Abstract：Northward deformation is monitored during the excavation period with various devices installed on the surface and inside the rock mass of the central pier between the double chambers of the permanent shiplock of the Three Gorges Project in China. In terms of the fundamental understanding of rock deformation for such a slope in hard rock，the northward deformation is believed to be linked to local factors such as presence of north-dipping geological structures in the pier，excavation and support sequences during the construction period. In details，the advanced proceeding in the north chamber in the early excavation stage did result in the northward deformation in general，but at a small magnitude with a few millimeters，thus it was not the major factor causing the occurrence of the northward deformation. With the proceeding of excavation to greater depths in the middle stage，dominant north-dipping geological structures started to play an important role in the northward deformation. In the later stage，the reinforcement on the potentially instable rock blocks controlled by the northward structures transfers partial loads to the southern rock mass simultaneity，which turns to be the main factor that induces the northward deformation. Based on the discontinuous mechanics，a three-dimensional distinct element code(3DEC) model was accordingly established and numerically validated the northward deformation.
