Experimental research on overlying strata movement and fracture evolution in pillarless stress-relief mining

In multiple seams mining, the seam with relatively low gas content （protective seam） is often extracted prior to mining its overlying and/or underlying seams of high gas content and low permeability to minimize the risk of high gas emission and outbursts of coal and gas. A key to success with this mining sequence is to gain a detailed understanding of the movement and fracture evolution of the overlying and underlying strata after the protective seam in extracted. In Zhuji mine, the No. 11-2 seam is extracted as a protective seam with the pillarless mining method by retaining goal-side roadways prior to its overlying No. 13-1 seam. An investigation has been undertaken in the panel 1111 （1） of Zhuji mine to physically simulate the movement and fracture evolution of the overlying strata alter the No. 1 I-2 seam is extracted. In the physical simulation, the displacement, strain, and deformation and failure process of the model for simulation were acquired with various means such as grating displacement meter, strain gauges, and digital photography. The simulation result shows that： （1） Initial caving interval of the immediate roof was 21.6 m, the first weighting interval was 23.5-37.3 m with the average interval of 33.5 m, and the periodic weighting interval of the main roof was in a range of 8.2-20.55 m and averaged at 15.2 m. （2） The maximum height of the caving zone after the extraction of No. 11-2 seam was 8.0 m, which was 4 times of the seam mining height and the internal strata of the caving zone collapsed irregularly. The mining-induced fractures developed 8-30 m above the mined No. 11-2 seam, which was 7.525 times of the seam mining height, the fracture zone was about 65° upward from the seam open-off cut toward the goaf, the height of longitudinal joint growth was 4-20 times of the mining seam height, and the height of lateral joint growth was 20-25 times of the mining seam height. （3） The ＂arch-in-arch＂ mechanical structure of the internal goaf was bounded by an expansion angle of broken strata in the lateral direction of the retained goaf-side roadway. The spatial and temporal evolution regularities of over- burden＇s displacement field and stress field, dynamic development process and distribution of fracture field were analyzed. Based on the simulation results, it is recommended that several goaf drainage methods, i.e. gas drainage with buried pipes in goaf, surface goaf gas drainage, and cross-measure boreholes, should be implemented to ensure the safe mining of the panel 1111 （1）.

Detection and treatment of water inflow in karst tunnel：A case study in Daba tunnel

In a karst tunnel, fissures or cracks that are filled with weathered materials are a type of potential water outlet as they are easily triggered and converted into groundwater outlets under the influence of high groundwater pressure. A terrible water inrush caused by potential water outlets can seriously hinder the project construction. Potential water outlets and water sources that surrounding the tunnel must be detected before water inflow can be treated. This paper provides a successful case of the detection and treatment of water inflow in a karst tunnel and proposes a potential water outlet detection(PWOD) method in which heavy rainfall(>50 mm/d) is considered a trigger for a potential water outlet. The Daba tunnel located in Hunan province, China, has been constructed in a karst stratum where the rock mass has been weathered intensely by the influence of two faults. Heavy rain triggered some potential water outlets, causing a serious water inrush. The PWOD method was applied in this project for the treatment of water inflow, and six potential water outlets in total were identified through three heavy rains. Meanwhile, a geophysical prospecting technique was also used to detect water sources. The connections between water outlets and water sources were identified with a 3-D graphic that included all of them. According to the distribution of water outlets and water sources, the detection area was divided into three sections and separately treated by curtain grouting.

An attribute recognition model for safe thickness assessment between concealed karst cave and tunnel

An attribute recognition model for safe thickness assessment between a concealed karst cave and a tunnel is established based on the attribute mathematic theory.The model can be applied to carrying out risk classification of the safe thickness between a concealed karst cave and a tunnel and to guarantee construction’s safety in tunnel engineering.Firstly,the assessment indicators and classification standard of safe thickness between a concealed karst cave and a tunnel are studied based on the perturbation method.Then some attribute measurement functions are constructed to compute the attribute measurement of each single index and synthetic attribute measurement.Finally,the identification and classification of risk assessment of safe thickness between a concealed karst cave and a tunnel are recognized by the confidence criterion.The results of two engineering application show that the evaluation results agree well with the site situations in construction.The results provide a good guidance for the tunnel construction.

Protection against water or mud inrush in tunnels by grouting:A review

Grouting is a major method used to prevent water and mud inrush in tunnels and underground engineering. In this paper, the current situation of control and prevention of water and mud inrush is summarized and recent advances in relevant theories, grout/equipment, and critical techniques are introduced. The time-variant equations of grout viscosity at different volumetric ratios were obtained based on the constitutive relation of typical fast curing grouts. A large-scale dynamic grouting model testing system （4000 mm × 2000 mm × 5 mm） was developed, and the diffusions of cement and fast curing grouts in dynamic water grouting were investigated. The results reveal that the diffusions of cement grouts and fast curing grouts are U-shaped and asymmetric elliptical, respectively. A multi-parameter real-time monitoring system （＆#981; = 1.5 m, h = 1.2 m） was developed for the grouting process to study the diffusion and reinforcement mechanism of grouting in water-rich faulted zone. A high early strength cream-type reinforcing/plugging grout, a high permeability nano-scale silica gel grout, and a high-expansion filling grout were proposed for the control of water hazards in weak water-rich faulted zone rocks, water inrush in karst passages, and micro-crack water inrush, respectively. Complement technologies and equipment for industrial applications were also proposed. Additionally, a novel full-life periodic dynamic water grouting with the critical grouting borehole as the core was proposed. The key techniques for the control of water inrush in water-rich faulted zone, jointed fissures and karst passages, and micro-crack water inrush were developed.

Cohesion variation during instability evolution of disaster medium in mud inrush of mountain tunnel

Mud inrush in mountain tunnel is an independent geological hazard type different from water inrush,landslide and debris flow.The intrinsic factor of mud inrush is the instability failure of disaster medium.Its essence is that when the cohesion decreases gradually with the increase of void ratio to the point where the movement of soil particles cannot be restricted,soil particles and groundwater form slurry and gush out.Thus,accurate calculation of cohesion with variable void ratios is crucial for analyzing the reliability of disaster medium.In this study,the disaster medium was regarded as graded soil and a structural model was established wherein soil particles were simplified as cubes and the inter-particle pores were represented by the clearance between cubes.On the basis of the structure model of disaster medium,a function between the soil particle distance and void ratio was derived.Cohesion is equivalent to the resultant force between soil particles per unit area;thus,a cohesion function was derived in which the void ratio is the main variable.This function considers the influence of gradation characteristics on cohesion variation and is generally applicable to various types of disaster medium.A series of direct shear tests were carried out to determine the cohesion variation for different types of disaster medium with variable void ratios.By comparing the variation of cohesion obtained through direct shear tests with those deduced by the proposed cohesion function,we verified the validity and general applicability of the cohesion function.It is of great significance because the cohesion function can accurately predict the variation of cohesion by using the void ratio,and effectively evaluate the possibility of mud inrush according to the initial mechanical properties of disaster medium.

Rheological properties of surrounding rock in deep hard rock tunnels and its reasonable support form

Second lining stability, which is the last protection in tunnel engineering, is critically important. The theological properties of the surrounding rock heavily affect second lining stability. In this work, we used laboratory triaxial compressive rheological limestone tests to study nonlinear creep damage characteristics of surrounding rock mass in construction projects. We established a nonlinear creep damage constitutive model for the rock mass, as well as a constitutive model numerical implementation made by programming. Second, we introduced a new foam concrete with higher compression performance and good ductility and studied its mechanical properties through uniaxial and triaxial tests. This concrete was used as the filling material for the reserved deformation layer between the primary support and second lining. Finally, we proposed a high efficiency and accuracy staged optimization method. The minimum reserved deformation layer thickness was established as the optimization goal, and the presence of plastic strain in the second lining after 100 years of surrounding rock creep was used as an evaluation index. Reserved deformation layer thickness optimization analysis reveals no plastic strain in the second lining when the reserved deformation minimum thickness layer is 28.50 cm. The results show that the new foam concrete used as a reserved deformation layer filling material can absorb creep deformation of surrounding rock mass, reduce second lining deformation that leads to plastic strain, and ensure long-term second lining stability.

Isotope Chronology and Geochemistry of the Lower Carboniferous Granite in Xilinhot, Inner Mongolia, China

Geochronological and geochemical analyses were performed on K-feldspar granites and monzonitic granites from the Xilinhot area, Inner Mongolia, China. Zircon U/Pb ages indicate that the two types granites were emplaced during the Lower Carboniferous. The K-feldspar granites （332 Ma） have the typical A-type granite characteristics of a post-collision setting. The monzonitic granites have an emplacement age of 323 Ma. Zircon tHf values of the former range from ＋12.8 to ＋14.2, with an av- erage TDM2 of 453 Ma. The latter have lower zircon ~Hf values, ranging from ＋5.4 to ＋10.7, with an av- erage TDM2 of 798 Ma. The strong, positive ~nf values of the zircon indicate that both sets of samples are from a juvenile crust formed in an oceanic crust subduction stage, although the monzonitic granite may have undergone a hybridization of crustal materials. These results indicate a younger post orogenic event. The Paleo-Asian Ocean had closed before the Early Carboniferous and the Xilinhot area started its post-orogenic evolution with an extensional tectonic environment durin~ the Early Carboniferous.