Abstract: It is difficult for traditional bolt-shotcrete support to meet the requirements of deformation control in soft rock tunnel, therefore yielding support becomes an important means to control deformation. The circumferential yielding support sets up the yielding device in the circumferential direction of the tunnel in order to realize the rigid-flexible-rigid characteristics of the supporting structure. From the perspective of energy transformation in tunnel excavation-support process, the principle of circumferential yielding support is clarified in this paper. The main factors affecting the deformation of the support structure are analyzed using the analytical method of structural mechanics, and the mechanical characteristics of traditional support and circumferential yielding support are compared using the finite element software ABAQUS. The following conclusions are drawn: the initial support is a typical compression-bending member. The circumferential yielding support causes the yielding device to buckle through the circumferential pressure, which balances with the internal force of the support structure. It is hence possible to achieve a certain support resistance while controlling the surrounding rock stress by shortening the circumference. The circumferential yielding device should be set at places where the bending stress is relatively small. The shear stiffness and bearing capacity of the device should be ensured with the characteristics of "strong shear and weak compression". The circumferential yielding support has the mechanical characteristics of rigid-flexible-rigid, which can be adapted to the rheological characteristics of soft rock with high geo-stress.

Key words: soft rock tunnel, large deformation, circumferential yielding support