Haijian Su

Set pair analysis for risk assessment of water inrush in karst tunnels

Considering the uncertain characteristics of water inrush in karst tunnels, the set pair analysis (SPA) method was newly applied in this study for risk assessment of water inrush. First, karst hydrology and engineering geological conditions were considered, and several main influencing factors were selected as evaluation indices, such as formation lithology, unfavorable geology, groundwater level and contact zone of dissolvable as well as other factors. Second, a set pair was established with the combination of the evaluation indices and the standards of risk grade. Then, the evaluation indices of water inrush were divided into two main types: economic and cost indices. Based on set pair analysis (SPA), the graded connection degrees of evaluation indices were calculated. Finally, the established risk evaluation model of water inrush with set pair analysis was applied to the Jigongling tunnel on the line of the Fanba expressway in China. The results not only are consistent with the results of the attribute mathematical theory, but also agree well with practical situations. In addition, the method of set pair analysis used in this study could provide relatively high accuracy when applied to risk assessment of water inrush in karst tunnels. Meanwhile, SPA is simple, feasible and easy to implement. The presented method has been validated as an effective method of risk assessment for water inrush, which also has good prospects for further engineering applications.

A novel cloud model for risk analysis of water inrush in karst tunnels

Water inrush is a serious geological hazard in underground engineering. The prediction of possibility and classification of water inrush risk has long been a global problem for the construction of deep-buried tunnels in karst areas. To solve the randomness and fuzziness in the evaluation process of water inrush risk, a novel comprehensive evaluation model was established based on the normal cloud theory. According to the systematic analysis of the influence factors of water inrush, seven factors were selected as evaluation indices, including formation lithology, unfavourable geological conditions, groundwater level, landform and physiognomy, modified strata inclination, contact zones of dissolvable and insoluble rock, and layer and interlayer fissures. Meanwhile, a hierarchy model of the influence factors was established for water inrush, and the analytic hierarchy process was adopted to determine the weighting coefficients for each evaluation index. The normal cloud theory was used to describe the cloud numerical characteristics for each evaluation index of risk classification for water inrush. Normal cloud droplets were generated to reflect the uncertain transformation between the risk levels of water inrush and the evaluation indices. Then, the synthetic degrees of certainty were calculated, and risk level of water inrush was determined. Finally, the proposed model was applied to two typical deep-buried tunnels in karst areas: Jigongling tunnel and Xiakou tunnel. The obtained results were compared with the relevant analysis results and the practical findings, and reasonable agreements were gained. The normal cloud model was found to be more accurate, feasible and effective for risk classification of water inrush prediction. It can not only meet the requirement of tunnel engineering, but also be extended to various applications.

Investigating the Roles of Included Angle and Loading Condition on the Critical Hydraulic Gradient of Real Rock Fracture Networks

List of symbols Q Volumetric flow rate w Width of a fracture bh Hydraulic aperture J Hydraulic gradient Jc Critical hydraulic gradient P Hydraulic pressure u Flow velocity vector ρ Fluid density E Nonlinear effect factor T Transmissivity T/T0 Normalized transmissivity xi X-coordinate of the fracture surface profile zi Z-coordinate of the fracture surface profile Z2 Dimensionless roughness parameter a Linear coefficient in the Forchheimer’s law b Nonlinear coefficient in the Forchheimer’s law l Fracture spacing Fx X-directional horizontal boundary load Fy Y-directional horizontal boundary load σfne Effective stress

CO2 Permeability Analysis of Caprock Containing a Single Fracture Subject to Coupled Thermal-Hydromechanical Effects

Coupled THM (thermal-hydromechanical) processes have become increasingly important in studying the issues affecting subsurface flow systems. CO2 permeability of the fracture in caprock is a key factor that affects sealing efficiency of caprock. A new model associated with coupled THM processes that shows a good reliability was derived. Then, based on the COMSOL multiphysics software, a series of numerical calculations were performed on caprock models with a single fracture subject to coupled THM effects. Transmissivity of the fracture as a function of fracture angle, overburden pressure, fluid pressure difference, injected CO2 temperature, and the initial fracture aperture was elucidated, respectively. Average transmissivity of the fracture undergoes an increase by 1.74 times with the fracture angle (45°–90°), 2-3 orders of magnitude with the fluid pressure difference (5–30 MPa), and 4-5 orders of magnitude with the initial fracture aperture (0.05–0.5 mm), while it decreases by 3-4 orders of magnitude as overburden pressure increases from 30 to 80 MPa. Injected CO2 temperature has a small impact on the fracture permeability. This work provides an alternative tool to enrich the numerical modeling for the assessment of CO2 caprock sealing efficiency.

Investigation on mechanical behavior and crack coalescence of sandstone specimens containing fissure-hole combined flaws under uniaxial compression

This study focuses on the effect of pre-existing flaw geometry on mechanical behavior and crack coalescence modes of sandstone specimens containing combined flaws with different fissure angle, ligament length and fissure length under uniaxial compression. The flaw geometry is a combination of a single hole and an inclined fissure underneath, which is generated by a high pressure water-jet cutting machine and is different from that reported in previous studies. The effect of flaw geometry on mechanical behavior of sandstone specimens is analyzed. Basically, mechanical parameters including the peak strength, peak axial strain, elastic modulus and secant Young’s modulus for the flawed specimens are lower than those for the intact specimens, with the reduction extent related to the fissure angle, ligament length and fissure length. Variation trends of the crack initiation stress for all tested cases are studied. Initiated crack types and cracking modes also depend on the combined flaws geometry. For the flawed specimens with a small fissure angle, ligament length or fissure length, cracking modes are generally characterized by cracks initiated from the hole-wall and evolved to the specimen boundary. However, when the fissure angle, ligament length or fissure length is increased, cracks initiated from both the hole-wall and fissure tips produce the main failure planes, accompanied by a free-standing “triangular prism structure” within the specimens. Numerical simulations using RFPA2D (Rock Failure Process Analysis in two dimensions) are carried out on the flawed sandstone specimens and agree well with the experimental results in the peak strength and overall cracking behavior.

Experimental Study on Stress-Dependent Nonlinear Flow Behavior and Normalized Transmissivity of Real Rock Fracture Networks

The mechanism and quantitative descriptions of nonlinear fluid flow through rock fractures are difficult issues of high concern in underground engineering fields. In order to study the effects of fracture geometry and loading conditions on nonlinear flow properties and normalized transmissivity through fracture networks, stress-dependent fluid flow tests were conducted on real rock fracture networks with different number of intersections (1, 4, 7, and 12) and subjected to various applied boundary loads (7, 14, 21, 28, and 35 kN). For all cases, the inlet hydraulic pressures ranged from 0 to 0.6 MPa. The test results show that Forchheimer’s law provides an excellent description of the nonlinear fluid flow in fracture networks. The linear coefficient and nonlinear coefficient in Forchheimer’s law generally decrease with the number of intersections but increase with the boundary load. The relationships between and can be well fitted with a power function. A nonlinear effect factor was used to quantitatively characterize the nonlinear behaviors of fluid flow through fracture networks. By defining a critical value of = 10%, the critical hydraulic gradient was calculated. The critical hydraulic gradient decreases with the number of intersections due to richer flowing paths but increases with the boundary load due to fracture closure. The transmissivity of fracture networks decreases with the hydraulic gradient, and the variation process can be estimated using an exponential function. A mathematical expression for decreased normalized transmissivity against the hydraulic gradient was established. When the hydraulic gradient is small, holds a constant value of 1.0. With increasing hydraulic gradient, the reduction rate of first increases and then decreases. The equivalent permeability of fracture networks decreases with the applied boundary load, and permeability changes at low load levels are more sensitive.

Effect of a Fault Fracture Zone on the Stability of Tunnel-Surrounding Rock

AbstractThe fault fracture zone is vital to the stability of the surrounding rock of tunnels in geological engineering. In this study, a three-dimensional numerical model was established for Wuzhuling Tunnel of Zhuji-Yongjia Highway in Zhejiang Province, China. The dynamic processes of tunnel excavation were simulated through the fault fracture zone. The deforming performance and stress distribution of surrounding rock were investigated. Moreover, the stability of surrounding rock in tunnels was analyzed with consideration of the slope angle and the width of the fault. The simulation results indicate that the fault fracture zone in the tunnel can reduce the stability of surrounding rock. The slope angle and the width of the fault all have obvious influences on the stability of surrounding rock in tunnels. Furthermore, the collapse processes of a tunnel in the construction steps were investigated in a laboratory model. Reasonable agreements can be obtained to validate the model presented here and the simul...

Strength degradation and anchoring behavior of rock mass in the fault fracture zone

Rock mass in the fault fracture zone has some characteristics such as low strength and poor self-stability, so the control mechanism of stability has been a difficulty in the research of underground engineering. A set of laboratory simulation method of fault fractured rock mass is developed to reflect the natural forming process of fault fracture zone. Compared with intact rock mass, the fault fractured rock mass has an obvious degradation in strength and deformation parameters, and the degradation index is between 22.79 and 84.06%. The bolt has a certain supporting effect on the fault fractured rock mass, and in the situation of end anchoring, the greater the pretightening force is, the better the enforcement effect will be. The stress field produced by high pretightening force can relieve the stress concentration around the bolt hole and make the initial cracks of rock mass away from the bolt plate. The evolution curve of bolt axial force in the process of uniaxial compression of large-scale specimen shows four stages, which are the initial compression stage, pre-peak joint load-bearing stage, post-peak joint load-bearing stage and the residual stage. Research results could provide some theory reference for the stability control of rock mass in the fault fracture zone.

An experimental study of the effect of fillings on hydraulic properties of single fractures

Fluid flow in single rock fractures considering the influences of fracture surface roughness, shearing process, normal loading, and so on has been extensively studied for several decades, yet the significant influence of fillings has not been systematically investigated due to the numerous difficulties such as determination of the physical parameters of fillings. The present study aims to estimate the hydraulic properties of single fractures filled with different graded and gap-graded fillings, based on a series of flow tests on rock-like samples using the MTS815.02 material testing system. With the increment of fracture aperture, the pressure drops before and after fillings are flowed away decrease, whereas the permeabilities before and after fillings are flowed away increase. When the ratio of mechanical aperture of fractures to maximum diameter of fillings decreases from 4 to 1.33, both pressure drop and permeability change significantly before the fillings are flowed away and then hold constant values after the fillings are flowed away. Due to the effects of fraction force and interlocking force between particles, the ratio of mechanical aperture to maximum diameter of fillings that equals to 2.67 is the inflection point, where the pressure drop has the maximum value and permeability has the minimum value. When the fractures are filled with gap-graded fillings, in which the ratio of mechanical aperture of fractures to mean diameter of fillings decreases from 5.76 to 1.45, the variations of both pressure drop and permeability before fillings are flowed away change more significantly than those after fillings are flowed away. The hydraulic aperture of fractures with fillings is approximately 2–3 orders of magnitude smaller than the mechanical aperture.

Effect of Thermal Treatment on the Dynamic Behaviors at a Fixed Loading Rate of Limestone in Quasi-vacuum and Air-filled Environments

Thermal damage in rock engineering occurs in the air‐filled and quasi‐ vacuum environments of rock mass located near or far from the free sur‐ face. Meanwhile, dynamic loads are encountered frequently in engineering practice. In this study, 39 limestone samples are prepared, and a series of laboratory tests, including split Hopkinson pressure bar SHPB , nuclear magnetic resonance NMR and optical microscopy analyses, are conduct‐ ed to investigate the effects of temperature and the environment on the dynamic mechanical properties of limestone. The results show that the macro‐physical and dynamic mechanical properties of limestone after thermal treatment can be divided into two stages by a critical temperature of 450°C, at which the thermal damage factor is 0.71 and 0.75 in the quasi‐ vacuum and air‐filled environments, respectively. In the first stage, with temperatures varying from 25°C to 450°C, the thermal damage due to ex‐ pansion and fracturing slightly influences the related parameters, except the P‐wave velocity. However, in the second stage, with temperatures rang‐ ing from 450°C to 900°C, the thermal damage caused by mineral decompo‐ sition and hydration leads to a remarkable decrease in the dynamic bearing and anti‐deformation capacities. The environment plays a negligible role in the first stage but an important role in the second stage, and the dynamic compressive strength and modulus of samples after thermal treatment in the air‐filled environment are much lower than those in the quasi‐vacuum environment. Both the temperature and environment of thermal treatment should be considered in engineering practice, especially when the temper‐ ature exceeds 450°C.