Effects of twin shield tunneling with different construction sequences and different relative locations on adjacent pipelines

A series of three-dimensional centrifuge model tests are carried out to investigate the effects of twin shield tunneling with different construction sequences and different relative locations on an existing buried pipeline in dry sand. Both the volume loss effect and the weight loss effect are simulated by using an improved tunnel model. Meanwhile, a displacement controlled method based on ground loss and an advanced hypoplasticity constitutive model are applied for the three-dimensional numerical back-analyses of the centrifuge tests and the other four extended conditions. It is shown that the twin tunnels with different construction sequences and different relative locations greatly affect the surface settlements, pipeline settlements and pipeline bending strains. The influences of the shield effect induced by the existence of the pipeline on the ground surface settlement become more obvious with the increasing greenfield ground surface settlement. The major influence zone on the pipeline induced by tunneling is within ±1.2D_T. For practical engineering, it is necessary to enhance the monitoring of the serviceability of pipelines during the second tunneling with shallow depth. Remarkably, it should not simply adopt the superposition principle to predict the greenfield surface settlements, pipeline settlements and pipeline bending strains due to twin tunnelling with different construction sequences and different relative locations. The effects of the accumulated shear strain and the shield effect due to the existence of the upper tunnel on the relative pipe-soil stiffness should be reasonably considered.