Long-term Stability of Clay Slopes

[Fourth Rankine lecture of the British Geotechnical Association] The values of the shear strength parameters as determined by conventional tests do not necessarily bear any relation to the values which must have been operative in the clay at the time of failure. This conclusion, which has now been established beyond the slightest doubt, is obviously one of immense practical significance. We must therefore endeavour to understand why, in certain cases, there is a wide discrepancy between our ordinary laboratory test results and the actual field value$ of shear strength. For such cases we must also try to develop modified laboratory procedures which give results of sufficient reliability to be used with confidence in engineering design. These two objectives form the substance of [this] Lecture. (A. Skempton)