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.

The role of polyurethane foam compressible layer in the mechanical behaviour of multi-layer yielding supports for deep soft rock tunnels

The polyurethane foam(PU)compressible layer is a viable solution to the problem of damage to the secondary lining in squeezing tunnels.Nevertheless,the mechanical behaviour of the multi-layer yielding supports has not been thoroughly investigated.To fill this gap,large-scale model tests were conducted in this study.The synergistic load-bearing mechanics were analyzed using the convergenceconfinement method.Two types of multi-layer yielding supports with different thicknesses(2.5 cm,3.75 cm and 5 cm)of PU compressible layers were investigated respectively.Digital image correlation(DIC)analysis and acoustic emission(AE)techniques were used for detecting the deformation fields and damage evolution of the multi-layer yielding supports in real-time.Results indicated that the loaddisplacement relationship of the multi-layer yielding supports could be divided into the crack initiation,crack propagation,strain-hardening,and failure stages.Compared with those of the stiff support,the toughness,deformability and ultimate load of the yielding supports were increased by an average of 225%,61%and 32%,respectively.Additionally,the PU compressible layer is positioned between two primary linings to allow the yielding support to have greater mechanical properties.The analysis of the synergistic bearing effect suggested that the thickness of PU compressible layer and its location significantly affect the mechanical properties of the yielding supports.The use of yielding supports with a compressible layer positioned between the primary and secondary linings is recommended to mitigate the effects of high geo-stress in squeezing tunnels.

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.

Tunnelling performance prediction of cantilever boring machine in sedimentary hard-rock tunnel using deep belief network

Evaluating the adaptability of cantilever boring machine(CBM) through in-depth excavation and analysis of tunnel excavation data and rock mass parameters is the premise of mechanical design and efficient excavation in the field of underground space engineering.This paper presented a case study of tunnelling performance prediction method of CBM in sedimentary hard-rock tunnel of Karst landform type by using tunneling data and surrounding rock parameters.The uniaxial compressive strength(UCS),rock integrity factor(Kv),basic quality index([BQ]),rock quality index RQD,brazilian tensile strength(BTS) and brittleness index(BI) were introduced to construct a performance prediction database based on the hard-rock tunnel of Guiyang Metro Line 1 and Line 3,and then established the performance prediction model of cantilever boring machine.Then the deep belief network(DBN) was introduced into the performance prediction model,and the reliability of performance prediction model was verified by combining with engineering data.The study showed that the influence degree of surrounding rock parameters on the tunneling performance of the cantilever boring machine is UCS > [BQ] > BTS >RQD > Kv > BI.The performance prediction model shows that the instantaneous cutting rate(ICR) has a good correlation with the surrounding rock parameters,and the predicting model accuracy is related to the reliability of construction data.The prediction of limestone and dolomite sections of Line 3 based on the DBN performance prediction model shows that the measured ICR and predicted ICR is consistent and the built performance prediction model is reliable.The research results have theoretical reference significance for the applicability analysis and mechanical selection of cantilever boring machine for hard rock tunnel.

Rockburst process and strength-weakening effect of the high-stress circular tunnel under internal unloading

To investigate the influence of unloading effect of a circular tunnel face on rockburst process,by innovatively combining rock drilling unloading devices and triaxial systems,the strain rockburst simulation under the entire stress path of“high initial stressþinternal unloadingþstress adjustment”(HUS test)was realized for the intact cubic red sandstone samples(100 mm×100 mm×100 mm).Comparative tests were conducted on cubic red sandstone samples with prefabricated circular holes(425 mm)under the stress path of“prefabricated circular hole+þhigh initial stress+stress adjustment”(PHS test),thereby highlighting the influence of internal unloading on rockburst failure.The test results revealed that with an increase in vertical stress,the sidewalls in both the HUS and PHS tests suffered strain rockburst failure.Compared with the PHS test,the initial failure stress in the HUS test is lower,and it is easier to induce sidewall rockbursts.This indicates that the internal unloading influences the sidewall failure,causing an obvious strength-weakening effect,which becomes more significant with an increase in buried depth.The strain rockburst failure was more severe in the HUS test owing to the influence of internal unloading.V-shaped rockburst pits were formed in the HUS tests,whereas in the PHS test,arcshaped rockburst pits were produced.It was also found that strain rockburst failure may occur only when the rock has a certain degree of rockburst proneness.

Numerical modeling of large deformation and nonlinear frictional contact of excavation boundary of deep soft rock tunnel

Roadways excavated in soft rocks at great depth are difficult to be maintained due to large deformation of surrounding rocks, which greatly influences the safety and efficiency of deep resources exploitation. During the excavation process of a deep soft rock tunnel, the rock wall may be compacted due to large deformation. In this paper, the technique to address this problem by a two-dimensional (2D) finite element software, large deformation engineering analyses software (LDEAS 1.0), is provided. By using the Lagrange multiplier method, the kinematic constraint of non-penetrating condition and static constraint of Coulomb friction are introduced to the governing equations in the form of incremental displacement. The numerical example demonstrates the efficiency of this technology. Deformations of a transportation tunnel in inclined soft rock strata at the depth of 1 000 m in Qishan coal mine and a tunnel excavated to three different depths are analyzed by two models, i.e. the additive decomposition model and polar decomposition model. It can be found that the deformation of the transportation tunnel is asymmetrical due to the inclination of rock strata. For extremely soft rock, large deformation can converge only for the additive decomposition model. The deformation of surrounding rocks increases with the increase in the tunnel depth for both models. At the same depth, the deformation calculated by the additive decomposition model is smaller than that by the polar decomposition model.

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.

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~

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.

Tunnel Surrounding Rock Deformation Characteristics and Control in Deep Coal Mining

In order to study the failure characteristics and control method of deep tunnel surrounding rock, based on the stress test, the structure and stress state of the main transportation tunnel surrounding rock in Mine Zhaogezhuang level 14 was analyzed, and it shows that the surrounding rock is exposed to an interphase hard and soft disadvantageous structure state and complex high stress repeated addition area;Through the theoretical analysis and the statistical data, the relation between the tunnel stress transformation and the surrounding rock deformation was proposed;Through the numerical simulation of the tunnel surrounding rock failure process with the help of RFPA procedure, the results show that the damage of the transportation tunnel level 14 mainly occurs in the bottom and the two coal ribs, and the failure process is spreading from the bottom to the two coal ribs, and the effect of the surrounding rock deformation control is obvious by using the way of 2.5 m anchor with 1.0 m grouting strengthening.

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.

Support pressure assessment for deep buried railway tunnels using BQ-index

Estimation of support pressure is extremely important to the support system design and the construction safety of tunnels.At present,there are many methods for the estimation of support pressure based on different rock mass classification systems,such as Q system,GSI system and RMR system.However,various rock mass classification systems are based on different tunnel geologic conditions in various regions.Therefore,each rock mass classification system has a certain regionality.In China,the BQ-Inex(BQ system)has been widely used in the field of rock engineering ever since its development.Unfortunately,there is still no estimation method of support pressure with BQ-index as parameters.Based on the field test data from 54 tunnels in China,a new empirical method considering BQ-Inex,tunnel span and rock weight is proposed to estimate the support pressure using multiple nonlinear regression analysis methods.And then the significance and necessity of support pressure estimation method for the safety of tunnel construction in China is explained through the comparison and analysis with the existing internationally widely used support pressure estimation methods of RMR system,Q system and GSI system.Finally,the empirical method of estimating the support pressure based on BQ-index was applied to designing the support system in the China’s high-speed railway tunnel—Zhengwan high-speed railway and the rationality of this method has been verified through the data of field test.

Real-time rock mass condition prediction with TBM tunneling big data using a novel rock-machine mutual feedback perception method

Real-time perception of rock mass information is of great importance to efficient tunneling and hazard prevention in tunnel boring machines(TBMs).In this study,a TBM-rock mutual feedback perception method based on data mining(DM) is proposed,which takes 10 tunneling parameters related to surrounding rock conditions as input features.For implementation,first,the database of TBM tunneling parameters was established,in which 10,807 tunneling cycles from the Songhua River water conveyance tunnel were accommodated.Then,the spectral clustering(SC) algorithm based on graph theory was introduced to cluster the TBM tunneling data.According to the clustering results and rock mass boreability index,the rock mass conditions were classified into four classes,and the reasonable distribution intervals of the main tunneling parameters corresponding to each class were presented.Meanwhile,based on the deep neural network(DNN),the real-time prediction model regarding different rock conditions was established.Finally,the rationality and adaptability of the proposed method were validated via analyzing the tunneling specific energy,feature importance,and training dataset size.The proposed TBM-rock mutual feedback perception method enables the automatic identification of rock mass conditions and the dynamic adjustment of tunneling parameters during TBM driving.Furthermore,in terms of the prediction performance,the method can predict the rock mass conditions ahead of the tunnel face in real time more accurately than the traditional machine learning prediction methods.

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.

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.

Viscoelastic Burger’s model for tunnels supported with tangentially yielding liner

This study proposed an analytical model for the tunnel supported with a tangentially yielding liner in viscoelastic ground.The efficiency of the developed analytical model was verified by comparing the calculated results with associated numerical simulation results.Using the analytical model,a comprehensive parameter sensitivity analysis was performed to examine the effects of the rate of tunnel face advancement,concrete liner thickness,installation time of liner,and strength and thickness of yielding elements on the tunnel responses.The results highlight the significant benefit of the tangentially yielding liner to relieve overstress in the tunnel liner and improve the stability of the tunnel.The yield efficiency of the tangentially yielding liner depends highly on the yielding strength and deformable capacity of the yielding elements and less on the installation time.

深埋岩溶隧道爆破开挖对围岩损伤及渗流特性的影响

深埋岩溶隧道开挖会造成围岩特殊的损伤破坏形式,并且损伤区也会影响围岩渗流场及隧道边界的涌水状况,对深埋岩溶隧道爆破开挖作用下围岩的损伤及渗流特性有重要的研究意义。为了研究爆破开挖对深埋岩溶隧道围岩损伤及渗流的影响规律,基于COMSOL Multiphysics软件建立数值模型并嵌入应力-渗流-损伤耦合方程式进行计算,采用解析法计算了简化条件下岩溶隧道开挖时围岩的应力分布,并与数值计算结果进行比较。结果表明:解析法与数值法计算的围岩应力分布有一致性,爆炸荷载使靠近溶洞侧隧道肩部及脚部区域会产生较大的拉应力;爆破后会在隔水岩柱形成“齿”状损伤区,同时引起肩部及脚部区域流速的增加,其可作为判断溶洞方位的参考依据;溶洞净距减小、洞径及水压增加会引起“齿”延伸倾角及最大涌水位置变化、隧道边界涌水量的增加,可根据“齿”延伸方向判断损伤区检测的合理方位,根据最大涌水位置变化、隧道边界涌水量的变化,合理调整涌水防治措施强度及重点防治部位。研究成果可为溶洞位置判定、隧道损伤区检测以及边界涌水防治措施提供参考。

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.