Characterization of the Mechanical Properties of Sensitive Clay by Means of Indentation Tests
Instrumented indentation tests were carried out on specimens of undisturbed clay, using various indenters at a constant rate of penetration of 0.5 mm/min. These tests were performed with 20-mm in diameter conical indenters with apical angles of 60o, 80o, 97o, and 144.3o, as well as with a Vickers indenter of semi-apical angle of 68.7o. Such tests allowed the determination of both the undrained shear strength and the deformation modulus of the clay. The undrained shear strength was found by assuming that a fully plastic stress field resulted from indenter penetration and by using slip-line theory. The deformation modulus was computed by assuming that the clay behaved elastically during initial penetration. The theory proposed by Love (1939) and later extended by Sneddon (1948, 1965) was used to obtain values of Young’s modulus. The expanding cavity model or ECM proposed by Johnson (1970, 1985) was also employed for the interpretation of test results obtained with both sharp and blunt indenters.
One of the most significant findings of the present study is that the structure of the undisturbed clay suffers severe and progressive breakdown with increasing penetration during indentation, which results in a dramatic decrease of the strength and deformation parameters of the clay. Comparison between the undrained shear strength deduced from the quasi-static indentation tests and the dynamic Swedish fall-cone tests, also indicates that Su obtained from the latter tests are much higher than the corresponding data derived from the indentation tests. It is believed that the possible cause of the overestimation of the undrained shear strength deduced from the Swedish fall-cone tests is related to the very high strain rate experienced by the falling cone.
The paper also presents a brief review of the most pertinent theories that are used for the interpretation of indentation tests.
KeywordsIndentation tests Conical and pyramidal indenters Sensitive clay Young’s modulus Undrained shear strength Comparisons
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