Research on the stability analysis and design of soil tunnel surrounding rock

The paper first analyzes the failure mechanism and mode of tunnel according to model experiments and mechanical calculation and then discusses the deficiency of taking the limit value of displacement around the tunnel and the size of the plastic zone of surrounding rock as the criterion of stability. So the writers put forward the idea that the safety factor of surrounding rock calculated through strength reduction FEM(finit element method) should be regarded as the criterion of stability,which has strict mechanical basis and unified standard and would not be influenced by other factors. The paper also studies the safety factors of tunnel surrounding rock (safety factors of shear and tension failure) and lining and some methods of designing and calculating tunnels. At last,the writers take the loess tunnel for instance and show the design and calculation results of two-lane railway tunnel.

Classification and Visualization of Surrounding Rock Mass Stability Based on Multi-Dimensional Cloud Model

The classification of the stability of surrounding rock is an uncertain system with multiple indices.The Multidimensional Cloud Model provides an advanced solution through the use of an improved model of One-dimensional Cloud Model.Setting each index as a one-dimensional attribute,the Multi-dimensional Cloud Model can set the digital characteristics of each index according to the cloud theory.The Multi-dimensional cloud generator can calculate the certainty of each grade,and then determine the stability levels of the surrounding rock according to the principle of maximum certainty.Using this model to 5 coal mine roadway surrounding rock examples and comparing the results with those of One-dimensional and Two-dimensional Cloud Models,we find that the Multi-dimensional Cloud Model can provide a more accurate solution.Since the classification results of the Multidimensional Cloud Model are difficult to be presented intuitively and visually,we reduce the Multi-dimensional Cloud Model to One-dimensional and Two-dimensional Cloud Models in order to visualize the results achieved by the Multi-dimensional Cloud Model.This approach provides a more accurate and intuitive method for the classification of the surrounding rock stability,and it can also be applied to other types of classification problems.

Numerical analysis of deformation and failure characteristics of deep roadway surrounding rock under static-dynamic coupling stress

In actual production,deep coal mine roadways are often under typical static-dynamic coupling stress(SDCS)conditions with high ground stress and strong dynamic disturbances.With the increasing number of disasters and accidents induced by SDCS conditions,the safe and efficient production of coal mines is seriously threatened.Therefore,it is of great practical significance to study the deformation and failure characteristics of the roadway surrounding rock under SDCS.In this paper,the effects of different in-situ stress fields and dynamic load conditions on the surrounding rock are studied by numerical simulations,and the deformation and failure characteristics are obtained.According to the simulation results,the horizontal stress,vertical stress and dynamic disturbance have a positive correlation with the plastic failure of the surrounding rock.Among these factors,the influence of the dynamic disturbance is the most substantial.Under the same stress conditions,the extents of deformation and plastic failure of the roof and ribs are always greater than those of the floor.The effect of horizontal stresses on the roadway deformation is more notable than that of vertical stresses.The results indicate that for the roadway under high-stress conditions,the in-situ stress test must be strengthened first.After determining the magnitude of the in-situ stress,the location of the roadway should be reasonably arranged in the design to optimize the mining sequence.For roadways that are strongly disturbed by dynamic loads,rock supports(rebar/cable bolts,steel set etc.)that are capable of maintaining their effectiveness without failure after certain dynamic loads are required.The results of this study contribute to understanding the characteristics of the roadway deformation and failure under SDCS,and can be used to provide a basis for the support design and optimization under similar geological and geotechnical circumstances.

Stability of coal pillar in gob-side entry driving under unstable overlying strata and its coupling support control technique

Considering the situation that it is difficult to control the stability of narrow coal pillar in gob-side entry driving under unstable overlying strata, the finite difference numerical simulation method was adopted to analyze the inner stress distribution and its evolution regularity, as well as the deformation characteristics of narrow coal pillar in gob-side entry driving, in the whole process from entry driving of last working face to the present working face mining. A new method of narrow coal pillar control based on the triune coupling support technique (TCST), which includes that high-strength prestressed thread steel bolt is used to strain the coal on the goaf side, and that short bolt to control the integrity of global displacement zone in coal pillar on the entry side, and that long grouting cable to fix anchor point to constrain the bed separation between global displacement zone and fixed zone, is thereby generated and applied to the field production. The result indicates that after entry excavating along the gob under unstable overlying strata, the supporting structure left on the gob side of narrow coal pillar is basically invalid to maintain the coal-pillar stability, and the large deformation of the pillar on the gob side is evident. Except for the significant dynamic pressure appearing in the coal mining of last working face and overlying strata stabilizing process, the stress variation inside the coal pillar in other stages are rather steady, however, the stress expansion is obvious and the coal pillar continues to deform. Once the gob-side entry driving is completed, a global displacement zone on the entry side appears in the shallow part of the pillar, whereas, a relatively steady fixed zone staying almost still in gob-side entry driving and present working face mining is found in the deep part of the pillar. The application of TCST can not only avoid the failure of pillar supporting structure, but exert the supporting capacity of the bolting structure left in the pillar of last sublevel entry, thus to jointly maintain the stability of coal pillar.

Dynamic Mechanical Behavior and Numerical Simulation of an Ancient Underground Rock Mass under Impact Loading

To study the dynamic mechanical properties of tuff under different environmental conditions,the tuff from an ancient quarry in Shepan Island was prepared.The impact damage to the rock was tested using a triaxial dynamic impact mechanical testing system(TDIMTS)with different ground stresses,temperatures,and groundwater pressures.The time-strain relationship,dynamic stress-strain relationship,energy dissipation law,energy-peak strain relationship,and the impact damage pattern of the tuff specimens under impact air pressures were investigated.The TDIMTS experiment on ancient underground rock mass under impact loading was also simulated using the finite element analysis software LS-DYNA based on the Holmquist-Johnson-Cook(HJC)material model.The dynamic failure process,failure pattern and peak stress of tuff specimen were calculated.The simulation results obtained using the above methods were in good agreement with the experimental results.The results of the dynamic experiment show that with the same local stress,groundwater pressure,and temperature,the damage to the tuff specimens caused by blasting and quarrying disturbances gradually increases as the impact pressure increases.Under the same local stress,groundwater pressure,and temperature,the energy required to rupture the tuffs in ancient underground caverns is relatively small if the impact pressure is low accordingly,but as the impact pressure increases,the damage to the tuff caused by quarrying disturbance gradually increases.The damage gradually increases and the degree of damage to the tuff and the strain energy exhibit asymptotic growth when the tuff specimens are subjected to the greater strain energy,increasing the degree of rupturing of the tuff.In addition,the average crushing size decreases with increasing strain energy.By comparing the simulation results with the experimental results,it was found that the HJC model reflected the dynamic impact performance of tuff specimen,and the simulation results showed an evident strain rate effect.These results of this study can offer some guidance and theoretical support for the stability evaluation,protection,and safe operation of the ancient underground caverns in future.

Integrated approach of predicting rock stability in high mountain valley underground caverns

High mountain valleys are characterized by the development of intricate ground stress fields due to geological processes such as tectonic stress,river erosion,and rock weathering.These processes introduce considerable stability concerns in the surrounding rock formations for underground engineering projects in these regions,highlighting the imperative need for rigorous stability assessments during the design phase to ensure construction safety.This paper introduces an innovative approach for the pre-evaluation of the stability of surrounding rocks in underground caverns situated within high mountain valleys.The methodology comprises several pivotal steps.Initially,we conduct inverse calculations of the ground stress field in complex geological terrains,combining field monitoring and numerical simulations.Subsequently,we ascertain stress-strength ratios of the surrounding rocks using various rock strength criteria.To assess the stability characteristics of the surrounding rocks in the 1^(#)spillway cave within our project area,we employ numerical simulations to compute stress-strength ratios based on different rock strength criteria.Furthermore,we undertake a comparative analysis,utilizing data from the 5^(#)Underground Laboratory(Lab 5)of Jinping II Hydropower Station,aligning the chosen rock strength criterion with the damage characteristics of Lab 50s surrounding rocks.This analysis serves as the cornerstone for evaluating other mechanical responses of the surrounding rocks,thereby validating the pre-evaluation methodology.Our pre-evaluation method takes into account the intricate geological evolution processes specific to high mountain valleys.It also considers the influence of the initial geostress field within the geological range of underground caverns.This comprehensive approach provides a robust foundation for the analysis and assessment of the stability of surrounding rocks,especially in high mountain valley areas,during the design phase of underground engineering projects.The insights derived from this analysis hold substantial practical significance for the effective guidance of such projects.

Effects of confining pressure on deformation failure behavior of jointed rock

For a deeper understanding of the deformation failure behavior of jointed rock, numerical compression simulations are carried out on a rock specimen containing non-persistent joints under confining pressure with the bondedparticle model. The microscopic parameters which can reflect the macroscopic mechanical properties and failure behavior of artificial jointed specimens are firstly calibrated. Then, the influence of joint inclination and confining pressure on stress-strain curves, crack patterns, and contact force distributions of jointed rock are investigated. The simulation results show that both the compressive strength and elastic modulus of the specimens increase with increasing confining pressure, and these two mechanical parameters decrease first and then increase with the increase of joints inclination. The sensitivity of strength and elastic modulus to confining pressure is not the same in different joints inclinations, which has the least impact on specimens with α=90°. Under low confining pressure, the failure modes are controlled by the joint inclination. As the confining pressure increased, the initiation and propagation of tensile crack are gradually inhibited, and the failure mode is transferred from tensile failure to shear-compression failure. Finally, the reinforcement effect of prestressed bolt support on engineering fractured rock mass is discussed.

Key rock mechanical problems of underground powerhouse in Shuibuya hydropower station

The complicated rock structures and the stability of surrounding rocks of the underground powerhouse were the key rock mechanical problems in Shuibuya hydropower station.In order to overcome the related rock mechanical problems encountered during its construction,a comprehensive research was carried out for the underground powerhouse in Shuibuya hydropower station based on a detailed geological survey.It covers the investigations on the initial in-situ stress distribution features,rock mechanical properties,engineering rock mass classifications by different methods,numerical modeling for stability and support analysis,proper measures for rock excavation and support.The results show that the rock excavations of the underground powerhouse under the given geological conditions can be controlled effectively.Some measures,suggested by the designers,are proved to be rational and effective.These measures mainly consist of:(1) the soft rock replacements by concrete in local area below the crane beam,(2) the shotcrete and reinforcement by rock bolts and anchor cables in surrounding rocks,and (3) 2 m concrete placement on the rock bench between adjacent tailrace tubes.The engineering practice shows that the treated surrounding rocks have a good overall stability.The deformation behaviors observed by safety equipments are within the designing limits.The research conclusions on the related rock mechanical problems,prior to the underground powerhouse excavations,are reliable.

Determining the rational layout parameters of the lateral high drainage roadway serving for two adjacent working faces

To determine the rational layout parameters of the lateral high drainage roadway(LHDR) serving for two adjacent working faces, a mechanical model of the LHDR under mining influence was established, and the overburden fissure, mining-induced stress distribution rules were analyzed. First, the development characteristics of mining-induced overburden fissure and the stress distribution law of the upper section of the working face were analyzed. Second, by analyzing the distribution law of vertical stress at different layers, the lateral distance of the LHDR was determined as 25 m. Third, by analyzing the surrounding rock deformation effect, stress distribution law, and overburden fissure distribution law of the LHDR at the heights of 20, 25, and 30 m away from the roof, the rational horizon of the LHDR was determined to be 25 m. Finally, an example of a LHDR located 25 m above the roof of the No. 2 coal seam and 25 m away from the No. 2-603 working face was presented. Results show that when the No. 2-603 coalface is being mined, the surrounding rocks lag 80 m or even further and the working face tends to be stable. The relative deformations of the roof and floor of the roadway and both of its walls were 583 and 450 mm,respectively. The reduction rate of the roadway section was 21.52%–25.32%. The section of the roadway was sufficient to extract the pressure relief gas in the overburden of the No. 2-605 working face. The average gas concentration and the pure volume at the branch pipeline were 24.8% and 22.3 m^3/min,respectively, showing that the position of high-level boreholes was reasonable.

Geological conditions and key rock mechanics issues in the Western Route of South-to-North Water Transfer Project

In terms of special geological conditions of the Western Route of South-to-North Water Transfer Project, the classification method for surrounding rocks is discussed by combining with the construction method of tunnel boring machine （TBM）. The classification standard of surrounding rocks is put forward on the basis of physical simulations and engineering practices. Damage, deformation and evolution of surrounding rocks induced by TBM excavation are discussed. Meanwhile, the long-term deformation mechanisms and stability of surrounding rocks are also studied. On this basis, a three-dimensional constitutive model for interbedded sandstone slate and a fiat shell-joint element-foundation system for calculating internal forces of segment lining are established. The deformation features of surrounding rocks of deep and steep interbedded sandstone slate and their influences on internal forces of segment lining are presented. Finally, the design methods of segment lining constructed in deep and steep flysch are proposed.