Search

The mechanism of surface subsidence induced by shallow buried excavation construction and the failure mode under ultimate state were not unclear. In view of this situation, a scaled model experiment based on particle image velocimetry(PIV)was conducted to simulate the construction process of shallow buried tunnels in sandy soil and Ⅴ-grade surrounding rock. PIV technology was used to analyze the deformation images of the surrounding rock during the construction process of the tunnel, and to obtain displacement cloud maps of geological deformation induced by construction disturbance. By analyzing the characteristics and gradual evolution of surface deformation induced by underground excavation construction disturbance, the shape and range of the failure surface and collapse induced by underground excavation tunnel construction in different strata under the limit state were summarized. The equation of the failure surface induced by the collapse of the arch top strata during underground excavation construction was derived by using the upper limit theorem of limit analysis and variational principle, and the collapse surface graph was drawn. The validity of the model test results was verified by comparing the range of collapse induced by underground excavation construction in the arch crown strata obtained from the model test with the theoretical calculation results. [ABSTRACT FROM AUTHOR]

Underground excavation is usually accompanied by complex soil-structure interaction problems in practical engineering. This paper develops a novel multi-scale approach for investigating the soil arching effect through trapdoor tests. This approach adopts the finite element method (FEM) and smoothed particle hydrodynamics (SPH) method to handle the particle-rigid body interaction in the trapdoor tests, incorporating a micromechanical 3D-H model to derive the nonlinear material response required by the SPH method. The variation of the earth pressure on the trapdoor in simulations exhibits good agreement with those of the experiments. Extensive parametric analyzes are performed to assess the effects of soil height and inter-particle friction angle on the evolution of load transfer and soil deformation. Three deformation patterns are observed under different buried conditions, including the trapezoid, the triangle, and the equal settlement pattern. Results indicate that the planes of equal settlement develop progressively with the trapdoor movement and then enter the range of experimentally observed values. Additionally, three failure mechanisms are identified that correspond to the three deformation patterns. Due to the advantages of the micromechanical model, mesoscale behavior is captured. The anisotropy of stress distribution in the plastic region is found during the arching process. [ABSTRACT FROM AUTHOR]

In the last few decades, rock mass classification had been used intensively in engineering design. The concern arises for the potential occurrence of localized or global rock mass failure due to geological (such as discontinuities, water, weathering) and non-geological (such as construction load, vibration) factors should be taken into consideration. The ability of a classification system to represent the rock mass condition in a straightforward and effective way can assist in engineering works. Two classification systems can be applied in underground construction such as tunneling and mining. In tunnel construction, the Rock Mass Rating (RMR), Q System, Japanese Highway (JH) Method, Geological Strength Index (GSI) and Rock Mass Quality Rating (RMQR) are commonly used. However, these systems cannot be applied for rock mass covered by shotcrete due to difficulty in observing the discontinuity planes. Therefore, a new investigation method and rock mass classification for rock mass covered by shotcrete is vital to overcome this limitation. [ABSTRACT FROM AUTHOR]

Construction of huge metro projects in congested areas in Cairo City, Egypt, calls for optimization of the underground excavation process. The soil profile in Cairo city, Egypt, is generally characterized as soft soil containing layers of clay and sand. To control the ground movements and shotcrete stresses, it is necessary to optimize the ground surface settlements and excavation step. In this paper, several tunnel sections were compared to illustrate how the tunnel shape affects the soil deformations and straining actions results. Results of numerical modeling using plane-strain finite elements are presented; different types of sequences for the tunneling excavation of each section are compared. Analysis results from different models show that using different tunnel section excavation sequences affects the soil behavior differently, and that suitability of the sequence is determined by the soil properties and the dimensions of the tunneling section. [ABSTRACT FROM AUTHOR]

The anisotropy combined with the time-dependency of rock mass is commonly observed in tunnel construction. In this study, a comprehensive analytical investigation is performed on analysing time-dependent ground responses induced by tunnel excavation in an anisotropic rheological rock mass. An innovative analytical solution for the anisotropic viscoelastic problems is first proposed, which can be applied to the general cases that the rheological behaviours in anisotropic principal directions are independent and arbitrary. Using the generalized corresponding principle, the analytical model, which can precisely and rapidly address the problem, is then presented for the deformation and stresses around tunnels in transversely isotropic viscoelastic rock mass, considering different viscoelastic characteristics, different anisotropic angles, and circular/elliptical tunnel shapes. The analytical solutions agree very well with the numerical predictions for the consistent models, and can also be reduced to the isotropic viscoelastic case. Furthermore, the application of analytical solutions in practical engineering is validated by the good agreements between analytical solutions and field measurements. A parametric analysis is then performed to investigate the time-dependency of stress, and the effects of anisotropy ratios and anisotropy angles on displacements and stresses. The proposed analytical solution has a valuable contribution to the field of rock rheological mechanics, as it offers a benchmark for anisotropic viscoelastic problems, insights into the complex behaviour of anisotropic rock masses and provides a more accurate prediction of the ground response that may be useful to optimize the design of tunnel excavation in anisotropic rock masses. [ABSTRACT FROM AUTHOR]

Highlights: The entire disturbing slope is divided into four regions. The critical sliding surface shows an L-shaped or a W (double L)-shaped broken line. When the advancement exceeds a certain value, the safety factor suddenly drops. This study offers a basis for selecting an appropriate working face stop line. In this study, the Lagrange element strength reduction method is used to explore slope stability and as an evaluation method of underground mining of end-slope coal in a rock-stability analysis. A numerical analysis model is established herein using the geological conditions for mining in a coordinated open pit with an underground mining area of the Anjialing Open-Pit Mine and Underground No. 2 Mine. Additionally, the evolution law of slope stability in open-pit end-slope mining is studied using the proposed numerical simulation method. According to our findings, the steps show obvious horizontal movement and deformation under the influence of underground mining disturbances. Taking the horizontal displacement at the slope tops of the steps as the deformation index, the entire disturbed slope is divided into four regions: upper, middle-upper, middle-lower, and lower steps. When a step is fully affected by underground mining, its subsidence value first increases rapidly and then slowly. An exponential function is used to reflect the change rule in the step-subsidence value as the working face advances. In the underground mining process, the critical sliding surface of the slope develops along the soft rock or coal seam, showing an L-shaped or a W (double L)-shaped broken line. As the working face advances, the initial position of the sliding mass is unchanged while the cutting position alternately changes up and down in the weak plane. The safety factor suddenly drops when the advancing distance exceeds a certain value. [ABSTRACT FROM AUTHOR]

Stress cycle disturbances induced by underground excavation can trigger fault activation and slip, leading to earthquakes and other dynamic hazards. This study investigates shear slip induced by cyclic unloading in sawed faults through laboratory experiments. We examine the effects of unloading rate, amplitude, and initial stress on fault slip behavior. Results suggest that cyclic unloading induces fault slip and modifies fault failure characteristics, leading to a gradual rise in confinement pressure during activation. Fault displacement correlates positively with higher unloading rates, amplitudes, and initial stress levels on the fault. Fault slip rate is mainly influenced by unloading rate, whereas initial stress primarily impacts the initial slip rate. Due to fault roughness, higher unloading amplitudes can induce several acceleration events on the fault. Moreover, data from all sub-slipping events demonstrate a positive correlation between the magnitude of stress drop and maximum slip rate. This study offers a theoretical framework for the rational selection and optimization of underground development strategies, aimed at effectively mitigating the dynamic instability of faults. • Cyclic unloading drive fault slip and alters activation conditions. • Increasing unloading rate enhances fault slip rate. • Increasing unloading amplitude drives fault multiple accelerations. • Lower initial stress enhances the both the displacement and rate of initial fault slip. • A positive correlation exists between fault stress drop and maximum slip rate under cyclic unloading driving. [ABSTRACT FROM AUTHOR]

Underground excavation is a necessary process for constructing mines, tunnels and depots in granite rock mass. In this study, the numerical granite specimens were established by the discrete element method and confirmed by laboratory experiments in order to investigate the peak stress, cracking development and failure properties of pre-holed granite under coupled biaxial loading and unloading conditions. The results show that, for the specimens containing D-type and square holes, the peak biaxial unloading strengths first decrease, then increase and finally decrease as the inclination angles of the holes increase. For the specimens with elliptical holes, the peak biaxial unloading strengths first decrease and then increase with the increases in the inclination angles of the holes. The biaxial unloading strengths of specimens containing elliptical, circular, D-type and square holes decrease in that order. The cracks initially appear near the crossover points between the X-type shear fracturing plane and the pre-hole in the center and gradually expand along the X-type shear direction, which indicates that the failure of pre-holed granite is primarily shear failure. When the overall length of cracks expanding along the X-type shear direction extends to the size of the pre-hole in the center, the failure of the pre-holed specimen occurs. When the existing pre-hole in the center of the granite specimen extends to connect with the shear slip in the vicinity of the hole, this triggers overall failure. [ABSTRACT FROM AUTHOR]

• A Burgers-viscoplastic strain-softening (BVSS) model has been implemented into CODE_BRIGHT. • The BVSS is capable to simulate the time-dependent plastic behavior of geomaterials. • A coupled solid-fluid model is used to simulate the interaction between solid deformations and fluid flow. • The coupled solid-fluid response of tunnel excavation in saturated time-dependent plastic rock masses has been analyzed. This article provides a general numerical approach for modelling the response of deep tunnels excavated in saturated time-dependent plastic rock masses, considering a coupled solid-fluid interaction and time-dependent plastic behaviour. In order to do that, a Burgers-viscoplastic strain-softening model has been developed and implemented into the finite element method software CODE_BRIGHT, and a coupled solid-fluid model is used to simulate the interaction between solid deformations and fluid flow. Parametric analyses are then performed to analyse the influence on the tunnelling response of different time-dependent models, different standstill times and different excavation rates. It has been observed that the time-dependent model selection is crucial to simulate the response of underground excavations. Additionally, the coupled solid-fluid results are significantly different from the purely mechanical ones. The liquid pressure build-up in the vicinity of the tunnel face and the overpressure dissipation with time due consolidation can be accounted for. Moreover, the higher the excavation rate, the larger build-up of liquid pressures occurs. [ABSTRACT FROM AUTHOR]

During underground excavation, rocks are generally subjected to weak dynamic disturbances caused by blasting, machine vibration, rockburst. To investigate the failure characteristics of hard rock subjected to weak dynamic disturbance, two types of tests, dynamic and static true triaxial tests, with different minimum principal stresses σ3 (0.5, 5, 10, 15, 20, 25, and 30 MPa) were conducted on granite specimens. In the former type of true triaxial test, a weak dynamic disturbance was included; while in the latter type of true triaxial test, only a static stress state was considered. Then, the strength and failure mode of the tested specimens were systemically investigated. The experimental results indicate that the rock strength from the dynamic true triaxial tests is lower than that from the static true triaxial tests, indicating a reduction effect of the weak dynamic disturbance on the rock strength. This reduction effect will gradually increase with σ3. In addition, the failure of the tested specimens from the dynamic true triaxial tests is much more serious than that of the tested specimens from the static true triaxial tests, which may be due to the promotion effect of the weak dynamic disturbance on the crack development. Specifically, when σ3 is low, the dynamic disturbance primarily promotes the development of subdominant cracks (vertical tensile cracks and short oblique tensile-shear cracks) within the tested specimens; when σ3 is high, the dynamic disturbance primarily promotes the development of predominant cracks (through oblique tensile-shear cracks). [ABSTRACT FROM AUTHOR]

Frequency-based criteria for control of blasting vibrations require methods to predict both peak particle velocity and dominant frequency. In this study, based on the theory of dynamics of structures a mechanical model is built firstly to derive vibration response to a cylindrical explosion source. According to model analysis, a theoretical formula is proposed to predict the frequency-dependent attenuation of vibration induced by cylindrical explosion source. With the application of dimensional analysis, a mathematical relationship is established between the dominant frequency and its principal influencing factors. The prediction formulae derived from model analysis, and dimensional analysis showed completely consistency in the formula form, indicated that the formula obeys both the method of dynamics of structures and the dimensional homogeneity. In the end the applicability of the proposed formula is verified by single-hole blasting experiments. The prediction accuracy of the proposed formula is compared with some other prediction formulae and the new proposed formula proved to be superior with a higher fitting degree and lower deviation. In addition, application of the prediction method in multi-hole blasting is discussed. With a simplified equation to predict dominant vibration frequency, the research would give a significant guiding path for future blasting engineering. [ABSTRACT FROM AUTHOR]

When the drill and blast technique is used to excavate a roadway in underground coal mines, the relationship between the carriage and roadway space affects the automated hole positioning of a computer-controlled drill jumbo. This leads to low efficiency in hole positioning and high safety risks. This study analyzes the effects of deviations in carriage positioning, left-stacked stones, and inappropriate hole-position design and proposes solutions. This method builds a functional relationship between the size of the drill-bit coverage area and the hole-position parameters based on the drill-arm workspace and its projection. Based on the real-time carriage positioning results, the program checks the alignment between the hole positions and drill-bit coverage area. The crucial angle parameters were adjusted based on the extent to which the blast holes exceeded the coverage area. The analysis showed that the carriage deviation from the midline of the roadway and the height of the remaining stacked stones should not exceed 35–45 cm. Hole drilling and blasting tests demonstrate that this strategy ensures continuous high operational efficiency of the jumbo, thereby safeguarding the blasting efficiency. [ABSTRACT FROM AUTHOR]

Pressure arch utilises the self-stabilizing capacity of surrounding rock and is benefit to the roof stability of underground excavations. The analysis of it provides a new stability assessment tool for underground engineering. In this paper, a series of numerical simulations were carried out to investigate the formation and properties of pressure arch and the self-stabilisation mechanism of surrounding rock. It is found that stress redistribution results in the formation of the pressure arch. The inner and outer boundary of arching area are determined as the principal stress trajectories through the inflection points of the principle stress-depth curves on the centre line of caverns. Furthermore, parametric analysis illustrates that in situ stress ratio and excavation size have more significant effect on the inner and outer boundaries and the arch thickness than roof shape and overburden depth. Finally, it can be observed that the various trends of the inner boundary height with different influence factors are same with the displacement of the crown, and it could be used as an indicator for stability assessment. Based on the observed phenomena, a design scheme is proposed to determine the maximum size of the underground openings cross-section without support in agreement with the geomechanical conditions. [ABSTRACT FROM AUTHOR]

Steel reinforced shotcrete lining (SRSL) support is the primary structure to maintain the stability and mobilize the self-bearing capacity of surrounding rock. However, the structural design of SRSL in underground excavation still relies on experience-based method and lack of quantitative mechanical analysis. This paper aims to propose a modified analytical model of support characteristic curve (SCC) that represents the mechanical behavior of SRSL structures in underground construction, through which the interface bond-slip behavior between steel arch and shotcrete layer is taken into consideration. Four-point bending test of SRSL composite beam was carried out to study the bearing mechanism and failure performance. Test results show that the shotcrete-steel interface is prone to slip failure which significantly reduces the overall strength of SRSL. The laboratory test is complemented by non-liner finite element parametric studies considering the bond-slip properties to clarify the design principles and to obtain the flexural stiffness of tunnel primary lining structures. Based on above studies, the simplified formulas for the SCC of SRSL is constructed. The research results provide a theoretical basis for the design and application of SRSL structure in related projects. [ABSTRACT FROM AUTHOR]

Weak dynamic disturbance caused by drilling and blasting, adjacent rockburst, and machine vibration is a key factor to rock failure/rockburst during underground excavation. Meanwhile, the propagation direction of weak dynamic disturbances varies when different underground constructions are encountered. To investigate the effect of the propagation direction of the weak dynamic disturbance on rock failure, a general true triaxial test (applying a loading state of "six stressed faces") and a modified true triaxial test (applying a loading state of "one free face and five stressed faces") are conducted. For the former test, Pd1 (weak dynamic disturbance along the direction of σ1), Pd2 (weak dynamic disturbance along the direction of σ2), and Pd3 (weak dynamic disturbance along the direction of σ3) are involved. For the latter test, only Pd1 and Pd2 are involved due to the limitation of the testing machine used. After testing, the rock strength and macroscopic failure characteristics of the tested specimens are systematically investigated. The experimental results indicate that the rock strength of the tested specimen without weak dynamic disturbance is higher than those with weak dynamic disturbance, indicating a reduction effect of the weak dynamic disturbance on the rock strength. Pd3 has the greatest reduction effect on rock strength due to its greatest promotion effect on crack development, while the reduction effect of Pd1 on rock strength, as well as its promotion effect on crack development, is smallest. In addition, for the tested specimens with a free face, the failure process is observed and investigated. Rockburst characterized by rock fragment ejection occurs during the testing process of these specimens. The kinetic energy of the ejected blocks without weak dynamic disturbance is higher than that with weak dynamic disturbance, showing that the weak dynamic disturbance has a reduction effect on kinetic energy. In addition, the kinetic energy of ejected blocks is lower under the Pd2 condition than that under the Pd1 condition; consequently, Pd2 has a greater influence on the kinetic energy of the ejected blocks, i.e., the intensity of rockburst. [ABSTRACT FROM AUTHOR]

With the increase in mining depth and the deterioration of surrounding rock conditions, large deformation failure of deep roadway surrounding rock is still very common. To solve the support problem, it is necessary to understand the deformation and failure mechanism of the deep roadway. This paper presents a case study on the deformation failure behavior and support design of a deep roadway in the Tangyang mine by field tests and numerical simulations. The rock mass properties were first evaluated based on field data and the mechanical parameters of rock specimens. Then, the numerical model of the deep roadway surrounding rock was established based on the calibrated microparameters. The deformation features and cracking behavior of the roadway were investigated in detail. The results show that the cracking process of crack initiation, crack propagation, and rock separation in the surrounding rock of this kind of roadway is a gradual process developing from a shallow to deep depth. A combined support "bolt‐cable‐mesh‐steel ladder" was proposed based on the failure characteristics of the roadway. Finally, a numerical simulation and field experiment were conducted to evaluate the rationality of the proposed support scheme, and the results show that the new support method can effectively control the surrounding rock. This study can provide valuable references for support design in deep underground engineering. [ABSTRACT FROM AUTHOR]

The stability of a fractured rock mass around a subsurface opening is critical to tunnel excavation. The traditional underground excavation analysis is based on a continuum description of randomly distributed discrete pre-existing fractures. In contrast to this, we have developed an improved hybrid finite element method (FEM) to investigate the stability of fractured rocks around an excavation by incorporating the outputs into the FEM codes. The proposed model consists of a discrete fracture network (DFN) model and cohesive zone model (CZM). The DFN model automatically generates a fracture network with a given fracture opening distribution and provides grid generation strategy to the FEM. The CZM captures material failure and intersection and surface contacts through the fracture element with different constitutive laws. As a case study, a DFN model was formulated for the underground excavation at Jinping Hydropower Station, in light of the input requirements of our model, and its deformation analysis was performed. The comparison analysis of rock deformation was made for excavation in both intact rock mass and fractured rock mass under the same boundary conditions. The numerical results show that two different modes of rock failure exist in these two rock masses and that intense deformation at the fractures intersected by the tunnel is responsible for fractured rock mass instability. The proposed approach was verified using data from field investigations. A larger displacement can be produced if the rock mass is weakened by a key block. A sensitivity analysis was carried out to investigate the effects of different model parameters on deformation variations. This study provides an insightful understanding of the deformation of fractured rock mass during tunnel excavation. [ABSTRACT FROM AUTHOR]

As our understanding and modeling capabilities evolve, an ever-increasing complexity of models representing the behaviour of geologic medium are analyzed. One way to evaluate these substantial problems is to optimize the underlying discretization of the governing differential equations by concentrating finite elements where solution accuracy counts the most. This paper develops and evaluates the performance of a priori local p-refinement method for finite element mesh improvement for stress analysis of underground excavations. This type of refinement entails a mesh with higher-order elements near the region of interest and lower-order elements elsewhere. The focus of the paper is the automated insertion of transitional elements at the interface of the two regions. The method relies on transitional finite elements in order to connect a mesh of quadratic interpolation order elements with a mesh of linear interpolation order elements. Four types of transitional elements were considered (4-node and 5-node triangles, 5-node and 7-node quadrilaterals). These were incorporated into a finite element code, and their performance was tested using representative problems such as a pressurized cavity or tunnelling through rock. For these problems the global stiffness matrix size was reduced on average by 85% and by 81% for the models using triangles and quadrilaterals, respectively, as a result, the calculation times were considerably shortened as well. While the average percentage of error with respect to the models without improvement, measured at critical points, was 0.04% and 0.02% in the case of triangular and quadrilateral elements, respectively. [ABSTRACT FROM AUTHOR]

As more and more TBM tunnels are constructed under complex conditions such as high stress and fault fracture zones, large deformation disasters of squeezing soft surrounding rocks occur frequently. However, TBM method shows poor adaptability to major engineering challenges. Equipment damage, track deviation and support structures failure are widely observed. It is important to propose an effective support scheme and to fully clarifying the mechanical properties of the support structures in TBM tunnel. In this case, this paper aims to conduct laboratory and numerical tests considering steel–concrete interface bond-slip behavior so as to verify the effectiveness of the proposed support scheme. Thus, the quantitative relationship between arch type, arch spacing, longitudinal connection strength, concrete strength, steel fiber content, and tunnel surrounding rock deformation can be obtained. The research results show that the support scheme proposed in this paper shows good applicability for deformation control of TBM tunnels under squeezing soft rock conditions, which provides a reference for TBM tunnel support under squeezing soft rock conditions. [ABSTRACT FROM AUTHOR]

Highlights • The relationship between Young's modulus and confining pressure was described as a non-linear function according the test results. • Considering the effect of confining pressure on Young's modulus, the stress and deformation of rock mass around tunnel was calculated by both analytical and numerical methods. • Tawara saka Tunnel in Japan was taken as an example to illustrate the influence of confining pressure-dependent Young's modulus. Abstract The actual convergence of an excavation located in fractured rock mass or the soft rock is largely different with the theoretical result in many cases. Experimental results showed that the influence of confining pressure on Young's modulus is very significant. This paper attempted to illustrate the influence of the confining pressure-dependent Young's modulus in the ground reaction analyses of mountain tunnel. Firstly, the relationship between Young's modulus and confining pressure was described as a non-linear function according to the test results. Based on the plane strain axial symmetry assumption and the incremental theory of plasticity, equilibrium equations and compatibility equations of rock mass around a circular tunnel were deduced theoretically. Based on fourth Runge-Kutta method, a semi-analytical solution was achieved. Considering the effect of confining pressure on Young's modulus, the stress and deformation of rock mass around tunnel was calculated by both analytical and numerical methods. The influence of confining pressure-dependent Young's modulus in surrounding rock was estimated quantitatively. Finally, Tawara saka Tunnel in Japan was taken as an example to explain the influence of confining pressure-dependent Young's modulus. The results showed that the error with respect to the monitoring data was largely reduced with the confining pressure-dependent Young's modulus model, which indicated the necessity of considering the non-uniform distribution of Young's modulus. [ABSTRACT FROM AUTHOR]

• Hole-shape effects on mechanical behavior of sandstone under uniaxial compression are revealed. • The initiation and growth of crack around holes are analyzed using DIC technique. • Fracturing mechanism of pre-holed sample is revealed by analytic stress solution around the hole. • The localization of compressive stresses dominates the macroscopic failure of pre-holed rock. Rock failure commonly initiates near the wall of openings with various geometries in underground excavation. In this study, to elucidate the hole-shape influences on the mechanical response and fracturing behavior on rock sample subjected to uniaxial compression, a thorough experimental–numerical-theoretical investigation is conducted on the intact and pre-holed rock samples with different hole shapes, including circular, elliptical, rectangular, and inverted-U shapes. Digital image correlation technology is employed to monitor the crack initiation and development around the hole during the loading process. The results show that the presence of different hole shapes leads to varying degrees of mechanical property deterioration in rock samples. The load capacity and stiffness of the tested samples follow the same order: intact sample > circular-holed sample > elliptical-holed sample > inverted-U holed sample > square-holed sample. Irrespective of the hole shape, the primary tensile crack initiates at the top and bottom of the hole, but its development is trapped. The growth and coalescence of shear cracks near the sidewalls eventually give rise to the sample failure. Stress analysis around the holes reveals a positive dependence of the reduction in load capacity on the maximum compressive stress concentration which is controlled by the shape of the hole. This indicates that the amplification of compressive stress induced by the hole governs the rock collapse. These findings offer valuable insights for the design and construction of underground engineering projects. [ABSTRACT FROM AUTHOR]

• A method for evaluating the effectiveness of tunnel support is proposed. • Arch instability and steel-shotcrete bond slip is considered in the research. • Field monitoring of a typical tunnel are carried out to validate the results. Multi-layer composite support structure is widely applied in tunnel excavation but shows complicated failure behavior under rock pressure. Local joint broken, out-of-plane instability and arch-shotcrete interface slip widely exist and highly weaken the overall strength of support structure, which is inconsistent with the ideal assumption, leading to sever engineering disaster. Based on above situation, this paper adopts laboratory and numerical methods to investigate the bearing mechanism and failure behavior of arch joint, arch frame and arch-shotcrete composite lining. Then, a modified safety evaluation method of arch is proposed based on convergence-confinement method. The research results proved that, component strength and arch critical instability strength check are necessary in arch design. On the other hand, compared with thin-walled steel arch, the concrete-filled steel tube arch shows higher critical instability strength and better ductility, effective for controlling large deformation of soft rock. The research conclusions can provide reference for related tunnel design and construction. [ABSTRACT FROM AUTHOR]

During underground excavations for civil or mining engineering purposes, the variations in rock mass quality are important, especially for the design of the most suitable support to be applied to ensure stability. The aim of this investigation is to model the expected behavior of the ground, and thus to predict the scenarios indicating potential variations in the quality of the rock mass during underground excavation. When considering the rock mass rating (RMR) values observed at the excavation face in six study cases, which together total more than 27 km in length of underground excavation by drilling and blasting (D&B), and based on the observed RMR values at the face, the most probable value (1-100) is estimate for the RMR index at the five subsequent front advances. It is concluded that, up to about 20 m ahead of the current face, the quality of the rock mass for the next advances is close to the quality observed at the present face, and that, with increasing distance, there is a greater deviation of RMR values with respect to the quality observed at the current face. [ABSTRACT FROM AUTHOR]

The Gongbei tunnel, as a critical part of the Zhuhai connection highway of the Hong Kong-Zhuhai-Macao Bridge project, is being constructed in Zhuhai, China. The tunnel, close to the Lingdingyang bay, is located in a very environmentally sensitive location with poor geological conditions and is extremely difficult to be constructed. To ensure no interruptions to customs clearance at Gongbei Port, an innovative long-distance curved pipe roof with soil freezing method is adopted to allow a super-shallow-buried large sectional double-decked six-lane tunnel to traverse under the Port. The area of excavation cross-section is up to 337 m 2 , however the burial depth is only 5–6 m and only 0.5 m away from the pile foundations of buildings at the Port. The pipe roof around the tunnel is made up of 36 steel pipes with a length of 257 m and diameter of 1620 mm at intervals of about 356 mm, and the soil freezing method is applied for sealing. There are daunting technical challenges such as controlling the jacking precision of the long pipe roof group, guaranteeing the precise thickness of long horizontal soil freezing, controlling the ground surface deformation of the underground excavation section, etc. The paper will elaborate on the selection of the construction method for traversing under Gongbei Port, on the basic principles of the pipe-roof and soil freezing underground excavation method, on the design of a long-distance and large sectional curved pipe roof, and the design of long-distance horizontal precise soil freezing curtain. [ABSTRACT FROM AUTHOR]

Arching effect has been attributed as a factor of roof stability of underground rock excavations, which affects excavation geometry and rock support. In this paper, a series of numerical investigations were performed using the 2D finite element method (FEM) to study the formation and features of pressure arches. Based on horizontal and vertical stress distributions, three characteristic lines of the arching area were defined, i.e., the outer boundary line, the inner boundary line and the centroid line. It is found that the height of the inner boundary of the arching area is an indicator of the roof stability of underground excavations. Furthermore, detailed parametric studies including the Geological Strength Index (GSI from 20 to 80), the overburden depth ( H from 40 m to 420 m), in-situ stress ratio ( k 0 from 0.8 to 3) and excavation roof rise-to-span ratio ( h /B ratio from 0.1 to 0.5) were conducted to discuss their influences on the arching area. The results show that the initial stress state ( H and k 0 ) has more significant effects on the roof stability of underground rock caverns than the GSI and cavern geometry ( h /B). A semi-circular roof is optimal design scheme for underground excavation in surrounding mass if only considering the cavern roof stability. [ABSTRACT FROM AUTHOR]

The paper investigates the mode I fracture propagation in brittle rocks using numerical simulations based on the discrete element method. The rock material is represented as an assembly of bonded particles interacting according to an elastic-brittle contact law. The formulation of the method includes the ability for near neighbour interactions to occur so as to control the density of interparticle bonds in the medium, and thus ensures the non-dependence of the model's predictions with respect to its discretization. The adopted model was calibrated to reproduce the behaviour of a granitic rock with the aim to focus more specifically on brittle failure. Numerical samples with different discretizations were generated and subjected to different loading paths. Firstly, the response of laboratory scale pre-cracked specimens subjected to direct tension and three points bending tests was studied. Then, the fracture process zone was characterized by evaluating the volume of the damaged zone as well as the energy dissipated by cracks for different discretizations. It is shown that the proposed approach ensures the fracture process zone characteristics to be independent on the discretization of the model. In addition, large scale simulations of a well-documented underground excavation were performed in order to assess the model's capability to describe brittle failure mechanisms at the structure scale. The results confirm the independence of the model with respect to its discretization and highlight the pertinence of the proposed discrete approach for the study of large scale boundary value problems. [ABSTRACT FROM AUTHOR]

Studying the behavior of the rock mass is essential for developing a geophysical monitoring system, which allows continuous timelapse monitoring of the state of the rocks environment. For example, rock mass monitoring might be a crucial part of a system for storing dangerous, especially nuclear waste in a geological environment. The goal of this project is developing a working system for long-term timelapse monitoring of a geological environment using non-destructive geophysical techniques. The monitoring system works with geoelectrical and seismic properties of the hard-rocks environment. The system carries out 4D monitoring of the rock properties behavior in the close vicinity of an underground excavation. For the initial experimental measurements, two geophysical methods were chosen - electrical resistive tomography (ERT) and ultrasonic microseismic. The ERT method uses a set of 48 stainless electrodes 20 cm apart (9.4 m length of the profile in total) and the frequency of the measurements is six hours (i.e., four measurements per day). Currently the seismic uses frequency of five minutes between gathering one seismic record. Attention is paid to the behavior of a significant fracture located in the middle of the measuring system. The monitoring device collects data that are being sent via the LAN connection to the project members' computers. [ABSTRACT FROM AUTHOR]

The present report highlights the methods used to monitor the rock mass behavior and performance of a cavern during excavation in order to verify and, if necessary, adjust the rock support to ensure safe construction at each stage of excavation. The present geotechnical monitoring practice for underground structures involves convergence monitoring with the help of optical targets and rock mass displacement with the help of borehole extensometers. More importantly, it involves detailed planning to finalize the position of each monitoring instrument, based on the location and orientation of geological features. It is equally important to ensure proper recording of monitoring data in order to analyze and take immediate action in case of any adverse situation. This requires dedicated high-end automated software, which can record, analyse and produce significant results from a large quantity of recorded data, to prevent what could otherwise be a catastrophic failure. Initially, geotechnical monitoring was carried out in the recently excavated zone of the cavern on a daily basis. Based on continuous monitoring data for at least 1 week, the frequency of subsequent monitoring can be decided. In most cases, the deformation of rock mass was considerably less than the alarming values which were calculated based on detailed design for different rock classes. [ABSTRACT FROM AUTHOR]

• The theoretical solutions of stress variations for rock mass subjected to multi perforation hydraulic fracturing (MPHF) under excavation condition were obtained. • A numerical model to simulate the MPHF under excavation was established using a flow-coupled discrete element method (DEM), and it is verified by the theoretical solutions. • Stress and fracture characteristics during MPHF under excavation condition were presented by considering different MPHF parameters. Performing multi-perforations hydraulic fracturing (MPHF) on the rock mass is a highly effective method for increasing the fracture network, in this paper, MPHF technology was considered to assist mechanical excavation for hard rock mass in underground engineering. To end this, the influences of MPHF on the stress and fracture characteristics of hard rock mass under excavation conditions were investigated theoretically and numerically by considering different perforation spacings, perforation heights, and injection pressures. Theoretical analyses were performed based on the stress field theory of hydraulic fracturing and analytical solutions for excavation stress redistribution. A numerical model was established using a flow-coupled discrete element method, and it was verified using theoretical solutions. The results show that the local maximum principal stresses are in the horizontal direction under different MPHF parameters. The local stress differences decrease as the increasing of perforation spacing and the decreasing of perforation height and injection pressure. With an increase in perforation spacing, fewer horizontal fracture trajectories were generated between the perforation zones, and the number of microcracks initially increased and then decreased, indicating there is an optimal perforation spacing in MPHF under excavation. Moreover, the increase in perforation height, injection pressure, and injection time promote the propagation of crack trajectories and increase the number of microcracks. However, in practice, they should not increase without limits owing to efficiency and economic considerations. Therefore, when using MPHF for preconditioning the rock mass to assist mechanical excavation in the hard rock mass, the project economics and performance can be enhanced by careful selection of appropriate MPHF parameters. [ABSTRACT FROM AUTHOR]

A 3D numerical model with strong discontinuities implemented within the Enhanced Finite Element Method (E-FEM) is developed to address multi-cracking problems. Two failure criteria are proposed for two (tensile and shear) fracture kinematics; namely an anisotropic Mohr–Coulomb criterion with sliding and an anisotropic principal strain criterion with pure opening. This model is used to reproduce induced fracture networks around drifts after an underground excavation. A transversely isotropic behaviour is considered for the host rock. The influence of the anisotropy of rock properties and the in situ stress field on the induced fractures and the convergence of drifts are also studied. [ABSTRACT FROM AUTHOR]

considered an integrating part of the new modular support, the anchored support for the consolidation of rocks, associated with the basic sliding metallic shield support, implies the drilling of holes and installation of anchors in the massif rock according to diagrams and designing principles appointed considering the geo-mechanical conditions of the rocks met while carrying out the underground excavations. The researches carried out up to the present in the field of anchor installation have highlighted that the technical and economic performance criteria for the execution and durability of underground works, namely mine opening works and works carried out for the preparation of the mineral oar, are judiciously carried out if the drilling of holes for the insertion of anchoring rods is given its rightful importance in order to correspondingly chose the equipment required for the execution as well as for a competitive use of anchors.In order to ensure the competitive characteristic of the reinforcement procedure of rocks using anchors for the execution of underground mine excavations, the paper deals with some of the most performant drilling solutions which suppose the use of installations capable of contributing to the full mechanisation of the drilling process and the installation of rigid or flexible metallic rods. [ABSTRACT FROM AUTHOR]

The paper concerns shape optimization of a tunnel excavation cross-section. The study incorporates optimization procedure of the simulated annealing (SA). The form of a cost function derives from the energetic optimality condition, formulated in the authors' previous papers. The utilized algorithm takes advantage of the optimization procedure already published by the authors. Unlike other approaches presented in literature, the one introduced in this paper takes into consideration a practical requirement of preserving fixed clearance gauge. Itasca Flac software is utilized in numerical examples. The optimal excavation shapes are determined for five different in situ stress ratios. This factor significantly affects the optimal topology of excavation. The resulting shapes are elongated in the direction of a principal stress greater value. Moreover, the obtained optimal shapes have smooth contours circumscribing the gauge. [ABSTRACT FROM AUTHOR]

This study develops a coupled elastoplastic damage model for brittle rocks within the framework of irreversible thermodynamics. In the model, degradations in the deformation and strength of rocks induced by damage are considered in the elastic stiffness matrix and plastic yield criterion. The thermodynamic force associated with damage is expressed as a state function of elastic strain and the internal plastic variable. The damage potential and criterion, as functions of the thermodynamic force, are constructed to describe the damage evolution of rocks, and to determine whether they are damaged, respectively. Constitutive relationships, and their numerical formulations of a return mapping algorithm for finite element method implementation, are deduced under three different loading conditions: damage, plastic and coupled plastic damage. For engineering application, the developed model is implemented in the finite element programme Abaqus as a user-defined material model (UMAT). The coupled model is then used for simulating the re-distributed stress fields, displacement and EDZs of the sidewall rock mass around the underground powerhouse of the Wudongde hydropower plant. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to analyze the convergence measurements in drifts of the Underground Research Laboratory (URL) of the French National Radioactive Waste Management Agency (Andra), excavated in Callovo-Oxfordian claystone. These measurements show an anisotropic closure, which depends on the drift orientations with respect to the horizontal in situ stresses. This anisotropic character of the deformation is taken into account by assuming that the drifts section evolves following an elliptical shape. The characteristics of the deformed elliptical section are evaluated following the methodology proposed by Vu et al. (Rock Mech Rock Eng 46:231-246, ). Then, using the semi-empirical law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech Abstr 24:145-154, ), the convergence evolution is fitted independently for each axis of the ellipse. This method allows to distinguish two effects: the face advance and the time-dependent behavior of the ground. The results for the two drift orientations (along the major horizontal stress and perpendicular to it) show very close values for the parameters describing the time-dependent properties of the ground, the distance of influence of the face, and the extent of the decompressed zone around the drift. Finally, the model is validated by keeping these parameters as constants and by simulating the convergence data on a new drift. It is shown that with a period of about 40 days of convergence monitoring, the model can provide valuable insights for predictions of the convergence evolution in the long term. [ABSTRACT FROM AUTHOR]

Responses and damage to the structures subjected to blast-induced vibration are highly dependent on vibration frequency. Understanding the vibration frequency characteristics and its formation mechanism is essential for the safe and economic design of mining and construction blasts. When millisecond delay blasting is used in excavation, a global frequency value of all of the delays of vibrations fails to present a true picture of the vibration frequency characteristics. In the present study, comparisons of frequency characteristics for the blast-induced vibration per delay were first conducted via an underground opening case excavated by the full-face millisecond delay blasting sequence. The results show that if the blasthole geometry and charge structure are kept the same in each blast delay, the frequencies of single-delay vibration signals will decrease as the dimensions of the equivalent blasting vibration sources increase. However, the cutting blast used for the headmost holes inside is an exception, and its vibration frequency is lower than that of the breaking blast outside. It is the authors’ belief that this counter-intuitive phenomenon is caused by the free faces created by the cutting blast for the breaking blast. A small-scale blasting test and the related numerical simulation were subsequently performed to demonstrate this belief. It is found that the vibration frequency from the holes in the free-face blast is higher than that in the confined blast. In terms vibration frequency, therefore, the vibration from the breaking blast is less harmful to structures compared to the cutting blast for a certain velocity, and decreasing the burden of breaking blastholes is beneficial to reduce the vibrational damage of structures. [ABSTRACT FROM AUTHOR]

Design of underground mining operations, in their preliminary stages, often requires obtaining geomechanical data from re-logged rock core initially drilled for mineral exploration. The degree of alteration of this core due to handling and storage time increases uncertainty in the geotechnical parameters assessed and in the estimated required excavation support. Uncertainty is primarily related to changes in observed joint infill characteristics during storage. This paper presents a simple approach to assess this uncertainty quantitatively, illustrated with a case study in the Peruvian Cordillera. The Q-System was adopted for such assessment and to estimate the required excavation support. From results of the case study presented in this paper, uncertainty associated with the use of stored core was not considered significant when compared to the natural variability of the rock mass. Numerical simulations of excavation geometries for different rock mass qualities were performed to validate estimated support requirements through the Q-System. These simulations required estimations of strength characteristics of the rock mass and discontinuities. These were estimated from measurements in all core available (old and fresh) and considered the uncertainty in the adopted strength envelopes quantitatively. The range of model results and sensitivities to effects of modeling validated the recommended support requirements. During excavation, support requirements would be based on an assessment of Q as the excavation proceeds (an observational approach). [ABSTRACT FROM AUTHOR]

A new analytical model that represents the behavior of reinforced ground near a circular underground opening in a homogeneous, uniform stress field has been developed by considering the interaction behaviors between the grouted rock bolt and ground. Axial force distribution along the rock bolt has been analyzed at first. The coupling and decoupling behaviors of the interface between the ground and rock bolt are taken into account in the model. Thereafter, the mechanical behaviors of the reinforced mass are also evaluated according to the elasto-plasticity concept and the strain softening characteristic of ground. It highlights the influence of the different bolting patterns on the extent yield zone and tunnel deformation. The derivation of the analytical model and the influence of bolts on the stress and displacement field near an opening are illustrated in this paper. The theoretical predictions are verified with the measurement data in field, e.g. a Shinkansen tunnel named as Tawara Zaga Tunnel, which is planned to be open in 2022. Moreover, the applications of the proposed approach to tunnel design are discussed further according to the rock bolting effects of the standard supporting patterns of Japan Highway Public Corporation (JH) in different ground conditions. [ABSTRACT FROM AUTHOR]

Designing a rock bolt reinforcement system for underground excavation involves determining bolt pattern, spacing, and size. In this paper, a topology optimisation technique is presented and employed to simultaneously optimise these design variables. To improve rock bolt design, the proposed technique minimises a displacement based function around the opening after bolt installation. This optimisation technique is independent of the material model and can be easily applied to any material model for rock and bolts. It is also extremely flexible in that it can be applied to any mechanical analysis method. To illustrate the capabilities of this method, numerical examples with non-linear material models and discontinuities in the host rock are presented. It is shown that the complexity of systems optimised using this approach is only restricted by limitations of the method used to analyse mechanical system responses. [ABSTRACT FROM AUTHOR]

Based on the approach using the method of splicing asymptotic expansions with the retention of only the dominant term of the asymptotics, in plane strain conditions, the authors model stress state of rock mass with a horizontal bed with a long slot-like excavation. The rock mass is assumed elastic, uniform and isotropic, whereas the bed undergoes elastic and, then, elastoplastic deformation. In the latter case, the length of the face zone of plastic deformation is supposed to exceed the bed thickness and to be less than the length of the excavation. [ABSTRACT FROM AUTHOR]

Closed-form solutions are of great interest for quick and accurate evaluation of stresses and displacements around underground excavations. Based on complex variable theory and on the method of conformal mapping, closed-form solutions for stresses and displacements around deep tunnels with arbitrary cross section in a homogeneous, transversely isotropic and linear elastic ground with non-isotropic far-field stress conditions are developed. The results obtained with the proposed analytical solution are compared with those obtained with the numerical code FLAC3D for some usual shapes of tunnel openings. [ABSTRACT FROM AUTHOR]

The concerns for a more efficient underground coal mining activity in the Jiu Valley continued on previous research results in domain of supporting for underground excavations and also in ensure stability for horizontal mine workings. Researches have been made for the introduction and assimilation of the competitive laminated profiles, for implementation of supporting and most appropriate models clamps for fastening elements. In this context, taking into account the situation encountered present in Romania with regard to the plight recorded in providing the necessary laminated profiles and clamps for implementation of metallic support for underground excavations, it is proposed and presented further in order to assimilation, a new metal building support, based on external acquisition and use of laminated profiles. similar in terms of static characteristics and resistance with the most common types designed and purchased from domestic. Besides, was studied the use of the right type of clamp for joining and tightening elements, having the shape and constr [ABSTRACT FROM AUTHOR]

Ground movements during deep excavations and tunnelling, especially in urban areas, may potentially have major impact on adjacent buildings, structures and utilities. This impact on buildings and structures needs to be assessed by considering the horizontal and vertical displacements induced by deep excavations to determine the necessary mitigation measures. One major factor affecting the degree of severity the impact due to deep excavation may have on the buildings and structures is the type of foundation systems. While methodology in determining the damage category for the buildings on shallow foundation has been quite well established, the methodology for assessing the impact on the pile foundation is not straightforward due to the geometry and complexity of soil structure interaction. Often simplified two-dimensional (2D) or comprehensive three-dimensional (3D) finite element analyses would be carried out for the stage excavation to predict the displacement and stresses in the piles. Suitable protective and preventive measures would need to be designed and implemented for the existing buildings/structures if the damage category falls within the unacceptable range. This paper discusses the analysis and methodology to assess the effect on the pile foundation of a high-rise building due to the deep excavation of the Down Town Line Stage 3 (DTL3) Tampines West (TPW) Station. The approach to assess the geotechnical capacity of the pile as a result of the deep excavation is presented in this paper. Based on the assessment of pile response, predicted movement, structural and geotechnical capacities of the pile, it was found to be within the acceptable limit and the pile foundation has adequate factor of safety with the deep excavation in close proximity. [ABSTRACT FROM AUTHOR]