Simple Shear of an Undisturbed Soft Marine Clay in NGI and Torsional Shear Equipment

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Courtesy of ASTM International

A series of undrained, simple shear tests were performed on undisturbed, soft, marine clay from the continental slope of Israel in a direct simple shear device (NGI) and in a new torsional shear apparatus. The new apparatus is described, and the problematics of simple shear testing in both devices are discussed. Most previous torsional shear testing on clays reported in the literature has been performed on remolded specimens. In order to prepare specimens from undisturbed samples of the soft, marine clay, whose natural moisture content is close to the liquid limit, a special preparation technique was developed. A comparison of the stress-strain curves obtained in the two devices indicates higher stiffness and strength of the specimens tested in the torsional equipment; a similar observation has been reported in the literature from tests on sands and remolded clays. It is the authors´ view that the main cause for the discrepancy is flexibility of the confining membrane in the NGI equipment. In view of the relevance of simple shear conditions to actual field problems, it appears important to further clarify this difference and to establish an accepted method for simple shear testing in the laboratory.

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