What Are The Strength Classes Of Rocks Used In Construction?

Based on the scale effects and geological conditions discussed in the previous sections, it can be seen that sliding surfaces can form either along discontinuity surfaces, or through the rock mass. The importance of the classification is that in essentially all slope stability analysis it is necessary to use the shear strength properties of either the discontinuities or of the rock mass, and there are different procedures for determining the strength properties as follows:

• Discontinuity shear strength can be measured in the field and the laboratory.
• Rock mass shear strength is determined by empirical methods involving either back analysis of slopes cut in similar geological conditions, or by calculation involving rock strength indices.

As a further illustration of the effects of geology on shear strength, relative strength parameters for three types of discontinuity and two types of rock mass are shown on the Mohr diagram. The slope of these lines represents the friction angle, and the intercept with the shear stress axis represents the cohesion

A description of these conditions on Figure 4.7 is as follows:

Curve 1 Infilled discontinuity: If the infilling is a weak clay or fault gouge, the infilling friction angle (φinf ) is likely to be low, but there may be some cohesion if the infilling is undisturbed.

Alternatively, if the infilling is a strong calcite for example, which produces a healed surface, then the cohesive strength may be significant.

Curve 2 Smooth discontinuity: A smooth, clean discontinuity will have zero cohesion, and the friction angle will be that of the rock surfaces (φr). The friction angle of rock is related to the grain size, and is generally lower in fine grained rocks than in coarse-grained rocks.

Curve 3 Rough discontinuity: Clean, rough discontinuity surfaces will have zero cohesion, and the friction angle will be made up of two components. First, the rock material friction angle (φr), and second, a component (i) related to the roughness (asperities) of the surface and the ratio between the rock strength and the normal stress.

As the normal stress increases, the asperities are progressively sheared off and the total friction angle diminishes.

Curve 4 Fractured rock mass: The shear strength of a fractured rock mass, in which the sliding surface lies partially on discontinuity surfaces and partially passes through intact rock, can be expressed as a curved envelope. At low normal stresses where there is little confinement of the fractured rock and the individual fragments may move and rotate, the cohesion is low but the friction angle is high.

At higher normal stresses, crushing of the rock fragments begins to take place with the result that the friction angle diminishes. The shape of the strength envelope is related to the degree of fracturing, and the strength of the intact rock.

Curve 5 Weak intact rock: Rocks that are composed of fine grained material that has a low friction angle. However, because it contains no discontinuities, the cohesion can be higher than that of a strong intact rock that is closely fractured. The range of shear strength conditions that may be encountered in rock slopes clearly demonstrates the importance of examining both the characteristics of the discontinuities and the rock strength during the site investigation.

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