Large Fracture Zones Research Articles

Research on energy-absorption active control method for large deformation of tunnel surrounding rock through multi-fault fracture zone

To study the problems of large deformation of surrounding rock and water-rich dense fracture zones during tunnel construction through fault fracture zone, the NPR (Negative Poisson’s Ratio) anchor cable control method is studied based on the Tabaiyi Tunnel. First, the mechanical deformation mechanism of surrounding rock is analyzed by geological conditions, lithology and strength of surrounding rock, and different surrounding rock zones are obtained. Then, based on the self-developed high-prestressed NPR anchor cable, the tunnel prevention and control measures are designed. The high-stress NPR anchor net technology is used to support the common large deformation and fault fracture zone, while in the water-rich dense fracture zones, the combined support of double gradient grouting technology and high-stress NPR anchor net technology is adopted. Finally, to evaluate the control effect of the support design scheme, the deformation of surrounding rock, the axial force of NPR anchor cable, and the pressure of steel arch are monitored in real time. The results show that the high-stress NPR anchor net technology can effectively control the large deformation of tunnel surrounding rock caused by fault zone. In addition, in the water-rich zones, the double gradient grouting technology can provide sufficient anchoring force for the NPR anchor cable. The two technologies control the maximum deformation of surrounding rock within 300 mm and realize the stability of tunnel primary support.

Calculation of thermal-hydraulic-mechanical response of a deep geological repository for radioactive used fuel in granite

The NWMO is considering siting a deep geological repository (DGR) for used fuel in a crystalline or sedimentary rock setting. One aspect of the long-term performance is the Thermal-Hydraulic-Mechanical (THM) response of the rock caused by the heat released from the high-level radioactive used fuel placed in the repository. In this paper, a method was proposed by building an integrated coupled THM model (Integrated model) to explore the importance of scale in modelling the host rock THM response in a crystalline rock setting. This integrated model included a site-scale domain of rock, with embedded large fracture zones (FZ). The full DGR geometry was represented by using large flat blocks to represent the bulk of the repository panels, with five placement rooms modelled explicitly. In one of these five placement rooms, a 9.24-m-long placement room section with 14 containers was modelled in detail. A preliminary study of the THM response of the rock mass around the proposed repository was performed using this model and its results were validated by comparing with accurate thermal response. The results from this integrated model were compared with the results from a near-field coupled THM model; and the comparison proves that the near-field model cannot give the correct THM response in a repository. To accurately model the THM response in a DGR, the proposed integrated model approach can be used to avoid the influence of the thermal, hydraulic and mechanical boundary conditions applied on near-field models.

Degradation of Typical Reverse Sand-Mudstone Interbedded Bank Slope Based on Multi-Source Field Experiments.

The bank slopes in the Three Gorges Reservoir area (TGRA) have experienced obvious deterioration under the action of the periodic fluctuations in the reservoir water level. Generally, laboratory tests have been used to reveal the evolution trend of the slope banks. However, this method has a certain degree of cross-scale problem, especially for the mechanical state in a complex environment. Therefore, in this study, we took the Yangjiaping bank slope in the TGRA as an example and proposed a comprehensive on-site detection method to further reveal the rock mass degradation phenomenon of this typical reverse sand-mudstone interbedded bank slope. Specifically, multi-scale laser scanning, cross-hole acoustic wave detection, and inclination measurements were performed to analyze the fractures, quality, and deformation of rocky banks. The results showed that the deterioration of the bank slope manifested as the expansion, deepening, and widening of the cracks, as well as the peeling off and loosening of rocky banks. Large-scale laser scanning revealed that the deterioration zone was deformed along large fracture zones and layers. Unlike limestone slopes, the intact sandstone underground might be degraded by changes in water. There are few inclinometers and no deformation or weak deformation, which requires long-term monitoring. The relevant research methods provide an important reference for determining the instability and failure trend of the reservoir bank slopes.

The Study and Application of In-situ stress Distribution in Ultra-deep Fractured-caved Carbonate Reservoirs

There are abundant oil and gas resources in carbonate reservoirs of Tarim Basin, and more than 90% come from fractured-caved carbonate. These fractured-caved reservoirs are formed by the superposition of large fracture zone and karstification. The fractured-caved bodies generally appear in strips along the fracture zone and are distributed irregularly in vertical and horizontal directions. In addition, the ultra-deep burial (6500-7500m) leads to strong heterogeneity of the reservoirs. This paper presents using horizontal wells to cross the fractured-caved bodies is an effective means to improve the economic profit of oil and gas field development. Based on geomechanical study, the development mechanism of high-quality fractured-caved bodies can be determined, the contradiction between multiple fractured-caved bodies and safe drilling can be solved, and the reservoir stimulation plan can be optimized. The study can provide quantitative data basis for well trajectory optimization, prediction of drilling safety window, determination of maximum build-up slope in deviated section, how to use one well to cross multiple fractured-caved bodies and optimization of reservoir stimulation program. The results show that the ultra-deep carbonate reservoirs are affected by four factors: faults, fractures, irregular karst and in-situ stress, resulting in strong heterogeneity and anisotropy of reservoir. Therefore, the probability of high-quality fractured-caved bodies encountered by well trajectory drilling in different directions and depths is different, and the stability of wellbore in different directions and deviation is also different. When the drilling trajectory crossed the fault zone, it experienced a large range of in-situ stress, so the stimulation effect varied greatly in different well sections. The geomechanical study of ultra-deep carbonate reservoirs shows that the strike of the fault is similar to the present in-situ stress, and it is easier to develop high-quality fractured-caved bodies in relatively weak stress environment. Through the study of the relationship between fracture system and in-situ stress, it provides a method to optimize the well trajectory in consideration of both high-quality reservoir and drilling engineering safety.On the basis of optimizing the best well trajectory, it can predict the safe drilling window more accurately, guide drilling through multiple fractured-caved bodies as much as possible, and implement reservoir stimulation effectively, laid a foundation for high-quality and high-yield drilling. So far, three carbonate reservoir oilfields’ 3D stress modelling have been completed and applied to well placement, drilling and development. The average oil production per well exceeds 300 tons under 6mm nozzle conditions.

Tectonic Structure and Relationship with Sinkhole Harzard in Bang Lung Area, Cho Don District, Bac Kan Province

By integration of remote sensing images analysis, geology, geomorphology, hydrogeology, geophysical method, and drilling data, the paper illustrates the structure tectonics, causes, and initial mechanism of a sinkhole forming in Bang Lung, Cho Don, Bac Kan province. The NE-SW normal slip faults are an essential fault system in the area, which created Bang Lung graben valley. This fault system also forms large fracture zones, creating advantage conditions for the groundwater runoff both vertically and horizontally to eroded and dissolved carbonate rock-forming underground karst caves. These are favorable natural conditions for forming a sinkhole. The sinkhole hazard in the Bang Lung area is initiated by some main factors such as tectonic activity, thickness, and characteristics of unconsolidated sediment layers, groundwater fluctuation, karst caves, and human activities. The most human impacts are mining exploitation and agricultural cultivation that promote sinkholes occurring faster and earlier. The horizontal and vertical movement of groundwater dragged the material on the ceiling karst caves into ground spaces. Thereby, weakening the cohesion of the unconsolidated sediment above caves leads to gravitational unbalance and creates a sinkhole. This study has also shown potential sinkhole areas in Bang Lung, which helps the authorities and local people in sinkhole prevention and mitigation mission.

Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small‐Scale Convection From the PI‐LAB Experiment

AbstractThe oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.

Determining the soil-bedrock interface and fracture-zone scope in the central urban area of the Jinan city, China, by using microtremor signals

AbstractTo explore and utilize the urban underground space, it is important for us to accurately obtain information on the near surface structures. Microtremor signals play a vital role in that they carry abundant information about the subsurface structure and can be easily observed under disturbance of human activities in urban areas. We collected microtremor signals at 24 sites along a survey line in central Jinan city, obtained the dispersion curves by using the Extended Spatial Autocorrelation method and the resonance frequencies by using the horizontal-to-vertical spectral ratio method, and inverted the near surface structures by using the forking genetic algorithm. The lateral variation of the soil–rock interface was delineated at a maximum depth of 33 m and a large fracture zone was determined at a width of approximately 300 m. The frequency peaks changed from 2.5 to 7.3 Hz for the fracture and stable zones. A frequency-dependent model of the sediment thickness was obtained as $h = 101{f_r}^{ - 1.13}$, where h is the depth with a unit of m and fr is the resonance frequency. These results may be useful for understanding near surface structures and then benefit exploration of urban underground space.

Mantle Flow as a Trigger for Subduction Initiation: A Missing Element of the Wilson Cycle Concept

AbstractThe classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and midmantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long‐term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the U.S. East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable colocation of subduction zones during at least the last 400 Myr.

Evaluation of two conceptual approaches for groundwater flow simulation for a rock domain at the block-scale for the Olkiluoto site, Finland

To evaluate conceptual approaches for simulating a rock domain, which is a rock unit outside a fracture zone, the block-scale groundwater flow system of the radioactive waste disposal site in Olkiluoto, Finland, was simulated using two approaches: the equivalent porous medium (EPM) approach and the hybrid approach. For the EPM approach, the rock domain was considered to be a homogeneous continuum with an equivalent hydraulic conductivity, while in the hybrid approach, it was conceptualized as a heterogeneous continuum from the stochastically generated background fracture network. In both approaches, several large fracture zones were included in the simulation models, which were calibrated by comparing the simulated hydraulic heads to the observed ones. The results showed that the simulated heads were in good agreement with the observed ones in both approaches, although the root mean square errors of the EPM approach were smaller than those of the hybrid approach. The estimated groundwater flow pathways in the hybrid approach generally had more complex geometries and longer travel times than those from the EPM approach. The simulations were evaluated by comparing the simulated flow rates at the boreholes in the site to the observed ones from the flow loggings, and the results show that the hybrid approach reproduced the observations more similarly than did the EPM approach.

The contribution of intragranular acicular ferrite microstructural constituent on impact toughness and impeding crack initiation and propagation in the heat-affected zone (HAZ) of low-carbon steels

We elucidate here the contribution of intragranular acicular ferrite (IAF) on impending crack initiation and propagation in the heat-affected-zone (HAZ) of low carbon steels based on Instrumented Charpy impact test. It was observed that both crack initiation energy and crack propagation energy of the steel in the presence of IAF were higher than steel that did not contain IAF, where the former situation was observed to have a large ductile fracture zone as compared to the latter. An analysis of the statistical distribution of voids in the cross-section of impact fracture indicated that IAFs increase the length of plastic zone at the tip of the main crack and impede crack initiation and enhance crack initial energy because of refined prior austenite grain and superior deformability. Furthermore, EBSD analysis of secondary cracks indicated that IAF impedes crack propagation and increases crack propagation energy by forming high angle grain boundaries with surrounding bainite and its own superior deformability.

A new rock mass classification system QHLW for high-level radioactive waste disposal

In this paper, a new rock mass classification system named QHLW system is proposed with the purpose of evaluating the suitability of the host rock for high-level radioactive waste (HLW) disposal. In the system, the rock suitability evaluation of two different scales is considered, namely the repository and tunnel scales. The system is developed on the basis of Q-system, and considers both the long-term safety and constructability requirement of the host rock for disposal. Thus, some additional parameters, including the fracture zone, groundwater chemistry and thermal effect are also taken into account in light of their significant influence on the long-term safety of HLW disposal. The avoidance strategy is highlighted in the proposed system by excluding the rock volume with unfavorable conditions, particularly adjacent to the large scale fracture zones. In the proposed system, the Q′ index (the product of the first four parameters in Q-method) is considered as the basic constructability index. By incorporating the influence of other parameters, the QHLW system is established in which the suitability of host rock is classified into three classes at the repository and tunnel scales. As a preliminary validation, the system is applied to the Beishan area, a potential region for HLW disposal in China. Using the classification method at the repository scale, the most suitable disposal site in Xinchang block of the Beishan area is identified. The rock classification at the tunnel scale is also carried out along two deep boreholes. The proposed system is found to be a usable tool in identifying suitable rock volume of different scales for HLW disposal.

Slug Test in a Large Dip Angle Fracture Zone: Model and Field Experiment

A single-porosity model is developed to deal with a fracture zone slug test in a large dip angle by assuming the fracture zone causes a downward regional flow. For the oscillatory response, a larger dip angle causes larger amplitude while introduces little impact on period. The effective water length, an important parameter necessary for analyzing the oscillatory response, is proven to be independent of the dip angle and can be evaluated using the available horizontal formation methods. The dip angle effect is more pronounced for a larger storage coefficient. An empirical relationship is developed to evaluate the limiting dip angle, below which the dip angle effect is negligible. Field data analysis of a slug test in a 47° dip angle fracture zone indicates that neglecting the dip angle can result in a 27% transmissivity over estimation and a 53% storage coefficient under estimation.

DETERMINATION OF SLIP SURFACES IN FRACTURE ZONE LANDSLIDES USING ORIENTED BOREHOLE CORE SAMPLES

In situ and laboratory observation, geophysical measurement and digital imaging analysis of oriented borehole core samples are performed to determine slip surfaces in two large fracture zone landslides in Shikoku, Japan. The following data are obtained from each oriented borehole: (1) a depth distribution of rock quality designation, magnetic susceptibilities, the Equotip hardness values and wet unit weight of core samples, (2) geometric orientation of geologic discontinuities (cracks, joints, faults, etc.), and (3) a depth distribution of numerical color values from digital imaging of borehole core samples. As a result, the rock quality designation, the Equotip hardness value and unit weight, and the orientation of cracks and joints showed a clear change near the slip surfaces respectively, but the digital color values clearly varied only in one of the landslides and no change of magnetic susceptibility of core samples was found at both sites. The results demonstrate that it is capable of locating the slip surface in a fracture zone landslide by using the above-mentioned data in combination.

Deformations of the northwestern Pacific plate

The style and age of deformations of the sedimentary cover and basaltic basement in the northwestern basin of the Pacific Ocean were studied based on seismic data coordinated with the results of deep-water drilling. The deformations are widespread not only on the oceanic slope of the trench as a plate boundary, but also within the plate especially close to the large Hokkaido, Tuscarora, and Seismikov fracture zones and the Hokkaido Swell. The most intense deformation took place at the Pliocene-Quaternary boundary. Abundant diapirs related to the plastic serpentinite layer at base of the crust indicate that gravity tectonics primarily develops here.

Effect of Heat Treatment on Strain Life of Aluminum Alloy AA 6061

The present work encompasses the investigation of Low Cycle Fatigue (LCF) of aluminum alloy AA6061 in three conditions, annealing (O), T4 and T651. AA6061-O has the higher value of transition fatigue life (NT) because it has the highest ductility, while the fatigue strength exponent and fatigue ductility exponent are within the range of metallic materials. The SEM morphology for specimens at strain closer to true tensile stress show multi-crack origins with secondary cracks and large final fracture zone. The striations are very clear for O-condition.

Multi-scale fracture network analysis from an outcrop analogue: A case study from the Cambro-Ordovician clastic succession in Petra, Jordan

A regional multi-scale fracture network is prominently exposed in the Cambro-Ordovician sedimentary succession in south-western Jordan. This succession consists of marine deposits, overlain by heterogeneous deltaic sediments, culminating in 500 m of massive to cross-bedded fluvial sandstones. A data set acquired from high-resolution satellite imagery and field measurements enables the quantification of the geometry and spatial characteristics of the fracture network in three dimensions.Five directional fracture sets are identified from the satellite data with mean strikes of 000°, 040°, 065°, 120°, and 145°. Large fracture zones (longer than 600 m), composed of tightly spaced joints confined in fracture swarms, exhibit a log-normal trace-length distribution and the fracture swarms are regularly spaced. The trace-length distribution of intermediate fractures (40–600 m long) exhibits power-law characteristics and the fractures are clustered, reflecting different fracture system styles and stratigraphic controls as a function of scale. The small-scale fracture network is strongly dependent on the distribution of bedding plane discontinuities throughout different parts of the succession. The multi-scale fracture network is categorized in a three-tier hierarchical fracture model that reflects underlying stratigraphic controls in the study area. This type of model can be used to improve the representation of fracture networks in fluid flow simulations of fractured reservoirs with hybrid models that combine elements of discrete fracture networks and continuum models.

Analysis on Seismic Damage Mechanism and Anti-Seismic Measures of Tunnels in Fault Fracture Zone

Fault fracture zone is an important factor that leads to tunnel seismic damage. In order to research failure mechanism and anti-seismic measures of tunnels across large fault fracture zone, theoretical analysis and numerical simulation has been done to Jiaozhou Bay submarine tunnel in Qingdao. The research results indicate that: in fault fracture zones, surrounding rocks instability resulting from ground motion, the huge earthquake inertial force, and the deformation energy by bedrock surface wave with macro energy all can cause damage to tunnel lining, yet the latter is the primary reason; when the differences of mass density and stiffness between tunnel lining and wall rocks become big, ductile tunnels with light weight will aggravate damage to tunnels rather than improve their anti-seismic capability; keeping stability of surrounding rocks and guaranteeing mass density and stiffness of tunnel lining to be the same as or similar to that of surrounding rocks could prevent tunnel damages in fault fracture zone, yet the latter is the most effective way. This research achievement can set particular examples for research on seismic damage mechanism and for anti-seismic design of tunnel structure in highly seismic regions.

Investigation of injection-induced seismicity using a coupled fluid flow and rate/state friction model

We describe a numerical investigation of seismicity induced by injection into a single isolated fracture. Injection into a single isolated fracture is a simple analog for shear stimulation in enhanced geothermal systems (EGS) during which water is injected into fractured, low permeability rock, triggering slip on preexisting large scale fracture zones. A model was developed and used that couples (1) fluid flow, (2) rate and state friction, and (3) mechanical stress interaction between fracture elements. Based on the results of this model, we propose a mechanism to describe the process by which the stimulated region grows during shear stimulation, which we refer to as the sequential stimulation (SS) mechanism. If the SS mechanism is realistic, it would undermine assumptions that are made for the estimation of the minimum principal stress and unstimulated hydraulic diffusivity. We investigated the effect of injection pressure on induced seismicity. For injection at constant pressure, there was not a significant dependence of maximum event magnitude on injection pressure, but there were more relatively large events for higher injection pressure. Decreasing injection pressure over time significantly reduced the maximum event magnitude. Significant seismicity occurred after shut-in, which was consistent with observations from EGS stimulations. Production of fluid from the well immediately after injection inhibited shut-in seismic events. The results of the model in this study were found to be broadly consistent with results from prior work using a simpler treatment of friction that we refer to as static/dynamic. We investigated the effect of shear-induced pore volume dilation and the rate and state characteristic length scale, [Formula: see text]. Shear-induced pore dilation resulted in a larger number of lower magnitude events. A larger value of [Formula: see text] caused slip to occur aseismically.

Lithospheric control on geochemical composition along the Louisville Seamount Chain

Major and trace element and Sr, Nd, and Pb isotope data for lavas from 12 seamounts along the western (older) 1500 km section of the Louisville Seamount Chain in the southwest Pacific show remarkably uniform compositions over a ∼30–40 Myr period of volcanism. All 56 samples analyzed are alkalic to transitional in composition. Unlike Hawaiian volcanoes, Louisville volcanoes appear not to pass through a sequence of evolutionary stages characterized by older tholeiitic basalts overlain by incompatible element enriched alkalic and silica-undersaturated lavas. The youngest lavas from a given Louisville seamount tend to have the least enriched incompatible element compositions. This unusual chemical evolution may be the result of re-melting of heterogeneous hot spot mantle that was partially depleted during the earlier, age progressive stages. The oldest Louisville seamounts were constructed close to the extinct Osbourn Trough spreading center, located north of the chain, but age-progressive lavas from these older seamounts are not significantly different to lavas from younger seamounts. This may indicate that spreading at this fossil ridge ceased several tens of millions of years before the oldest Louisville seamounts were constructed. Large fracture zones apparently had no significant effects on the composition of Louisville magmatism. However, lavas from the central part of the Louisville Seamount Chain, where volcanoes are smaller and more widely spaced, tend to have more variable and more enriched compositions. We suggest this may reflect smaller degrees of melting resulting from greater lithosphere thickness, and hence a shorter melting column for this section of the Louisville Seamounts.

Radionuclide Geomicrobiology of the Deep Biosphere

This review summarizes research into interactions between microorganisms and radionuclides under conditions typical of a repository for high-level radioactive waste in deep hard rock environments at a depth of approximately 500 m. The cell–radionuclide interactions of strains of two bacterial species (i.e., Shewanella putrefaciens and Desulfovibrio aespoeensis) with Cm, Pm, and Pu were investigated in vitro and the results were found to agree with literature data. Siderophores are capable of binding actinides strongly and need to be considered in terms of radionuclide mobility in the subsurface. Siderophores and other bioligands were found to have a generally very strong mobilizing effect on Am, Cm, Fe, Np, Pm, Pu, Th, and U. Where reduced groundwater enters an aerobic environment, such as a large open fracture or fracture zone (e.g., in tunnels), there is the possibility of rapid aerobic bacterial metabolism, microbial proliferation, biofilm development, and iron oxide formation. In these environments, the stalk-forming bacterium Gallionella may act as a scaffold for iron oxide precipitation on biological material. In situ work in the Äspö Hard Rock Laboratory tunnel indicated that the concentrations of biological iron oxides, lanthanides, and actinides correlated positively with Gallionella biomass, a finding that compares well to literature data. In deep oligotrophic subsurface granitic rock environments, fracture biofilms reach a threshold of approximately 2–5 × 106 cells cm−2. The cells in these biofilms are spatially distinct and are surrounded by an extracellular polysaccharide matrix that constitutes up to 60% of the total organic carbon. Calcium-rich amorphous masses are associated with this base layer of cells and organic exudates. In situ, these biofilms have been found to influence the adsorption and immobilization of Am, Np, Pm, Th, and U. This review demonstrates that microorganisms can influence, and sometimes even control, the migration behavior of radionuclides in deep geological environments typical of future sites for radioactive waste repositories.