Guojun Wu

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.

Formation mechanisms of water inrush and mud burst in a migmatite tunnel: a case study in China

This paper presents a case study of water inrush and mud burst occurring in a migmatite tunnel to study its formation mechanisms. The geological investigation and mineralogical analysis showed that water inrush and mud burst in the migmatite was closely related to the component of the host rock. High content of soluble minerals, e.g., calcite and dolomite, would make the migmatite rock prone to be fragmentized, isintegrated and eventually form different sorts of connected or semi-connected veins. The field exploration revealed most cavities in the magmatite tunnel were eroded by groundwater and formed large interconnected networks. The two faults and the dike in the magmatite tunnel became the preferred paths and provided great convenience for plenty of precipitation and mud slurry. Due to high water pressure and blast disturbance, the cavities can soon connect each other as well as all sorts of veins, forming a complex ground channel for water inrush and mud burst. To estimate the potential occurrence of water inrush and mud burst, the water bursting coefficient was employed. The results showed the water bursting coefficient of the magmatite tunnel was much bigger than the threshold values and it can be used to explain the accident of water inrush and mud burst occurring in the magmatite tunnel.

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.

A discussion on analytical and numerical modelling of the land subsidence induced by coal seam gas extraction

Coal seam gas (CSG) is an increasingly important source of natural gas all over the world. Although the influence of conventional oil and gas extraction on surface subsidence has been widely recognized and studied, few studies are carried out on the surface subsidence in coal seam gas fields and its impact on surface infrastructure and the environment. This paper discusses modelling of the surface subsidence associated with coal seam gas production by applying both analytical and numerical methods. By comparison of results from the numerical model and two analytical models, i.e. the disc-shaped reservoir model and the uniaxial compaction model, the analytical solutions cannot describe the complex process of water and gas extraction and have the limitations to predict the surface subsidence, while the numerical model can be better used in prediction of subsidence. After applying the numerical model in numerical analysis, the deformation characteristics of coupled fluid flow, and the effects of permeability change of coal seam, associated overlying and underlying layers, and depressurization rates on surface subsidence are investigated. The results demonstrate that the proposed model can simulate the production of water and gas from coal seams and the associated surface subsidence.

Elastic-Plastic Analysis of Peak Stress of Anchor in Circle Tunnel

In this paper,the rule of direct proportion between anchor stress and creep strain of rock mass is taken as a starting point. After tunnel excavation,stress distribution of anchor in perfect circle tunnel is analyzed,then an important conclusion that the location is coincident between peak stress of anchor and plastic zone border of rock mass can be drawn in terms of theoretical derivation when rock mass is in plasticity.

Application of Digital Image Correlation Technique in Experimental Study of the Creep Behavior and Time Dependent Damage of Natural Rock Salt

The paper presents an experimental study on the creep behavior of a natural rock salt by means of digital image correlation (DIC) technique. To determine the creep rate and understand the creep mechanism of the rock salt, the time-dependent behavior of the material was studied at various scales (100 μm–1 cm) under different mechanical loading conditions. The results showed that the rock salt deformed at low constant stress. The strain rate at the macroscopic scale (cm) was about 10−9 s−1 at 11 MPa and it increased by an order of magnitude at 18 MPa. The strain rate at the mesoscopic scale (mm) depended on the microstructure, the boundary of subgrains deformed more rapidly than the subgrain, and the strain rate reached 10−7 s−1 at the interface of grains. The damage was identified by analyzing the full-field strain map and it was mainly caused by the open of cracks and it developed with the propagation of the cracks during creep tests.

Constitutive model of anchorage interface and its application on timeliness analysis in underground engineering

Abstract The creep property of anchorage interface is one of most important problems in analysing time effect of anchorage in underground engineering. In this paper, aiming at the defects of hard contact formulation in normal direction and discontinuity from bond to slip in tangential direction at anchorage interface when studying on the anchoring mechanism, a contact formulation which is an exponential softened pressure–overclosure relationship in normal direction and a continuous non‐linear constitutive model in tangential direction are proposed. The model solves the problem of converging difficultly and reflects the whole process of shear creep behaviour truly and reasonably because of its consideration of normal stress. By the further development of ABAQUS, the non‐linear creep model of anchorage interface is applied to analyse time effect of anchorage engineering. The main achievements are conducted as follows: anchor stress will increase with creeping effect of anchorage interface, and the location of anchorage peak stress will change and move towards the far end of anchor with the lapse of time. This paper provides the theory basis for deeply researching the reliability of anchorage in underground engineering.

Numerical Simulation of Humidity-Stress in Unsaturated Argillite Tunnel

An extension of Hoek-Brown criteria to include unsaturated behavior of argillite in porous medium is presented. The model is applied to simulate evolution of saturation degree in argillite and concrete in an experimental tunnel where field investigation of thermo-hydro-mechanical response of argillite and concrete will be done. Based on the laboratory experimental data, two different flow rules of water relative permeability and water saturation degree were suggested. The general evolution rules of saturation degree in argillite and concrete with time considering the effects ventilation are obtained.