Static Stability Impact:
The seismic stability of slopes is affected by their static stability; slopes with low static safety factors require less dynamic stress for instability.
Analysis Methods:
Approaches for static stability analysis:
Culmann method: assumes plane failure.
Wedge methods: assume failure along 2 or 3 planes.
Circular or log-spiral methods for homogeneous slopes.
Soil above potentially unstable planes subdivided into slices for equilibrium consideration.
Page 6: Failure Surface Geometries
Common Failure Surface Geometries:
(a) Planar
(b) Multiplanar
(c) Circular
(d) Noncircular
Page 7: Factor of Safety Considerations
Factors of Safety:
Must be greater than 1.0; acceptable values depend on uncertainties in:
Model accuracy.
Input parameters.
External loading magnitude and duration.
Consequences of slope failure.
Typical minimum safety factors: 1.5 for long-term loading, 1.3 for temporary conditions.
Page 8: Stress-Deformation Analyses
Common Methods:
Finite element method (FEM) used for stress-deformation analyses to predict stresses and movements during construction.
Accuracy influenced by the stress-strain model; hyperbolic model is often effective.
Page 9: Seismic Slope Stability Analysis
Key Considerations:
Analyze dynamic stresses from earthquakes and changes in material behavior due to seismic loading.
Types of Effects:
Inertial effects: dynamic stresses exceed soil shear strength.
Weakening effects: soil undergoes liquefaction or softening, destabilizing under dynamic stresses.
Page 10: Inertial Instability Analysis
Stresses Induced by Earthquakes:
Earthquake motions create dynamic stresses along potential failure surfaces, which can lead to instability.
Techniques for inertial instability analysis vary in how they represent earthquake motion and slope response.
Significance of Seismic Effects:
Knowledge of seismic forces helps evaluate embankment stability with pseudo-static approaches, linking factor of safety and displacement assessment.
Page 11: Pseudo-Static Approach
Method Overview:
Pseudostatic analysis estimates safety factors by treating seismic forces as static forces added to existing forces.
Calculation Components:
Incorporates horizontal and vertical pseudostatic accelerations to assess slope stability.
Page 12: Calculating Pseudostatic Forces
Calculating Magnitudes:
For seismic forces:
Horizontal forces: Fh = kh * W
Vertical forces: Fv = kv * W
Factors used include Mohr-Coulomb shear strength parameters (c and φ) and failure plane length.
Page 13: Effects of Forces on Factor of Safety
Impact of Forces:
Horizontal force decreases factor of safety by reducing resisting force while vertical influence is less significant.
Analytical Applications:
Pseudostatic factors of safety evaluated across various surface geometries (planar, circular, noncircular).
Page 14: Calculating Overturning Moments
Overturning Moment Analysis:
Static overturning moment from weight of soil above failure surface.
Pseudostatic conditions moment includes static and added pseudostatic forces.
Page 15: Limitations of Pseudo-Static Approach
Analysis Limitations:
Simplifying dynamic effects to a constant pseudostatic acceleration can lead to inaccuracies, particularly in soils that develop significant pore pressure.
Page 16: Newmark Sliding Block Analysis
Purpose:
Provides insight into permanent displacements, contrasting with limit equilibrium methods that only yield factor of safety.
Evaluation:
Dropping factor of safety below 1.0 indicates the potential for failure acceleration.
Page 17: Analogy in Sliding Block Failure
Sliding Block Analogy:
Helps conceptualize destabilization under seismic conditions, similar to block dynamics on an inclined surface.
Page 18: Forces Acting on a Sliding Block
Force Dynamics:
Forces acting on a block include static and dynamic conditions, influencing stability during seismic events.
Page 19: Yield Coefficient and Instability
Yield Coefficient:
Identifies minimum pseudostatic acceleration for block instability; dynamic factor of safety decreases as yield coefficient increases.
Page 20: Factor of Safety Variation
Safety Factor Variations:
Factor of safety changes as a function of horizontal pseudostatic coefficients under different conditions.
Page 21: Stress-Deformation Analysis Techniques
Stress-Deformation Analyses:
Dynamic finite-element analyses predict seismic-induced permanent strains, providing insights into post-earthquake behavior.
Page 22: Weakening Instability Analysis
Weakening Mechanisms:
Includes pore water pressure generation reducing shear strength, leading to flow and deformation failures.
Page 23: Types of Failures
Flow Failures:
Driven by static stresses; can cause large deformations and damage.
Deformation Failures:
Smaller deformations than flow failures; result from dynamic shear stresses exceeding reduced shear strengths.
Techniques for prediction stem from empirical methods due to the complexity of mechanisms.
Page 24: Hamada et al. Approach
Method Details:
Considers geotechnical and topographic conditions affecting permanent ground displacements; empirical relationships established.
Equation:
D(m) = 0.75H^(1/2)(^1/3) (thickness and slope parameters yield predicted displacements).
Page 25: Example of Post-Liquefaction Behavior
Case Study:
Post-liquefaction impact of the Sardis Dam illustrating large strains; measures taken include driving compaction piles to mitigate risk.
Page 26: Youd and Perkins Approach
Liquefaction Severity Index (LSI):
Index based on observed displacements to estimate ground displacement and response in specific conditions, aiding probabilistic analysis.
Page 27: LSI Variability with Magnitude
Effects of Earthquake Magnitude and Distance:
Relationship seen in datasets, providing foundations for seismic hazard mapping.
Page 28: LSI and Ground Effects
Table of Ground Effects by LSI:
Different LSI values correlate with specific characteristics and occurrences of liquefaction effects in grounded deposits.
Page 29: Byrne Approach for Displacement Estimation
Model Framework:
Permanent displacement from liquefied soil using work-energy principles with an empirical model representing soil behavior during seismic events.
Page 30: Estimation of Limiting Shear Strain
Approach Details:
Incorporates soil behavior observations to qualify predictions for various scenarios, adjusting for liquefaction effects.
Page 31: Upper San Fernando Dam Analysis
Finite-Element Mesh Analysis:
Illustrates significant failure response as observed in the 1971 dam failure, with significant displacements in liquefied zones.
Page 32: Liquefied Sand Displacement Problem
Example Problem:
Estimation of permanent displacement in a slope scenario using the Bryne approach due to liquefaction.
Page 33: Calculating Permanent Displacement
Outcome from Scenarios:
Given parameters indicate a calculated displacement under specific shear strain conditions.
Page 34: Baziar et al. Approach
Seismic Displacement Modeling:
Sliding block analysis generative equations help determine permanent lateral displacements under various seismic conditions.
Page 35: Displacement Predictions Comparison
Comparison of Approaches:
Correlations made between predicted displacements with case histories suggesting reliability of proposed models under varying conditions.
Page 36: Empirical Displacement Models by Bartlett and Youd
Model Development:
Empirical expressions derived from extensive case history databases; equations established for predicting lateral displacement based on multiple influencing factors.
Page 37: Ground-Slope Deformation Models
Equation Applications:
Ground-slope model predicting lateral ground displacement incorporating key parameters like moment magnitude, slope conditions, and distance from seismic sources.
Page 38: Estimation of Permanent Displacement Problem with Bartlett and Youd
Scenario Problem:
An assessment to predict displacement based on specific earthquake magnitudes and surface distance revelations utilizing the Bartlett and Youd model.
Page 39: Parameters for Displacement Calculation
Parameters Summary:
Extract parameters needed from scenario to assess permanent displacement outcomes with further note on estimations.