Abstract: The soft rock mass of deep-buried tunnel often has obvious rheological characteristics and time-weakening effect. After excavation, the parameters of surrounding rock gradually decrease with time, resulting in the increase of rheological deformation of surrounding rock, which easily leads to the cracking of first lining and secondary lining of tunnel, and seriously affects the safety of tunnel construction and operation period. In order to ensure safety of the tunnel, a buffer layer is set between secondary lining and surrounding rock, which can absorb rheological deformation of surrounding rock. The buffer layer has ideal elastic-plastic characteristics. Its special mechanical characteristics make it compatible with deformation of surrounding rock, absorbing long-term deformation energy of surrounding rock, and realizing the long-term stability of soft tunnel surrounding rock. Therefore, in recent years, the design of buffer layer yielding support has gradually been recognized by many scholars and engineering construction units, but at present, buffer layer yielding support has not been widely used, because the design of yielding deformation and yielding stress of buffer layer support has always been a difficulty in engineering construction. Based on elastic-plastic theory, the design method of buffer layer yielding support for tunnel engineering is proposed considering the time-effect weakening of surrounding rock parameters and the resistance of buffer layer support. The rationality of the buffer layer yielding support is verified by the support design of V-grade soft rock tunnel.

Key words: soft rock, aging weakening, yielding support, buffer layer, parameters of support