抄録
A large continuous sample of a saturated sand to a depth of about 10 m was successfully obtained in Niigata, Janan, by means of an in situ freezing method, and so-called undisturbed samples of the sand were obtained by a conventional triple-tube sampler. Unidirectional freezing tests were conducted in the laboratory to determine the minimum confining pressure for preventing expansion during freezing. Undrained cyclic triaxial tests were conducted on the specimens prepared from the samples of dense sand from a depth of about 9.5 m after they had been thawed under isotropic stress conditions as well as on specimens reconstituted at the same density as the in situ frozen sample. An attempt was made to estimate the undrained cyclic shear strength in the field based on the laboratory tests, and the result was compared with probable field behavior at the site during the Niigata earthquake of 1964. Based on the above studies it was shown that (1) the quality of the in situ frozen sample was judged quite good ; (2) compared with the undrained cyclic strength of the in situ frozen sample, that of the triple-tube sample was only about 30%, and that of the reconstituted samples was about 30 to 60% ; and (3) the ratio between the undrained cyclic strength and consolidation pressure for the in situ frozen sample markedly increased with a decrease in the consolidation pressure.
