Deep-level Tunnel Research Articles

Mechanical problems for the long-term stability of rocks surrounding deep level underground tunnels

Based on the research of Academician Tan Tjong-Kie on the long-term stability mechanics of underground tunnel and considering Academician Sadovsky’s structural hierarchy theories on complex geological rock masses, the inherent statistical mechanical properties of inhomogeneous structure and closed stress in deep rock masses were investigated. Two mechanical problems were mainly studied, i.e. characteristics of inherent non-uniform deformation and closed stress of rock masses, and long-term stability of deep tunnels. The quantitative mathematical characterization of inherent non-uniform deformation and closed stress of rock masses were given using the method of statistical mechanics. Based on the law of mass conservation, a general calculation method of long-term stability and deformation of rock masses surrounding deep level tunnels was proposed. The Maxwell model was used to calculate the threshold of splitting. A post-peak failure model of rock was established to estimate the in-situ stress at which rock undergoes post-peak failure. With the help of the theory of the hirerachical structure of rock masses, the maximum value of rock displacement due to splitting dilatation was obtained. A dimensionless energy factor was introduced to define the extent of zonal disintegration of surrounding rock masses. It was concluded that the splitting and dilatancy deformation is the main part of the deformation of surrounding rocks. The reasons why unloading splitting failure is more likely to take place in rock masses surrounding deep level tunnels were explained. The split evolution pattern of rock masses surrounding deep level tunnels and the calculation method of dilatancy displacement were obtained. The calculation results of the range of loosening zone, the location of the rupture zone in rock masses surrounding deep level tunnels, and the displacement of the sidewall were compared with the existing monitoring data in the underground powerhouse of the Jinping Ⅰ Hydropower Station, and the agreement is good.

Modeling Deformation Wave in Rock Mass near Deep-Level Tunnel

In this paper deformation wave is studied on the basis of continuous phase transition theory. Relative shear strain is taken as an order parameter. The potential energy is obtained by analogy with the Ginzburg–Landau expansion of free energy. Hamilton’s variational principle is used to obtain the motion equation by which several types of deformation waves can be modeled. The results show that the proposed model is effective in modeling deformation waves.

Прочностные и теплофизические свойства бетона с полипропиленовой фиброй в условиях температурного режима стандартного пожара

The paper discusses the problems of protection of reinforced concrete tunnel structures from brittle (explosive) destruction of concrete tubbing lining of the tunnel. The relevance of the research is attributed to increasing pace of construction of deep-level tunnels. Fires in such facilities could be catastrophic, often resulting in massive loss of life and great material losses, and their suppression requires involvement of considerable forces and assets. During the construction and operation of road and subway tunnels, the protecting structures - reinforced concrete lining blocks have a higher moisture content, which in the event of fire in the early stages can lead to brittle failure of concrete tubing and the premature loss of their load-bearing capacity. To reduce the effects of brittle fracture of concrete in the protective layer of concrete structures anti-spall mesh is installed, or fire retardant coating is used which reduces the intensity of heating of concrete during fire. However, recent studies have shown that the most effective way of protecting against brittle fracture of concrete from the point of view of labor and material cost is the use of additives in the concrete mixture in the form of polypropylene fibers. Earlier, experiments were carried out in VNIIPO to determine the actual limits of fire resistance of tunnel tubing and the influence of polypropylene fibers additives in concrete mix on the likelihood of brittle fracture of concrete. However it seems impossible to assess the fire resistance of similar structures using numerical methods due to the lack of baseline data on the strength and thermo-physical properties of concrete with polypropylene fibers. To achieve this goal, studies were conducted of concrete strength under axial compression with the addition of polypropylene fibers in the amount of 1 kg/m 3 and experimental data of thermal characteristics of fiber-reinforced concrete at high temperatures were obtained. The paper presents the results of experiments on the samples of fiber concrete under axial compression when exposed to temperature in the range 20-800 °C. Graphics show the process of the strength change of concrete with and without additives during heating. Analytical dependencies for determination of strength of concrete under compression were obtained with two types of polypropylene fibers at high temperatures.A comparison of the strength properties of the investigated concrete mixtures was carried out. It was established experimentally that when using the polypropylene fibers, the strength characteristics of fiber-concrete are reduced on average by 16 %, compared to the concrete without fiber additives, both at normal and high temperatures. As a result of processing of the experimental data by regression analysis the analytical dependencies were obtained for determination of strength characteristics of concrete under axial compression with the addition of domestic and imported fibers when exposed to high temperatures. Experiments to determine the thermal properties of concrete with the addition of polypropylene fibers, were conducted during one-sided heating of board samples on the temperature regime of “standard fire”. In the presence of experimental data, by solving the inverse heat conduction problem using the previously developed computer program, the thermophysical characteristics (thermal conductivity and heat capacity) of fiber-reinforced concrete at elevated temperatures were defined. With increasing temperature, the thermal conductivity decrease is more intensive in concrete with added polypropylene fibers than that of concrete without additives. At the same time, the addition of fiber does not affect the intensity of increase of the heat capacity of concrete. The obtained dependences of thermophysical properties of concrete with domestic and imported polypropylene fibers on temperature increase make it possible to carry out calculations of heating of concrete structures with selected additives on a temperature regime of “standard fire”. The conducted studies on the effect of temperature on the strength and thermal properties of concrete with addition of polypropylene fiber reinforcement can be used in calculation of the fire resistance of load-bearing and enclosing structures made of this type of fiber-reinforced concrete.

Strain Gradient Model of Zonal Disintegration of Rock Mass near Deep-Level Tunnels

This paper presents one strain gradient model of zonal disintegration of rock mass near deep level tunnel. The governing equations and boundary conditions of the model are established. Numerical methods (quasi-Newton method and shooting method) are adopted to solve the obtained fourth-order equilibrium equations with higher order boundary conditions in terms of displacement. The stress field in elastic and plastic zones is obtained. The effects of model parameters on stresses distribution in surrounding the tunnel rock mass are examined. The necessary conditions for the formation of zonal disintegration are elucidated.

Modeling the zonal disintegration of rocks near deep level tunnels by gradient internal variable continuous phase transition theory

AbstractThe occurrence of alternating damage zones surrounding underground openings (commonly known as zonal disintegration) is treated as a “far from thermodynamic equilibrium” dynamical process or a nonlinear continuous phase transition phenomenon. The approach of internal variable gradient theory with diffusive transport, which may be viewed as a subclass of Landau’s phase transition theory, is adopted. The order parameter is identified with an irreversible strain quantity, the gradient of which enters into the expression for the free energy of the rock system. The gradient term stabilizes the material behavior in the post-softening regime, where zonal disintegration occurs. The results of a simplified linearized analysis are confirmed by the numerical solution of the nonlinear problem.

One plastic gradient model of zonal disintegration of rock mass near deep level tunnels

In the present paper zonal disintegration phenomenon is investigated within the framework of gradient theories of elastic-plastic solids. Gradient of effective plastic strain is introduced as additional internal variable. Equilibrium equation and boundary conditions are obtained by using virtual work principle. Evolution equations for internal variables are obtained by using Clausius—Duhem inequality. For circular tunnels the governing equation for effective plastic strain is obtained from the above model. Solution of the governing equation for ideal brittle rock mass model is obtained. The obtained solution may describe zonal disintegration phenomenon very well.

Evolution of the deformation and fracturing in rock masses near deep-level tunnels

The temporal zonal disintegration process is studied on the basis of irreversible thermal dynamics, the theory of continuous phase transition, and elastoplasticity theories. A spatial and temporal analytical solution to the problem of zonal disintegration near deep-level tunnels is given, and some laws of the zonal disintegration evolution are demonstrated.

Gallium-arsenide deep-level tunnel diode with record negative conductance and record peak current density

Tunnel diodes utilizing deep-levels in low-temperature-grown (LTG) gallium-arsenide (GaAs) are demonstrated. These tunnel diodes achieve peak-to-valley current ratios as high as 22, a record negative-conductance-per-area of 1/226 Ω μm 2, and a record peak current density of 16 kA/cm 2, all at room-temperature.