Research On Large Deformation Mechanism Of Soft Rock Of Baozhen Tunnel

ABSTRACT: It has been greatly concerned to study the stability of underground caverns and theirs surrounding rock masses under very complex geological environments. High earth stress, deep-buried, rich groundwater and very weak and cracked rock masses make self-stability of the tunnel be unfavorable, it is difficult to excavate, support, and maintain. During construction, The high deformation rate, intense and long-time deformation are the basic characteristics; At the same time, the deformation shows asymmetrical features and uniformities. Surrounding rock masses mentioned as above have remarkable rheological property, but previous researchers consider that the rheology of rock masses resulted from rocks and that structure of rock masses is determinate and static and does not change in spite of any condition, so their researches had been carried out by rheology theory of continua. Engineering practices show that these studies do not reveal the key of deformation and failure of surrounding rock masses with above-mentioned properties and those supports adopted on the basis of their research results can not be take effects.Baozhen tunnel is the only soft rock tunnel and the key project in the Yi-wan railway. The very complex geological environments, such as high earth stress, deep-buried, rich groundwater, very weak and cracked rock masses and bias pressure along the rock seam, make self-stability of the tunnel be unfavorable. During construction, The high deformation rate, intense and long-time deformation are the basic characteristics; At the same time, the deformation shows asymmetrical features and uniformities. Carrying out a series r of laboratory testing research on Buzhen tunnel engineering practice:(1) According to the environment of surrounding rock occurrence, different physical and mechanical parameters of rock were measured, the Impact law of confining pressure and saturated time on peak strength, residual strength and post-peak volume change was obtained;(2) According to the rock laminated composite structures which tunnel face section distribute weak interlayer in large deformation sector, the law between interlayer thickness and rock strength was researched through conventional triaxial compression tests, the relation between interlayer and surrounding rock was put up before and after the interlayer damaged according to the specimen failure mode, the dynamic process of surrounding rock damaged caused by interlayer failure was studied, mechanical model of structural system was established to study the mechanism of deformation and yield;(3) According to test results and monitoring data of stress on supporting structure, a series of triaxial compression tests were conducted under conditions of low confining pressure, and explore the influence law of support resistance to the soft rock strength and the volumetric strain, in controlling aspects of large deformation of surrounding rock pointed out that the "strong support" was unreasonable support method in theory, in practice not feasible, put forward the best reasonable time for lining was at the end stage of post-peak softening and residual phase begins.Take Baozhen tunnel as an example, on the basis of detail site investigation and lots of data tested, measured and monitored in laboratory and on site, it is found that rheology and other characteristics of jointed and soft surrounding rock in high stress area resulted from not rocks but strata structure. It is complicated time-dependent deformation of structural distortion that results in large deformation of rock masses. Based on calculation and analysis by these data, three kinds of types of deformation were put forward:rock property controlling type, strata structure controlling type and environment in which surrounding rocks existed controlling type, pointed out that the second type was the key type which provoking large deformation. The large deformation of strata structure controlling type is defined as the phenomena or process in which surrounding rocks put up rheology feature due to the complex time-dependent behaviors and response of strata structure in the course of stress redistribution by excavation of soft jointed rock mass in high stress area.By analyzing spatial and temporal characteristics of displacements and failure of surrounding rock, the strong rheology and other correlated properties during dynamic evolutionary process of surrounding rocks are discussed. The mechanism and its characteristics and pattern of deformation were also discussed. On this basis, the dynamic characteristics of the surrounding rock and deformation types were summed up, the mechanical models of deformation were given respectively according to different strata structures, the principles and methods of controlling large deformation in soft tunnel were put up. At last, systemic technologies for building the tunnel in high earth stress and soft rock area were formed.From the viewpoint of large deformation provoked by strata structure failure, the dynamic evolutionary process and deformation mechanism were understood, which modifies the classic knowledge completely. The research results obtained form the author of this dissertation applied to Baozhen tunnel ensure various measures of controlling large deformation done in advance, and can also be used and referred for other similar underground works under complicated conditions.