Direct shear tests performed on model tension fractures have provided a very realistic picture of the behavior of unfilled joints at the roughest end of the joint spectrum. The peak shear strength of rough-undulating joints such as tension surfaces can now be predicted with acceptable accuracy from a knowledge of only one parameter, namely the effective joint wall compressive strength or JCS value. For an unweathered joint this will be simply the unconfined compression strength of the unweathered rock. However in most cases joint walls will be weathered to some degree. Methods of estimating the strength of the weathered rock are discussed. The predicted values of shear strength compare favourably with experimental results reported in the literature, both for weathered and unweathered rough joints. The shear strength of unfilled joints of intermediate roughness presents a problem since at present there is insufficient detailed reporting of test results. In an effort to remedy this situation, a simple roughness classification method has been devised which has a sliding scale of roughness. The curvature of the proposed strength envelopes reduces as the roughness coefficient reduces, and also varies with the strength of the weathered joint wall or unweathered rock, whichever is relevant. Values of the Coulomb parameters c and o fitted to the curves between the commonly used normal stress range of 5-20 kg/ appear to agree quite closely with experimental results. The presence of water is found in practice to reduce the shear strength of rough unfilled joints but hardly to affect the strength of planar surfaces. This result is also predicted by the proposed criterion for peak strength. The shear strength depends on the compressive strength which is itself reduced by the presence of water. The sliding scale of roughness incorporates a reduced contribution from the compressive strength as the joint roughness reduces. Based on the same model, it is possible to draw an interesting analogy between the effects of weathering, saturation, time to failure, and scale, on the shear strength of non-planar joints. Increasing these parameters causes a reduction in the compressive strength of the rock, and hence a reduction in the peak shear strength. Rough-undulating joints are most affected and smooth-nearly planar joints least of all.

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  • Supplemental Notes:
    • Reprinted From Engineering Geology (Amsterdam), Volume 7, 1973 pp 287-332,
  • Corporate Authors:

    Norwegian Geotechnical Institute

    P.O. Box 40 Tasen
    Oslo 8,   Norway 
  • Authors:
    • Barton, N
  • Publication Date: 1974

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  • Accession Number: 00095912
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jul 2 1975 12:00AM