3D DEM simulation of hard rock fracture in deep tunnel excavation induced by changes in principal stress magnitude and orientation

To achieve the loading of the stress path of hard rock,the spherical discrete element model(DEM)and the new flexible membrane technology were utilized to realize the transient loading of three principal stresses with arbitrary magnitudes and orientations.Furthermore,based on the deep tunnel of China Jinping Underground Laboratory II(CJPL-II),the deformation and fracture evolution characteristics of deep hard rock induced by excavation stress path were analyzed,and the mechanisms of transient loading-unloading and stress rotation-induced fractures were revealed from a mesoscopic perspective.The results indicated that the stressestrain curve exhibits different trends and degrees of sudden changes when subjected to transient changes in principal stress,accompanied by sudden changes in strain rate.Stress rotation induces spatially directional deformation,resulting in fractures of different degrees and orientations,and increasing the degree of deformation anisotropy.The correlation between the degree of induced fracture and the unloading magnitude of minimum principal stress,as well as its initial level is significant and positive.The process of mechanical response during transient unloading exhibits clear nonlinearity and directivity.After transient unloading,both the minimum principal stress and minimum principal strain rate decrease sharply and then tend to stabilize.This occurs from the edge to the interior and from the direction of the minimum principal stress to the direction of the maximum principal stress on theε1-ε3 plane.Transient unloading will induce a tensile stress wave.The ability to induce fractures due to changes in principal stress magnitude,orientation and rotation paths gradually increases.The analysis indicates a positive correlation between the abrupt change amplitude of strain rate and the maximum unloading magnitude,which is determined by the magnitude and rotation of principal stress.A high tensile strain rate is more likely to induce fractures under low minimum principal stress.

In-situ observations of damage-fracture evolution in surrounding rock upon unloading in 2400-m-deep tunnels

The damage-fracture evolution of deep rock mass has obvious particularity,which is revealed in 2400-mdeep tunnels by field tests.The evolution of the excavation damaged zone depth is consistent with that of the fractured zone depth.The ratio of the excavation damaged zone depth to the excavation fractured zone depth is greater than 2.0 in a rock mass with both high strength and good integrity,but less than1.5 in a rock mass with lower strength or poor integrity.Zonal disintegration in a rock mass with high strength and fair integrity is more likely to occur when it contains more than two groups of primary fractures in damaged zones.Fractures develop outward in zonal disintegration but are totally different from the single-zone fracture,in which the fractures develop inward,and it is the starting position of the fractured zone when the excavation surface of the middle pilot is 7–9 m close to the pre-set borehole and it stops after the excavation surface of the baseplate is 11–14 m away.The most intense evolution occurs around 2–4 m from the pre-set borehole in the sidewall expansion stage.The research results provide a reference for the monitoring scheme and support design of CJPL-Ⅲin its future construction.

Numerical simulation of the effect of coupling support of bolt-mesh-anchor in deep tunnel

The mechanical effects of bolt-mesh-anchor coupling support in deep tunnels were studied by using a numerical method, based on deep tunnel coupling supporting techniques and non-linear deformation mechanical theory of rock mass at great depths.It is shown that the potential of a rigid bolt support can be efficiently activated through the coupling effect between a bolt-net support and the surrounding rock.It is found that the accumulated plastic energy in the surrounding rock can be sufficiently transformed by the coupling effect of a bolt-mesh-tray support.The strength of the surrounding rock mass can be mobilized to control the deforma-tion of the surrounding rock by a pre-stress and time-space effect of the anchor support.The high stress transformation effect can be realized by the mechanical coupling effect of the bolt-mesh-anchor support, whereby the force of the support and deformation of the surrounding rock tends to become uniform, leading to a sustained stability of the tunnel.

Effect of layered joints on rockburst in deep tunnels

The existence of joints in the surrounding rock mass has a considerable efect on tunnel rockbursts.Herein,we studied the efect of layered joints with diferent inclination angles and spacings on rockburst in deep tunnels and investigated the failure area,deformation process of the surrounding rock mass,stress change inside the surrounding rock mass,velocity of the failed rock,and the kinetic energy of the failure.The failure type of the surrounding rock mass can thus be determined.The results showed that the intensity of rockburst increases as rock quality designation(RQD)decreases,while the deformation rate of the surrounding rock mass frst increases and then decreases.The deformation rate exhibits a turning point between RQD=50 and 70,below which the deformation rate of the surrounding rock mass gradually decreases,ultimately ceasing to be a rockburst.Rockburst always occurs perpendicular to the direction of the joint.Whenσ_(x)=σ_(y),as the joint inclination angle changes from 45°to 90°,the intensity of a rockburst frst decreases(from 45°to 60°),and then increases(from 60°to 90°).When combined with the evolution law of stress and strain energy,the rockburst process can be divided into four stages.

Zonal disintegration phenomenon in enclosing rock mass surrounding deep tunnels——Elasto-plastic analysis of stress field of enclosing rock mass

The zonal disintegration phenomenon （ZDP） is a typical phenomenon in deep block rock masses. In order to investigate the mechanism of ZDP, an improved non-linear Hock-Brown strength criterion and a bi-linear constitutive model of rock mass were used to analyze the elasto-plastic stress field of the enclosing rock mass around a deep round tunnel. The radius of the plastic region and stress of the enclosing rock mass were obtained by introducing dimensionless parameters of radial distance. The results show that tunneling in deep rock mass causes a maximum stress zone to appear in the vicinity of the boundary of the elastic and the plastic zone in the surrounding rock mass. Under the compression of a large tangential force and a small radial force, the rock mass in the maximum stress zone was in an approximate uniaxial loading state, which could lead to a split failure in the rock mass.

Zonal disintegration phenomenon in rock mass surrounding deep tunnels

Zonal disintegration is a typical static phenomenon of deep rock masses. It has been defined as alternating regions of fractured and relatively intact rock mass that appear around or in front of the working stope during excavation of a deep tunnel. Zonal disintegration phenomenon was successfully demonstrated in the laboratory with 3D tests on analogous gypsum models, two circular cracked zones were observed in the test. The linear Mohr-Coulomb yield criterion was used with a constitutive model that showed linear softening and ideal residual plastic to analyze the elasto-plastic field of the enclosing rock mass around a deep tunnel. The results show that tunneling causes a maximum stress zone to appear between an elastic and plastic zone in the surrounding rock. The zonal disintegration phenomenon is analyzed by considering the stress-strain state of the rock mass in the vicinity of the maximum stress zone. Creep instability failure of the rock due to the development of the plastic zone, and transfer of the maximum stress zone into the rock mass, are the cause of zonal disintegration. An analytical criterion for the critical depth at which zonal disintegration can occur is derived. This depth depends mainly on the character and stress concentration coefficient of the rock mass.

Analytical algorithm for longitudinal deformation profile of a deep tunnel

To investigate the longitudinal deformation profile(LDP)of a deep tunnel in non-hydrostatic condition,an analytical model is proposed in our study.In this model,the problem is considered as a superposition of two partial models,and the displacement field of the second partial model is the same as that of the concerned problem.Therefore,the problem can be solved by a model with simple boundary conditions.We obtain the solutions for the stress and displacement fields of an infinite body caused by arbitrary surface tractions on the boundary of the coming tunnel(zone inside the tunnel before excavation)by integrating the extended Kelvin solution over the boundary.The obtained stress solution is used to solve the specific surface tractions,which can satisfy the boundary conditions of the second partial model,on the boundary of the coming tunnel in an infinite body.Then,the specific surface tractions are substituted into the obtained displacement solution to solve the displacement on the wall and face of the tunnel.Therefore,the LDP can also be calculated.The proposed solution is verified by both numerical simulation and the LDP functions recommended by other researchers.The major advantage of our analytical model is that it can consider the effects of the axial and horizontal lateral pressure coefficients.It is revealed that the horizontal lateral pressure coefficient majorly affects the LDP behind the tunnel face,while the axial lateral pressure coefficient dominates the LDP ahead of the tunnel face.Furthermore,the deformation characteristics of the LDPs ahead of the face and the unexcavated core are investigated.The axial displacements of the excavation face can be used to predict the crown displacements ahead of the face.

Numerical Analysis of Three-Layer Deep Tunnel Composite Structure

To date,with the increasing attention of countries to urban drainage system,more and more regions around the world have begun to build water conveyance tunnels,sewage pressure deep tunnels and so on.However,the sufficient bearing capacity and corrosion resistance of the structure,which can ensure the actual service life and safety of the tunnel,remain to be further improved.Glass Fiber Reinforced Plastics(GFRP)pipe,with light weight,high strength and corrosion resistance,has the potential to be applied to the deep tunnel structure.This paper proposed a new composite structure of deep tunnel lined with GFRP pipe,which consisted of three layers of concrete segment,cement paste and GFRP pipe.A new pipe-soil spring element model was proposed for the pipesoil interaction with gaps.Based on the C3D8R solid model and the Combin39 spring model,the finite element numerical analysis of the internal pressure status and external pressure stability of the structure was carried out.Combined with the checking calculation of the theoretical formula,the reliability of the two finite element models was confirmed.A set of numerical analysis methods for the design and optimization of the three-layer structure was established.The results showed that from the internal GFRP pipe to the outer concrete pipe,the pressure decreased from 0.5 to 0.32 MPa,due to the internal pressure was mainly undertaken by the inner GFRP pipe.The allowable buckling pressure of GFRP pipe under the cover of 5 GPa high modulus cement paste was 2.66 MPa.The application of GFRP pipe not only improves the overall performance of the deep tunnel structure but also improves the construction quality and safety.The three-layer structure built in this work is safe and economical.

Attenuation characteristics of impact-induced seismic wave in deep tunnels:An in situ investigation based on pendulum impact test

The radiated seismic energy is an important index for the intensity assessment of microseismic(MS)events and the early warning of dynamic disasters.However,the energy of MS signals is significantly attenuated due to the heterogeneity and viscous damping of rock media.Therefore,the study on attenuation characteristics of MS signals in underground engineering has practical significance for the accurately estimation of radiated seismic energy.Based on a pendulum impact test facility and MS monitoring system,an in situ investigation was carried out to explore attenuation characteristics at a deep tunnel.The results show that the seismic energy and peak particle velocity(PPV)attenuation are exponentially related to the propagation distance.The attenuation coefficient of energy is larger than that of PPV.With the increase in the input impact-energy,the seismic energy attenuation coefficient decreases as a power function.An empirical relationship between energy attenuation coefficient and wave impedance of rock mass was established in this scenario.Moreover,the time-frequency characteristics and energy distribution laws of impact-induced signals were investigated by the continuous wavelet transform(CWT)and wavelet packet analyses,respectively.The dominant frequency of signals decreases gradually as the propagation distance increases.Based on the energy attenuation characteristics,a new method was proposed to calculate the released source energy of MS events in the field.This study can provide an insight into energy attenuation characteristics of seismic waves and references for attenuation correction in seismic energy calculation.

Zonal disintegration test of deep tunnel under plane strain conditions

To investigate the zonal disintegration form of the surrounding rock in deep tunnels,model tests were performed in the simulation set-up of fracture mechanism and support technology of surrounding rock in deep tunnel.The test results illustrate that the first fracture of the surrounding rock occurred at the intersection of the tunnel floor and the side wall.After more serious destruction,the side wall and the vault were destroyed.Although the fracture width of each surrounding rock mass was distinct,they were relatively uniform with a nearly continuous fracture form.The width of the split bodies of the model tunnels(i.e.,the annular zonal disintegration area)developed with an increasing load.It was observed from the fitting curves of the data that all radial strain values of the surrounding rock were more symmetric with a smooth fitting curve,and the maximum value occurred near the tunnel wall before reducing instantly.The circumferential strain values were dispersed and the data were inconsistent with the fitting curve,which caused some data to be unreliable.The phenomenon of zonal disintegration was primarily caused by radial tension strain of the surrounding rock.This phenomenon would not extend indefinitely as the rupture range would be limited to a certain extent,because the maximum radial tension strain of the surrounding rock was less than the limiting value.

Application of steel fibre reinforced sprayed concrete to a deep tunnel in weak rocks

Based on an engineering background of a deep tunneling in weak rocks, the numerical modeling is used to compare different support schemes of tunnel at great depth in this paper. Focused on the general behaviors of weak rocks at great depth, a tunneling scheme with rock bolting and steel fibre reinforced sprayed concrete is proposed. This scheme is practiced successfully at a deep tunnel in weak rocks in Coal Mine No 10 of Hebi Coal Mining Administration.

Analysis of mechanical behavior of soft rocks and stability control in deep tunnels

Due to the weakness in mechanical properties of chlorite schist and the high in situ stress in Jinping II hydropower station, the rock mass surrounding the diversion tunnels located in chlorite schist was observed with extremely large deformations. This may significantly increase the risk of tunnel instability during excavation. In order to assess the stability of the diversion tunnels laboratory tests were carried out in association with the petrophysical properties, mechanical behaviors and waterlweakening properties of chlorite schist. The continuous deformation of surrounding rock mass, the destruction of the support structure and a large-scale collapse induced by the weak chlorite schist and high in situ stress were analyzed. The distributions of compressive deformation in the excavation zone with large deformations were also studied. In this regard, two reinforcement schemes for the excavation of diversion tunnel bottom section were proposed accordingly. This study could offer theoretical basis for deed tunnel construction in similar geological condition~

Reducing risk in long deep tunnels by using TBM and drill-and-blast methods in the same project-the hybrid solution

There are many examples of TBM tunnels through mountains, or in mountainous terrain, which have suffered the ultimate fate of abandonment, due to insufficient pre-investigation. Depth-of-drilling limitations are inevitable when depths approach or even exceed l or 2 km. Uncertainties about the geology, hydro-geology, rock stresses and rock strengths go hand-in-hand with deep or ultra-deep tunnels. Unfortunately, unexpected conditions tend to have a much bigger impact on TBM projects than on drill-and-blast projects. There are two obvious reasons. Firstly the circular excavation maximizes the tangential stress, making the relation to rock strength a higher source of potential risk. Secondly, the TBM may have been progressing fast enough to make probe-drilling seem to be unnecessary. If the stress-to-strength ratio becomes too high, or if faulted rock with high water pressure is unexpectedly encountered, the ＂unexpected events＂ may have a remarkable delaying effect on TBM. A simple equation explains this phenomenon, via the adverse local Q-value that links directly to utilization. One may witness dramatic reductions in utilization, meaning ultra-steep deceleration-of-the-TBM gradients in a log-log plot of advance rate versus time. Some delays can be avoided or reduced with new TBM designs, where belief in the need for probe-drilling and sometimes also pre-injection, have been fully appreciated. Drill-and-blast tunneling, inevitably involving numerous ＂probe-holes＂ prior to each advance, should be used instead, if investigations have been too limited. TBM should be used where there is lower cover and where more is known about the rock and structural conditions. The advantages of the superior speed of TBM may then be fully realized. Choosing TBM because a tunnel is very long increases risk due to the law of deceleration with increased length, especially if there is limited pre-investigation because of tunnel depth.

Amplification effect of near-field ground motion around deep tunnels based on finite fracturing seismic source model

Dynamic failure of rock masses around deep tunnels,such as fault-slip rockburst and seismic-induced collapse,can pose a significant threat to tunnel construction safety.One of the most significant factors that control the accuracy of its risk assessment is the estimation of the ground motion around a tunnel caused by seismicity events.In general,the characteristic parameters of ground motion are estimated in terms of empirical scaling laws.However,these scaling laws make it difficult to accurately estimate the near-field ground motion parameters because the roles of control factors,such as tunnel geometry,damage zone distribution,and seismic source parameters,are not considered.For this,the finite fracturing seismic source model(FFSSM)proposed in this study is used to simulate the near-field ground motion characteristics around deep tunnels.Then,the amplification effects of ground motion caused by the interaction between seismic waves and deep tunnels and corresponding control factors are studied.The control effects of four factors on the near-field ground motion amplification effect are analyzed,including the main seismic source wavelength,tunnel span,tunnel shape,and range of damage zones.An empirical formula for the maximum amplification factor(a_(m))of the near-field ground motion around deep tunnels is proposed,which consists of four control factors,i.e.the wavelength control factor(F_(λ)),tunnel span factor(F_(D)),tunnel shape factor(F_(s))and excavation damage factor(F_(d)).This empirical formula provides an easy approach for accurately estimating the ground motion parameters in seismicityprone regimes and the rock support design of deep tunnels under dynamic loads.

Axisymmetric alternating direction explicit scheme for efficient coupled simulation of hydro-mechanical interaction in geotechnical engineering-Application to circular footing and deep tunnel in saturated ground

Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical(H-M) interaction of fluid flow and deformation induced by structures built above and under saturated ground, i.e. circular footing and deep tunnel. However, the technique is only conditionally stable and requires small time steps, portending its inefficiency for simulating large-scale H-M problems. To improve its efficiency, the unconditionally stable alternating direction explicit(ADE)scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain flow conditions. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving flow problems in nonuniform grids and under axisymmetric conditions. The new scheme is derived by performing a fourthorder finite difference(FD) approximation to the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps,giving rise to a new axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code fast Lagrangian analysis of continua(FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. Application of SEA-4-AXI for solving axisymmetric consolidation of a circular footing and of advancing tunnel in deep saturated ground shows that SEA-4-AXI reduces computer runtime up to 42%-50% that of FLAC’s basic scheme without numerical instability. In addition, it produces high numerical accuracy of the H-M solutions with average percentage difference of only 0.5%-1.8%.

Mechanism of zonal disintegration phenomenon in enclosing rock mass around deep tunnels

In order to study the mechanism of the zonal disintegration phenomenon(ZDP),both experimental and theoretical investigations were carried out.Firstly,based on the similarity law,gypsum was chosen as equivalent material to simulate the deep rock mass,the excavation of deep tunnel was modeled by drilling a hole in the gypsum models,two circular cracked zones were measured in the model,and ZDP in the enclosing rock mass around deep tunnel was simulated in 3D gypsum model tests.Secondly, based on the elasto-plastic analysis of the stressed-strained state of the surrounding rock mass with the improved Hoek-Brown strength criterion and the bilinear constitutive model,the maximum stress zone occurred in vicinity of the elastic-plastic interface due to the excavation of the deep tunnel,rock material in maximum stress zone is in the approximate uniaxial loading state owing to the larger tangential force and smaller radial force,the mechanism of ZDP was explained,which lay in the creep instability failure of rock mass due to the development of plastic zone and transfer of the maximum stress zone within the rock mass.Thirdly,the analytical critical depth for the occurrence of ZDP was obtained,which depended on the mechanical indices and stress concentration coefficient of rock mass.

Assessment of strain bursting in deep tunnelling by using the finite-discrete element method

Rockbursting in deep tunnelling is a complex phenomenon posing significant challenges both at the design and construction stages of an underground excavation within hard rock masses and under high in situ stresses. While local experience, field monitoring, and informed data-rich analysis are some of the tools commonly used to manage the hazards and the associated risks, advanced numerical techniques based on discontinuum modelling have also shown potential in assisting in the assessment of rockbursting. In this study, the hybrid finite-discrete element method(FDEM) is employed to investigate the failure and fracturing processes, and the mechanisms of energy storage and rapid release resulting in bursting, as well as to assess its utility as part of the design process of underground excavations.Following the calibration of the numerical model to simulate a deep excavation in a hard, massive rock mass, discrete fracture network(DFN) geometries are integrated into the model in order to examine the impact of rock structure on rockbursting under high in situ stresses. The obtained analysis results not only highlight the importance of explicitly simulating pre-existing joints within the model, as they affect the mobilised failure mechanisms and the intensity of strain bursting phenomena, but also show how the employed joint network geometry, the field stress conditions, and their interaction influence the extent and depth of the excavation induced damage. Furthermore, a rigorous analysis of the mass and velocity of the ejected rock blocks and comparison of the obtained data with well-established semi-empirical approaches demonstrate the potential of the method to provide realistic estimates of the kinetic energy released during bursting for determining the energy support demand.

Upper bound analysis for deep tunnel face with joined failure mechanism of translation and rotation

A joined failure mechanism of translation and rotation was proposed for the stability analysis of deep tunnel face, and the upper bound solution of supporting force of deep tunnel was calculated under pore water pressure. The calculations were based on limit analysis method of upper bound theory, with the employment of non-associated Mohr-Coulomb flow rule. Nonlinear failure criterion was adopted. Optimized analysis was conducted for the effects of the tunnel depth, pore water pressure coefficient, the initial cohesive force and nonlinear coefficient on supporting force. The upper bound solutions are obtained by optimum method. Results are listed and compared with the previously published solutions for the verification of correctness and effectiveness. The failure shapes are presented, and results are discussed for different pore water pressure coefficients and nonlinear coefficients of tunnel face.

A comparison of seismic response to conventional and face destress blasting during deep tunnel development

A novel design of development face destress blasting was implemented during the construction of an experimental tunnel at great depth.A second tunnel was developed nearby using conventional blasting as a control.The tunnels were developed parallel to one another and perpendicular to a high subhorizontal stress.High resolution seismic monitoring was used to record and compare the seismic response generated by each excavation.Analysis of the seismic data from the conventionally blasted tunnel indicated that the seismogenic zone of stress-driven instability extended up to 3.6 m ahead of the face.Destress blasting within the corresponding zone of the adjacent tunnel had the effect of reducing the rock mass stiffness,primarily due to weakening of the pre-existing natural discontinuities.The reduction in rock mass stiffness was inferred from the spatial broadening of the seismogenic zone and associated reduction in the measured spatial density of events,radiated energy and seismic potency ahead of the face.High strain gradients around the unsupported portion of the conventionally blasted excavation were implied by the rate at which the spatial density of seismicity changed with respect to the tunnel face position.In contrast,the change in the spatial density of seismicity around the destressed development face was much more gradual.This was indicative of lower strain gradients in the rock there.A reduction in rock mass stiffness following destress blasting was also indicated by the much wider variety of seismic source mechanisms recorded adjacent to the destressed tunnel.Seismic source mechanisms associated with destress blasting were also more clearly characteristic of compressive overstressing with fracture closure.The source mechanism data also indicated that destress blasting induced instability on all natural joint sets.When compared to conventional development blasting,destress blasting typically reduced violent strain energy release from the rock mass and the associated seismicity,but not always.

Inverting the rock mass P-wave velocity field ahead of deep buried tunnel face while borehole drilling

Imaging the wave velocity field surrounding a borehole while drilling is a promising and urgently needed approach for extending the exploration range of the borehole point.This paper develops a drilling process detection(DPD)system consisting of a multifunctional sensor and a pilot geophone installed at the top of the drilling rod,geophones at the tunnel face,a laser rangefinder,and an onsite computer.A weighted adjoint-state first arrival travel time tomography method is used to invert the P-wave velocity field of rock mass while borehole drilling.A field experiment in the ongoing construction of a deep buried tunnel in southwestern China demonstrated the DPD system and the tomography method.Time-frequency analysis of typical borehole drilling detection data shows that the impact drilling source is a pulse-like seismic exploration wavelet.A velocity field of the rock mass in a triangular area defined by the borehole trajectory and geophone receiving line can be obtained.Both the borehole core and optical image validate the inverted P-wave velocity field.A numerical simulation of a checkerboard benchmark model is used to test the tomography method.The rapid convergence of the misfits and consistent agreement between the inverted and observed travel times validate the P-wave velocity imaging.