Weak Zones Research Articles

Influence of Farallon Slab Loading on Intraplate Stress and Seismicity in Eastern North America in the Presence of Pre‐Existing Weak Zones

AbstractDespite the stability of the continental interior, eastern North America has hosted many significant historical earthquakes. Seismicity concentrates within tectonically inherited structures, which can act as weak zones where stress accumulates. Within these zones, systematic stress rotations may be explained by long‐wavelength sources. We test the hypothesis that mantle‐flow driven by the Farallon slab contributes to intraplate seismicity via the reactivation of pre‐existing faults. We model the stress field using seismically constrained global high‐resolution finite‐element flow models with CitcomS. To isolate the slab's effect, we vary its buoyancy between a case of neutrality and a case with full negative thermal buoyancy derived from tomography. Low‐viscosity lithospheric weak zones located at failed rifts, loaded by a mass anomaly at depth, transmit elevated stresses to the overlying crust. The sinking of the Farallon slab drives localized mantle flow beneath the central‐eastern US, generating a large stress amplification of 100–150 MPa peaking over the New Madrid Seismic Zone (NMSZ). This stress amplification exerts a continent‐wide clockwise rotation on the stress field, which in the presence of weak zones reproduces some observed deviations of the seismically inferred SHmax from the regional borehole SHmax, bringing optimally oriented faults, closer to failure, some of which are associated with major historical earthquakes, including the Reelfoot Fault in the NMSZ and the Timiskaming Fault in Western Quebec. However, stronger lithospheric viscosity gradients, shallower weak zones, or weaker faults are still needed to fully reproduce the observed stress field in some areas.

Integrated Geophysical Methods to Discriminate the Locations of the Subsurface Weakness Zone Areas in the State Company for Glass and Refractories at Al-Ramadi City, Iraq

Integrated Geophysical Methods to Discriminate the Locations of the Subsurface Weakness Zone Areas in the State Company for Glass and Refractories at Al-Ramadi City, Iraq

Integration Of 3G Data (Geomagnetic, Gravity And Geology) To Identify Geothermal System Controlled By Geological Structure Of Telomoyo Plateau (Study Case Of Candi Umbul Area)

As the technology development, there are more and more new innovations that utilize existing resources to support the energy needs and to fulfill the consumer needs. The energy source that is being discussed at this time is geothermal energy. Geothermal energy sources are considered effective because they are renewable and environmentally friendly when compared to other energies such as fossil energy. In Indonesia itself, geologically, it has a complex series of volcanoes that can be used as a heat source in this new energy innovation. One area that thought to have geothermal potential is the telomoyo mountain area, which is indicated by the manifestation of hot springs on the surface, precisely around Candi Umbul. However, it is necessary to conduct subsurface studies to identify the presence of subsurface structures such as faults or intrusions as heat sources, where this can be overcome using geophysical methods. This research was conducted using the integration of two geophysical methods, namely the geomagnetic method to determine the direction of the fault continuity and the gravity method to determine the regional heat source, and in the other side geological data as a reference for interpretation. From geomagnetic measurements, 163 rock magnetism data were obtained which indicated the existence of a fault continuity with northeast-southwest orientation in the direction of the river flow and cut the manifestation of hot springs. Meanwhile, from 176 gravity topex data, a high complete boguer anomaly value was obtained as an indication of an intrusion in the northeast that was cut by the fault recorded in the geomagnetic data. Based on geophysical data analysis and correlation with geological data from previous studies, it can be assumed that there is a geothermal system in the study area with andesitic intrusion as the heat source, fault structure with northeast-southwest orientation as a weak zone for meteoric water migration, andesite lava as caprock, unit tuff rock as a reservoir and the telomoyo plateau as a recharge area to supply meteoric water from the geothermal system.

СТЕПЕНЬ ВЛИЯНИЯ НЕОТЕКТОНИКИНА СЕЙСМИЧНОСТЬ АБИНСКОГО СЕГМЕНТА СЕВЕРО-ЗАПАДНОГО КАВКАЗА

Relief is one of the sources of information about neotectonic activity. Using structural-geomorphological analysis, it is possible to identify weak zones, which often reflect hidden low-amplitude faults and fracture zones under the surface of Quaternary deposits. Using structural-geomorphological analysis, planation surfaces were identified, displacement amplitudes along faults and the magnitude of neotectonic movements were determined for certain periods of time. The results obtained were compared with seismicity and the degree of inheritance was revealed. 3D computer geological modeling of the stress state also helps in identifying zones of increased neotectonic activity, as the most dangerous zones in terms of seismicity. The stress state and directions of the compression axes in the horizontal plane were calculated using the RMS Roxar software. Also, for a more detailed study of the latest geodynamics of the Abinsk segment, the frequencies of earthquakes and the amount of seismic energy released were analyzed, and frequency graphs were plotted. When comparing the growth deformations of neotectonic uplift structures and the values of the latest stress state with seismicity, their significant correlation was revealed.

Fastening in concrete vs. rock mass—Comparative determination of pull‐out loads for artificially created discontinuities in concrete

AbstractPossible discontinuities in the subsurface have a major impact on the load‐bearing behavior of post‐installed fastenings. For concrete, the behavior of post‐installed anchors and thus their design can be clearly addressed as concrete is assumed as a homogeneous fastening substrate. This is valid except, for example, in the area of building joints and at joints of precast concrete elements, where the formation of structural joints inevitably occurs and which basically correspond to a separation surface. In contrast, rock mass is characterized by the rock type, but is generally also significantly influenced by its discontinuities. These play a decisive role concerning rock mass stability and show a great impact on the load‐bearing capacity of rock, especially for fasteners with a shallow embedment depth. It is to be assumed that the same also applies to separating surfaces in concrete. Furthermore, the question arises as to the non‐destructive preliminary detectability of such weak zones. For carrying out a comparative study under controlled conditions, artificial interfaces of different geometries were generated in concrete in laboratory tests by inserting PTFE layers at different positions of the test member. Pull‐out tests of post‐installed fastening systems were carried out in their vicinity to determine the load transfer as well as the failure mode. It could be shown that discontinuities have a negative effect on pull‐out loads not only in the rock mass, but also in concrete. However, detection of these by means of a rebound hammer was only possible in the rock mass.

Modelling of welded aluminium connections in large-scale analyses

This article addresses large-scale analyses of welded aluminium connections where the weak zones are to be represented by a few shell elements. A combined experimental and numerical study on the behaviour and modelling of welded aluminium connections is presented, where a shell-element modelling framework applicable in large-scale analyses is proposed, accounting for geometrical instability, thinning, and ductile fracture. The proposed modelling framework is calibrated and validated using cross-weld tension tests. The test campaign involves tension testing of two Al–Mg–Si alloys and two welding techniques widely used in industrial applications. The shell-element simulations represented the response of the tests reasonably well.

Design, preparation, and mechanical properties of glass fiber reinforced thermoplastic self‐anchor plate cable exposed in alkaline solution environment

AbstractThe reliable anchorage and long‐term durability of high‐performance fiber reinforced polymer composites were the decisive factors for engineering applications owing to the anisotropy of materials and the complexity of service environment. In this paper, an innovative glass fiber reinforced polypropylene (GFRPP) self‐anchor plate cable was developed to utilize the high toughness/durability of thermoplastic resin and self‐anchor load‐bearing system. The effects of different reinforcement methods and cable arc angles (10°, 20°, and 30°) on mechanical properties of plate cable were investigated. Water absorption behavior and mechanical properties immersed in alkali solution were tested to evaluate its long‐term service behavior. The thermogravimetric and surface morphology analysis were conducted to reveal the performance evolution mechanism. The results showed through the combination of secondary melting, carbon fiber winding confinement and epoxy reinforcements, the splitting failure of transition zone and interlaminar cracking failure of straight zone for plate cable were effectively avoided. The tensile strength retention of plate cable for arc angles of 10°, 20° and 30° were 45.6%, 41.3%, and 34.0% of GFRPP plate, respectively. Larger arc angle increased the curvature of arc‐straight transition zone and stress concentration near the straight section, leading to interlaminar splitting failure of plate cable at weak transition zone. The tensile strength of plate cable with the immersion time deteriorated obviously until the lowest retention of 29.7%. The degradation mechanism was mainly due to the etching of glass fibers in alkali solution and the formation of pores and internal defects, including fiber/resin interface debonding and resin swelling. The research results were of significance for solving the anchoring problems and promoting the long‐term service reliability in bridge applications.Highlights The mold can realize reliable molding of glass fiber reinforced polypropylene cables after the reinforcements. The tensile strength retention of plate cable was up to 34.0%–45.6% of plate. Larger arc angle increased the stress concentration near the straight section. Non‐polar polypropylene plate was proved to have better hydrophobic performance. The degradation mechanism of plate cable in alkali solution was revealed.

An alternative experimental configuration to generate wrench zone above a viscous layer

Analogue modelling of wrench tectonics typically utilizes a rigid basement with a velocity discontinuity under a brittle or brittle-viscous cover, such as in Riedel experiments, which confines fault localization in the overlaying model. However, such a set-up is hardly compatible with modeling brittle-ductile systems such as the upper and lower crust or a brittle sedimentary cover overlying a viscous evaporitic layer. To achieve a more realistic experimental approach, Bruno Vendeville designed an alternative experimental set-up decoupling the basement from the brittle overburden with a viscous layer in which the basement is not involved. In this configuration, strike-slip movement is driven laterally rather than from the base up, facilitated by “weak zones” that preferentially localize the deformation during shortening and enable sliding between compartments. This original approach provides greater flexibility for modeling complex strike-slip settings, allowing for more freedom for strike-slip structures to form and evolve through time.Although the experiments described in this work were conducted in the late 1990s, the co-authors have chosen to revisit and adapt this earlier work for this Special Issue to underscore Bruno's influence on another aspect of salt tectonics and his pioneering foresight in the field of analogue modelling.

Cyclic behavior of high-strength steel beam-to-column welded flange-bolted web connections

Four Q690 high-strength steel beam-to-column moment connections, which use welded joints between beam and column flanges as well as a bolted beam web to the column flange, were tested under cyclic loading. The effects of beam-to-column welding details and panel zone strength were studied. Two pairs of welded flange details were incorporated, including the Chinese code-specified complete-joint-penetration (CJP) welded connections with backing plates where the bottom backing plate is reinforced by a fillet weld, and enhanced CJP welded connections with backing plates removed, weld roots backgouged, and further reinforced by fillet welds. Three panel zone thicknesses were designed to characterize strong, intermediate and weak panel zones, respectively. The test results show that backing plates should be removed in these moment connections to prevent brittle weld fracture, but still rendering only limited plastic hinge rotations in the order of 0.01–0.02 rad in the beam end before ductile fracture of the beam flange. The panel zone, however, survived even after a plastic shear rotation of 0.03 rad, demonstrating much higher plastic deformation capacity than the beam plastic hinge. This suggests that ductility can be exploited in 690 MPa high-strength steel panel zones, but not reliably in 690 MPa steel beams.

Multiscale fracture, physical and mechanical properties of stromboli volcano (Italy) edifice

The physical, mechanical and fracture properties at Stromboli volcano have been integrated at multiple scales to understand whether the interplay between a presumed NE/SW rift zone and the Sciara del Fuoco (SDF) depression has resulted in a zone of weakness able to promote fracturing prone to flank instability. Multiscale fracture quantification by imaging via FracPaQ toolbox both fractures and sample scale fractures has been integrated with rock physics and rock mechanics experiments on cm-scale samples belonging to the Paleostromboli, Vancori, Neostromboli, Pizzo and Present Deposit volcanic cycles that have been taken from within and outside the rift zone. The structural changes to the edifice have been quantitively assessed by mapping at different scale fracture properties such density and orientation within and outside the rift zone allowing to identify the potential damaged zones that could reduce the edifice strength.Results indicate that basalt textures, microfracture density, porosity, chemical zoning and preferential alignments, despite lithologically dependent, can be related to the NE/SW zone of weakness at the regional scale and to collapsed volumes that have been subject to continuous intrusive activity. Numerical inversion models have been performed to cross correlate fracture density in the basalts at multiple scales.A link between microfracture density and seismic velocities has been also established via numerical modelling, allowing to interpret in terms of degree of fracturing the results of seismic tomographies at the field scale, providing a novel method to image crack damage evolution within the inner structure of the volcano edifice.

Discrete element modeling of distal deformation propagation in thrust wedge and implications for early deformation on northern Tibetan and Iranian Plateaus

Coulomb critical wedge theory predicts that thrust wedges would grow sequentially from the hinterland to the foreland, meaning that distal deformation occurs last. However, in the northern Tibetan and Iranian Plateaus, far away from the southern collision zones, widespread deformation occurs soon after collisions of Arabia and India with Eurasia. Additionally, despite the prevalence of weak lower crust and distal pre-existing faults or weak zones, their relationship to early distal deformation remains poorly understood. For this reason, we run systematic experiments of discrete element models involving basal décollement layer as well as distal pre-existing fault. Our model results reveal that (1) the presence of pre-existing faults is necessary for the occurrence of early distal deformation; (2) the early deformation of distal pre-existing fault is dependent on basal décollement strength and independent of model width; (3) strong basal décollement fails to activate the distal pre-existing faults, instead weak basal décollement can deform them at the early stage; (4) in the presence of weak basal décollement, a slower shortening rate not only facilitates greater shortening absorption by the distal pre-existing fault at the early stage but also results in a more pronounced deviation from sequentially-forward deformation propagation. These findings demonstrate that the preferential reactivation deformations of distal pre-existing faults are mechanically controlled by a weak basal décollement layer. Together with geological and geophysical observations, we suggest that the early deformations of northern Tibetan and Iranian Plateaus may be the result of the reactivation of pre-existing faults due to the existence of weak lower crust soon after collisions.

Macroscopic properties and microscopic characterisation of an optimally designed anti-cracking stone base course filled with cement stabilised macadam

Stone base course filled with cement stabilised macadam (SFC) is a promising solution for addressing reflection cracks in semi-rigid base asphalt pavement due to its satisfactory crack resistance. However, its limited strength and construction challenges have impeded its widespread application. To overcome these challenges, an optimised mix proportion design method of SFC was proposed. Natural crushed limestones with specific particle sizes were selected to serve as skeletal coarse aggregates; fine aggregates and their proportions were step-by-step determined using aggregate and unconfined compressive strength tests. The multi-scale analysis integrating the performance tests and the microscopic detections was performed to evaluate the effectiveness of the proposed SFC, and compared with common cement stabilised macadam (CSM). The results indicate that the material exhibits satisfactory mechanical strength through a tightly interlocked skeleton structure and excellent crack resistance by dissipating strain energy and retarding cracking of SFC through the weak interface transition zone (ITZ).

Seismic and radon signatures: A multiparametric approach to monitor surface dynamics of a hazardous 2021 rock–ice avalanche, Chamoli Himalaya

AbstractThe observation of precursory signals of the 2021 Chamoli rock–ice avalanche provides an opportunity to investigate the multidisciplinary analysis approach of rock failure. On 7 February 2021, a huge rock–ice mass detached from the Raunthi peak at Chamoli district in Uttarakhand, India. The tragic catastrophe resulted in more than 200 deaths and significant economic losses. Here, we analyse radon concentration and seismic signals to characterise the potential precursory anomalies prior to the detachment. Continuous peaks of radon anomalies were observed from the afternoon of 5 to 7 February and decreased suddenly after the event, while a cumulative number of seismic tremors and amplitude variations are more intensified ~2.30 h before the main event, indicating a static to dynamic phase change within the weak zone. This study not only characterises abnormal signals but also models the rock failure mechanisms. The analysis unveils three time‐dependent nucleation phases, physical mechanisms of signal generation and a complete scenario of physical factors that affected the degree of criticality of slope failure. The results of this study suggest gradual progression of rock cracks/joints, subsequent material creep and slip advancement acceleration preceded the final failure. Furthermore, the study highlights the importance of an early warning system to mitigate the impact of events like the 2021 Chamoli rock–ice avalanche.

Analogue modelling of a salt ridge growth in M'Rhila-Labaied-Trozza fault relay zone, central Tunisia

Tunisia, located in the Northern African margin, was subjected to Late Permian-Early Cretaceous N–S extension subsequent to Pangea breakup and leading to the Tethyan opening and widening. During the Mid- and Late Cretaceous, the NE–SW crustal extension that established the Pantelleria-Malta-Sirt grabens system underlining the eastern margin stretched also the Tunisian Atlassic domain, creating and/or reactivating many NW-SE extensional structures as much as the grabens in central Tunisia and leading to volcanism and halokinesis along several weak zones. Neogene compressional tectonics inverted the earlier extensional structures and impacted most of the halokinetic features established within the post-Triassic cover. The M'Rhila-Labaied-Trozza fault relay zone, located in the middle of central Tunisia, shows both extensional and compressional structures with Triassic extrusive features and seems to be a key feature for understanding the geodynamic development of the area. In this work, we combined field and geophysical data together with analogue models to decipher the structure and the kinematic evolution of the study area. The results showed that the studied structures are associated with reactive Mesozoic salt ridges established along an extensional fault relay zone that utilized NW-SE, E-W and NE-SW inherited fractures. These initially created extensional corridors allowing Triassic evaporite extrusion and accumulation, then influenced the deformation of the area in transpressional and local pure compressional regimes depending on the fractures and stress orientations during tectonic inversion. Analogue guided interpretation of the successive deformational stages of the study area from salt ridge growth to its tectonic inversion.

Gravity‐Driven Jointing in Layered Rocks: Mechanical Analysis and Numerical Modeling

AbstractThe origin of open‐mode fractures or joints, common in sedimentary basins (reservoirs), remains incompletely understood. These fractures require effective tensile stress for their formation, yet they usually develop at depths where effective compressive stresses dominate. Through 2‐D finite‐difference elastoplastic modeling, we demonstrate that effective tensile stresses and fracturing can be internally generated in stiff competent layers separated by compliant incompetent layers. Two primary factors drive this phenomenon: the layered structure of sedimentary units with varying mechanical properties of the layers and the lithostatic compression of the unit, leading to its gravity‐driven horizontal widening or stretching. This stretching occurs when the average horizontal stress over the unit’s thickness is compressive. Various mechanisms can accommodate the stretching, including preexisting faults or weak zones, free borders like canyons, and horizontal extension of tectonic or gravitational origin. Tensile horizontal effective stresses σxx occur within the competent layers, while the incompetent layers generally remain under effective compression. The magnitude of σxx increases with the lithostatic stress or burial depth, as does the tensile strength σt of incompletely lithified sediments. Jointing occurs within depth intervals where |σxx| > σt. In our modeling approach, effective tensile stresses σxx are generated due to lithostatic compression σv. In the previous numerical modeling studies, these stresses resulted from extension directly applied to the lateral boundaries of the layers and did not depend on σv. We provide an analysis of the distinctions between these two approaches.

Three‐Dimensional Electrical Resistivity Structure Beneath a Strain Concentration Area in the Back‐Arc Side of the Northeastern Japan Arc

AbstractIntraplate earthquakes occur more frequently in the Japanese islands than in other regions. Large intraplate earthquakes in the island arc preferentially occur in strain concentration zones detected by geological and geodetic studies. Crustal heterogeneity plays a crucial role in generating large intraplate earthquakes and strain concentrations. Thus, we elucidated the crustal heterogeneities beneath a strain concentration area on the back‐arc side of the northeastern Japan Arc based on electrical resistivity, which is sensitive to weak zones in the crust. By deploying wideband magnetotelluric surveys, we revealed the three‐dimensional electrical resistivity structure in the crust, suggesting the coexistence of two different types of strain‐concentration mechanisms in the strain‐concentration area. The shallow conductive layers and lower‐crustal conductors appear to act as low‐elastic‐modulus and low‐viscosity areas, respectively, and are responsible for the strain concentration. We found shallow and lower‐crustal conductors in the strain concentration zone revealed by geological studies. Those conductive areas are considered to act as mechanically weak zones and cause stress loading on the brittle pars of pre‐existing faults, resulting in large intraplate earthquakes. The resultant earthquakes presumably contribute to strain accumulation on a geological timescale. In addition, a spatial correlation between the epicenters of large intraplate earthquakes and edges of lower‐crustal conductors implies the contribution of the fluids in the lower crust to the generation of large earthquakes. We also identified vertical conductors ranging from the lower crust to Quaternary volcanoes, which may indicate trans‐crustal magmatic systems under these volcanoes.

Experimental study and multi-objective optimization of the shear mechanical properties of recycled aggregate concrete with hybrid fibers and nano SiO2

Structural imperfections (microcracks, micropores, and weak interfacial transition zones) impact the shear mechanical properties of recycled aggregate concrete (RAC). Fibers and nanoparticles can mitigate these issues, enhancing the overall quality of recycled aggregates (RAs). Various hybrid systems, including basalt fiber (BF), polypropylene fiber (PPF), and nano-SiO2 (NS), were added to the RAC for double shear testing. Regression models were developed using the response surface method to optimize the hybrid systems. The study showed that the NS-BF-PPF hybrid system achieved the optimal shear strength, shear strain, shear toughness index, and shear fracture energy (6.99 MPa, 9.58 × 10−4, 0.716, and 34.603 J, respectively). These values were superior to those of the BF-PPF hybrid system and single system. The three-dimensional reticulated supporting structure devised by the BF-PPF effectively remedied the structural defects in the RA at various levels. Furthermore, the fibers integrated into the hybrid system act as bridges, absorbing shear energy during the pullout process. The incorporation of nanoscale NS stimulated the hydration response of the cement, resulting in a more compact pore structure within the RAC. Nonlinear multivariate regression models were fabricated utilizing the response surface method to maximize strength. The optimal contents of BF and PFF in the BF-PPF hybrid system were 0.344 % and 0.3 %, respectively. The optimal contents of NS, BF, and PPF in the NS-BF-PPF hybrid system were determined to be 3.5 %, 0.5 %, and 0.1 %, respectively. The combination of fibers and nanoparticles elicits a synergistic effect, significantly augmenting the shear performance of RAC.

The effect of the stress equilibrium factor on the composite case of the solid rocket motor

In this paper, based on the composite structure design and analysis software ANSYS ACP, the layup design for the composite cases of the equal pole hole solid rocket motor with different stress equilibrium factors is carried out. The finite element software ANSYS Workbench is used to study in detail the effects of stress equilibrium factor on the stress level, failure degree, and burst pressure prediction of the case under a certain internal pressure. The results show that reducing the value of the stress equilibrium factor can reduce the stress level of each winding layer along the main direction of the material in the motor case and gradually transfer the maximum stress of the case along the fiber direction from the weak stress zone to the cylinder section. This is beneficial for improving the burst pressure of the case and making the burst point located in the cylinder section. However, whether the case failure is related to the selected failure theory and strength criterion, the failure results and burst pressure obtained using different failure theories and strength criteria may be different, which indicates that each failure criterion has its scope of application and limitations. Under the Tsai-Wu criterion, using the stress equilibrium equation to calculate the stress equilibrium factor can just ensure that each winding layer of the motor case does not fail under the working pressure. In summary, when designing a composite case and predicting its failure, in addition to considering whether the stress equilibrium factor taken is conducive to meeting the design requirements for the strength of the case, it is also necessary to consider the use of multiple failure criteria to study the failure of the case. Obviously, arbitrarily selecting the stress equilibrium factor is not appropriate in the above process. By analyzing the influence of the stress equilibrium factor on the strength and failure condition of the motor case, it can provide an effective reference for the design of a composite case for SRM.

Constraints on Growth and Stabilization of the Western Superior Craton From Inversion of Magnetotelluric Data

AbstractA data set consisting of 376 broadband and long‐period MT measurements was used to generate the first ever 3D resistivity model of the Archean western Superior Craton. The modeled resistivity structure is compared to coincident seismic reflection data. The observed geophysical signatures are interpreted within the context of the late stages of crustal growth and cratonization of the region via the progressive accretion of terranes against the initial cratonic core. The northern‐most terranes comprising the cratonic core exhibit a nearly homogenous highly resistive crust. The lower crust of the southern terranes contains largely continuous low resistivity bands which run subparallel to major terrane boundaries and corresponding fault systems. In some cases, low resistivity features are coincident with dense packages of sub‐horizontal to listric reflections within the mid‐ to lower crust. These resistivity structures are inferred to represent preserved geoelectric signatures of late to post‐orogenic magmatic pulses likely related to delamination of locally overthickened crust. Increased mantle heat flow resulted in partial melting of the lower crust and upper mantle and upward migration of CO2‐rich melts and fluids through crustal weak zones corresponding to shear and/or suture zones formed during terrane amalgamation. Thermal softening of the mid‐ to lower crust led to orogenic collapse and reactivation of the crustal shear zones, resulting in formation and interconnection of graphitic films which were preserved within the stable craton. These results have implications for the tectono‐magmatic history of the western Superior Craton, as well toward the understanding of the geodynamic regime of the Archean Earth.

A case study on the stability of a big underground powerhouse cavern cut by an interlayer shear zone in the China Baihetan hydropower plant

AbstractThe big underground powerhouse cavern of the China Baihetan hydropower plant is 438 m long, 34 m wide, and 88.7 m high. It is cut by a weak interlayer shear zone and its high sidewall poses a huge stability problem. This paper reports our successful solution of this problem through numerical simulations and a replacement‐tunnel scheme in the detailed design stage and close site monitoring in the excavation stage. Particularly, in the detail design stage, mechanical parameters of the shear zone were carefully determined through laboratory experiments and site tests. Then, deformation of the surrounding rocks and the shear zone under high in situ stress conditions was predicted using 3 Dimensional Distinct Element Code (3DEC). Subsequently, a replacement‐tunnel scheme was proposed for the treatment on the shear zone to prevent severe unloading relaxation of surrounding rocks. In the construction period, excavation responses were closely monitored on deformations of surrounding rocks and the shear zone. The effect of local cracking in the replacement tunnels on sidewall stability was evaluated using the strength reduction method. These monitoring results were compared with the predicted numerical simulation in the detailed design stage. It is found that the shear zone greatly modified the deformation mode of the cavern surrounding rocks. Without any treatment, rock mass deformation on the downstream sidewall was larger than 125 mm and the shearing deformation of the shear zone was 60–70 mm. These preset replacement tunnels can reduce not only the unloading and relaxation of rock masses but also the maximum shearing deformation of the shear zone by 10–20 mm. The predictions by numerical simulation were in good agreement with the monitoring results. The proposed tunnel‐replacement scheme can not only restrain the shear zone deformation but also enhance the safety of surrounding rocks and concrete tunnels. This design procedure offers a good reference for interaction between a big underground cavern and a weak layer zone in the future.