In the process of assessment of the stability of a slope, the following factors have to be taken into account:

  1. Geology of the slope area

  2. Type of movements involved

  3. Method of analysis

  4. Strength properties of the material

  5. Method of calculation

The order of the factors listed above is also the chronological order in which a study of slope problems should be carried out.

The geologic reconnaissance should give:

  1. The engineering classification of soils and rocks.

  2. The space distribution of the different classes of materials.

  3. Structure of the rock masses.

  4. Distribution and regime of the ground water.

  5. Mapping of surface disturbances.

Proper geotechnical studies should be able to produce:

  1. A classification of the types of slope movements found in the area of interest.

  2. An analysis of the mechanism involved in each type of land movement.

  3. Strength parameters determining the stability of the slope for each type of movement,

  4. The most reliable method of calculation capable of producing a numerical indication of the degree of stability of natural and cut slopes.

Abundant literature is available to the project engineer suggesting ways to perform the different stages of a complete assessment of the stability of a slope (e.g. Zaruba and Mencl 1969). Howevcr, one cannot expect to obtain, neither in the laboratory nor through field tests alone, reliable strength parameters for rocks and residual soils. The principal reason for it is that the mechanical properties of rocks vary widely and randomly throughout the rock mass.

Carrying out numerous laboratory tests on small rock samples wouldn't solve the problem, unless the samples were all taken along the failure zone, which is not known in advance.

The most reliable way to obtain a statistically mean value of shear strength parameters of a non-homogeneous material in an extended slope is by back-calculation. The approach is based on the following assumptions:

  1. The geometry of the slope before failure is known.

  2. The mechanism of land movement is known,

  3. A condition of static equilibrium at the point of failure (limit equilibrium) exists at the time of failure.

  4. Failure of the slope occurs instantaneously when normal and shear stresses equal the tensile and shear strength of the material along the whole surface of rupture.

Homogeneity and isotropy are not necessary conditions. What is obtained by back-calculation is a weighed mean value of the strength parameters along the failure surface.

Backcalculations of one of the strength parameters have been successfully performed for many years. To the author's knowledge, the first complete, reliable method to calculate the stability of a slope assuming a circular slip surface was published by Petterson in 1916. Many other methods have been used ever since to calculate either the friction angle (¢) or the cohesion (c). More recently (Sancia 1979,Sancio and Goodman 1979) have introduced the tensile strength (To) as a third factor affecting the stability of a slope.

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