Abstract: Presently, traditional strengthened support is still the main choice in soft rock large deformation projects. In this study, the necessity of yielding support for large squeezing deformation is analyzed. The traditional rigid support and retractable steel arch are not feasible for controlling the large squeezing deformation of the tunnel, especially when the deformation δ is greater than 500 mm. 〖HJ1.7mm〗The relevant results of the yielding anchor bolt and prestressed yielding anchor cable in five projects confirm their effectiveness in controlling the large squeezing deformation of soft rock. First, the design concept and working principle, are introduced Second, (yielding anchor bolt and prestressed yielding anchor cable are compared. The prestressed yielding anchor cable is recommended owing to its construction and force performance). The anchor bolt and anchor cable parameters, anchor head form, and anchor cavity position of yielding support are introduced. Third, the economic benefit of yielding support is discussed; the comparison and estimation results indicate its costefficiency. Fourth, some problems in calculating and analyzing the soft rock tunnel with large squeezing deformation are explained: (1) the mode selection of twoand threedimensional analyses; (2) the geometric nonlinearity; (3) the coupling interaction between creeptime effect and rheological mechanical properties of nonlinear nonsteady viscoelastic surrounding rock; (4) development and application of specific software for analyzing the coupling effect between geometric and material nonlinearity. Fifth, the research works on the coupling effect are summarized. Finally, the following suggestions are put forward for further research. (1) Vast amount of data should be collected from rheological tests and tunnel field measurements, and numerical simulation should be conducted to provide more feasible methods for predicting large squeezing deformation. (2) The threedimensional nonlinear viscoelastic rheological mechanical characteristics should be introduced and expanded. Furthermore, the appropriate rheological constitutive model should be adopted. (3) The test methods, numerical calculating methods, and theoretical calculation should be integrated. A more detailed study on nonlinear rheological mechanical characteristics of squeezing deformation of different soft surrounding rocks should be conducted. (4) The multifield coupling should be considered when conducting indepth analysis on large squeezing deformation. (5) The study is based on soft rock, and the proposed theory is based on continuous medium, which does not suit for large deformation failure of soft clay tunnel. However, it should be considered an extended research aspect.

Key words: Central Yunnan water diversion project, water conveyance tunnel, large squeezing deformation of surrounding rock, yielding support, yielding anchor bolt, prestressed yielding anchor cable, design concept of support while yielding and flexible support goes before rigid support, yielding deformation, enlarged excavation