Abstract: Parameter values adopted in railway engineering analysis at present are generally obtained through triaxial tests, whereas the remarkable difference between the actual stress path during train passage and the loading path in cyclic triaxial tests may lead to inaccurate predictions and even engineering problems. Published researches show that the actual stress path probably contains principal stress rotation(PSR), which has a sisgnificant influence on the mechanical response of soil. However, the conventional approach, i.e. cyclic triaxial test is not efficient in simulating this complicated stress path involving PSR. Instead, hollow cylinder test with cyclic shear stress has been proved to be an excellent selection for PSR study. In order to obtain reliable stress path for guiding hollow cylinder tests, three-dimensional calculation for a certain urban railway is carried out; and the distribution regularity of cyclic stress amplitudes, cycle numbers and PSR under train loads is analyzed. Furthermore, a series of cyclic torsional shear tests on hollow cylinder samples is employed to simulate the complex stress path and to research the deformation and pore pressure accumulation properties of saturated soft clay. Considering stress in foundation soil decreasing along with the depth increasing, various cyclic stress amplitudes corresponding to different depths are adopted in this experiment study. Three-dimensional calculation shows that the major principal stress axis will rotate from -90° to 90° during the train loading passage as well as the number and amplitude of stress waves reduce gradually with the increase of depth. It is also shown that both PSR and stress are quite sensitive to train loads as the upper soil element can identify a single wheel load while the lower soil element can just identify a bogie and even the whole train load. And through the cyclic torsional shear tests, it’s obvious that the rotation of principal stress can promote the accumulation of pore pressure and strain in soft soil foundations significantly; and in the case when vertical cyclic stress amplitude equals to 15 kPa, the pore pressure and strain in cyclic torsional shear test are 77% and nearly 50% higher than that in cyclic triaxial test respectively. What’s more, as the cyclic stress amplitude increases, the gap of cumulative pore pressure and strain between that two types of tests will enlarge further, which may even result in essential difference that samples in triaxial tests show only small strain whereas failure in torsional shear tests.

Key words: cyclic loading, principal stress rotation, soft clay, mechanical response