Study On The Creep Properties Of Water-bearing Siltite And Its Application In Soft Rock Tunnel Engineering

Rheology is the most important mechanical characteristics of rock material, and many geotechnical-engineering problems closely relates to the rheological properties of rocks. As the phenomenon of rock rheology is widespread, people have begun to realize the impact of rheological behavior to the projects. Although people have made a great and important progress in describing and processing time effect and rheology of materials, however, there are still many problems need to solve. Additionally, water is a very important factor in affecting the rheological properties of tunnel surrounding rock. During the tunnel excavation, disturbance because of excavation will cause physical breakdown of original rock and increase the exposed area of rock. As a result, the seepage path of rock will increase and surrounding rock will become weaker because of the contacts with ground water. With the time increasing, the phenomenon that surrounding rock may become unstable, which may cause the occurrence of major engineering accident. Therefore, it has very important significance in practice to understand and master the rheology of rocks (especially the soft rock) in the groundwater environment.In view of this, researches carried out in this paper will use a combination method including testing research, theoretical analysis and numerical simulation. According to uniaxial creep tests of T2b2 siltite under different moisture conditions and the non-linear rheology theory and damage theory, the creep properties of siltite under different moisture conditions is analyzed, nonlinear creep model of siltite with considering water damage is present and also programmed in computer. Based on the theoretical study results, researches on the effectiveness of soft rock tunnel dynamic construction mechanics under different moisture conditions are carried out. The main contents and results of research in this paper are as follows:①According to the mechanics and deformation tests of the T2b2 siltite in different moisture conditions, which is collected freshly from the Tao Shu Ya tunnel site, the stress-strain curves under 4 kinds of moisture conditions are obtained. Based on the creep tests of T2b2 siltite under different moisture conditions, uniaxial strain-time curves in five load levels are obtained under different moisture conditions, and the variation law of creep curves with the change of stress and moisture content is analyzed.②According to the basic nonlinear rheology theory of rock and the relevant researches on stress-strain relationship of T2b2 siltite, a revised Kelvin model is put forward, and the rheological constructive equation of it is present. Through comparing the characteristics of the revised K model and common K model, it is indicated that the revised K model has many special characteristics and has a better adaptability in describing the rheological properties of rocks.③In response to the problems of creep model identification of T2b2 siltite, a swath model, which can be expressed as a pulse function, is put forward firstly, and by installing the swath model with N model in parallel, a revised N model is established too. Through connecting revised N model with revised K model and H model in the form of series, an MBurgers model, which can well describe the nonlinear creep property of rock, is established. According to the nonlinear least squares theory and uniaxial creep test results of T2b2 siltite, the parameters of MBurgers model are obtained by inversion caculation. By substituting the parameters obtained on behalf of the inversion into the model and calculating the theoretical results, it is shows that theoretical curve and test curve is well anastomotic, which indicates that MBurgers model can well describe the creep of T2b2 siltite.④In response to the problems of creep properties of siltite effected by the changes of moisture content, the concept water damage and two damage variable instantaneous elastic damage D (w), creep damage D′( w) are introduced. Through deriving damage evolution equation, MBurgers model with water damage effect is established, what's more, the characteristics of it are analyzed in the case that moisture content is invariable or variable over time.⑤With analyzing the degradation of MBurgers with water damage effect in different assumptions, several other rheological models are established. In case of not considering the parameterαeffect but consider the water damage effect, a new creep model KBurgers model can be established, and it can also be extended to three-dimensional format in the assumptions of invariable Poisson'ration and invariable bulk modulus. In addition, the relationships among the model parameters under different modeling ideals are studies too. Through analyzing the damage evolution of model parameters with moisture content changing under the assumption of invariable bulk modulus, the damage evolution equations of instantaneous elastic damage and creep damage are obtained. By connecting the M-C model with KBurgers model in serial and assuming the strain rate components of viscoelastic and viscoplastic are compatible, a new model KBurgers-MC model which can describe the visco-elastic-plastic characteristics of deviator and elastic-plastic characteristics of bulk is established. Furthermore, the new model not only has the characteristics of instantaneous elastic, attenuation creep, steady creep and water damage, but also has the characteristics of plastic and unstable creep.⑥Based on the difference theory and good redevelopment platform of the software FLAC3D, the finite difference scheme of KBurgers-MC is present, and the computer program of it is obtained too. Combining the computer program and the actual construction process of Tao Shu Ya tunnel, deformation and stress of surrounding rock and supports are simulated by FLAC3D under different times and different moisture conditions. Finally, stability of surrounding rock under different times and different moisture conditions is judged, the best construction time for secondary lining under different moisture conditions is suggested, and the mechanism of larger deformation occurrence in Tao Shu Ya tunnel is also further expound.