Study On The Small Duct Pre-supporting Technology Of Tunnel In The Shallow Broken Rock Sections

When tunneling in shallow broken rock sections, in order to overcome the disadvantageous impact of unstable strata, supportive measures are often taken prior to excavation in the front of the working face to ensure the stability of surrounding rock. This thesis summarizes previous works and conducts in-depth study on technology of mall duct pre-supporting. The main research contents and achievements are as follows.(1)It describes the characteristics of surrounding rock under shallow broken geological conditions and points out that the shear strength of broken rock is mostly determined by weak structural plane or weak intercalation;(2)It analyses the reinforcement mechanism and action effect of small duct pre-supporting and demonstrates that pre-supporting helps improve the stress state of the surrounding rock in front of the working face and increases the stability of surrounding rock. The interrelationship among various pre-supporting units can be clarified by building the serial parallel connection model of small duct pre-supporting system. Only when each unit matches in the optimum can the overall system work the best;(3)It explores the tunnel pre-supporting principle in depth and points out that the pre-supporting force is composed of the self-bearing capacity of surrounding rock and the bearing capacity of supporting structures. As the bearing force of the tunnel Surrounding Rock and Supporting structure, pre-supporting force, in order to ensure stable surrounding rock in tunnel, is always supposed to be greater than the initial internal force that maintains stable rock masses before construction;(4)By comparing the impact on vertical displacement of vault under different working conditions, it illustrates that small duct grouting support plays an important role in controlling vault settlement and maintaining the stability of surrounding rock. In addition, It discusses the influence of the diameter of small duct, the annular space, the duct length, the excavation step, the tunnel depth, etc on the effect of small duct pre-supporting and reaches the following conclusions:â‘ Without grouting, neither changing the diameter and the length of small duct nor the annular space would affect the displacement of working face;â‘¡The optimal length of small duct exists;â‘¢With a given tunnel depth, the influence of grouting length on vault displacement is limited; when the tunnel depth reaches an certain amount, small ducting pre-supporting can not meet the needs to control surrounding rock deformation, which requires increasing some parameters of small duct pre-supporting or changing some other supporting methods.