Although the shear strength expression proposed by Coulomb (1776) for masonry, brick and earth, made up of cohesion and friction components, is still in general use today, it was not until Terzaghi (1936) presented the principle of effective stress that it could be used with confidence in most soils and soft or jointed rocks. The Griffith (1921, 1924) crack theory, while inadequate in itself, has provided a starting point for developing empirical expressions for the shear strength of hard intact rocks. Many factors influence the shear strength of soils and rocks, such as drainage conditions, rate of strain or shear displacement, degree of saturation, stress path, anisotropy, fissures and joints. A brief review is attempted here of the more salient advances which have been made in understanding and quantifying the influence of these various factors.


Until the 20th Century such assessments were necessarily qualitative, based mostly on experience or rule of thumb. The problem is that soil and rock come in an infinite number of guises, and while experience and rule of thumb may often have sufficed in the past, there were no doubt many instances, mostly long forgotten, when they proved inadequate and even misleading, leading to failures. The advances in understanding the strength of engineering materials which occurred in the 19th Century did not extend to soils and rock masses. A fundamental understanding of the strength of soils and soft rocks had to await an understanding of effective stress, first expressed formally by Terzaghi in 1936 and for which he is justifiably regarded as the Father of Soil Mechanics. A different, more empirical approach has been required in the case of hard rocks for which it is recognised that their strength is dominated by the presence of imperfections.

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