Abstract: To study the calculation methods for surrounding rock pressure of tunnels with small spacing in weak stratum and the influence of different tunnel spacing and buried depth on surrounding rock pressure, the theory of three collapse arch is introduced and the boundary depth of deep and shallow tunnels is established. Then, a tunnel load calculation model is established based on the road tunnel design specification, the theoretical calculation formula of surrounding rock pressure is deduced, and the influence of different spacing and buried depth on pressure of surrounding rock inside and outside the tunnel is discussed. Finally, a case study is conducted, and effective reinforcement measures are proposed for the weak middle rock pillars. The results show the following. (1) The surrounding rock pressure of extremelyshallow and shallowburied tunnels with small clear spacing can be calculated by entire soil column and Xie Jiaxiao′s theoretical formulas in the standard, that of deepburied tunnel can be calculated by modified Bill Bowman calculation method. (2) The asymmetrical pressure characteristics of tunnel with small clear spacing are prominent, and the surrounding rock pressure is approximately linear with the buried depth. (3) The interaction between the left and right tunnels decreases with the increase of the clear spacing. When the clear spacing is greater than one time of the tunnel span, the surrounding rock pressure can be calculated as a single tunnel. (4) The middle rock pillar is the key and weak part of the tunnels with small clear spacing in weak stratum. The hollow grouting bolts and reasonable anchor rod length can effectively restrain the development of plastic strain, and improve the strength of tensile and shear strength of surrounding rock. 

Key words: smallspacing tunnel, weak stratum, surrounding rock pressure, theoretical calculation, middle rock pillar