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Mechanical Properties of Rock Mass by Loading and Unloading Tests and Its Engineering Application

Author: ChenXing
Tutor: LiJianLin
School: Three Gorges University
Course: Geotechnical Engineering
Keywords: sandstone rebound longitudinal wave velocity unloading anisotropy high temperature Mohr-Coulumb Hoek-Brown FLAC3D
CLC: TU452
Type: Master's thesis
Year: 2011
Downloads: 162
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Abstract


The anisotropy regular of banding rock mass in unloading conditions is significant in guiding the engineering design and construction.The research on loading and unloading failure of sandstones with notable parallel bedding or vertical bedding in Yichang shows that there is dependence in rebound data,longitudinal wave velocity and strength of rock mass.The basic law is that,the larger of the rebounding data or longitudinal wave velocity,the higher strength of the rock ,and the mechanical property regular of the sandstones with parallel bedding or vertical bedding is basical in accordance to the conclusions from formula of rebounding data and longitudinal velocity. Whether in the failure mode of loading or unloading, the strength of rock mass with vertical bedding is normally higher than that with parallel bedding in the same confining pressure, and whether sandstone with parallel bedding or vertical bedding, when in low confining pressure, the unloading failure peak strength of which is higher than that in loading condition,but in high confining pressure, it is opposite. There is a big difference between shear resistance parameters of the rock mass in unloading condition and that in loading condition,which is closely related to the anisotropy property of the rock mass.For parallel bedding rock mass, the cohesion c of the rock mass after unloading is larger,while the internal friction angle is lower than those in loading condition,but for vertical bedding rock mass,the property is opposite. The loading and unloading failure modes of the sandstone are mainly shear failure, practically without axial or circumferential cracks, which is likely related to the load controlling mode(loading axial pressure and unloading confining pressure), low velocity of unloading and the high survival confining pressure after unloading.The forecast failure angle of the rock specimens in Mohr-Coulomb criterion is largely close to the measured value.Loading and unloading failure planes of the sandstones with parallel bedding or vertical bedding are all running through the sedimentary weak plane, but seen from the form of actual fracture line, the fracture trace is not affected by the sedimentary weak plane.Using the pilot program of unloading, this paper have researched the longitudinal wave velocity-temperature and mechanical properties- temperature variation of the sandstone after high temperature.The results show that: with the increasing of baking temperature, the velocity of longitudinal wave of the sandstone become lower, and the velocity decrease more while the temperature becomes higher. With the increasing of baking temperature, rebound value of the sandstone is not a simple monotone increasing or decreasing. The rule about the elastic modulus of loading section with the change of confining pressure has a discrete variation. But from the basic trend, the elastic modulus of sandstone increases with the increasing of confining pressure. Under the same confining pressure, the elastic modulus of sandstone after high temperature baking decreased. Mechanical properties of sandstone strength are changing under interaction between frictions and cement properties, and friction characteristics strengthened greatly after high temperature baking. Due to the friction, its strength increased significantly in the larger confining pressure. Longitudinal wave velocity, rebound value and strength don’t have inexorable law after high temperature baking. Under low confining pressure, natural air-dried rock samples are unloading damage, it still showed a significant compression and shear failure modes. The axial splitting phenomenon of 300℃, 600℃, 900℃baked rock samples strengthens gradually, and analysts believe that tensile strength of rock samples after high-temperature baking decrease significantly. The sandstone interior produce the thermal stress after heat treatment, that inducing thermal cracking of the surface and internal micro-cracks is the essential reason that caused the tensile strength decreased.Analysis of rock mass strength under unloading condition has great significance for the security of projects. Based on loading and unloading test of two typical rocks which are Three Gorges granite and Yichang sandstone, the following conclusions are obtained: For unloading test of Three Gorges granite, Hoek-Brown criterion is accurate than Mohr-Coulumb, in terms of fitting correlation or coincidence of envelope. Hoek-Brown criterion has shown a good applicability of describing the tension shear failure and compression and shear failure. For test of confining pressure unloading of layer or high temperatured Yichang sandstone, the curve of Hoek-Brown is more consistent with the discrete points than Mohr-Coulumb. According to the prediction of Hoek-Brown criterion, with the confining pressure increasing, the differences between rock mass with parallel and vertical layer gradually weakened. Based on strength criterion of loading and unloading of rock mass, constitutive model of rock mass was established by research of deformation, failure characteristics and mechanisms.Surrounding rock of the cavern erecting bay of Shenxigou Hydropower Station was classified with the“RMR”method. A finite difference model based on the newest Hoek-Brown criterion was used to analysis the stability of the erecting bay and the calculation results was compared with the monitoring data. It was demonstrated that according to the RMR classify method there was surrounding rock of classⅣandⅢin every section of the erecting bay. Terrane Zbdn1 belonged to surrounding rock of classⅣwhich was of poor quality, and terrane Zbdn2、Zbdn3 belonged to surrounding rock of classⅢwhich was of average quality. Based on the Hoek-Brown criterion, the parameter of rock mass was estimated, in which the influence of the structure of the rock mass、the strength of rock block、state of stress and other factors were taken into account. It was efficient, fast and economic. Digging process of the erecting bay was simulated using finite difference technique FLAC3D, whose calculation results was compared with the monitoring data and showed to be compatible with the monitoring data.

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CLC: > Industrial Technology > Building Science > Soil mechanics,foundation engineering > Rock ( rock ) mechanics and rock testing > Rock mechanical properties and stress theory
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