Dynamic behaviour of Tagus River sand including liquefaction

In the framework of a third Tagus River crossing, through an immersed tunnel, advanced laboratory tests were performed on its highly liquefiable foundation sand. To characterize the cyclic behaviour of the sand and to support the use of advanced constitutive models in the design of the tunnel, key to simulate liquefaction realistically in such a complex project, five cyclic undrained torsional tests were executed. These tests, in which imposed strain increases progressively until and during liquefaction, considering strain amplitude levels with a limited number of cycles, allow a more thorough study of the phase after initial liquefaction than the usual test with constant stress amplitude and the necessary number of cycles to attain liquefaction. Additionally, they provide high quality data, essential to calibrate advanced constitutive models. From their analysis, it is concluded, as expected, that the number of cycles until initial liquefaction increases with relative density and confining pressure. Moreover, while before initial liquefaction maximum excess pore pressure amplitude is due to sand softening and consequent pore pressure build-up, after initial liquefaction it usually happens at the first cycle, being related to an initial higher dilation tendency, before particles loose contact. Finally, the damping ratio increases until a peak value, and then, just before initial liquefaction, generally decreases till the end of the test.