Abstract
The problem of large deformation is very prominent in deep-buried tunnel excavation in soft rock, which brings serious potential safety hazards and economic losses to projects. The knowledge of deformation law and support measures is the key to ensure the rational design and safe construction in a large deformation tunnel of soft rock. This paper describes rock physical and mechanical tests and field monitoring is employed to investigate the cause and development process of large deformation in Dongsong hydropower station in Sichuan Province, China. The results show that the free expansion rate of the rock sample is 20.0%, the average expansion stress of the rock sample is 11.0 kPa, and the expansibility of the rock is low. Large deformation of surrounding rock mainly comes from the dilatancy effect with high geostress and relaxation deformation with weak support. Shotcrete sealing exposed surrounding rock, and early strength support avoiding water immersion are useful to deal with the three main factors (weathering, water and confining pressure) that affect the strength of surrounding rocks. The second lining applied in time can effectively limit the further development of stress and deformation of initial support, and prevent the cracking and large deformation of concrete. Clearance convergence is suggested to be the main monitoring work in construction, because of its advantages of intuitive results, easy quality assurance of instrument installation and high accuracy.
Highlights
In recent years, with the rapid development of the Chinese economy and an increase in investment in infrastructure construction, all kinds of tunnel projects have been rapidly developed [1,2,3,4,5,6].The increasing number of long, large and deep buried tunnels has brought great challenges to the construction and operation of projects, in which large deformation of the soft rock mass is a prominent problem [7,8,9,10,11,12,13]
The causes of large deformation of soft rock mass are complex and can be generally classified into two categories: one is that the stress redistribution during excavation exceeds the strength of surrounding rock and causes plasticization; the other is that some minerals and water react and cause expansion [31,32,33,34]
The large deformation of surrounding rock mainly comes from the dilatancy effect with high geostress and relaxation deformation with weak support
Summary
With the rapid development of the Chinese economy and an increase in investment in infrastructure construction, all kinds of tunnel projects have been rapidly developed [1,2,3,4,5,6]. Society of Rock Mechanics in 1995 [26] It is a time-dependent deformation behavior, usually occurring around the excavation surface of underground space. It is generally caused by creep due to the instability of ultimate shear stress. The causes of large deformation of soft rock mass are complex and can be generally classified into two categories: one is that the stress redistribution during excavation exceeds the strength of surrounding rock and causes plasticization; the other is that some minerals and water react and cause expansion [31,32,33,34]. The findings can provide a useful reference for theoretical basis and technical support of the construction of large deformation tunnel constructions
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