Fault Fracture Zone Research Articles

Comprehensive monitoring for rockburst at a pumped storage power station in northeast China

Rockburst is characterised by the wide occurrence areas and the long delayed time at a pumped storage power station in Heilongjiang province of China. For the study, a new method is proposed to identify the induced rockburst based on the combination of microseismic (MS) and electromagnetic radiation (EMR) monitoring methods. EMR monitoring method is introduced to make up for the shortcomings of the monitoring blind area caused by the propagation path of wave velocity during MS monitoring and a beneficial attempt is carried out for the comprehensive application of the two methods to monitor the rockburst. Firstly, a MS monitoring system is established around the underground powerhouse to recognize the propagation of micro-fracture during the deep excavation, while the typical five parameters including the frequency of the microseismic events, the total energy of daily events, energy index (EI), cumulative apparent volume (CAV) and microseismic signal b-value are analyzed to reveal the temporal and spatial evolution law of microseismic events. Secondly, an EMR monitoring system is also established near the MS events clusting area to study the characteristics of EMR produced from the fractured rock mass. The temporal and spatial variation of the two parameters including electromagnetic radiation intensity and pulse number are also analyzed and the distribution of higher values are compared with the geological structure. Finally, the correlation of electromagnetic signals, microseismic signals and local stress is studied based on theories of geophysics, electromagnetism and rock mechanics, and some credible results are obtained. Here are the following resluts: (1) The microseismic events are characterized by spatial clustering, and the joint action of the excavation disturbance and the weak structure body of the surrounding rock mass are the main factors for rockburst. The trend of sharp decrease of energy index and sudden increase of cumulative apparent volume generally indicates the occurrence of rockburst or macro-fracture. The magnitude of b value indicates that the rockburst failure type in the project belongs to fracture-slip rockburst. (2) The sudden change of electromagnetic radiation intensity in the fault fracture zone of the underground powerhouse can be used as the precursor information of rockburst or large-scale rupture events.(3) The micro-fracture activities revealed by EMR and MS methods are consistent in spatial distribution. The distribution of the peak values of electromagnetic radiation pulse number are basically consistent with the direction of the apparent stress migration disturbed by the field construction situation. So we can infer that it will play an important role to improve the accuracy of rockburst prediction based on the analysis of the multi-source singals.

An analytical model for face stability of tunnels traversing the fault fracture zone with high hydraulic pressure

The issue of tunnel face stability traversing the fault fracture zone with high hydraulic pressure has been addressed based on a new three-dimensional limit equilibrium model. First, the finite element numerical simulation is conducted to reveal the effects of the differences in permeability coefficient of strata on both sides of the geological interface on the distribution of seepage flow field nearby tunnel face. With this discovery, an existing universal approximate formula for hydraulic head nearby tunnel face is modified to accurately calculate the hydraulic head distributions under this scenario. Second, introducing the effect of geological interface, a new limit equilibrium model that consists of a trapezoid and a prism is proposed. The distributed loading exerted by the prism upon the trapezoid is deduced by considering the effects of seepage force and geological interface. Then, limit support pressure is derived according to the limit equilibrium state of the trapezoid. Finally, the comparison of the predicting results of the proposed limit equilibrium model with the results of the numerical simulations and the existing failure mechanism is performed, which indicates that the proposed trapezoid-prism model can well predict the tunnel face stability traversing the fault fracture zone with high hydraulic pressure.

The highest potential transmissivities of fractures in fault zones: Reference values based on laboratory and in situ hydro-mechanical experimental data

The transmissivity of a fracture can be related to fracture roughness (JRC0), initial aperture (E0), effective normal stress (σ'n), and tensile strength (σt) of the intact rock, based on the Barton–Bandis model and their data, and the transmissivity (or E0) can increase by shear-induced dilation. Previous studies revealed that the transmissivities of fractures in fault zones, detected as flow anomalies (highly transmissive zones) during borehole investigations at six sites, decrease uniformly with an increasing effective mean stress normalized to σt. If this uniform change in transmissivity is explained by σ'n-dependent fracture-normal displacement following the Barton–Bandis model, those transmissivities represent the upper limit of transmissivities of fractures in fault zones that can increase by shear-induced dilation. To verify this possibility, the E0 of fractures was estimated using those transmissivities, σt, and possible JRC0 and σ'n. Then, using this estimated E0, the changes in transmissivity were simulated, varying σ'n. The results reproduced very well the observed uniform change in transmissivity. The estimated values of E0 are tens of micrometers to a few millimeters, which can occur by slight shear displacements (e.g., 0.05–2.00 mm) during shear-induced dilation, easily achievable in fault zones. Thus, the requirements for the highest transmissivities are slight shear displacements and no/limited fracture-sealing rather than large shear displacements. Transmissivities in fault zone fractures that have already reached the highest transmissivities do not change significantly by shear displacement, while the transmissivities of fractures sealed by mineral filling can increase by orders of magnitude, as confirmed by recent fault-stimulation-experiments.

Analysis of Water and Mud Inrush in Tunnel Fault Fracture Zone—A Case Study of Yonglian Tunnel

Water and mud inrush disaster is easily induced during tunnel construction through water-rich fault fracture zones. In this paper, based on the field data, the process of water and mud inrush in tunnels is introduced in detail, and generation conditions and evolution mechanisms have been analyzed. Results show that the key factors of water and mud inrush include poor strata lithology, abundant groundwater and tunnel excavation disturbance. These key factors provide material conditions, source power and start-up conditions for water inrush, respectively. The evolution process of water and mud inrush can be divided into three stages: generation, development and occurrence. During generation stage, a seepage channel expands continuously, with water flow increasing gradually until a large range of loose zone is formed. During the development stage, a large amount of groundwater and a large range of softened muddy rock mass accumulates around the tunnel. During the occurrence stage, the “protective layer” and initial supports suddenly lose stability and are destroyed. These research results could provide some references for the effective prediction and forewarning of similar engineering disasters.

Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

Failure Mechanism Analysis and Support Technology for Roadway Tunnel in Fault Fracture Zone: A Case Study

This paper introduces a case study on the failure mechanism and support design of a roadway tunnel in the fault fracture zone of the 106 mining area in the Yuandian no.2 coal mine. Based on the on-site geological conditions (in-situ stress test, borehole television imaging, and lithological analysis), the failure mechanism of the roadway tunnel in the fault fracture zone was studied. The test results showed that the high tectonic stress, fractured rock, and poor lithology are the primary reasons for the roadway instability. According to the support principles of grouting reinforcement, pre-reinforced support, and rational support range, a new type of combined support technology was proposed, including advanced grouting, grouting bolts, and grouting anchor cables. A 100 m roadway section was selected for field testing using the new support scheme, and detailed deformation monitoring was performed. Monitoring results showed that the roadway deformation under the new support was significantly reduced. During the roadway excavation process, no roof collapse phenomenon occurred, and the safety of roadway excavation was ensured. This successful case provides an important reference for similar roadway projects in the fault fracture zone.

Effect of the Location of Fault Fracture Zones on the Stability of Symmetrical Submarine Tunnels

In this paper, we aim to reveal the influence of fault fracture zones on the stability of submarine tunnels and the surrounding rock under different water and drainage measures. Firstly, four typical working conditions of submarine tunnels intersecting with fault fracture zones were selected. On the basis of the typical cross section of the intersections of submarine tunnels and faults, they were divided into four working conditions. Then, the displacement and plastic zones of the surrounding rock of the tunnel were studied, and the stability of the rock surrounding the submarine tunnel was discussed. This research structure indicates that the bending moment and axial force of the lining structure of the submarine tunnel increase with increasing sealing degree, but the safety factor exhibits a downward trend. When the fault fracture zone goes through the section above the tunnel axis, the bending moment and axial force at the lining vault are greater than the other working conditions, and the displacement of the surrounding rock at the vault and spandrel is prominent. When the fault fracture zone completely passes through the tunnel, the safety factor of the lining structure is at its lowest, and the displacement of the surrounding rock at the arch waist develops laterally. When the fault fracture zone passes through the part below the tunnel axis, the arch foot displacement converges significantly, and the surrounding rock displacement exhibits a downward inclination. In addition, the plastic zone is mainly developed in the arch and the shoulder. These research results provide a reliable reference for tunnel design and excavation support.

Characteristics of Fault Fracture Zones of the Activated Shionohira Fault and the Non-Activated Kuruma Fault of the 2011 Fukushima-ken Hamadori Earthquake, Northeast Japan

Characteristics of Fault Fracture Zones of the Activated Shionohira Fault and the Non-Activated Kuruma Fault of the 2011 Fukushima-ken Hamadori Earthquake, Northeast Japan

Evolution of delayed water inrush in fault fracture zone considering time effect

The flow properties of a water inrush from a fault fracture zone are important for mining engineering applications. Numerous fault-delayed water inrush cases were analyzed, and it was concluded that the delayed groundwater bursting evolution in the fault fractured zone can be divided into two phases: (i) the activation of faults and (ii) the fault-weakening seepage. Three flow equations that indicate the flow variability of three flow stages in the second phase were established. A finite-element numerical model was established, and the evolution rules of the groundwater flow pattern under the time effect were obtained. Finally, the water inflow over time was calculated. The results were consistent with in situ measurements of the water inflow, which verifies the rationality of the theory.

Crustal Structure and Seismogenic Background Beneath Zhumadian, Henan, China: Evidence from Magnetotelluric Data

Earthquakes threaten people’s lives and property, especially when cities are affected. Thus, detecting the crustal structure and seismogenic background beneath cities is important if we are to lessen losses. Because the city of Zhumadian in Henan Province, China, is a well-developed area with a large population, more attention should be paid to earthquake detection. We therefore collected data from 15 magnetotelluric (MT) stations along a 55-km survey line across Zhumadian. Then, subsequent to acquisition, processing, and data inversion, we developed a preferred electrical resistivity model. We accurately relocated original earthquakes during the period between 1981 and 2019 within the study area using the double-difference earthquake location algorithm. The relocated earthquakes were projected onto our preferred MT model. The new resistivity model presented here reveals that saturated rocks, including rocks with porosities of 4.31–19.69% and moderate salinities of 10 g/L pore fluid and/or graphite, form conductive features in the upper crust. We also show that aqueous fluids derived from metamorphic dehydration within the lower crust and mantle, not melts, constitute mid-crustal conductors and facilitate detachment structures. The data show that the relocated earthquakes are located adjacent to the boundaries between conductive and resistive areas as well as at the bottom of fault fracture zones. These outcomes imply that the combination of rock failure and fault slipping triggers earthquakes.

Seismic response of tunnel near fault fracture zone under incident SV waves

This study investigated the impact of a non-causative fault on the dynamic response of a nearby lined tunnel under the incidence of plane SV waves using the indirect boundary element method. The effects of several critical parameters, such as the incident frequency, the inclination degree of the fault, the distance between the fault and the tunnel on the hoop stress of the lined inner and outer walls, were explored intensively. The numerical results indicated that the non-causative fault could significantly change the hoop stress distribution of inner and outer surfaces of the tunnels. In general, for the vertically incident seismic waves, when the tunnel was located in the foot wall (under the fault), the hoop stress within the tunnel was significantly greater than that of the tunnels in the non-fault half space, with an amplification factor of up to 117%. The amplification effect became more pronounced as the fault dip angle increased. However, when the tunnel was located in the hanging wall (above the fault), the non-causative fault could produce a significant shielding effect on the dynamic response of the tunnel under high frequency wave incidence, with the reduction of hoop stress being up to 81%. For low-frequency waves, though, the fault could lead to an increase of the hoop stress of the tunnel of up to 152%. The research results will provide a reference for the seismic design and safety protection of underground structures in non-causative fault sites.

Ant Colony Optimization Inversion Using the L1 Norm in Advanced Tunnel Detection

During the construction of the tunnel, there may be water-bearing anomalous structures such as fault fracture zone. In order to ensure the safety of the tunnel, it is necessary to carry out advanced tunnel detection. The traditional linear inversion method is highly dependent on the initial model in the tunnel resistivity inversion, which makes the inversion results falling into the local optimal optimum rather than the global one. Therefore, an inversion method for tunnel resistivity advanced detection based on ant colony algorithm is proposed in this paper. In order to improve the accuracy of tunnel advanced detection of deep anomalous bodies, an ant colony optimization (ACO) inversion is used by integrating depth weighting into the inversion function. At the same time, in view of the high efficiency and low cost of one-dimension inversion and the advantages of L1 norm in boundary characterization, a one-dimensional ant colony algorithm is adopted in this paper. In order to evaluate the performance of the algorithm, two sets of numerical simulations were carried out. Finally, the application of the actual tunnel water-bearing anomalous structure was carried out in a real example to evaluate the application effect, and it was verified by excavation exposure.

Risk assessment for landslide of FAST site based on GIS and fuzzy hierarchical method

In the context of circular high steep slopes within a 500-m aperture spherical radio telescope (FAST) site in Pingtang, Guizhou Province in southwest China, a pertinent small-area karst landslide risk assessment is carried out based on geographic information system and fuzzy analytical hierarchy process. Results show the following: (a) The AUC value of hazard assessment is 0.826, which means a good adaptability and a relatively higher accuracy, and the AUC value of risk assessment is 0.803, which means the accuracy of landslide risk assessment is within acceptable level. (b) The slopes of FAST area are generally at the medium or low-hazard level, with high-hazard region accounting for only 3.17% of the total area. High-hazard regions are primarily found in two locations, one is located in nearby the Guangmingding slope where the dangerous rock masses were basically cleared away, the other is located in nearby feed tower 5H where potentially unstable rock masses were reinforced to improve the stability of slope. (c) High-vulnerability area of FAST accounts for approximately 43.62%. As a direct result, the high-risk area of FAST site accounts for 34.36% based on the overlap of vulnerability and hazard. The high risk is concentrated in the surrounding areas such as the feed tower, support pillar, and telescope mirror, and the high-risk area overlaps most of the fault fracture zone in the study area. Our findings provide meaningful references for landslide prevention and monitoring of FAST areas and are instructive in theory and practice.

Tunnel Collapse Mechanism and Its Control Strategy in Fault Fracture Zone

Complex geological conditions such as fault fracture zone will have a significant adverse impact on tunnel engineering, and collapse, large deformation, and other problems are prominent. The research on the large deformation mechanism and control of tunnel crossing fault fracture zone can provide guidance for tunnel safety construction. Based on the Jingzhai tunnel, combined with geological analysis, theoretical research, numerical simulation, and other means, this paper studies and analyzes the large deformation mechanism of the tunnel. The control effect of different advanced reinforcement measures is studied.

Three-dimensional resistivity structure in the hydrothermal system beneath Ganzi Basin, eastern margin of Tibetan Plateau

The Ganzi geothermal system located in the eastern margin of Tibetan Plateau, is found to distribute the strongly active surface manifestations, such as boiling springs, and fumaroles. The resistivity structure of high-temperature hydrothermal system beneath Ganzi Basin has been obtained from the three-dimensional (3-D) magnetotelluric inversion of 103 broadband MT stations in period range from 320 Hz to 1000s. The MT stations have an average spacing of 500 m and completely cover the Ganzi Basin. The detailed 3-D resistivity structure which the authors found has been described characteristics of the shallow and deep geothermal reservoirs as well as to correlate with the structural features present in the Ganzi Basin. The shallow geothermal reservoir is mainly composed of the sedimentary layer, with a depth of 300−800 m, while the deep geothermal reservoir mainly consists of the fault fracture zone with a depth of 1−5 km. The porosity of the geothermal reservoirs is in the range 5–10 %, estimated based on the resistivity model. The Ganzi fault provides the pathway for transport heat and the circulation of geothermal fluids. The possible heat source for the Ganzi geothermal system has been attributed to tectonic deformation heating in the crust, radiogenic heat production derived from the decay of U, Th and K, and heat from the mantle.

Study on Mechanical Properties and Control Technology of Surrounding Rock in the Fracture Zone of a Roadway

We aim at the problem of the large deformation and difficult control of surrounding rock when passing through a fault fracture zone in the centralized rail transportation lane along the south wing of Xinyi Coal Mine; the stress environment and failure mechanism of surrounding rock are analyzed through field investigation, numerical simulation, and field industrial test. The instability of the surrounding rock in the fault fracture zone was considered to be the result of the joint effect of the surrounding rock fracture development, lithology differences, water gushing occurrences, low strength of the original support, high in situ stress, and fault-related tectonic stress. Rock blocks are collected on site at the fracture zone, and the remoulded samples are prepared for mechanical experiments in the laboratory. The basic mechanical parameters of the roadway passing through silty mudstone, sand-mudstone interlayer, and fine sandstone were analyzed. A three-dimensional model is established to analyze the distributions of the stress, deformation, and plastic area in the surrounding rock mass after the tunnel passes through, considering both a single-rock mass and a multilayer-rock mass. Based on the above analysis, the “closed support + shotcrete + grouting + anchor mesh cable coupling support” is proposed. Three stations were arranged on site to observe the mine pressure, and the field industrial test shows that, within the 100 days of observation, the maximum roof-to-floor convergence is 38 mm, while the maximum horizontal convergence is 56 mm. The overall reinforcement effect of the roadway is good, the surface is smooth, and there is no phenomenon of concrete cracking and bolt fracture.

Study on anti-seepage treatment and seepage control effect of core dam foundation curtain of the fault fracture zone in Xinjiang province

The treatment effect of dam foundation fracture zone is related to dam safety, A case is provided base on the fault-fracture zone with large permeability in a core dam foundation in Xinjiang province, The dam foundation seepage model is established with the seepage finite element method base on The test results of post-grouting in the fault-fracture zone, simulation working conditions of different curtain depth and permeability indicate that significant impervious effects of compound grouting and depth-curtain at fault zone, leakage is less in the dam foundation and seepage stability meet the requirement on the riverbed overburden in the downstream, The results are high reference value for similar projects.

Progressive failure and friction motion characteristics of contact surface of composite rock mass

The structural planes existing in natural rock mass can control the failure process of rock mass. Based on the progressive response-stability failure process of a single fracture interface, the relationship between progressive response and material failure of composite rock mass is discussed. The method of friction contact plane analysis in PANDAS numerical analysis software is applied to explore the correlation between the movement and mechanical properties of composite rock mass under external forces. The motion index is mainly represented by the sliding speed and distance of the contact surface, while the mechanical properties are mainly explained by the normal contact force, the friction of the contact surface, the friction coefficient and the material damage. The relationship between these six variables illustrates the progressive response relationship during the movement of the composite rock mass. Based on the static and progressive characteristics of the progressive response process of deep rock mass engineering, the failure evolution law and energy dissipation law of composite rock mass loading process and the synergistic failure characteristics of composite rock mass are discussed. The scientific problems that need to be studied in the structure, deformation and failure of deep rock mass are put forward. The reference is provided for the failure law of fractured rock mass in water inrush process under the influence of mining, as well as fault plate material and the study of mechanical state of rock mass in fault fracture zone.

Water Inrush Risk Assessment Based on AHP and Advance Forecast Approach: A Case Study in the Micangshan Tunnel

Water inrush is a serious geological disaster in tunnel. For the effective prevention and control of the occurrence of water inrush, a static‐dynamic water inrush risk assessment method is proposed by considering the Micangshan tunnel as an example. First, four possible types of water inrush phenomenon are identified based on the geological and hydrogeological conditions of the tunnel: water inrush in water‐bearing cracks, fault fracture zones, karst pipelines, and karst caves. Next, evaluation indexes that affect water inrush are determined. By combining the index weight value calculated by analytic hierarchy process (AHP) with the index quantitative value, the static water inrush disaster evaluation model is established, which provides a basis for tunnel design. Finally, with the combination of the static evaluation model and advanced forecast method, a dynamic risk prediction method of water inrush is established, which provides guidance for safe construction. The results confirm that the proposed method is a reliable theoretical basis for early assessment and prediction of tunnel water inrush disasters.

Quantitative Study on Empirical Strength Parameters of Extremely Fractured Phyllite Based on Fractal Theory

Extremely broken phyllite is widely distributed and is easily seen in engineering construction. Aiming at the problem that the empirical strength parameters of extremely broken surrounding rock have intense subjectivity and significant difference, based on the characteristics of the existence of particle group state in the highly broken surrounding rock, the fractal theory and large‐scale direct shear test are used, the fractal description of polar broken phyllite samples with different pile numbers in fault fracture zone of Qinyu Tunnel is carried out, and the fractal dimensions and empirical strength parameters of each sample are determined. Based on a fractal description and large shear test, the functional relationship between fractal dimension and empirical strength parameter of extremely broken phyllite in fault fracture zone is established, and the quantitative value method of empirical strength parameter determined by fractal dimension of extremely broken phyllite is given, which provides ideas for the quantitative value of empirical strength parameter of the similar extremely broken surrounding rock.