Slope stability
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This document provides an overview of slope stability and analysis. It defines different types of slopes as natural, man-made, infinite and finite. Common causes of slope failure like erosion, seepage, drawdown, rainfall, earthquakes and external loading are described. Key terms used in slope stability are defined, including slip zone, slip plane, sliding mass and slope angle. Types of slope failures are identified as face/slope failure, toe failure and base failure. Methods for analyzing finite slope stability, like Swedish circle method, Bishop's simplified method and Taylor's stability number are introduced. Infinite slope analysis is described for cohesionless, cohesive and cohesive-frictional soils. Example tutorial problems on slope stability calculations are
Slope stability
- 3. Introduction • Why need to understand the slope failures?
- 6. Himalayan Region 50 million cubic yards
- 9. Slopes and its necessity • An exposed ground surface that stands at an angle (b) with the horizontal is called slope. • construction of highway and railway • Embankments • earth dams • levees and canals.
- 10. Types of Slopes Infinite Slopes • They have dimensions that extend over great distances and the soil mass is inclined to the horizontal. Finite Slopes • A finite slope is one with a base and top surface, the height being limited. • The inclined faces of earth dams, embankments and excavation and the like are all finite slopes.
- 11. Types of slopes
- 12. Types of slopes • Natural : Formation due to geological features of the earth • man made: Construction activity like cutting, filling etc •
- 13. Why to understand slopes? • Failure of natural slopes (landslides) and man-made slopes has resulted in much death and destruction. • Civil Engineers are expected to check the safety of natural and slopes of excavation. • Slope stability analysis consists of determining and comparing the shear stress developed along the potential rupture surface with the shear strength of the soil. • Attention has to be paid to geology, surface drainage, groundwater, and the shear strength of soils in assessing slope stability.
- 14. Slope Failure Triggering Mechanisms • Intense Rain-Fall • Water-Level Change • Seepage Water Flow • Volcanic Eruption • Earthquake Shaking • Human activity
- 15. Causes of Slope failure • Erosion: The wind and flowing water causes erosion of top surface of slope and makes the slope steep and thereby increase the tangential component of driving force. • Steady Seepage: Seepage forces in the sloping direction add to gravity forces and make the slope susceptible to instability. The pore water pressure decrease the shear strength. This condition is critical for the downstream slope. • Sudden Drawdown: in this case there is reversal in the direction flow and results in instability of side slope. Due to sudden drawdown the shear stresses are more due to saturated unit weight while the shearing resistance decreases due to pore water pressure that does not dissipate quickly. • Rainfall: Long periods of rainfall saturate, soften, and erode soils. Water enters into existing cracks and may weaken underlying soil layers, leading to failure, for example, mud slides.
- 16. Causes of Slope failure …. • Earthquakes: They induce dynamic shear forces. In addition there is sudden buildup of pore water pressure that reduces available shear strength. • External Loading: Additional loads placed on top of the slope increases the gravitational forces that may cause the slope to fail. • Construction activities at the toe of the slope: Excavation at the bottom of the sloping surface will make the slopes steep and there by increase the gravitational forces which may result in slope failure
- 17. Definition of Key Terms
- 18. • Slip or failure zone: It is a thin zone of soil that reaches the critical state or residual state and results in movement of the upper soil mass. • Slip plane or failure plane or slip surface or failure surface: It is the surface of sliding. • Sliding mass: It is the mass of soil within the slip plane and the ground surface. • Slope angle : It is the angle of inclination of a slope to the horizontal. • The slope angle is sometimes referred to as a ratio, for example, 2:1 (horizontal: vertical).
- 19. Types of failure • Broadly slope failures are classified into 3 types as 1. Face (Slope) failure 2. Toe failure 3. Base failure
- 20. Face (Slope) Failure: Face (Slope) Failure: This type of failure occurs when the slope angle is large and when the soil at the toe portion is strong. A B
- 21. Toe Failure: In this case the failure surface passes through the toe. This occurs when the slope is steep and homogeneous.
- 22. Base Failure: In this case the failure surface passes below the toe. This generally occurs when the soil below the toe is relatively weak and soft.
- 23. Analysis of slopes : Factor of safety • Factor of safety of a slope is defined as the ratio of average shear strength (tf ) of a soil to the average shear stress (td) developed along the potential failure surface.
- 26. Finite Slopes: Analysis • A finite slope is one with a base and top surface, the height being limited. • The inclined faces of earth dams, embankments, excavation and the like are all finite slopes. • Investigation of the stability of finite slopes involves the following steps – a) assuming a possible slip surface, – b) studying the equilibrium of the forces acting on this surface, and – c) Repeating the process until the worst slip surface, that is, the one with • minimum margin of safety is found.
- 27. Analysis methods: finite slope • Swedish Circle/Arc Method/Method of slices/Standard method • Bishop’s Simplified method • Taylor’s stability No
- 28. The Swedish method of slices for a cohesive – frictional (c-Ø ) soil • Adopted for general type of soil which have the combined grains of C and Ø soils. • For a c-Ø soils the undrained strength envelope shows both c and Ø values. • The total stress analysis can be adopted.
- 33. Bishop’s Simplified method • Swedish circle method ignores the rotational failure of each slides, but consider overall moment equilibrium equation. • In Bishop’s method the equilibrium is consided for all forces at the each slide . •
- 36. Infinite Slopes: Analysis (Slope stability of earthen dam for different conditions) • Infinite slopes have dimensions that extended over great distances and the soil mass is inclined to the horizontal. • If different strata are present strata boundaries are assumed to be parallel to the surface. • Failure is assumed to occur along a plane parallel to the surface.
- 38. Analysis cases • Case (i) Cohesionless soil • Case (ii) Cohesive soil • Case (iii) Cohesive-frictional soil.
- 39. Infinite slopes in Cohisionless soils Consider an infinite slope in a cohesionless soil inclined at an angle to the horizontal as shown. Consider an element ‘abcd’ of the soil mass.
- 41. Applying the concept of limit equilibrium “The maximum inclination of an infinite slope in a cohesionless soil for stability is equal to the angle of internal friction of the soil”.
- 43. Infinite slope in pure cohesive soil
- 46. Infinite slope in cohesive frictional soil
- 48. 3) Compute average pore pressure at base of each slice
- 49. Problem 1
- 57. Problem 2
- 58. Solution
- 59. Problem 3
- 63. Problem 4
- 67. Problem 5