| Research Abstract |
A practical methed to estimate the non-linear stiffness of soils and rocks in the field was studied by means of a comprehensive series of laboratory tests, field tests and analyses of field observations and the comparison of the stiffness values obtained from the above-mentioned various methods. First, a unique method was developed with which in a compression test, a very precise strain for an extremely wide range from about 0.0001% to about 10% can be continuously measured from the local axial compression along the lateral surface of specimen. It was revealed that for stiff geometerials including sedimentary soft rocks, the axial strain obtained by the conventional method (i. e., from the axial displacement of loading piston or the specimen cap) considerably underestimates the true stiffness and is, therefore, is totally unreliable. It was found that this factor mentioned above and the fact that in the current practice of laboratory tests, the stiffness is defined at a relatively larg
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e strain are two major causes for that many field cases have been reported where the estimated displacement be much larger than the observed ones. Then, it was found, common for a wide rage of geomaterials from soft clays to soft rocks, that the deformation properties at strains less than about 0.001% are virtually linear and elastic. Namely, in the laboratory tests, the Young's modulus defined at this small strain level is the same among static monotonic and cyclic loading tests and dynamic tests. Further, the elastic Young's modulus precisely determined by any laboratory test could be very close to the value determined from the corresponding field shear wave velocity measurement. On the other hand, the field stiffness determined from the conventional methods of pressuremeter tests and plate loading tests are much smaller than the elastic Young's modulus, because of much larger strain levels involved in these field loading tests.
Based on the above-mentioned findings, the following method was proposed. Namely, first, the elastic Young's modulus (or shear modulus) is obtained from field shear wave velocity measurements. Next, the stiffness values at relatively large strain levels are obtained from pressuremeter tests. Besides, a continuous stress-strain relation from a strain range from 0.0001% to that at the peak (at about 10%) is obtained from an accurate laboratory monotonic loading test (or tests) using high-quality undisturbed samples. When needed, cyclic loading tests are performed. Combining the above information, the field value of stiffness as a function of strain is estimated. Less
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