In 1970 Stimpson & Walton reported on the presence of thin clay bands, tentatively (and perhaps inappropriately) named ‘clay mylonites’, in core recovered from the Northumberland and Durham Coalfield, England. The geotechnical properties of these layers were investigated and the results reported at the First International Congress of the International Association of Engineering Geology in Paris. Apart from the low direct shear strengths of these thin layers and the excellent drilling and careful core logging that must be undertaken in order to detect them, the most scientifically interesting aspect of the laboratory test data was a relationship between the thickness of the ‘clay mylonite’ and its shear strength (Fig. 1); the thicker layers had a greater shear strength than the thinner layers. A possible model to explain this feature is to consider the shear zone as comprising a series of parallel surfaces stacked one upon the other, each of which undergoes some shear displacement. The total shear displacement of the shear zone is the sum of all the internal shear displacements. According to this model, a thin layer would have fewer internal surfaces and, hence, for the same total shear displacement each internal surface would suffer a greater displacement than in a thick clay mylonite. Thus, it would be anticipated that a thin clay mylonite would have a lower shear strength than a thick clay mylonite, i.e. it is closer to its ultimate shear strength.
The magnitude of displacement along the clay mylonite bands is a matter of