An experimental study of the dynamic response of shield tunnels under long-term train loads

In this paper, results are presented from a series of physical tests aimed at studying the influence of long-term train loads on the dynamic response of a tunnel lining and the surrounding soil. In the tests, the train loads were constantly applied at the tunnel invert by an electromagnetic shaker. The dynamic response of the tunnel lining and surrounding soil were measured initially and after 160,000, 330,000, and 600,000 loading sequences (each sequence representing the passage of a model train). The peak particle acceleration and frequency response function (FRF) of the model were calculated at each of these loading cycles. Two tunnel lining models were used in the experimental tests: stagger-jointed and uniform. The results show that the peak particle acceleration of the tunnel is amplified only after long-term train loads are applied. However, the peak particle acceleration of the soil shows an almost linear increase with loading cycles. The frequency domain results show that the dynamic characteristics of the soil could be significantly affected by the long-term train loads. Due to the variation of confining stress induced by long-term train loads, a clear 'shift' of soil FRFs with loading cycles was found. Test results also show that it is important to consider segmental joints when studying long-term train load effects on tunnel response.