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The zonal disintegration phenomenon (ZDP) is investigated by proposing a criterion based on fracture mechanics. Accordingly, the mechanism is revealed to be the circulation of the fracture of the elasto-plastic boundary caused by the peak SIF (Stress Intensity Factor) and the constant formation of a “new” plastic boundary. Consequently, the reason ZDP occurs in deep caverns is also interpreted based on this theory. In addition, a formula is proposed to predict and determine the post-mortem features of ZDP, including the numbers ( N ) and sizes ( r ) of the fractured zones. The subsequent sensitivity analysis shows that the impact factors are the geo-stress and parameters of the rock mass. The variation laws of the post-mortem feature versus these impact factors are also determined. A 3-D geo-mechanical model test is conducted based on an analogous material. A deep tunnel in the PANJI coal mine, where zonal disintegration is observed, serves as the prototype engineering project. This model presents the forming process of ZDP. The criterion and post-mortem features proposed by the formula are validated through the model test. Furthermore, the results confirm that the wavy deformation law can be used to predict the ZDP. [ABSTRACT FROM AUTHOR]

A series of three dimensional (3D) shaking table tests were carried out to investigate the mechanism and effect of seismic measures of mountain tunnel using a scaled model based on a real tunnel. Key technical details of the experiment, including similarity relations, seismic measures simulation, boundary conditions, sensor layout, modeling methods, and ground motion input were presented. Main seismic measures, including reinforcing surrounding rock with anchors, increasing lining flexibility with steel wire mesh, and installing seismic isolation layer between reinforced surrounding rock and tunnel lining, were investigated in this study. Experiment results show that: (1) adding a layer of steel wire mesh in the tunnel lining can improve the flexibility and seismic performance and also may effectively prevent radial cracks from crossing the lining; (2) installing a geofoam isolation layer between the reinforced surrounding rock and the tunnel lining reduces dynamic earth pressure by 70–90% for the lining without a seismic isolation layer; (3) the flexible joints can effectively avoid global failures of tunnel lining for they reduce dynamic strain and bending force in the tunnel lining and decrease the seismic energy transmission along the lining in axial direction; (4) reinforcing surrounding rock with anchors significantly reduces dynamic earth pressure and strain of the lining by about 50%. In addition, the length of seismic reinforcement for general mountain tunnel portal is recommended to be 50 m from the tunnel portal along the axial direction. [ABSTRACT FROM AUTHOR]

Highlights: [•] The major principal stress can be divided into a stress relaxation zone, an increased stress zone and a balanced stress zone. [•] The large deformation characteristics of the Zhegu mountain tunnel were studied. [•] The geostress, rock strength, rock mass structure and groundwater are main factors of the large deformation. [•] The large deformation mechanisms of the Zhegu mountain tunnel are categorized into three modes. [Copyright &y& Elsevier]

An accurate and real-time identification of the quality grades of tunnel surrounding rocks is essential for efficient tunneling and geological hazard prevention when using tunnel boring machines (TBMs). In this study, an intelligent surrounding rock classification (SRC) model based on TBM operational parameters was constructed using the simultaneous optimization (SO) of the network structure and optimizer hyperparameters. First, a high-quality database for SRC was constructed after abnormal data elimination, data correlation analysis, and data discretization. Six input parameters were selected: advance rate, total thrust, cutterhead torque, cutterhead rotational speed, and pitch and roll angles of the gripper shoes. Second, the SO method was proposed using the Bayesian optimization (BO) algorithm to search for the optimal structure and optimizer hyperparameters of the high-resolution neural network (HRNet), and an HRNet–BO model was constructed for SRC. The evaluation results of the accuracy, precision, recall, micro-F1 score, macro-F1 score, and receiver operating characteristic (ROC) curve showed that the HRNet–BO model had greater robustness in learning and generalizing than the VGG–BO and ResNet–BO models. The proposed SO method and HRNet–BO model can effectively support TBM tunneling and real-time SRC. [ABSTRACT FROM AUTHOR]

The first order reliability method (FORM) is efficient, but it has limited accuracy; the second order reliability method (SORM) provides greater accuracy, but with additional computational effort. In this study, a new method which integrates two quasi-Newton approximation algorithms is proposed to efficiently estimate the second order reliability of geotechnical problems with reasonable accuracy. In particular, the Hasofer–Lind–Rackwitz–Fiessler–Broyden–Fletcher–Goldfarb–Shanno (HLRF–BFGS) algorithm is applied to identify the design point on the limit state function (LSF), and consequently to compute the first order reliability index; whereas the Symmetric Rank-one (SR1) algorithm is nested within the HLRF–BFGS algorithm to compute good approximations, yet with a reduced computational effort, of the Hessian matrix required to compute second order reliabilities. Three typical geotechnical problems are employed to demonstrate the ability of the suggested procedure, and advantages of the proposed approach with respect to conventional alternatives are discussed. Results show that the proposed method is able to achieve the accuracy of conventional SORM, but with a reduced computational cost that is equal to the computational cost of HLRF–BFGS-based FORM. [ABSTRACT FROM AUTHOR]

Abstract: The Mounigou tunnel crosses complex geological strata, mostly with surrounding rock of poor quality with an IV or V classification. Failure in the form of collapses or squeezing deformation occurred many times during the tunnel construction. Especially, in the soft and hard rock contact zone of phyllite and meta-sandstone, the deformation failure was most obvious. This paper uses the FLAC-3D modeling analytical software to establish a 3D-numerical model, on the basis of a field tracing survey and laboratory tests to analyze the stress and strain changes before and after excavation of the tunnel. After excavation there are obvious stress concentrations and differentiations near the interface of the soft-and-hard rock stratum. Stress transfers and concentrates towards the hard rock stratum while in the soft rock zone stress release could be observed to some degree. The range of plastic deformation is obviously larger in the soft rock zone than in the hard rock zone. Both differences in principal stress and principal stress ratio increase after excavation. The amplification of the difference in principal stress is larger than the augmentation of the principal stress ratio. Moreover these principal stress amplifications are smaller in the soft rock zone than in the hard rock zone. The difference in principal stress amplifications between the soft and hard rock zones, together with the fact that soft rock strength is far lower than that of hard rock, causes local “relatively high geostress” effects in the soft rock. It is the main controlling factor of the large deformation of soft rock in the tunnel. It is concluded that the large deformation in the soft and hard rock contact zone in combination with the surrounding rock deformation failure in the Mounigou tunnel and the damage of the supporting structure, belongs to the type of soft rock plastic-squeezing failure. [Copyright &y& Elsevier]

The Yagan area of the southernmost Sino–Mongolian border is characterized by an extensional structure where a large metamorphic core complex (Yagan–Onch Hayrhan) and voluminous granitoids are exposed. New isotopic age data indicate that the granitoids, which were previously regarded as Paleozoic in age, were emplaced in early and late Mesozoic times. The early Mesozoic granitoids have 228±7 Ma U–Pb zircon age, and consist of linear mylonitic quartz monzonites and biotite monzogranites. Their chemical compositions are similar to those of potassic granites and shoshonitic series, and show an intraplate and post-collisional environment in tectonic discrimination diagrams. Their fabrics reveal that they experienced syn-emplacement extensional deformation. All these characteristics suggest that the adjustment, thinning and extensional deformation at middle to lower crustal levels might have occurred in the early Mesozoic. The late Mesozoic granitoids have a U–Pb zircon age of 135±2 Ma, and are made up of large elliptical granitic plutons. They are high-K calc-alkaline, and were forcefully emplaced in the dome extensional setting. Both the early and late Mesozoic granitoids have #x03B5;Nd (t) values of -2.3 to +5, in strong contrast with the negative #x03B5;Nd (t) values (-11) of the Precambrian host rocks. This suggests that juvenile mantle-derived components were involved in the formation of the granitoids. The similar situation is omnipresent in Central Asia. This study demonstrates that tectonic extension, magmatism and crustal growth are closely related, and that post-collisional and intraplate magmatism was probably a significant process for continental growth in the Phanerozoic. [Copyright &y& Elsevier]

Examines the structures of mylonitic granites of Yagan metamorphic core complex in the Sino-Mongolian border in China. Shape of the mylonitic granitic plutons; Similarity in foliation and lineation; Mechanism of the mylonitic granitic plutons.

The vertical dynamic response of a pile embedded in radially inhomogeneous soil is investigated while allowing for the vertical wave effect of soil. For most engineering applications, the soil adjacent to pile may be hardened or softened due to the construction disturbance effect of pile, leading to the radial inhomogeneity of soil which has a significant influence on the vibration characteristics of pile. To consider this problem, this paper introduced the vertical shear complex stiffness transfer model to simulate the radial inhomogeneity of the surrounding soil and underlying soil of pile. Then, the governing equations of pile-soil system subjected to vertical dynamic loading are established based on the fictitious soil pile model. By means of Laplace transfer method, vertical shear complex stiffness transfer method and impedance function transfer method, the analytical solution of the vertical dynamic impedance of pile head is derived in the frequency domain. The present solution is verified by comparing it with existing solution. Based on the present solution, a parametric study is conducted to study the influence of the construction disturbance effect of pile surrounding soil and underlying soil on the vertical dynamic response of pile head. The results show that, even if the surrounding soil of pile is hardened or softened within a very small range, the construction disturbance effect still has a remarkable influence on the vertical dynamic response of pile head. • The present solutions can take both the inhomogeneity of pile surrounding soil and pile end soil into account. • The vertical shear complex stiffness transfer method is used to simulate the inhomogeneity of soil in the radial direction. • The disturbance influences the vertical dynamic response of pile head largely even if the surrounding soil deforms a little. [ABSTRACT FROM AUTHOR]

[Display omitted] • A bimodal convolutional network based on the VGG is constructed for waveform multi-classification. • Features from both the time and frequency domains are enhanced by the attention mechanism. • Noises highly similar to microseismic signals are well distinguished. Microseismic monitoring systems deployed in deep underground engineering can capture massive waveform signals in real time. However, some noise signals are highly deceptive and similar to microfracture signals. This problem usually requires engineers to compare the signal characteristics from different domains and poses a challenge for the rapid detection of microseismic data. To address this problem, we establish a new waveform multiclassification model based on the multimodal feature extraction technology. The proposed model constructs a dual-branch convolutional neural network to learn the bimodal features from the time and frequency domains. Furthermore, the attention mechanism (channel and spatial attention) with skip connection is used to increase the focus on the prominent features of the waveform and reduce the interference caused by irrelevant information. We conduct experimental studies using microseismic data from deep tunnel projects under complex geological conditions and classify the recorded data into three target types: microseismic, blast, and noise waveforms. The results show that the proposed approach achieves good multiclassification performance for noise signals highly similar to microseismic signals as well as for noisy microseismic signals with different signal-to-noise ratios. In summary, this study lays the foundation for further exploration of the potential application of multimodal fusion in seismic engineering and various rock engineering fields. [ABSTRACT FROM AUTHOR]

Tunnel portals are often located in rather weak rock. Also the ground surface of tunnel portal section is often inclined to form slopes. These characteristics in engineering geology make this section easy to be damaged during earthquakes both in slope and in structures. A model with single free surface slope is built to analyze the dynamic response of portal section of a mountain tunnel. First, the theory of elastic wave propagation is used to deduce the solution of free field ground motion on the axis of tunnel under incident SH wave without considering the existence of the tunnel. Then, the ground motions are putting on tunnel structure, where the 3-D elastic shell theory is employed in order to provide strains of the structure. The analytical solutions of the model are verified by the results of shaking table test. In this way, the analytical solutions do not only consider the amplitude-time–frequency effects, but also derives analytical expressions of axial, hoop and shear strain distribution on tunnel cross-sections. These results could be references for seismic design of tunnel portal section in mountain tunnels and have positive effects on reasonable understandings of the seismic response characteristics in this section. [ABSTRACT FROM AUTHOR]

There are many nonpersistent joints in natural rock masses, and they play an important role in the mechanical and failure characteristics of rock masses. To investigate the shear failure behaviour of 3D nonpersistent jointed rock masses, laboratory tests and numerical simulations were conducted in this paper. The 3D nonpenetrating joints are created using a 3D printer, and five persistence ratios are considered. The experimental results show that the shear strength, cohesion and friction angle decrease with increasing joint persistence. A 3D DEM model containing 3D nonpersistent joints is established considering the joint persistence. The flat-joint model and smooth-joint model are used to model rock-like materials and prefabricated joints, respectively. The numerical results are consistent with the laboratory results in both shear strength parameters and failure characteristics. Combined with numerical results, the crack initiation, propagation and coalescence on 3D nonpersistent joint specimens were analysed at the microscale. [ABSTRACT FROM AUTHOR]

• Compressive-shear tests were conducted on double-notched sandstone rock specimens with different freeze–thaw cycles. • The influence of F-T treatment on the mode II fracture toughness are analyzed. • The 3D surface topography and fractal dimension value of fracture surface was obtained. • The 3D JRC value for 3D fracture surfaces was calculated. For jointed rock masses in cold regions, freeze–thaw cycles cause damage to the rock matrix and degradation of the strength parameters. To further understand the deterioration mechanism of freeze–thaw cycles, substantial efforts have been devoted to the investigation of the failure mechanism of rock after freeze–thaw treatments. Until now, little attention has been given to the degradation of fracture toughness and the change in the fracture surface morphology of rocks after cyclic freeze–thaw treatments, especially for planar shear fracture characteristics. In this research, experiments on double-notched sandstone specimens after different freeze–thaw cycles subjected to compression-shear tests were conducted to further investigate the effect of the freeze–thaw treatment on the mode II fracture toughness and fracture surface morphological characteristics. The results show that freeze–thaw cycles have an important role in the mode II fracture toughness and that the fracture toughness shows an obvious decreasing trend with an increase in freeze–thaw cycles (N). Moreover, the 3D surface topography was reconstructed by using 3D laser scanner scanning. Parameters such as fractal dimension, asperity height and slope angle show that the fracture surface becomes rougher with increasing N. In addition, based on the 2D contour lines extracted from fracture planes, the 3D joint roughness coefficient (JRC) values of 3D fracture surfaces were calculated. The results indicate that more freeze–thaw cycles will produce rougher fracture surfaces. [ABSTRACT FROM AUTHOR]

Most back analysis methods for geotechnical engineering are based on the measured displacement. However, before the monitoring sections are assembled, the displacement—termed the displacement loss—has already been induced; this displacement is difficult to determine, and thus, it is not considered in the back analysis. In the present study, a novel displacement back analysis method considering the displacement loss is developed, that can obtain not only the reasonable mechanical parameters of rock masses but also the displacement loss. To reduce the computational cost of back analysis, a new hybrid optimization algorithm based on the Gaussian process (GP) and particle swarm optimization (PSO) is presented. The GP is used as an inexpensive fitness evaluation surrogate to predict the global optimum solution and accelerate the local search of PSO, which is employed to determine the best mechanical parameters for the model. Combined with FLAC3D numerical analysis, a novel back analysis method called GP-PSO-FLAC3D is proposed. The results of a case study demonstrate that this method can effectively predict more reasonable mechanical parameters and displacement loss using the monitored displacement. An engineering application in the auxiliary tunnel of the Jinping II hydropower station indicates that the elastic deformation of the surrounding rock increases rapidly after excavation, especially for deep tunnels, thereby resulting in a large displacement loss. The back analysis results for the main powerhouse of the Taian pumped storage power station indicate that the displacement loss also exists in engineering processes involving ordinary geostress conditions. Therefore, the displacement loss of a surrounding rock mass cannot be ignored in the stability evaluation or back analysis of underground engineering, especially for deep underground rock engineering. [ABSTRACT FROM AUTHOR]

Water inrush hazard is a major threat to safe mining and tunnel construction. During the water inrush process, nonlinear properties of rock mass can change gradually due to the erosion of flowing water. As a special rock mass, red sandstone subjected to water is prone to be disintegrated, resulting in the severe erosion. To study effects of erosion seepage on hydraulic properties of broken red sandstones, a series of seepage-induced particle erosion experiments were carried out. Forchheimer coefficient was used as an index to investigate the influencing mechanism of erosion on the nonlinear hydraulic properties. Experimental results show that, as erosion seepage processes, porosity and permeability of all samples increase, while non-Darcy factor of that decreases. Base on the variation of Forchheimer coefficient, the erosion seepage process can be divided into four stages, including the growth period, stationary period, decline period and stagnation period. Dual impacts of the erosion process on the Forchheimer coefficient were observed. On the one hand, the flow velocity is increased during the seepage process, and Forchheimer coefficient is promoted. On the other hand, the porosity of the rock mass is increased by the mass loss, and the growth of Forchheimer coefficient of the fluid is inhibited due to the erosion. The large particle rearrangement in the early stages is the main reason for the sharp rise of nonlinear hydraulic properties, and the particle loss can induce the decrease of nonlinear hydraulic properties. Prevention measures in the first two periods are more effective for mitigating water inrush hazard. Last but not least, with the increase of initial porosity and water pressure and the decrease of Talbot index, there are more rising stages of nonlinear hydraulic properties, and the greater difference between the mean value and the actual value. [ABSTRACT FROM AUTHOR]