Deep Tunnel Excavation Research Articles

Effects of longitudinal ventilation and GHEs on geothermal energy extraction and HRC in high geothermal tunnels

With the excavation of deep tunnels, the problem of high geothermal heat is becoming more and more prominent. However, high geothermal heat can be utilized as a renewable energy source. In order to effectively reduce thermal damage and utilize geothermal energy, it is necessary to study the effects of longitudinal ventilation and ground heat exchanges (GHEs) on the heat-regulating circle of the surrounding rock. In this study, an experimental platform was designed and constructed in a laboratory to test different geothermal heat exchanger flow rates and ventilation air velocities and to monitor the heat extraction from the geothermal heat exchanger, the ventilation heat extraction, and the temperature field of the surrounding rock. The experimental results show that when the temperature field tends to be stabilized, the flow rate and ventilation velocity of the GHEs are closely related to the temperature distribution of the surrounding rock. The initial heat storage and heat replenishment capacity of the enclosing rock have a great influence on the heat extraction efficiency. Ventilation has a great influence on the heat extraction efficiency of GHEs, and the optimal wind speed range is 1.5∼2 m/s considering the cooling and heat extraction efficiency. When the wind speed is less than 1.5 m/s, the heat extraction efficiency of GHEs can be improved, and the total heat transfer can be increased. When the wind speed is greater than 1.5 m/s, the situation is reversed. The thermal entropy theory for circular tunnels is not fully applicable to arched tunnels containing GHEs.

Mechanised deep tunnel excavation in saturated clayey soils: a pre-design hydro-mechanically coupled method for the assessment of both spoil and face volume loss

Mechanised tunnel excavation in soils causes over-excavations, potentially leading to large amounts of spoil and settlements at ground level. An accurate estimation of over-excavations is crucial in the pre-design phase for assessing costs, determining the appropriate excavation method and choosing the muck management strategy. Currently, the estimation is based on experience and data from similar projects, but this becomes difficult when project conditions are heterogeneous. As an alternative, finite-element analyses are time-consuming and not suitable for early design stages; therefore, simplified tools are needed. In this paper, the authors present a simplified approach putting in relation face extrusion with estimated spoil mass and face volume loss. This approach, conceived for deep tunnels, is the extension to the case of mechanised tunnelling of a hydro-mechanical coupled meta-model derived from finite-element numerical analyses for tunnels in clayey soils excavated by using conventional techniques (i.e. without any use of tunnel-boring machines). The model has been validated against field data relative to a case study. The approach can be used in the early design process to identify tunnel-boring machine characteristics and provide preliminary cost estimates. In addition, during the construction phase, the method can be employed to interpret monitoring data and pre-design mitigation measures for unforeseen soil profile variations.

Reliability Evaluation and Prediction of Deep Buried Tunnel Based on Similarity Theory and Model Test

Deep buried tunnel has a complex stress environment and large stress level. At present, it is difficult to dynamic monitoring for such tunnels. In order to make the safety evaluation and prediction of tunnel closer to the engineering practice, this paper deduces the tunnel reliability evaluation algorithm. The time-varying reliability of deep tunnel excavation process is analyzed and predicted. Based on the similarity theory, the actual deep tunnel excavation is simulated by indoor model test, the stress and deformation are monitored. The results show that after the tunnel excavation, the reliability of the structure does not decrease gradually, but increases slowly first and then decreases sharply. With the increase of the buried depth of the tunnel, the structural reliability decreases gradually. When the buried depth reaches a certain depth, the structural reliability will decrease rapidly. The influence of different parameters on the structural reliability has different sensitivity. The sensitivity of friction angle is the highest, and the sensitivity of gravity is the lowest. The sensitivity has little effect with the change of tunnel excavation time and tunnel buried depth. The accuracy of this method is verified by the comparative analysis of model box tests.

Study on the Creep Characteristics and Fractional Order Model of Granite Tunnel Excavation Unloading in a High Seepage Pressure Environment

The creep associated with unloading surrounding rock during the excavation of deep tunnels seriously affects the stability of the tunnel, and a high seepage pressure will aggravate the strength attenuation and structural deterioration of the surrounding rock. Based on the background of the excavation-induced unloading of the surrounding rock of a deeply buried granite tunnel with high seepage pressure, in this paper we carry out a triaxial unloading seepage creep test that considers the effects of both excavation disturbance and seepage pressure. We also analyze the mechanism of unloading and seepage pressure leading to sample failure and construct a fractional creep damage constitutive model that considers the unloading effect. The results include the following findings, firstly, seepage pressure will affect the creep deformation of rock for a long time, and the circumferential expansion of the granite creep process is more obvious than the axial expansion. Secondly, a high seepage pressure will reduce the rock bearing capacity. Under 0, 2 and 4 MPa seepage pressures, the long-term strength of the samples are 193.7 MPa, 177.5 MPa and 162.1 MPa, respectively. Thirdly, the rock damage factor increases with increasing seepage pressure, time and deviatoric stress. Finally, the rationality of a fractional-order model that considers the effect of unloading and seepage is verified by the test data. These research results may provide some reference for the stability analysis of surrounding rock during excavation in environments under high-stress and high-seepage-pressure.

A post-peak dilatancy model for soft rock and its application in deep tunnel excavation

The dilation angle is the most commonly used parameter to study nonlinear post-peak dilatancy (PPD) behavior and simulate surrounding rock deformation; however, simplified or constant dilatancy models are often used in numerical calculations owing to their simple mathematical forms. This study developed a PPD model for rocks (rock masses) based on the Alejano–Alonso (A–A) dilatancy model. The developed model comprehensively reflects the influences of confining pressure (σ3) and plastic shear strain (γp), with the advantages of a simple mathematical form, while requiring fewer parameters and demonstrating a clear physical significance. The overall fitting accuracy of the PPD model for 11 different rocks was found to be higher than that of the A–A model, particularly for Witwatersrand quartzite and jointed granite. The applicability and reliability of the PPD model to jointed granites and different scaled Moura coals were also investigated, and the model was found to be more suitable for the soft and large-scale rocks, e.g. deep rock mass. The PPD model was also successfully applied in studying the mechanical response of a circular tunnel excavated in strain-softening rock mass, and the developed semi-analytical solution was compared and verified with existing analytical solutions. The sensitivities of the rock dilatancy to γp and σ3 showed significant spatial variabilities along the radial direction of the surrounding rock, and the dilation angle did not exhibit a monotonical increasing or decreasing law from the elastic–plastic boundary to the tunnel wall, thereby presenting the σ3-or γp-dominated differential effects of rock dilatancy. Tunnel deformation parabolically or exponentially increased with increasing in situ stress (buried depth). The developed PPD model is promising to conduct refined numerical and analytical analyses for deep tunneling, which produces extensive plastic deformation and exhibits significant nonlinear post-peak behavior.

Stability Index of Surrounding Rock during Deep Rock Excavation Considering Energy Release Speed

Rockburst is a kind of dynamic rock failure process that is easily induced by the excavation of a high-stress rock mass. However, from an energy perspective, the existing indexes for rockburst tendency have a limitation in that they do not consider the energy release speed. In this study, energy release effectiveness was proposed based on the local energy release rate while considering the influence of the energy release speed. The index can evaluate the stability of surrounding rock during the excavation of high-stress rock. The index can be obtained by recording the strain energy density of every element in the model during excavation and identifying the maximum strain energy density (Eimax), the minimum strain energy density (Eimin), the maximum time (timax), and the minimum time (timin) to calculate energy release speed and energy release effectiveness. A case study of the excavation of an experimental tunnel, namely, the URL of AECL, was adopted to validate the index. The results indicated that the proposed index can clearly identify the location and strength of the impact tendency area, and it can be effectively applied to the stability analysis during the excavation of deep tunnels.

Sensitivity Analysis of Factors Affecting the Stability of Deep Buried Tunnel

The excavation of deep tunnels has significant spatial and temporal effects on the stress distribution of the surrounding rock. Accurately describing the distribution characteristics of the surrounding rock stress state is a key issue in analyzing the failure mechanism of the surrounding rock. Based on the numerical analysis, a study on the influencing factors of the surrounding rock stress state during the excavation of deep tunnels was conducted. The results show that (i) the surrounding rock was divided into stress mutation, disturbed, and stables zones using the quantitative index: the peak and stable value of the stress triaxiality, which can reflect the influence of the principal stress orientation; (ii) the evolution law of the stress path of the surrounding rocks in different areas was obtained, and the detailed loading method of laboratory test is proposed, which could consider the influence of the principal stress magnitude and orientation during tunnel excavation; and (iii) five variable indices were used to conduct sensitivity analysis on the influencing factors of the surrounding rock stress state. Specifically, the direction of in situ stress and lateral pressure coefficient considerably influences the disturbance range, and the depth the tunnel is buried affects the deformation and failure characteristics of the surrounding rock. This paper provides a modified and quantified test method for tunnel stability analysis.

Elastoplastic Solutions for Deep Tunnel Excavation in Weak Rocks with High Geostress Considering Different Stress Release Measures

The Deformation Reserved (DR) method and the Pilot Heading (PH) method are two commonly used stress release methods in the construction of weak rock tunnel under high geostress. The ground pressure exerted on the support would be reduced to a certain extent with the stress release measures according to the Convergence–Confinement principle. The elastoplastic solutions of tunneling with DR method and PH method are derived, respectively, considering the support delay. By introducing the concept of stress release ratio, the equivalent relationship between two methods is studied. The effects of these two methods on reducing the contact pressure between surrounding rock and support are compared. Furthermore, the stress release effects of DR method and PH method under different conditions are explored. The results show that DR method is much more suitable when the geostress is not very high and surrounding rock is intact with high strength. Contrarily, if the geostress is quite high and the surrounding rock is very weak, PH method is suggested. For DR method, the stress release effect abates gradually with the increase of initial geostress, but the effect of stress release by PH method is much better under high geostress. Though both DR method and PH method are effective in reducing the contact pressure between surrounding rock and lining, the reserved deformation and the radius of the pilot tunnel are not suggested to be as large as possible.

On the Influence of Direction-Dependent Behavior of Rock Mass in Simulations of Deep Tunneling Using a Novel Gradient-Enhanced Transversely Isotropic Damage–Plasticity Model

In engineering practice, numerical simulations of deep tunneling are commonly based on isotropic linear–elastic perfectly plastic rock models. Rock, however, commonly exhibits highly nonlinear and distinct direction-dependent mechanical behavior. The former is characterized by irreversible deformation, associated with strain hardening and strain softening, and the degradation of stiffness; the latter is due to the inherent rock structure. Nevertheless, the majority of the existing rock models focuses on the prediction of either the highly nonlinear material behavior or the inherent anisotropic response of rock. The combined effects of nonlinear and direction-dependent rock behavior, particularly in the context of the numerical simulations of tunnel excavation, have rarely been taken into account so far. Thus, it is the aim of the present contribution to demonstrate the influence of both effects on the evolution of the deformation and stress distribution in the rock mass due to deep tunnel excavation on the example of a well-monitored stretch of the Brenner Base Tunnel (BBT). To this end, the recently proposed gradient-enhanced transversely isotropic rock damage–plasticity (TI-RDP) model, is employed for modeling the surrounding rock mass consisting of Innsbruck quartz-phyllite. The material parameters for the nonlinear transversely isotropic rock model are identified by means of three-dimensional finite element simulations of triaxial tests on specimens of Innsbruck quartz-phyllite, conducted for varying loading angles with respect to the foliation planes and different confining pressures. Subsequently, the results of the nonlinear 2D finite element simulations of tunnel excavation are presented for different anisotropy parameters and different orientations of the principal material directions with respect to the tunnel axis. The capabilities of the TI-RDP model are assessed by comparing the numerically predicted results with those obtained by the isotropic version of the RDP model.

Composition and Characteristics of Rock Vibration Generated in Blasting Excavation of Deep Tunnels

During blasting excavation in deep rock masses, the in situ stress initially exerted on blast-created free surfaces is rapidly released along with rock cracking by blasting. The rapid stress release can initiate seismic waves transmitting through the medium. In addition to explosion loading, the rapid stress release occurring on blast-created free surfaces is another excitation source of the rock vibration generated in blasting excavation of deep rock masses. In this paper, a theoretical model of seismic wave radiation from a circular blasting excavation in a deep rock mass is first developed to study the frequency differences between explosion seismic waves and stress release-induced seismic waves. Based on this, variational mode decomposition (VMD) is then introduced to separate explosion seismic waves and stress release-induced seismic waves from coupled vibration signals in the frequency domain. By utilizing the VMD separation, the composition and the amplitude and frequency characteristics of the rock vibration monitored in an actual deep tunnel blasting are investigated. The theoretical analysis and field investigation show that the vibration frequency of stress release-induced seismic waves is significantly lower than that of explosion seismic waves. Due to the existence of stress release-induced seismic waves with lower frequency, the coupled vibration amplitude is increased and vibration frequency is reduced. The monitored rock vibration in the near field is dominated by explosion seismic waves. However, in the far field, stress release-induced seismic waves become the major component due to their lower frequency and slower attenuation with distance. Extra care should be taken for the stress release-induced seismic waves in the far field. The stress release-induced seismic waves can be effectively reduced through shortening blast-created free surface sizes and increasing blasthole lengths moderately.

Engineering Properties of Soft Rock with High Geostress and the Performance Under Excavation of Deep Tunnel

Engineering Properties of Soft Rock with High Geostress and the Performance Under Excavation of Deep Tunnel

Quantitative Threshold of Energy Fractal Dimension for Immediate Rock Burst Warning in Deep Tunnel: A Case Study

Abstract Rock bursts are a serious geological disaster occurring in deep underground engineering operations, which will cause casualties and economic loss. The quantitative threshold of energy fractal dimension for immediate rock burst warning in a deep tunnel was studied. The study was conducted based on the immediate rock burst cases in the deep tunnels of Jinping Hydropower Station, China. Firstly, a fractal dimension calculation method was proposed for deep linear tunnels associated with microseismic monitoring to explore the energy fractal dimension during the 37 immediate rock bursts and their development to events of different intensities (intense and moderate events). On this basis, a mechanism analysis was undertaken to assess the distribution range and evolution of the energy fractal dimension in the development of the immediate rock bursts. Then, the energy fractal dimension, as a quantitative threshold, was taken as a criterion for judging the rock burst risk. Furthermore, the corresponding warning index and dynamic control method were established. This index and method were applied in the subsequent construction process. The results can be used as a guide to establish a dynamic warning system based on the microseismicity monitored and provide a scientific basis for the prediction, warning, and risk-control standard of rock burst disasters during excavation of deep tunnels.

Rockburst Prediction From the Perspective of Energy Release: A Case Study of a Diversion Tunnel at Jinping II Hydropower Station

As a man-made engineering hazard, it is widely accepted that the rockbursts are the result of energy release. Previous studies have examined the unloading of in-situ stress resulting from deep tunnel excavation as a quasi-static process but the transient stress variation during excavation has received less attention. This research discusses rockbursts that happened during the construction of a diversion tunnel at Jinping II hydropower station. The brittle-ductile-plastic (BDP) transition property of Jinping marble was numerically described by the Hoek-Brown strength criterion, and the dynamic energy release process derived from the transient unloading of in-situ stress was studied using an index, local energy release rate. Studies have shown that, due to transient unloading, the strain energy of the surrounding rock mass goes through a dynamic process of decreasing at first, increasing second, then reducing before finally stabilizing. The first decrease of strain energy results from elastic unloading waves and does not cause brittle failure in rock masses, which is consistent with the elastic condition but the secondary reduction of strain energy is because the accumulated strain energy in rock masses exceeds the storage limit, which will inevitably trigger the brittle failure in the rock mass. Thus, the shorter the distance to the tunnel wall the bigger and more intense the energy release. Finally, a relationship between the average value of the local energy release rate and the rockburst intensity was established to assess the risk of rockburst induced by the blasting excavation of a deep tunnel.

Effects of In-situ Stress on Blasting Damage during Deep Tunnel Excavation

Effects of In-situ Stress on Blasting Damage during Deep Tunnel Excavation

Research on similar test device of deep roadway impact

With the continuous increase of coal mining depth, the impact of ground pressure disaster is showing more and more serious development trend, which poses a great threat to the safety production of coal mine and the life safety of miners. The design and construction of similar test bench for deep tunnel excavation has become one of the difficult problems of scholars at home and abroad When the impact of high altitude falling hammer on the tunnel is tested, the drop hammer hits the pressure column and will be bounced, causing two shocks and affecting the experimental results. Based on the disaster mechanism of roadway rock burst, and simulating the damage caused by the immediate release of rock burst, we designed a test system that can fall smoothly when the falling hammer falls, and it will be held when the falling hammer starts to fall again. The device prevents the second impact of the falling hammer in the impact test of the roadway, and realizes the dynamic loading of the roadway.

Damage zones induced by in situ stress unloading during excavation of diversion tunnels for the Jinping II hydropower project

During the excavation of deep tunnels with high in situ stress, the stress unloading path caused by the rock mass excavation has important influences on the formation of the EDZ (excavation damage zone). In this study, the project background of the Jinping II diversion tunnels was firstly introduced. Then, the influences of stress unloading path induced by blasting excavation and tunnel boring machine (TBM) excavation on the EDZ were analyzed based on the acoustic data. Finally, the effects for different unloading paths of in situ stress induced by different excavation methods on the EDZ were studied by theoretical calculation and numerical simulation. The results show that compared with the quasi-static unloading, the transient unloading amplifies the effects of radial stress unloading and the tangential stress loading in surrounding rock masses. The EDZ depths of the blasting excavation tunnel are about 1.0 m greater than those of the TBM excavation tunnel. Moreover, the depths of the low-velocity band of the blasting excavation tunnel account for more than 50% of the total EDZ depths, while the depths of the low-velocity band of the TBM excavation tunnel account for about 30% of the total EDZ depths. The distribution of the low-velocity band along the excavation contour is closely related to the secondary stress field, indicating that the transient unloading of in situ stress is one of the important causes of the low-velocity band. After considering the post-peak mechanical properties of marble, the EDZ extents induced by different excavation methods can be estimated through numerical simulation.

150 years of plans, geological survey and drilling for the Fréjus to Mont Blanc tunnels across the Alpine chain: an historical review

Since Roman age, people living on both sides of the Alps had been seeking different ‘north-west passages’, first overriding the mountains and then moving under them. The first idea of a tunnel under the Mont Blanc was envisaged by de Saussure in 1787. In the 19th century a growing railway network played a fundamental role for the Industrial Revolution, but was hampered for the southern countries by the barrier of the Alps, so that modern transalpine railways became essential for the Reign of Sardinia. This paper presents an historical review of first suggestions, projects, field survey, failed attempts and successful drilling works across the Alps, from the Frejus (1871), San Gottardo (1882) and Simplon (1906) railway tunnels to the Grand St Bernard (1964) and Mont Blanc (1965) highway tunnels, relived within the advances of regional geology and mapping. The Fréjus tunnel was conceived by Medal, projected by Maus between Modane and Bardonèche and approved by a ministerial commission, but it was abandoned due to the insurrection of 1848. Later, it was taken up again, developed by Sommeiller, promoted by Cavour himself, and approved in 1857. Technical management was assigned to Sommeiller, Grattoni and Grandis. Fréjus was the first, longest and innovating railway tunnel in the world: drilling began in 1857 and ended in December 1870, thanks to a new drilling machine powered by compressed-air. Its inauguration took place on September 1871. When the Fréjus drilling was still in progress, John Fell built a mountain railway between Piedmont and Savoy, through the Mont Cenis, following the Napoleonic road. During its four years of work (1868- 1871) it managed to transport about 100,000 travellers, but it was forced to close down when the Fréjus railway tunnel was opened. The geological advances and mapping in the Western Alps by the mid-19th century provided information and reliable tools for better planning of great engineering works, but tunnel designs were not always based on a geological survey: the main exceptions were represented by GiorDano (Gottardo, 1881) and Baretti (Mont Blanc, 1880). The former tunnel was drilled, the latter was not accomplished, as other projects were instead preferred across the Western Alps, Colle di Tenda (1898) and then Simplon (1906). After new hopes, the geological study on four project tunnels under the Mont Blanc by Franchi, Kilian and Jacob (Franchi et alii, 1908), and further disappointments, decisive moves to achieve the underground connection between Courmayeur and Chamonix were made by Dino Lora Totino who requested a project to Vittorio Zignoli. Meanwhile, a geological survey for radioactive minerals was promoted by CNRN on the Italian side of the Mont Blanc: field work was performed by Baggio and his staff, and their best result was the discovery of mylonitic-cataclastic shear zones crossing the tunnel route. Their work continued during the drilling inside the Italian part of the Mont Blanc motorway tunnel, started in 1959 and ended in 1962: systematic geological, structural and application data were acquired by Mezzacasa during excavation and studied by Baggio, Elter and Malaroda, whereas the French side was surveyed by Gudefin and studied by Vittel and others after the end of excavation. This paper is meant to be a grateful tribute to all scientist and technicians that, thanks to geological studies, maps and sections, allowed deep mountains to be metaphorically transparent, thus making it easier and safer to physically go through them.Geology and tunneling have close interactions and mutual benefits. Preventive geology allow to optimize the technical design, reduce costs and minimize bitter surprises. Conversely, systematic survey during excavation of deep tunnels has provided innovative data for the advance of geosciences.

Experimental Investigation on Deformation and Strength Behavior of Marble with the Complex Loading-Unloading Stress Path

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading-unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high-stress underground engineering.

Analysis of Stress-Strain States in the Vicinity of Mining Excavations in a Rock Mass with Variable Mechanical Properties

Rock mass is a medium created through processes that have lasted over four billion years, and its intrinsic feature is the variability of the parameters that describe it. A particularly high variability of mechanical parameters is observed in Carboniferous sedimentary and metamorphic rocks. The mechanical properties, especially the rock mass strength, are essential for the design and maintenance of the stability of excavations conducted in it and for the safe conduct of mining operations. This article presents the test results confirming the random variability of the mechanical parameters of the rock mass. The second part of the article presents the results of numerical simulations mapping the Carboniferous rock mass along with a 1000 m deep tunnel excavation protected by steel arch supports. Numerical simulations were carried out for models with different variability coefficients of strength and deformation parameters, and the obtained results were analyzed in terms of damage zones and rock mass stress state as well as axial forces in the modelled supports. The results of the simulation demonstrate the impact of the variability of rock mass properties on its state in the vicinity of the excavation and, consequently, on the internal forces in the steel supports and their uneven loading along the length of the excavation. This fact should be taken into account in their design.

Driving force of rockburst in deep tunnels with geological structures based on nonequilibrium evolution

Data from field tests show that rockburst often occurs during the excavation of deep tunnels with geological structures. In the prediction and avoidance of rockburst, the driving force and evolution of the broken rock zone are more important than the final failure state. Deformation reinforcement theory is based on the minimum plastic complementary energy principle and reflects the nonequilibrium evolution for continuous entities. The residual unbalanced force of elastoplastic iteration describes the distance between the balanced and unbalanced states if the equilibrium and yield conditions cannot be satisfied simultaneously. On the basis of the unbalanced stress distribution that cannot be transferred and eliminated in the elastoplastic iteration process, the unstable zone can be distinguished and employed to predict rockburst. The distribution and direction of the unbalanced force represent the possible failure location and mode of the unstable structure. The magnitude of the unbalanced force indicates the severity of the failure. The unbalanced force criterion is compared with the traditional stress criterion through numerical simulations. Numerical simulations show that the worst condition with different geological structures can be addressed with an unbalanced force distribution. An unbalanced force can be employed as the driving force and criterion of rockburst.