Stability of Earth Masses and Slopes

Stability of Earth Masses and Slopes

Copyright: © 2015 |Pages: 61
DOI: 10.4018/978-1-4666-6505-7.ch009
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Abstract

The design of open-cut slopes and embankments, foundations, levees, and earth-dam cross-sections is based primarily on stability considerations. There are many causes and types of earth instability. There are also many ways of analyzing the stability of slopes. The chapter considers the limit equilibrium approach, which aims essentially to determine a factor of safety, F, that would ensure a slope does not fail. The chapter considers the analysis of stability of infinite slopes based on translational type of failure and the analysis of finite slopes using the Swedish Method, Method of Slices, Bishop Simplified Method, Friction Circle Method, and the Translational Method. The solution of equations developed for the analysis of stability of slopes can be tedious and time consuming. A way of reducing the amount of calculation required in slope stability studies is by use of charts based on geometric similarity. The chapter discusses how Taylor (1948) and Janbu (1964) charts are used in stability analysis of slopes. Finally, the chapter discusses ways to reduce the risk of instability in slopes.
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9.1 Types And Causes Of Earth Movements

The safety of an earth mass against failure or movement is termed its stability. It must be considered not only in the design of earth structures but also in the repair and correction of slope failures. The design of open-cut slopes and embankments, foundations, levees and earth-dam cross-sections is based primarily on stability considerations.

9.1.1 Causes of Earth Failure

Earth movements occur when the shear strength of the soil is exceeded by the shear stresses over a relatively continuous surface. Failure at a single point in the mass does not necessarily mean that a soil mass is unstable. Instability results only when shear failure has occurred at enough points to define a surface along which the movement can take place. Anything that causes a decrease in soil strength or an increase in soil stress contributes to instability. Failures in foundations for embankments, buildings or earth structures can occur due to an increase in applied load without a corresponding increase in the shear strength of the soil or due to a decrease in the foundation shear strength.

Increase in stresses may be caused by (a) external loads such as the weight of buildings, water and snow; (b) increase in unit weight of the soil due to increased water content; (c) removal of part of soil mass by excavation; (d) undermining caused by tunneling, collapse of underground caverns or seepage erosion; (e) shock caused by earthquake or blasting; (f) tension cracks in cohesive soils; (g) water pressure in cracks; and (h) changes in slope profile resulting in added driving weight.

Decrease in strength may be caused by (a) swelling of clays by adsorption of water; (b) increase of ground water pressure resulting in decrease of frictional resistance in cohesionless soils or swell in cohesive soils; (c) breakdown of loose or honey-combed soil structure; (d) time dependent decrease in shear strength due to weathering, leaching, mineralogical changes, opening and softening of fissures, or continuing gradual strain; (e) hair cracking from alternate swelling and shrinking of cohesive soils; (g) thawing of frozen soil or frost lenses; (h) deterioration of cementing material which gives cohesion to soils; and (i) vibration of loose granular soils.

In the analysis of stability, the gravitational, seepage and other forces, which tend to cause slippage and produce failure, are called disturbing forces while the forces which resist failure (e.g. shear resistance of the soil) are referred to as restoring forces. Stability analysis is based on theoretical considerations and practical observations. Some of the factors, which affect the stability of earth masses, are very complex and difficult to evaluate. Therefore a number of simplifying assumptions are usually made for carrying out the analysis. Some of these assumptions relate to the geometry of the failure surface, the strength characteristics of the soil, the variability and heterogeneity of the soil properties and the seepage conditions in the soil.

Although there are multiple types of causes of landslides, the three that cause most of the damaging landslides around the world are these:

  • 1.

    Geological cause:

    • a.

      Weak or sensitive materials,

    • b.

      Weathered materials,

    • c.

      Sheared, jointed, or fissured materials,

    • d.

      Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth),

    • e.

      Contrast in permeability and/or stiffness of materials.

  • 2.

    Morphological causes:

    • a.

      Tectonic or volcanic uplift,

    • b.

      Glacial rebound,

    • c.

      Fluvial, wave, or glacial erosion of slope toe or lateral margins,

    • d.

      Subterranean erosion (solution, piping),

    • e.

      Deposition loading slope or its crest,

    • f.

      Vegetation removal (by fire, drought),

    • g.

      Thawing,

    • h.

      Freeze-and-thaw weathering,

    • i.

      Shrink-and-swell weathering.

  • 3.

    Human causes:

    • a.

      Excavation of slope or its toe,

    • b.

      Loading of slope or its crest,

    • c.

      Drawdown (of reservoirs),

    • d.

      Deforestation,

    • e.

      Irrigation,

    • f.

      Mining,

    • g.

      Artificial vibration,

    • h.

      Water leakage from utilities.

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