Fractures In Strata Research Articles

Multi-source seismic geological ahead-prospecting for TBM tunneling in fractured strata: improved strategies and case studies in a water conveyance project, China

Multi-source seismic geological ahead-prospecting for TBM tunneling in fractured strata: improved strategies and case studies in a water conveyance project, China

Detect Multiple-Set Fractures by Crosshole Seismic Tomography for use in Environmental and Engineering Geophysics

Crosshole seismic tomography (CST) could be used to reconstruct the velocity section of media between two holes. One commonly used attribute of fractures reflected in seismic waves is the anisotropy of velocity. Therefore, in this study, the multiple symmetry axes of anisotropic media are utilized to simulate the multiple-set fractures in strata. The anisotropic algebraic reconstruction technique (AART) and anisotropic simultaneous iterative reconstruction technique (ASIRT), extensions of isotropic ART and SIRT, are developed to detect the medium with multiple symmetry axes and strong anisotropy. Full coverage of ray paths in azimuth is needed for these techniques. The performances of these techniques are tested by numerical and physical modeling. Testing results show that the proposed AART and ASIRT methods can successfully detect the multiple-set fractures when the observed data exceeds the estimated parameters. However, in complexly anisotropic and heterogeneous media, the estimation error will significantly increase when using these techniques if the number of estimated parameters is equal to or greater than the number of observed data.

Study on the Synergistic Effect of Primary Support and Surrounding Rock of Large Buried Depth Tunnel in Soft and Fractured Strata

The soft and fractured strata can cause significant deformation of surrounding rock during tunnel excavation. This study analyzes field monitoring test results and compares numerical simulations from the third bid project of the Dali I section construction within the water diversion project in central Yunnan to address the issue of significant deformation following tunnel excavation in soft and fractured strata. It proposes an optimized support scheme consisting of a densified steel arch and enhanced initial support strength and stiffness. In addition, the research investigates support effectiveness considering varying support strengths and steel arch ring spacing. The study findings indicated the following: (1) The tunnel traverses soft and fractured strata, causing unevenly distributed vertical convergence deformation around the cavern. The maximum settlement occurs at the crown, showing pronounced nonlinearity. (2) The maximum stress in the steel arch is concentrated at the arch crown, measuring −19.02 MPa. The arch remains compressed, with stress decreasing from the crown to the waist. (3) The axial force in the anchor bolt reduces from the crown to the arch’s waist on both sides. As the depth of the rock mass increases, the axial force in each anchor bolt decreases and the tension state is maintained. The maximum axial force reaches 46.57 kN. (4) The maximum displacement decreases from 4.21 to 0.15 cm after the optimized support structure is implemented, demonstrating the optimization scheme’s effectiveness. Future constructions can refer to this scheme and make necessary adjustments based on various terrain conditions to ensure safety.

Research and Practice of Full Plugging Groundwater Technology in Water-Rich TBM Diversion Tunnel

To solve the problem of water discharge in the tunnel when the tunnel passes through the low-pressure water-rich fractured stratum, the grouting plugging technology of the low-pressure water-rich tunnel is studied. The grouting plugging scheme is optimized through the field test. The post-validation research is numerically simulated and the parameters of the complete sealing grouting ring are analyzed. The results show that: 1) Full plugging grouting can effectively improve the funnel effect of precipitation with good water plugging effect. With the decrease of the permeability coefficient and the increase of the thickness of the grouting ring, the flow velocity around the tunnel and the water discharge in the tunnel both decreased significantly, with a trend of “rapidly decreasing-slowly decreasing”. Considering various factors, an optimal grouting ring thickness between 1.5 m and 2.5 m and a reasonable range for permeability coefficient between 2.07 × 10−6 cm/s and 4.14 × 10−6 cm/s are considered appropriate. 2) The use of cement-water glass system full plugging grouting for water plugging, can significantly reduce the discharge of water, only partial dripping or even dripping state. Using polyurethane foam single point method to systematically block grouting and water plugging can only changes the discharge path and hardly attenuates water discharge.

A rational construction method and deformation control system of tunnelling in extremely soft and fractured chlorite schist medium

The large deformation disaster is a major engineering problem that has not been sufficiently addressed in tunnelling in soft rock mass. Previous investigations essentially focused on large deformation control methods or measures for soft rock masses, with little attention to extremely soft and fractured rock masses. As a result, we here conduct a comparative field test of the cross diaphragm (CRD) method and the three-bench annular excavation method with reserved core soil. The case study site is the Lianchengshan Tunnel of the Baoji-Hanzhong Highway in Shaanxi Province and we could establish an appropriate construction methodology for the chlorite schist stratum. Following that, the mechanism of large deformation of extremely soft and fractured surrounding rocks is revealed, and the compatibility of the main methods of controlling large deformation in very soft and fractured layers is discussed. Thereafter, a suitable large deformation control approach and support system for extremely soft and fractured rock strata is proposed. Based on engineering practice, we also elaborate on the cooperative bearing principle of each component of the support system in detail. Our findings indicate that the CRD method has no outstanding effect on the deformation control of the Lianchengshan Tunnel. The three-bench annular excavation method with reserved core soil outperforms the CRD method in controlling the settlement of the tunnel arch, reducing the construction risk, and ensuring the construction progress. In general, it works better than the CRD method and should be preferred in tunnel construction. Considering the “soft” and “fractured” geological features of chlorite schist in the Lianchengshan Tunnel, a multi-layered and staged support system is developed with strengthening support as the core and stress release, as well as section optimization as the assistance. Under the support system, the tunnel deformation is effectively controlled. Compared to the single-layered primary support scheme via the same constructional approach, the maximum values of settlement and convergence of the measured section are reduced by 51 % and 56 %, respectively. The achieved results reveal that the overall stiffness of the single-layered primary support consisting of HW200 × 200 steel rib is about 3 times that of I22b steel rib, and the overall stiffness of the double-layered primary support consisting of HW200 × 200 steel rib is about 2.7 times that of a single layer. The results of this study not only contribute to the control concept and method of large deformation, but also provide a novel pathway and a substantial reference to solve the problem of control of large deformations of tunnels in extremely soft and fractured strata.

Extension of discontinuous deformation analysis method for coupled reactive transport problems

Fluid flow and the accompanying reactant transport in fractured rock masses are key concerns for understanding the performance and safety assessments of subsurface engineering problems, particularly in waste disposal and tunneling project. Nevertheless, predicting solute transport behavior remains a great challenge due to computational sophistication and numerical instability if numerous discontinuities and chemical reaction is taken into consideration. In this study, the partial differential equation describing the reactive transport of solute is solved and coupled with the Discontinuous Deformation Analysis (DDA) method. First, the extended DDA method is validated by simulating an indoor dissolution test on the single rock fracture. Then, it is applied to establish a discrete fracture network (DFN), and be further employed to investigate the reactive transport behavior and geochemical evolution in a case of acidic mine drainage in a fractured stratum. The study analyzes the effects of stress field, reaction speed, and diffusion coefficient on the reactive transport. Numerical results indicate that the conclusions reached are consistent with current findings, combined with its prominent features in simulating the mechanical behavior of discontinuities, demonstrating that the extended method is a promising tool for the coupled hydro-mechanical-chemo analysis.

Key Technology of TBM Excavation in Soft and Broken Surrounding Rock

Tunnel Boring Machine (TBM), as the most advanced construction equipment for rock tunnels, has been widely used in railway tunnel engineering, water conservancy tunnel engineering, urban rail engineering, coal mine roadway engineering, and other fields. However, the disadvantage of TBM lies in its poor adaptability to complex geological conditions, resulting in a significant reduction in tunneling efficiency and even major accidents due to improper TBM construction measures, especially in weak and fractured strata, which pose a great challenge to TBM construction. Therefore, it is necessary to study the key technologies of TBM construction in weak and fractured strata in order to improve the efficiency of TBM construction. Based on the background of the TBM construction of the Gaoligongshan Tunnel on the Southwest Darui Railway, this paper provides a reference for safe and efficient TBM excavation in weak and fractured areas through research on equipment, excavation parameter control, and construction measures.

A new method for predicting S-velocity, anisotropy and fracture parameters in horizontal transverse isotropy media

S-velocity, anisotropy and fracture parameters are of great importance in the development of horizontal transverse isotropy (HTI) reservoirs for the fracture attribute analysis and determination of fracture location. Increasing attention is gradually being paid to the accuracy of predicting them, especially for some well logs missing these parameters. Rock physics modelling is the one of the effective tools. For complex background media, simple spherical pores are no longer applicable. Multi-types of pores, the main component of the complex isotropic background, need to be considered using effective media model. For anisotropic media, Hudson and Schoenberg models are the classic theories for HTI fractures. However, the pitfall of Hudson model resides in the restrict of small crack density assumptions, that is not useful for most fractured stratum with higher crack density. To improve the applicability and quality of Hudson model, Padé approximation, based on Hudson first and second order expansions, is introduced. With the combination of the approximation and Schoenberg model, the new relationship of the fracture parameters and stiffness matrix are derived. On basis of this relationship, S-velocity, anisotropy and fracture parameters can be calculated. Compared to the Hudson combination method, the new proposed method shows significant advantages on the accuracy and validity of the estimated parameters for the model analysis and real well data in higher crack density, which can provide favourable data support for the exploration and development of fractured reservoirs.

Study on fracture characteristics of high key stratum and development law of water and gas transport channels in Datong mining area

AbstractThe study aimed to examine the problem of the abandoned mine water and harmful gas downflow from the overlying Jurassic of Tongxin coal mine in the Datong mine area, China. A theoretical analysis, physically similar simulation, and field measurement were used. A fracture aperture mechanical model based on key stratum fracture was used to analyze the main influencing factors of high key stratum fracture and rotation. We also compared the fracture characteristics, displacement changes, and fracture development of the high‐key stratum. The results showed that: (1) the main influencing factors affecting the fracture characteristics of high key stratum are the thickness of coal seam mining, the thickness of high key stratum, and the distance between high key stratum and 3–5# coal seam. (2) When both 8309 and 8207 working faces reached critical mining, the high key stratum was fractured and the mining‐induced fracture was developed to the 14# coal seam goaf, which coincided with the field tracer gas measurement results of the 8309 working face, indicating that when the distance between 3 and 5# coal seam and the overlying 14# coal seam goaf is less than 220.9 m, 8309 and 8207 working faces will form the channel for water and gas inrush. (3) The fracture length of the high key stratum above the working face of 8207 is smaller than that of 8309, and the pipeline of water and gas transport channel formed after the critical mining of its working face is larger and wider, which makes it more prone to the disaster of water and gas inrush. Before mining the extra‐thick coal seam, it is necessary to detect and pump the water and gas in the 14# coal seam goaf, so as to avoid the water and gas inrush hazard during the coal seam mining.

Investigation of the Evolution of Stratum Fracture during the Cavity Expansion of Underground Coal Gasification

The evolution of fracture zone controls the safety of underground coal gasification (UCG) in terms of gas emission and water leakage. In order to understand the fracture propagation in the confining rock of a UCG cavity with various influence factors, this paper implemented a set of numerical models based on different geological and operating conditions. Analysis was implemented on the mechanism of fracture propagation and its evolution characteristics, suggesting that (a) continuum expansion of the cavity leads a near-field fracture circle in confining rock initially, followed by the roof caving and successive propagation of shear band. (b) The key observed influence factors of fracture propagation are the grade of confining rock, overburden pressure, dimension of the cavity and gasifying pressure, the linear relationships between them, and the fracture height. Additionally, the fracture depth in the base board was mainly caused by tensile fracture. (c) A model was proposed based on the evolution of fracture height and depth in roof and base board, respectively. Validation of this model associated with orthogonal tests suggests a good capacity for predicting fracture distribution. This paper has significance in guiding the design of the gasifying operation and safety assessment of UCG cavities.

Fracture distribution in overburden strata induced by underground mining

AbstractCoal‐mining activities give rise to a series of ecological environmental problems, such as ground settlement and groundwater pollution. In fact, they are mainly caused by mining‐induced fractures. Hence, it is necessary to study the mining‐induced fracture distribution to identify the behavior of rock mass movement. However, the fractures in overburden strata cannot be directly measured owing to the special condition. Therefore, the majority of previous studies are based on experiments or experience. For this reason, this study first used a discrete element method to simulate the shape of mining‐induced fractures in overburden strata. Then, a geophysical tool of transient electromagnetic method (TEM) was used to investigate the mining‐induced fracture distribution. Based on the low‐resistivity anomaly area, the water‐rich area in overburden strata was analyzed to be mainly caused by fracture seepage. Through the mutual authentication between numerical simulation and TEM results, the mining‐induced fractures in overburden strata were explored. This study can enhance the understanding of mining‐induced fracture distribution on the one hand and guarantee the coal mining safety on the other, thus guiding the coordinated development between coal mining and environmental protection.

Approximate equations of PP-, PS1- and PS2-wave reflection coefficients in fluid-filled monoclinic media

SUMMARY Studying the seismic reflection characteristics of fractured strata can provide a theoretical basis for the exploration of unconventional oil and gas reservoirs. When several (more than 1) sets of oblique vertical fractures that are rotationally non-invariant are embedded in an isotropic or vertical transversely isotropic (VTI) background, the stratum exhibits equivalent monoclinic anisotropy. However, the exact algorithm of the reflection and transmission coefficients of monoclinic media is algebraically complicated and difficult to use in theoretical analysis and application. In this paper, under the assumption that fractures are filled with fluid, based on the linear-slip theory, we solve the phase velocities of monoclinic media from the Christoffel equations by using the first-order perturbation method and then derive the recursive equations of the PP-, PS1-, and PS2-wave reflection coefficients. The theoretical analysis results show that for an interface of monoclinic media, when setting a series of fracture densities for the lower medium, the relative error of the proposed approximate reflection coefficient of the PS2 wave is less than 5.7 per cent, and those of the PP and PS1 waves are generally less than 2.5 per cent for a wide range of anisotropy strength and impedance contrast; when setting a series of fracture densities for the upper medium, the relative error of the proposed approximate reflection coefficients of the PP, PS1 and PS2 waves are generally less than 6.4, 5.1 and 4.5 per cent, respectively. When degenerated to the horizontal transversely isotropic model, compared with Rüger's approximation, at incidence angles of 1–30°, the two results are almost coincident, but the proposed approximations are closer to the exact solutions.

Indosinian Supergene Karsts in the Qaidam Basin, NW China: Evidence from Paleo‐tectonics, Paleoclimates and Paleo‐fluids

AbstractIn the Qaidam basin, Northwest China, karstic fissures and caves are developed in many places of the Lower Carboniferous Chengqianggou Fm. and Huaitoutala Fm. and the Upper Carboniferous Keluke Fm., in which are found typical karstic marks. Several suites of transgressive carbonate rocks formed during the Late Paleozoic epoch are preserved as reservoir properties in the Qaidam basin, with paleokarst‐related cave‐fractured diagenetic structures, which provided the basis for cave‐fractured development. Carbonate and clastic samples covering or infilling caves and fractures in Carboniferous strata were collected in the eastern Qaidam basin to focus on petrological and geochemical analyses of karst and infilling materials. Based on this study of the Caledonian diagenetic sequence, and bitumen‐infilled inclusion temperature and burial history, there existed a continuous atmospheric freshwater leaching process, which played a critical role in the construction of the Carboniferous supergene paleokarsts. Investigation and survey of the tectonic setting, plate paleolatitude and river development intensity also proves that there was abundant rainfall during the Indosinian period, after which the strata undergo a shallow burial process. Because Indosinian period is earlier than the key timing of hydrocarbon‐generation of the Carboniferous source rocks, this kind of karst reservoir has potential significance for oil‐gas resources in this area.

Building a geothermal formation model using microtremor array measurement

A comprehensive understanding of the internal structure and construction of a geomechanical formation model play an important role in developing and using geothermal resources. Formation models help in identifying the channel and cycling modes of the heat flow. Due to urban sprawl and development, constructing a formation model of geothermal resources based on data from traditional geophysical methods is challenging. The microtremor survey method (MSM) is adopted to obtain critical information in Jimo, which is famous for rare seawater geothermal resources in China. Three microtremor survey lines are deployed to identify subsurface structures up to 2 km into the ground. Dispersion curves of Rayleigh waves with frequencies from 0.4 to 10 Hz are extracted using the spatial autocorrelation method. An empirical equation is adopted to obtain the apparent shear wave (S-wave) velocity of each survey point and to plot the apparent S-wave velocity sections. The obtained sections reveal the development of two interacting faults, which form a channel for the heat-flow cycle. Two conceptual models are established to depict the formation and cycling modes of seawater geothermal resources in Jimo, based on the results and analysis. Our model will help verify the geothermal system and scientifically guide the development of unique geothermal resources. Moreover, the developed model verifies that MSM is effective and dependable for identifying fracture zones and strata.

Research on Structural Evolution and Microseismic Response Characteristics of Overlying Strata during Repeated Mining of Steeply Inclined and Extra Thick Coal Seams

Abstract Repeated mining is the main factor that leads to development, propagation, and eventual deformation of the overlying strata fissures in the steeply inclined and extra thick coal seams (SIETCS). The evolution of the overlying strata structure is closely related to microseismic events in a mine. As the mining depth increases, the evolution rules of the overlying strata structure become more complicated and can easily induce dynamic disaster accidents. To solve these problems, this paper established a physical similarity simulation model. Microseismic monitoring equipment was used to study the relationship between the evolution of the overlying strata structure and the energy-frequency of microseismic events. On the basis of the principle of quantitative seismology, the response relationship between the overlying strata structure and the microseisms at different mining stages was compared and analyzed from a quantitative perspective. The characteristics of cumulative apparent volume, energy index, and microseismic b value were used to reveal the precursor characteristics of overburden instability and failure. The results showed that due to the occurrence characteristics of coal seam, the distribution characteristics of rock stratum stress, and the effects of mining disturbances, the energy accumulation-release period after instability failure of the overlying strata induced by shallow mining was longer than the energy accumulation-release period induced by deep mining. And the deep coal and rock mass had a periodic “balance-instability-rebalance-instability again” dynamic evolution process under the disturbance of repeated mining. In the working face mining, the slope of the accumulative apparent volume ΣVA curve suddenly increased, and the energy index EI gradually decreased at the late peak period, which indicated the deformation and failure of overburden. However, the b value of the microseismic event presented the precursory characteristics of rock stratum fracture that gradually increased and then changed drastically.

Environmental geochemical maps of harmful trace elements in Chinese coalfields

Coal resource utilization and environmental protection is a critical global issue. This study aims to address the need for geochemical maps of harmful trace elements (HTEs) in Chinese coalfields and to extract scientific information from these maps. Based on data extracted from the Trace Elements in Coal of China database, geochemical maps of As, Cd, Cr, F, Hg, Ni, Pb, and Se in Chinese coalfields were generated, for the first time, using the ArcGIS platform. Differences in regional HTE concentrations were attributed to multiple factors, including the type of coal-forming environment, terrigenous debris, and groundwater effect. However, on a national scale, the spatial distribution pattern of HTEs in coal is affected by the abundance of elements in the earth's crust. Herein, the enrichment anomaly of HTEs in coal were found to be significantly correlated with fault locations, and hydrothermal fluid action was characterized as the primary causal factor. HTE abundance in coal is the result of geochemical cycles in the earth's crust. Additionally, stratum fracture zones may serve as conduits and material sources for the migration of HTEs from deep layers to shallow layers, including coal seams. This study provides an essential reference for extensive map applications and coal environmental management while advancing our understanding of the spatial distribution patterns of chemical elements in coal.

Thief-zone plugging mechanism and application of calcite particles in fractured formations

Lost circulation in fractured strata during drilling incurs additional costs and leads to difficulties in promoting drilling safety and efficiency. Plugging-while-drilling is a feasible means to address lost circulation in fractured formations. The particle size distribution (PSD) of plugging particles is determined empirically; therefore, it is often not matched with the fracture sealing requirement. This study investigates the lost circulation mechanism of fractured strata and identifies a calcite particle-based material as the preferred plugging agent. The plugging mechanism and the design of PSD and particle concentration are demonstrated. Based on the concentration, a plugging-while-drilling technique was developed for fractured strata. The results show that calcite particles tend to form the filling layer at the fracture inlet, which cuts off the leakage of the drilling fluid into the fracture, eliminating the drilling fluid pressure applied on the fracture surface. Thus, a stable sealing for the fractured formation is achieved, and the pressure-bearing capacity of the borehole wall is increased. The result also reveals the optimal mixing and concentration models for calcite particles with various diameters. The plugging technique based on calcite particles for fractured formations is applied in a field experiment. The results confirm that the technique can improve the chance of lost circulation prevention and thief-zone plugging in fractured strata and remarkably reduce both the event quantity of lost circulation and the volume of circulation loss. The findings of this research lay a theoretical basis to address lost circulation in fractured formations and, thus, have important practical significance.

Effects of Ground Fracturing with Horizontal Fracture Plane on Rock Breakage Characteristics and Mine Pressure Control

Ground fracturing is a new and effective method for controlling high-level hard roofs. In underground coal-seam mining, the maximum principal stress is horizontal stress, along with the direction of hydraulic fracture propagation. In this study, the control mechanism of the horizontal fracture plane formed by ground fracturing on the strata breaking span was analyzed theoretically, and the effect of the horizontal fracture plane on the structural characteristics, stress distribution, and impact effect of the high-level thick and hard strata (hereinafter referred to as THS) were numerically investigated. To verify the results, the 8218 working face in the Tashan coal mine of the Datong mining area was selected as a case study. The fractured stratum was 20 m thick and was separated vertically from the coal seam by 110 m. After ground fracturing, the extension range of the fracture plane was 216 m in length and 30–100 m in width, covering the working face. In situ monitoring of the mine pressure revealed that the bearing strength of the supports in the working face was weakened by the ground fracturing, and that the roadway deformation was significantly reduced, indicating that ground-fracturing technology plays an important role in controlling high-level THS and improving the mining environment of the working face.

Nonlinear deformation behaviors and a new approach for the classification and prediction of large deformation in tunnel construction stage: a case study

Reasonable evaluation and prediction of squeezing condition to avoid large deformation disaster in tunnels have been an important research issue. In order to accurately and rapidly predict the large deformation under complex geological conditions, this paper proposes a classification and prediction method for quick identification of large deformation in tunnel construction stage in a case study of Muzhailing Tunnel. Previous prediction methods of large deformation are usually based on the stages of geological survey and engineering design, and there are great difficulties in obtaining some prediction indexes in soft and fractured strata, such as uniaxial compressive strength. In this paper, the nonlinear deformation behaviors of surrounding rock were analyzed. The geo-stress condition, strata occurrence, rock strength, rock intactness and groundwater condition were chosen as evaluation indexes (including eight sub-indexes). The classification of large deformation was carried out based on the deformation statistics in Muzhailing Tunnel. And the fuzzy prediction was conducted and compared with other methods in engineering. The comparison results show that the new method has higher accuracy and applicability in predicting the large deformation of weak rock mass. This study provides a new approach for rapid identification and prediction of large deformation in tunnel construction stage.

Force chains in top coal caving mining

During the top coal caving mining, rock mass becomes fractured, and the physical status of top coal and overlying strata in the vicinity of the working face changes from continuous to discontinuous medium. It is considerably difficult to explain in theory the transition and action of mining-induced pressure in discrete top coal and fractured overlying strata. Force chain is a proper modality to describe loading transition in discontinuous and discrete media. By creating the photoelastic experiment apparatus of granular materials, trajectories of force chains in discrete top coal and overlying strata were observable by means of digital image processing technology. The characteristics and evolution of force chain network were elaborated on the basis of the theory of topology. Three configurations of force chains in overlying strata and top-coal were identified, namely, force chain, force loop and coupled force chain-loop, which were described in terms of Betti numbers of topological geometry. The study shows that there was a complicated force chain network composed by initial force in fractured strata. Top coal caving mining destroyed the equilibrium of the initial force chain network. Force chains developed and grew in the overlying strata in an arch-like form over the mining area, which transmitted the overlying loads to the front of the coal seam with advancing the working face. After mining the intensity of force chain dropped promptly inside of the goaf instead. According to the intensity of force, force chains were graded into strong, sub-strong and weak force chains. The strong force chains determined the load bearing capacity of the network, while the stability of the force chain network relied on the existence of weak force chains. The loading mechanism of top coal on the supports clarifies that low-pressure phenomena in top caving mining face were caused by the loss of laterally developed weak force chains, resulting in the reduction of strong force chains. Besides, the statistical features, such as force chain intensity, the force chain azimuth and contact force in the top coal and overlying strata, were manifested during the mining process. The study will help understanding of the mechanism of force chain formation and evolution in the fractured overlying strata and cracked top coal due to top coal caving mining.