High Geostress Research Articles

Experimental simulation study of rockburst characteristics of Sichuan–Tibet granite: A case study of the Zheduoshan tunnel

Rockburst poses a great threat to the construction safety of deep tunnels under high geostress conditions. Recently, numerous deeply buried tunnels have been constructed in China, especially in the Sichuan–Tibet railway area. In this study, two rockburst experiments (strainbursts and impact-induced rockbursts) were conducted under different initial geostresses using the strainburst experimental apparatus and impact-induced rockburst experimental apparatus equipped with an acoustic emission (AE) monitoring system. A video acquisition system was used to capture the rockburst process. The results demonstrated that the rockburst process is similar under different initial geostresses, including grains ejection, rock plates spalling/buckling, and fragments ejecting violently. Based on the result of the AE characteristics, the large- amplitude AE signals were concentrated in the lower frequency band and the rockburst intensity was increased gradually with increase in the initial geostress. The b-value and βt-value fluctuated slightly at first, subsequently increased steadily, and then decreased abruptly when rockburst occurred. Furthermore, the displacement and divergence fields on the specimen surface were calculated using digital image correlation (DIC) technology. The result shows that the zone of large deformation and high divergence is consistent with the rockburst zone. Moreover, 80%σpeak is a critical stress state where rocks begin experiencing slight damage.

Engineering Properties of Soft Rock with High Geostress and the Performance Under Excavation of Deep Tunnel

Engineering Properties of Soft Rock with High Geostress and the Performance Under Excavation of Deep Tunnel

Mining Stress Evolution Law of Inclined Backfilled Stopes Considering the Brittle-Ductile Transition in Deep Mining

To study the mining stress evolution law of inclined backfilled stope in deep mining, this paper first proposes a method for determining the parameters of the brittle-ductile transition model corresponding to the Hoek–Brown criterion and Mohr-Coulomb criterion under high geostress. Then, a model composed of inclined backfilled stopes with different depths is established to simulate the sequential mining process of ore bodies with varying depths from shallow to deep. The numerical model’s stratum displacement, rock mass stress distribution, and risk factors show that the mining-induced stress will move to the upper stopes and the stratum below the deepest stope. The transfer range and influence degree of mining-induced stress will increase with the increase of the deep mining, resulting in the most dangerous backfilled stope occurring one to two layers above the deepest stope and the apparent stress concentration area occurring below the deepest stope. To prevent disasters caused by mining stress, pillars in inclined deep stopes should have large safety factors. Replacing low-strength backfills with high-strength backfills can reduce the stress concentration in the stratum below the deepest stope.

Statistics and construction methods for deep TBM tunnels with high geostress: A case study of Qinling Tunnel in Hanjiang-Weihe River Diversion Project

Rockbursts pose serious threats to the engineering safety. As a typical deep TBM tunnel with high geostress, the Qinling Tunnel of Hanjiang-Weihe River Diversion Project has encountered a large number of rockbursts. Based on this project, a detailed analysis of the rockburst data was used to investigate the size and locations of single rockfalls, and volume of a single blast. Moreover, the relationships between rockbursts and lithology, and between rockbursts and buried depth were established, and the form of rockburst damage was analysed. In addition, specific construction methods under different rockburst levels were summarised, drawing on the successful experience of this project. A method for replacing the cutter head under rockburst risk after long-distance tunnelling was also proposed.

A New Numerical Finite Strain Procedure for a Circular Tunnel Excavated in Strain-Softening Rock Masses and Its Engineering Application

The small strain theory underestimates the self-bearing capacity of rock masses, especially for a soft rock tunnel under high geostress. To perform an efficient and accurate calculation and provide a reference for the stiffness design of a tunnel, the finite strain solution for a circular tunnel in Mohr–Coulomb strain-softening rock masses with a non-associated flow rule was derived as three sets of differential equations under the Lagrangian coordinate, which are in the residue region, the softening region, and the elastic region, respectively. Based on the bisection method, an iteration procedure for solving the finite strain solution was proposed to approximate the boundary condition at infinity, the values of two adjacent boundaries, and the initial values on the excavation boundary. This numerical procedure was verified by comparing with self-similar solutions, recursive solutions, and FLAC simulation results. In the calculation example, the relative error on boundaries can be decreased to less than 10−8 after only 10 times iteration and the time for each calculation is less than 15 s. Applying this procedure on the sensibility analysis and stiffness reliability design for the Zhongyi tunnel, a support stiffness of 4.3 MPa/m is recommended to guarantee a tunnel displacement lower than 0.5 m.

Discussion on Reasonable Clear Spacing of Twin-Tunnels in Weak Surrounding Rock: Analytical Solution and Numerical Analysis

Due to the limitations of the terrain and other reasons, twin-tunnels with small clear spacing are becoming more and more common. The reasonable clear spacing of twin-tunnels in weak surrounding rock is an urgent problem. In this paper, based on pressure-arch theory (PAT), the method used to calculate the loosening pressure on the surrounding rock of twin-tunnels under three cases was derived by considering the additional disturbance of the excavation on the middle rock pillar through the amplification factors of the sliding angles, k1 and k2. Taking the weak surrounding rock as an example, we discussed the effect of the clear spacing variation on the loosening pressure of the surrounding rock, and obtained the dangerous clear spacing of twin-tunnels. The accuracy and applicability of this approach were verified by the in-site data that were measured. Considering the limitation of the pressure-arch theory on a deep-buried tunnel under high geo-stress, based on the Jiuzhai valley-Mianyang highway in China, the reasonable clear spacing of twin-tunnels in weak surrounding rock at different buried depths, i.e., 400 m–1,600 m, is discussed using the numerical simulation method. The analytical solution indicated that the loosening pressure of the inner side of the first tunnel was greater than the one of the second due to the amplification factors of the sliding angles, k1 and k2. The in-site data showed that in the weak surrounding rock mass, when k1 and k2 are taken as 1.15 and 1.3, respectively, the loosening pressure distribution law is closest to the actual situation.

Investigation on the Properties of Gneiss under Different Ground Stresses.

Initial geostress has great influence on the properties of gneiss. The physical and mechanical properties of gneiss vary considerably due to different initial geostresses, which exert a huge effect on the stability of underground engineering. In order to explore the influence of initial ground stress on the properties of gneiss. Changes in the physical properties (e.g., P-wave velocity and volumetric weight), mechanical properties (e.g., compressive strength, elastic modulus, and residual strength) and failure mode of gneiss are analyzed by conducting physical and mechanical tests on gneiss in different ground stress areas. The results show that high geostress can improve the pre-peak mechanical properties of gneiss, and weaken its post-peak mechanical properties. When the initial geostress is greater, the pre-peak mechanical properties are better, and the post-peak mechanical properties are worse. The failure mode of gneiss under high ground stress is primarily brittle failure. When the initial ground stress is greater, brittleness is stronger. According to the research results of this paper, it can provide the basis for the optimization and improvement of underground engineering support in gneiss strata with high geostress. The research results have important reference value and guiding significance for underground engineering construction in high geostress gneiss areas.

Large Deformation Disaster Mechanism and Control Technique for Deep Roadway in Faulted Zone

In order to study the mechanism and control technique of deep roadways in faulted zones, the 11-2 transport roadway of Guqiao coal mine in Huainan was taken as a case. The research roadway encountered two faults named F97 and F98 and suffered a severe large deformation disaster (including floor heaving, side cracking, steel arch bending, and section shrinkage) after excavation. Then, laboratory tests, in situ stress measurement, and numerical simulation results indicate that the lower strength and easy to soften property of the surrounding rock is an immanent cause, while high geo-stress is an external inducement of large deformation. The continuous behaviors of opening, moving, and rotation of rock blocks under the above factors finally caused large deformation disaster. Finally, a control technique named stages and combination control method (SCCM) has been proposed. This technique contains two steps, namely, primary (including prestressed anchor bolt, shotcrete, and grouting) and secondary (including prestressed cable) support stages. Numerical calculation and in situ monitoring results indicate that the SCCM technique can stop the failure of surrounding rock and control the displacement of the surrounding rock effectively.

Control technologies for the deformation of a tunnel excavated in steeply inclined layered phyllite under high geo-stress

Control technologies for the deformation of a tunnel excavated in steeply inclined layered phyllite under high geo-stress

Damage and Failure Characteristics of Surrounding Rock in Deep Circular Cavern under Cyclic Dynamic Load: A True Triaxial Experiment Investigation

For ensuring safety and efficiency during the construction of deep engineering, it is essential to explore the failure mode of the surrounding rock mass under dynamic disturbance and high geo-stress. We conducted true triaxial load tests for rock-like material with a preexisting circular hole, and monitored the acoustic emission (AE) signal during the whole test. The result demonstrates the evolution characteristics of damage and failure mode with different cyclic dynamic load amplitudes and intermediate principal stress. With the increase in cyclic dynamic load amplitude or the decrease in intermediate principal stress, the failure source mainly occurs at the two horizontal side walls of the surrounding rock where the failure patterns change from the slabbing to wall caving and, finally, to rockburst. The former failure mode can actually serve as an important precursor for the latter. Based on such mechanisms, the precursor can be indirectly detected in forms of AE signal released by microcracking. The research can provide a reliable guidance for the rock stability control and faithfully forecasting the larger-scale failure during the excavation of deep circular cavern.

Experimental investigation and theoretical analysis of indentations on cuboid hard rock using a conical pick under uniaxial lateral stress

Stress conditions are critical in deep hard rock mining and significantly influence hard rock cuttability. The peak cutting force (PCF), cutting work (CW), and specific energy (SE) can reflect rock cuttability and determine the feasibility and saving of mechanized mining to some extent. In this paper, the influence of uniaxial lateral stress on rock cuttability was investigated by an indentation experiment on cuboid rock using a conical pick, and a theoretical model was proposed to analyze the PCF and associated factors. The PCF, CW, and SE were used as indices to measure hard rock cuttability. The regression analyses show that rock cuttability presents as decreasing followed by increasing as uniaxial lateral stresses increases. The theoretical model was established by simplifying rock fragments into three-dimensional ellipse cones, and a formula was derived based on the elastic fracture mechanics theory. The error between the calculated and experimental values is 3.8%, which confirms the accuracy of the prediction formula. Finally, rock fragmentation by using conical picks was successfully applied on the field mining stope by inducing high geostresses to promote adjustments in stress and improve ore-rock cuttability.

Large deformation characteristics and treatment effect for deep bias tunnel in broken phyllite: A case study

The support structure of deep buried tunnel often encountered bias load of during construction in weak layered strata. This paper studied the support structure failure behavior and surrounding rock deformation mechanism with Wuduxi Tunnel as an engineering case. The tunnel presents the damge phenomenons such as primary support cracking, steel arch distortion, secondary lining sidewall extrusion under asymmetrical load and high geostress. Firstly, the laboratory test was producted to research the engineering characteristics of the broken phyllite. The field test results show that the surrounding rock pressure is unevenly distributed around the tunnel, the primary support is all under pressure and the secondary lining is partial in tension. The surrounding rock pressure distribution shows an ellipsoidal distribution consistent with the trend of the rock stratum. The maximum vault settlement and horizontal convergence are 165.58 mm and 282.56 mm, respectively, and the convergence rate is far more than the stable deformation rate in the specification, which proves that the existing specification is not applicable to the large deformation deep buried bias highway tunnel with soft rock. Furthermore, the construction methods of “double primary support-secondary lining” are proposed to control the large deformation of surrounding rock. The field test is carried out to verify the feasibility of the double layer primary support. The field test show that the large deformation can be effectively controlled, and the support structure was stable without crack and damage, which ensured the construction safety of the deep buried bias tunnel in broken phyllite.

Research on Influence Factors of Bearing Capacity of Concrete-Filled Steel Tubular Arch for Traffic Tunnel

When a traffic tunnel passes through special strata such as soft rock with high geo-stress, expansive rock, and fault fracture zones, the traditional supporting structure is often destroyed due to complicated loads, which threatens the construction and operation safety of tunnel engineering. Concrete-filled steel tubular (CFST) structure gives full play to the respective advantages of steel and concrete and has better bearing capacity and economic benefits than traditional support structure, which has achieved good results in some underground engineering applications. In order to promote the application of CFST in the construction of traffic tunnels with complex geological conditions and improve the bearing capacity of the initial supporting structure of tunnels, the influencing factors of the bearing capacity of CFST arch were studied by numerical simulation. The main achievements are as follows: (1) The load-displacement curves of CFST members under different material parameters are basically consistent. CFST members have significant restrictions on displacement in the elastic stage and have high ultimate bearing capacity. Although the bearing capacity decreases obviously after reaching the peak, it shows good extension performance. (2) The height of the steel tube section, the thickness of the steel tube wall and the grade of the core concrete have an approximately linear positive correlation with the bearing capacity of CFST arch, but the influence of these three factors on the bearing capacity of CFST arch decreases in turn, and when the grade of core concrete increases above C50, it has no significant effect on the bearing capacity of members.

Damage mechanism and stability analysis of rock mass in the high geo-stress tunnel subjected to excavation

High geo-stresses and complicated geological conditions are two typical characteristics in the traffic tunnel of the tailrace surge chamber at the Shuangjiangkou hydropower station, Southwest China. Rock failures such as spalling and rockbursts induced by excavation were encountered repeatedly in this tunnel. To evaluate the stability of rock mass of the tunnel during excavation, the high-precision microseismic (MS) monitoring technology was introduced. Through analysing the tempo-spatial distribution characteristics of MS events, the main damage areas and their influencing factors were studied. The rock fracturing mechanisms of MS clusters were revealed based on the source parameters of MS events. A three-dimensional model was established to obtain the stress and deformation characteristics of rock mass. Results indicated that the MS events mainly occurred between July 10 and 26, 2017 and the energy release was higher during this period. The main damage areas of rock mass were located at the Stake K0 + 110m–K0 + 170m. Weak structures and multi-cavern effect aggravated rock mass damage subjected to excavation-unloading. Tensile failure was the main fracturing type of rock mass. Rock mass damage influenced by weak structures was especially prominent. The numerical simulation results are consistent with the MS monitoring results. The results could provide significant references for safety evaluation and construction design of similar tunnels.

Progressive failure characteristics and energy accumulation of granite with a pre-fabricated fracture during conventional triaxial loading

Deformation and failure of rocks under the loading effect is an energy-driven progressive process of damage evolution, and the abrupt energy release from rocks storing strain energy under high geostress conditions is a determinant inducing catastrophic effects. Therefore, studying the progressive fracture mechanism of rocks from the perspectives of energy accumulation and dissipation contributes to prediction and evaluation of catastrophic effects such as a rockburst. Fractured samples with different fracture dip angles were prepared using granite collected from the underground powerhouse (under maximum geostress of about 37.82 MPa) in Shuangjiangkou Hydropower Station in Dadu River Basin in China. By using these samples, the progressive failure characteristics and energy accumulation and evolution in fractured rocks during triaxial loading were studied under different stress conditions. In addition, the functional relationships of the crack initiation strength, damage strength, and peak strength of the fractured granite with the confining pressure and fracture dip angle in the loading process were established. The evolution of volumetric strain upon cracking corresponding to the peak stress with the confining pressure and fracture dip angle was discussed. Furthermore, the research explored the intrinsic relationships between the acoustic emission (AE) events and mechanical characteristics of the fractured granite during fracturing thereof. The correspondence of the AE count rate and cumulative energy count with various stages in the progressive failure of the rock was studied, and the evolution of the elastic strain energy and dissipated energy in the rock can be predicted based on the strength, deformation, and AE characteristics. Besides, effects of the confining pressure and the dip angle on the AE characteristic signals were investigated. Finally, the accumulation and evolution of the input energy, elastic strain energy, and dissipated energy of fractured granite in the pre-peak stage during triaxial loading were revealed using an energy analysis. Moreover, the relationships of the eigenvalues of elastic strain energy and dissipated energy of fractured granite with the confining pressure and fracture dip angle were determined.

Microseismic Monitoring of Rock Mass Fracture Response to Blasting Excavation of Large Underground Caverns Under High Geostress

Microseismic Monitoring of Rock Mass Fracture Response to Blasting Excavation of Large Underground Caverns Under High Geostress

Field test study on large deformation control of strongly weathered carbonaceous slate tunnel in high geostress

Field test study on large deformation control of strongly weathered carbonaceous slate tunnel in high geostress

Research on Deformation Mechanisms of a High Geostress Soft Rock Roadway and Double-Shell Grouting Technology

In this study, the deformation characteristics and mechanical properties of coal and rock mass in the S2N5 working face of the Xiaokang coal mine are analyzed to address the problem of large deformation of soft rocks with high in situ stress surrounding roadways. Through a newly developed grouting pipe, a double-shell grouting technology, consisting of low-pressure grouting and high-pressure split grouting, is proposed for the Xiaokang coal mine. In addition, the effect of grouting is evaluated by borehole peeping and deformation monitoring. The results show that the double-shell grouting technology can effectively improve the overall mechanical properties of the surrounding coal and rock mass, preventing the large deformation and failure of the roadway. This technology can be useful when analyzing and preventing large deformation of soft rock roadways.

Countermeasures against large deformation of deep-buried soft rock tunnels in areas with high geostress: A case study

Large squeezing deformation has been a main concern in the construction of deep-buried tunnels with soft surrounding rocks in high-geostress areas, threatening the safety of tunnel construction and delaying the construction progress. In this study, the Dapingshan tunnel, where a large deformation occurred, was taken as an example to study this issue. First, the geological and deformation characteristics of the large deformation area were analyzed. Subsequently, the creep model and creep parameters of the surrounding rock in the large deformation area were identified through laboratory tests. Finally, considering the time-dependent effect of tunnel construction, the geostress in the large deformation area was measured using the displacement back-analysis method, and the feasibility of the optimization scheme was verified. The results showed that: (1) The occurrence of large deformation is due to the combined action of the high geostress and poor self-stability of argillaceous sandstone. (2) The creep characteristics of argillaceous sandstone are consistent with the creep characteristics of the CVISC model. (3) The calculation result of the displacement back-analysis method considering the influence of the time-dependent effect of tunnel construction is found to be reliable. (4) The optimization scheme can help reduce the tunnel arch convergence and horizontal convergence by 50% and 60%, respectively, which confirms that the optimization scheme can help eliminate the large deformation of the Dapingshan tunnel.

Key Technologies for Controlling Large Deformation of Soft Rock Tunnels with High Geostress（高地应力软岩隧道大变形控制关键技术）

Key Technologies for Controlling Large Deformation of Soft Rock Tunnels with High Geostress（高地应力软岩隧道大变形控制关键技术）