Excavation Disturbance Research Articles

Influence of Three-Dimensional Stress Field Variation on Fracture Evolution Characteristics of a Roof

Excavation disturbance on the dynamic variation of the three-dimensional stress field is the main cause for the dynamic disasters of the surrounding rock mass of the roof. The stress condition in the surrounding rock mass of the roof during entry excavation and its impact on entry stability are systemically studied in this study. It is found that the surrounding rock mass of the roof is mainly influenced by the combined effect of the stress unloading and stress transference induced by entry excavation. A servo-controlled true triaxial material testing system is used to conduct the true triaxial loading and unloading experiments of rocks under different stress paths. The influence of different stress paths, especially the variation of the principal stress direction, on the mechanical characteristics and fracture characteristics of rocks is investigated. The results indicate that the variation of the principal stress direction has a significant impact on the macroscopic fracture characteristics of the rock. The main macroscopic fracture plane of the rock highly depends on the intermediate principal stress. The fracture evolution of the roof rock mass during entry excavation is analyzed. The results show that the change of the three-dimensional stress field induces the formation of complex fracture networks in the surrounding rock mass of the roof. The roof is likely to dislocate horizontally and collapse. The corners of the entry are seriously damaged. Based on the above findings, a support scheme is proposed to maintain the stability of a gob-side entry. The field experience suggests that the support scheme can achieve good results.

Stability analysis of open-pit gold mine slopes and optimization of mining scheme in Inner Mongolia, China

The geological structure of the Changshanhao open-pit mine in Urad Middle Banner, Inner Mongolia, China is extremely complicated, and slope instability has frequently occurred in various forms, such as wedge sliding, bedding sliding, and toppling failure. In order to study the failure mechanisms of these slopes, the geological structure and mechanical rock properties were investigated based on field investigations and laboratory tests. Numerical models for the present mining area and final mining area of the original scheme were established to analyze slope stability. The results showed that the unique geomorphological characteristics of the mining area were generated by geological tectonism, and the north side of the stope is an anti-dip layered rock slope and the south side is a dip layered rock slope. Slope failure is the consequence of endogenic and exogenic integration, including physical and mechanical properties of the rock mass, geological structures such as faults and joints, and human-caused factors such as blasting and excavation disturbances. Then the original excavation scheme was redesigned mainly by optimizing the slope angle and decreasing the final mining depth to maintain slope stability. Finally, the Monte Carlo method was used to analyze the reliability of the slope angle optimization scheme. The open-pit mine excavation plan that meets the stability requirements was obtained eventually.

Analysis of Cooperative Control Effect of Pressure Relief and Long Bolt Support for Deep Roadway Under Strong Mining Disturbance of Adjacent Working Face

The stability of mining roadways is pivotal to safe mining in deep underground coal mines. In order to control the stability of mining roadways in deep mines, this paper studies the cooperative control effect of pressure relief and long bolt support for deep roadway under strong mining disturbance of adjacent working face in Wenjiapo Coal Mine in West China. The numerical calculation software FLAC3D was used to simulate the distribution law of the displacement, stress and plastic zone around the roadway under the disturbance of excavation and working face mining with three support schemes: “short bolt + long cable”, “long bolt + short cable”, and “long bolt + short cable + drillholes and cutting grooves pressure relief”. The results show that extending the length of bolt can ensure the supporting effect while reducing the consumption of anchor cables in the roof, and can reduce the roadway floor heave by 5.5%. Moreover, the peak lateral pressure of the roadway is reduced by 12.5%, the peak value is shifted to the depth by 6 m, and the floor heave of the roadway is reduced by 36% when the countermeasures of pressure relief in the ribs and floor are adopted. This indicates that the schemes of pressure relief and long bolt support has a significant beneficial effect on decreasing the high stress environment and controlling the deformation of roadway.

Study on the seepage characteristics of deep buried tunnels under variable high-pressure water heads

Well-connected seepage channels are the key components of tunnel water inrush. To study the trends of the pore water pressure in the rock mass in a seepage channel, the physical experiment and corresponding numerical model were established. First, in the seepage experiment under excavation disturbance, the propagation rate of the high-pressure water source and the growth rate of the pore water pressure in the rock mass were analyzed by considering the different water pressures. Second, under the same model size, parameters, and working conditions, the finite element method was used to simulate the seepage process of water in the rock mass. From the microlevel, the evolution process of the pore water pressure isosurface was obtained. The results showed that the pressure water head is positively correlated with the pore water pressure in the rock mass, which displays multiple approximate relationships. Moreover, when the water pressure is high, excavation disturbance can significantly increase the pore water pressure in the rock mass. The disturbance caused by the first excavation is larger than the subsequent excavations. In addition, the research results were compared with previous results. Reasonable agreements that would provide a reference for the conditions of high water-pressure disasters in deep buried tunnel construction were obtained.

Stress-induced cracking performance of hard basalt in a large underground cavern based on multi-information observation

Overcoming the brittle cracking and the corresponding hard rock has become the key bottleneck challenge in the excavation of underground engineering with high geostress. To further understand the onsite cracking’s characteristics and the basic mechanism of hard rock, we carried out a detailed field monitoring action for basalt breaking behaviors in a large underground powerhouse by in situ investigation, digital borehole camera, multi-point deformation measurement, and real-time micro-seismic monitoring. On the basis of our observations, the inner cracking performances of surrounding rock embodied as discontinuous appearance and open of cracks during excavation, and these fractures were often parallel to the outline of the cavern and extended to the inner side because of the subsequent excavation disturbance. Corresponding numerical simulation indicated that the redistribution of rock stress induced local stress concentration and would result in the splitting break of basalt. Field complicated proof indicated that this stress-induced cracking of basalt belonged to the tensile break, and the macroscopic deformation of the surrounding was the result of the accumulation of abundant open deformation of discontinued cracks. This observed achievement can enrich our understanding of the cracking mechanism of hard rock and provide some key cures for optimization design for underground engineering construction under high geostress conditions.

Stability Monitoring and Analysis of High and Steep Slope of a Hydropower Station

Slope deformation and failure are major challenges for hydropower station engineering. Taking the left bank slope at the exit of the flood discharge tunnel of the Wudongde Hydropower Station in China as an example, the deformation mechanism of the high-steep rock slope was studied. The height of the slope is about 200 m, which is affected by slope excavation, rainfall, and atomized rain. The results of multipoint displacement meters, surface deformation monitoring, and anchoring stress meters showed that the deformation and deformation rate of the slope have increased dramatically. Through the comprehensive analysis of the slope, it is found that the overall lithology of the slope is poor, the excavation disturbance causes the redistribution of the stress in the slope, and the excavation surface is relatively steep, which provides space for the deformation of the slope rock mass. Unloading relaxation leads to a large number of new fissures in the slope rock mass. These new fissures and fault fracture zones provide convenient conditions for rainfall and atomized rain infiltration. The rainwater infiltrated along the slope surface, formed the seepage field in the slope body, and weakened the rock-soil mass parameters. Meanwhile, saturated runoff is formed on the slope, causing large deformation of the slope rock mass. However, the migration of water in the slope has a time effect, and its influence on the stability of the slope also has a time effect. It is difficult for traditional monitoring methods to monitor the resulting changes in internal sliding force of the slope. Therefore, a remote monitoring and early warning system for landslide anchor cable force was introduced to monitor the slope stability changes caused by the impact of water flow and rainfall infiltration, which provided a reasonable and scientific reference for subsequent slope construction.

Study on Rock Burst Prediction of Deep Buried Tunnel Based on Cusp Catastrophe Theory

Deep coal mining changes the original rock state, stress concentration occurs in the process of self-stabilizing adjustment of surrounding rock. At this time, rock burst is very easy to occur, which brings great threat to mine safety production. Based on catastrophe theory, the energy catastrophe criterion, displacement mode catastrophe criterion and entropy catastrophe criterion for surrounding rock system of deep buried tunnel are established in this paper. And the criteria are applied to the stability judgment of surrounding rock in Binchang coal mine. The analysis results show that the energy eigenvalue, displacement eigenvalue and entropy information eigenvalue for surrounding rock of deep buried tunnel suddenly jump and the surrounding rock loses stability when the excavation reaches the fourth step. The accuracy of the criterion is verified by laboratory test and FLAC3D numerical simulation. The above criteria can be used to judge the possibility of instability from different aspects according to the system mutation conditions, which can provide some reference for preventing local surrounding rock system instability in the process of excavation disturbance of deep buried tunnel.

Mechanism Analysis of Uncovering Coal in Crosscut and Gas Outburst Based on Flac3D

Zhao, Z. and Lei, Y., 2020. Mechanism analysis of uncovering coal in crosscut and gas outburst based on Flac3D. In: Yang, Y.; Mi, C.; Zhao, L., and Lam, S. (eds.), Global Topics and New Trends in Coastal Research: Port, Coastal and Ocean Engineering. Journal of Coastal Research, Special Issue No. 103, pp. 333–338. Coconut Creek (Florida), ISSN 0749-0208.Gas outbursts occur frequently in the process of uncovering coal in crosscut, so it is of great significance to study the outburst conditions of coal and gas in the process of tunneling. By establishing the model of these gas outbursts, we can obtain the function of the thickness of stone gate, gas pressure and gas outburst. With or without high pressure gas, the impact of high pressure gas during gas bursts is analyzed through the numerical simulation method. Based on the velocity, displacement, maximum principal stress and time curve of coal and rock interface in the center of roadway under two kinds of working conditions, the main conditions of gas bursts have been successfully verified. According to the results, in coal and gas outbursts, necessary conditions include the plastic zone penetration caused by high pressure gas storage and excavation disturbance, while sufficient conditions include the mutation of velocity field, displacement field and stress field in plastic zones.

Numerical simulation and analysis of the pile underpinning technology used in shield tunnel crossings on bridge pile foundations

Currently, the pile foundation underpinning technology is widely used when underground transportation infrastructure passes through existing buildings or structures in urban areas. This study aims to investigate stress transfer mechanisms in pile foundations during an underpinning process as well as the influence of shield tunnel construction on pile stability. To this end, the pile foundation underpinning technology used in China’s Shenzhen Metro Line 10 crossing through the bridge pile foundation group of the Guangzhou–Shenzhen highway was analyzed in detailed. The refined numerical simulation of the pile foundation underpinning and shield tunnel construction processes were conducted using the fast Lagrangian analysis of continua in 3 dimensions (FLAC3D) software. The results demonstrate that after the pile foundation underpinned, the previous bridge load system of bridge panel → pile foundation → bearing soil would transform into a bridge panel → existing pile foundation → new underpinning pile → deep bearing soil stratum structure. The overlying load on the underpinned pile could be effectively transferred to a new underpinning pile. In the process of underpinning and tunnel excavation, the settlement and deformation of the foundation can improve the tip resistance and shaft friction of piles, which in turn, can reduce the maximum principal stress in the pile foundation group. The deformation of the bridge pile foundation is mainly caused by ground loss and excavation disturbance generated during shield tunneling as the settlement induced by pile foundation underpinning accounts for approximately 20%–30% of the total settlement. The reduction effects of settlement deformation, lateral displacement, and principal stress are mainly manifested in underpinning piles, while the non-underpinning pile exhibits minimal variation. Meanwhile, the deformation of the segment lining structure of the shield tunnel primarily occurs near the underpinning area of the pile foundation, and it is mainly settlement deformation with a small horizontal displacement.

Physical experimental study on excavation disturbance of TBM in deep composite strata

Physical experimental study on excavation disturbance of TBM in deep composite strata

Experimental Research of Excavation Disturbance Effect Due to Trenchless Construction by Using Distributed Strain Sensing Technique

In this paper, an assessment method of excavation disturbance effect is proposed by using distributed optical-fiber strain sensing technique. Model tests of trenchless construction under different parameter conditions are carried out in the laboratory. In the test, the excavation was simulated by using the deflate of pre-burying air bag, soil displacement is measured by settlement mark, at the same time, The soil strain of overburden layer is obtained based on Brillouin Optical Time-Domain Analysis (BOTDA). Through the analysis of the experimental data, the partition theory of excavation disturbance zone is proved and verified through test data. The correlation between ground settlement and soil strain distribution due to trenchless construction are compared and analyzed. The mechanical response of overlying pavement are further discussed. Research results indicate that soil strain of overburden layer is symmetrically distributed about the excavation surface, which is compressed near the centerline and tensioned on both sides due to the excavation disturbance impact. The width of excavation disturbance zone derived from the test data shows good accordance with the calculation results based on the partition theory, the maximum error is less than 10%. A certain consistency exists between the distribution law of strain curve and settlement trough. Due to the uneven distribution of support force caused by foundation settlement, the pavement model shows notable bending deflection after trenchless excavation, and the maximum increment ratio of support force exceed 50%. The combined impact of ground traffic load and excavation disturbance is considered to be more unfavorable to the safety of pavement structure. The conclusions can be suggestive and helpful to trenchless excavation control.

Quantitative analysis of rockburst in the surrounding rock masses around deep tunnels

Rockbursts in deep tunnels subjected to high crustal stress were investigated using a general particle dynamics (GPD) code with the Holmquist–Johnson–Cook damage model (HJC). Under dynamic excavation disturbance, several numerical tests were carried out to investigate the dynamic failure patterns of surrounding rock masses with different preexisting cracks around deep tunnels. The numerical results showed that the GPD code can satisfactorily simulate the phenomenon of rockburst caused by crack evolution, including predicting the ejection velocity, rockburst location and ejected rock block volume. Moreover, the analysis of the circumferential stress in the surrounding rock of deep tunnels at the Jinping II Hydropower Station shows the efficiency of the GPD code. In addition, four typical rockburst, which occurred under different geological conditions at the Jinping II Hydropower Station, were analyzed by using the GPD code. The numerical simulations suggested that the GPD code can efficiently reproduce the 2D and 3D crack growth in the surrounding rock, which eventually leads to the occurrence of rockburst.

Dynamic Unloading Instability Mechanism of Underground Cavern Based on Seepage-Damage Coupling

The seepage-damage coupling effect will aggravate the instability of the surrounding rock during the unloading process of underground cavern excavation. Considering this coupling effect and excavation disturbance, the theoretical solution of the stress state of surrounding rock is derived by using the elastic-brittle damage model. The dynamic criterion of the instability and water inrush is presented. Based on the theoretical derivation, the calculation program for the seepage-damage analysis of the surrounding rock under dynamic unloading is programmed, and the seepage flow and the radius of the damage zone of the surrounding rock are calculated. By analysing the variation of radius of the damaged zone with pore water pressure and excavation radius under different calculation conditions, the influence of dynamic unloading disturbance on the damaged zone of the surrounding rock is discussed. The radius of the damaged zone increases with the pore water pressure and excavation radius. Considering the effect of dynamic unloading, the calculation result of the damaged zone radius and seepage discharge of underground cavern are much larger than the theoretical calculation and coupling calculation of seepage-damage without dynamic unloading. Research methods and results can provide guidance and reference for similar engineering research.

Conceptualization and preliminary study of engineering disturbed rock dynamics

Many large engineering projects, e.g., the Sichuan–Tibet Railway, inevitably cross the earthquake active areas and the geology complicated zones, facing the challenges of dynamic disturbances and disasters. In view of this, the conceptualization of engineering disturbed rock dynamics is proposed in this paper, aiming to systematically study the rock dynamic behavior and response subjected to engineering disturbances, to establish the 3D rock dynamic theory, and to develop the disaster prevention and control technical measures. The classification standards of rock loading states based on strain rate are summarized and analyzed. The engineering disturbed rock dynamics is defined as the theoretical and applied science of rock dynamic behaviors, dynamic responses and their superposition caused by dynamic disturbances during engineering construction and operation periods. To achieve the goals of the proposed engineering disturbed rock dynamics, a combined methodology of theoretical analysis, laboratory experiment, numerical simulation and in situ tests is put forward. The associated research scopes are introduced, i.e., experimental and theoretical study of engineering disturbed rock dynamics, wave propagation, attenuation and superposition in rock masses, rock dynamic response of different loading conditions, dynamic response of engineering projects under construction disturbance and disaster mitigation techniques, and dynamic response of major engineering projects under operation disturbance and safety guarantee measures. Some theoretical, experimental and field preliminary studies were performed, including dynamic behavior of disturbed rock at varied depth and strain rates, dynamic response of rock mass subjected to blasting excavation disturbance and dynamic drilling disturbance, and disturbance of rock mass subjected to TBM excavation. Preliminary results showed that the rock masses are significantly disturbed by dynamic disturbances during construction and operation periods of engineering projects. The innovative conceptualization of engineering disturbed rock dynamics and the expected associated outcomes could facilitate establishing the 3D rock dynamic theory and offering theoretical fundamentals and technical guarantees for safety and reliability of the design, construction and operation of modern large engineering.

Study on Excavation Disturbance Characteristics of High Ground Stress Soft Rock Tunnels

Based on the Xianglushan tunnel in The Central Yunnan Water Diversion Project, the stress, deformation, damage range and supporting structure bearing characteristics of the surrounding rock of the tunnel under different initial stress field are studied. It is shown that the stress, deformation, rupture range and supporting structure load of the surrounding rock are characterized by zonal distribution after tunnel excavation. The maximum principal stress transfer profile and deformation profile of the surrounding rock are basically perpendicular, and the deformation at the location of high stress concentration is small, while the deformation in the unloading zone is large. For the system anchor, the axial force of the anchor in the stress concentration zone is relatively small, and the axial force of the anchor in the unloading zone is relatively large. Therefore, according to the bearing characteristics of the supporting structure, the surrounding rock of the tunnel can be divided into different zones for different support. The related understanding and conclusions are of certain theoretical and engineering significance.

Research on the Large Deformation Prediction Model and Supporting Measures of Soft Rock Tunnel

The weak surrounding rock has the characteristics of easy softening, poor integrity, low mechanical strength, etc., which makes it easy to induce different degrees of deformation and damage under excavation disturbance and then seriously affects the stability of the tunnel. Carrying out soft rock tunnel deformation prediction research and designing the supporting structure according to the predicted value is of great significance to engineering construction and design. Based on the grey theory, the large deformation of the vault, shoulder, and waist of the soft rock tunnel are predicted, and then the specific bolt support is designed in the maximum predicted value (Smax·R) area. The control effects of different bolts, spacing (d), length (L) on the maximum displacement (Smax·M), and maximum stress (σmax·M) the surrounding rock are analyzed by numerical simulation. Results show that the gray model has high prediction accuracy, the best prediction time is one week, and the maximum error is only 2.99%; with the decrease in d, resin bolt support has a significant supporting effect compared with mortar bolt support, with Smax.M and σmax·M reduced by 64.38% and 10.35%, respectively; as the L of bolt increases, compared with the mortar bolt support, the resin bolt support has a more obvious restraining effect on the surrounding rock deformation, and Smax·M and σmax·M are reduced by 28.20% and 10.00%, respectively; when 4.5 m < L < 6.0 m and 0.6 m < d < 0.7 m, resin bolt support should be adopted; in other ranges, mortar bolt support or resin bolt support has a less significant difference in controlling surrounding rock deformation.

Instability Mechanism of Cavity‐Bearing Formation under Tunnel Excavation Disturbance

Urban subway construction inevitably causes disturbances to the rock strata. It can even cause ground collapse accidents when construction encounters a bad geological body in the stratum. To verify the influence of cavities on surface settlement and the mechanism of formation instability, the instability mechanism of cavity‐bearing strata under tunnel excavation disturbance was herein studied by tests using self‐designed indoor models and numerical simulations. This study was based on the concrete project case of Qingdao Metro. Two groups of experiments (with and without cavities) were designed to simulate the four‐step excavation by staged unloading of an airbag. The results show that the settlement value in the cavity state was about twice as much as that in the nonvoid state at the same stage. Besides the first step of excavation, in the numerical simulation, the settlement value of the same stage is about 1.3 times that of the model test. Simulating the deformation process of the surrounding rock in model tests shows that, in the excavation of the tunnel, the collapse surface of the tunnel arch roof will be connected with the sliding surface of the formation cavity. The cavity will gradually change from a regular circle to an ellipse or may even close, resulting in a sudden increase in land subsidence or even ground collapse. This indicates that disturbance during tunnel excavation would cause greater instability in a stratum when bad geological bodies, such as cavities, exist in the stratum.

Comprehensive risk assessment and engineering application of mine water inrush based on normal cloud model and local variable weight

ABSTRACT Mine water inrush disaster occurs after the critical risk degree is reached under the non-linear dynamic coupling action of hydrogeological conditions, water hazard sources and excavation disturbance of artificial machinery. In this paper, the risk assessment unit is divided from the time and space dimensions, and the risk assessment evaluation index set is determined based on the “4M1E” management system, taking into account the fuzziness, dynamics, and balance of the evaluation indicators. This method changes the defect of the index weight in the previous comprehensive evaluation method as the constant weight. The game theory set model is selected to construct the initial weight vector of the evaluation index, and the final weight vector is determined by the local variable weight equalization function combined with the indicator state value. The normal cloud model is used to process the evaluation indicator status values, and the comprehensive evaluation matrix is determined to solve the ambiguity and randomness of the evaluation indicators. Finally, a comprehensive assessment model for mine water inrush disasters is formed, and a dynamic assessment framework is used to conduct dynamic risk assessment of mine water inrush. The research results show that the dynamic comprehensive evaluation model provides a theoretical basis for the early warning and prevention of mine water inrush disasters.

Parallel Offset Crack Interactions in Rock under Unloading Conditions

Rock slopes contain numerous nonpenetrating intermittent joints which maintain stability under excavation disturbance. The tip interaction coefficient (IC) of parallel offset double cracks in a typical rock mass under unloading conditions was calculated in this study based on the superposition principle and fracture mechanics to determine the meso-influence law of intermittent joint interaction in the slope under the action of excavation. The influence of many factors on the said interaction was also analyzed theoretically. Lateral unloading tests were conducted on rock-like specimens with parallel offset cracks in addition to RFPA2D numerical simulation and theoretical analysis. The results show that a smaller length of rock bridge or staggered distance between the cracks results in more severe and sensitive interactions at the crack tip. The Type I interaction strength of the tip of the crack is not affected by the inclination angle of the crack, but shear failure gradually weakens as the angle changes. The shear failure of the tip of the crack is more sensitive to changes in the inclination angle when the cracks are closer to each other; the change is the most intense when α is about 60°. Lateral unloading test and RFPA2D numerical simulation results are in close agreement with the theoretical analysis, which validates the theoretical results. The current study shows the interaction of the parallel offset cracks in rock under unloading conditions and is conducive to the study of the meso-failure mechanism of the jointed rock slope in an open-pit mine under the action of excavation.

Deformation and mechanic response of surrounding rock mass during shallow buried large-section tunnel excavation

The stability of underground tunnel is closely related to the stress status of surrounding rock mass. Aiming at a typical large-span hydraulic tunnel, the variation of deformation and stress of rock mass during excavation was investigated. Results show that the peripheral convergence of surrounding rock increases with buried depth, and the crown settlement is more sensitive to depth. The horizontal displacement of surrounding rock decreases with the distance to tunnel side wall and the excavation disturbance is about 2 times of the tunnel width. There are obvious time and spatial effects of displacement because of the existence of working face. The restrain distance is about 1.5 times of tunnel width. The excavation of the tunnel changes the stress status of surrounding rock mass. Stress concentration first forms above the crown of the tunnel and then a continuous stress arc develops around the support. The stress arc is favorable to the stability of tunnel.