Weizhong Chen

SYSTEMATIC NUMERICAL SIMULATION OF ROCK TUNNEL STABILITY CONSIDERING DIFFERENT ROCK CONDITIONS AND CONSTRUCTION EFFECTS

This paper outlines the construction process mechanics (CPM) principle for analysing the stability of rock tunnels and presents finite element method (FEM) numerical simulation and prediction on the deformation and failure of the rock masses surrounding tunnels under various rock mass properties and excavating and support conditions. Based on numerical modelling, a series of predicting curves for rock mass response and deformation are obtained, which provides the basis of guiding the design and construction of rock tunnels in Taiwan.

A Transversely Isotropic Damage Model for Boom Clay

Boom clay can be considered as a transversely isotropic geomaterial. However, due to lack of experimental evidence and data base, it is still difficult to describe the transversely isotropic plastic behavior of this argillaceous rock. In this paper, we present first, by means of an experimental approach, the main features of the mechanical properties of Boom clay. Then, combining the transversely isotropic elastic model and the modified Mohr–Coulomb criterion, a suitable constitutive model is introduced so as to fully describe the mechanical behavior of the studied material, in which, an elastic damage law which takes into consideration, the transversely isotropic effect, a plastic hardening law and a plastic damage law were introduced to describe the nonlinear elastic, hardening and softening behavior of Boom clay. As a preliminary step, the evolution law of both elastic moduli and Poisson’s ratio during the elastic stage was obtained by direct analysis of the test data. The synchronism of the elastic damage in both transversal and axial directions was proved by this method. Some of the parameters of the model in the elastic stage were also determined by direct analysis method and further verified by back analysis. Other unknown parameters in the model were determined by back analysis.

Experimental Study of Ultralight (<300 kg/m3) Foamed Concrete

A type of ultralight (<300u2009kg/m3) foamed concrete (FC), which can be used as a new energy-conservation and environmental-protection building material and is particularly suitable for the thermal-insulation engineering of building external walls, was produced. The influences of different mixing amounts of fly ash, fly ash activator, WC (WC) ratio, and foaming agent (FA) on the compressive strength of FC were reported. The experimental study indicated that (1) the addition of fly ash reduced the strength of the FC and that the appropriate mixing amount of fly ash in this ultralight FC system should not exceed 45%; (2) with the increasing of fly ash activator, the strength of the FC sample is notably enhanced and the appropriate mixing amount of fly ash activator is 2.5%; (3) the optimized proportion of WC ratio is 0.45, and the FC that was produced according to this proportion has relatively high compressive strength; (4) by increasing the mixing amount of FA, the compressive strength of the FC notably decreases, and the optimal mixing amount of FA in this experiment is 3.5%.

A fully coupled thermo-hydro-mechanical model for unsaturated porous media

Abstract In examining potential host rocks for such purposes as the disposal of high-level radioactive wastes, it is important to understand the coupled thermo-hydro-mechanical (THM) behavior of a porous medium. A rigorous and fully unified coupled thermo-hydro-mechanical model for unsaturated porous media is required to simulate the complex coupling mechanisms involved. Based on modified Darcy’s and Fourier’s laws, equations of mechanical equilibrium, mass conservation and energy conservation are derived by introducing void ratio and volumetric liquid water content into the model. The newly derived model takes into account the effects of temperature on the dynamic viscosity of liquid water and void ratio, the influence of liquid flow on temperature gradient (thermo-osmosis), the influence on mass and heat conservation equations, and the influence of heat flow on water pressure gradient and thermal convection. The new coupled THM constitutive model is constructed by a finite element program and is used to simulate the coupled behavior of a tunnel during excavation, ventilation and concrete lining stages. Oil and gas engineering, underground disposal of nuclear waste and tunnel engineering may be benefited from the development of the new model.

Experimental and numerical analysis of the time-dependent behaviour of argillaceous red sandstone under high in situ stress

Understanding the time-dependent behaviour of soft rock under high in situ stress is essential to the evaluation of the long-term stability of the deep-buried tunnels in expressways or coal mines. This paper presents an experimental and numerical study of the time-dependent behaviour of argillaceous red sandstone under high in situ stress. First, several triaxial creep tests for strongly and moderately weathered specimens under the confining pressure of 20–40xa0MPa were conducted, and the variation of time-dependent damage with time was obtained by investigating the evolution of volumetric strain during the creep process. The test results verify that creep damage has a similar effect on both axial strain and lateral strain of argillaceous red sandstone. Second, a creep damage model that is able to describe nonlinear variation in creep strain and volume expansion for sandstone under high in situ stress was established. Last, the parameters of the proposed model were determined by a back analysis method. The results of back analysis show that the model is able to describe the nonlinear variation in creep strain and volume expansion during the creep process very well.

Groundwater control and curtain grouting for tunnel construction in completely weathered granite

During tunnel construction, groundwater inrush from completely weathered granite strata is a significant challenge to geotechnical engineers. Up to the present, prevention of water inrushing hazards is almost exclusively based on the experience of engineers. This paper presents a coupled seepage–erosion water inrush model to investigate the characteristics of seepage–erosion properties. The proposed model is based on classical theories of solute transport and fluid dynamics in porous media. In the model, changes of porosity link permeability with the accumulation of particle loss in the seepage–erosion process. The coupled seepage–erosion model was applied to examine the influence of curtain grouting thickness on the seepage–erosion process. The results showed that the seepage–erosion process was attenuated as thickness increased. The results also showed that the porosity and permeability visibly changed and the water inflow clearly exceeded the acceptable engineering criterion when the thickness was less than 6xa0m. However, with a further increase in thickness, the seepage–erosion process was suppressed and little changes of the relative parameters were showed. The numerical results demonstrated that a curtain grouting thickness of 6xa0m was suitable for curtain grouting in completely weathered granite. Field investigation of Cenxi tunnel verified the effectiveness of the thickness determined by the proposed model.

Investigating the Permeability of Marble under Moderate Pressure and Temperature

The permeability of intact marble samples collected from the depth of 1.6u2009km in southwestern China is investigated under moderate confining pressures and temperatures. No microcracks initiate or propagate during the tests, and the variation of permeability is due to the change of aperture of microcracks. Test results show a considerable decrease of permeability along with confining pressure increase from 10 to 30u2009MPa and temperature increase from 15 to 40°C. The thermal effect on the permeability is notable in comparison with the influence of the stress. A simple permeability evolution law is developed to correlate the permeability and the porosity in the compressive regime based on the microphysical geometric linkage model. Using this law, the permeability in the compressive regime for crystalline rock can be predicted from the volumetric strain curve of mechanical tests.

Application of the orthogonal design method in geotechnical parameter back analysis for underground structures

The back analysis method has been widely used as an indirect method of determining geotechnical parameters based on field measurements. The number of parameters and their initial values greatly influence the reliability and efficiency of back analysis. Therefore, sensitivity analysis is often employed to select high sensitivity parameters that have more greater impact on measured back analysis values. The orthogonal design method was first utilized to select geotechnical parameters for back analysis. The optimized parameter values obtained from an orthogonal design table can be used as the initial back analysis values, so as to avoid optimisation algorithm searching in local parameter spaces. By introducing a penalty function to the objective function, back analysis of the geotechnical parameters is changed into an unconstrained optimisation problem, whereby the Nelder–Mead method can then be employed. To verify the feasibility of the proposed back analysis method, a case study was conducted to determine the rock mass parameters for the Houziyan underground powerhouse complex.

Numerical evaluation of a yielding tunnel lining support system used in limiting large deformation in squeezing rock

Large deformation in squeezing soft rock is a significant challenge that complicates the safety of underground construction engineering. A yielding tunnel support system allows a certain amount of over-excavation, thereby accommodating large deformation in severely squeezing rock. In this study, a special yielding support system has been developed with a type of newly developed foamed concrete material which has a cushion effect. The special yielding support uses pre-cast foamed concrete blocks which are mounted in the primary lining, and an in situ cast foamed concrete layer which is placed between the primary lining and the secondary lining. The effect of the special yielding support on squeezing rock tunnels has been validated by comparing the numerical results with those of a lining-strengthened stiff support system. The incorporation of the foamed concrete blocks can both reduce the maximum and minimum principal stress in the primary lining. Relative to the stiff support, the maximum and minimum principal stress in the primary lining are about 50 and 60% of those of the stiff support, respectively, thereby improving the stress state of the primary lining. Further, compared with that in the stiff support, the plastic zone in the secondary lining in this yielding support is significantly improved, and the deformations at the roof and the sides of the secondary lining are 40 and 46% less than that of the stiff support, respectively, resulting in a better stress state and less deformation in the secondary lining.

Nonlinear seepage–erosion coupled water inrush model for completely weathered granite

ABSTRACT Water inrush from completely weathered granite has presented a significant challenge for tunnel construction, suggesting the urgency and importance of revealing the water inrush mechanism. In this paper, a seepage erosion model is proposed to describe the water inrush. Assuming that completely weathered granite consists of solid grain phase, fluid phase, and fluidized grain phase, the three-phase interaction was constrained by mass balance equations, and the erosion of fluidized grains was described by a modified porosity evolution equation. The fluid flow is governed by a coupled Darcy–Brinkman/Navier–Stokes equation, which responds to the variation of the flow pattern in the evolution process. Then, the validity of the model has been proved, and the superiority has been studied by comparing with the previous models. The comparison results showed that the flow pattern has a significant impact on pore pressure, water velocity, and mutation time, and the proposed model can more accurately predict the development of velocity. Furthermore, this model was used to simulate the development of water inrush and achieved good results in predicting the direction, channel size, and whole evolution process of water inrush. The research findings from the paper can benefit tunnel engineering in the case of water inrush disasters.