Numerical study on tunnel damage subject to blast-induced shock wave in jointed rock masses

In this study, numerical modeling on the damage of existing circular tunnel subject to blast-induced shock wave was carried out with DEM-based code UDEC. The disturbed zones including failure zones, open zones and shear zones around circular tunnel and peak particle velocities (PPVs) at tunnel surface are employed to analyze the damage of tunnel. The effects of joint spatial and mechanical properties, initial stress of rock mass, and magnitude of shock wave amplitude to damage of tunnel were evaluated in this study. The difference of damage between non-supported circular tunnel and bolt-supported circular tunnel subject to the same blast-induced shock wave was also studied. It is found that the orientations of joints in rock mass around the tunnel have great effects on tunnel damage. The initial stress around tunnel has relatively small influence on tunnel damage. The bolt support could greatly increase the stability of tunnel by changing the vibration form of particle velocity rather than the decreasing of PPV.     

Influence of blasting load directions on tunnel stability in fractured rock mass

Tunnels in fractured rock masses are typically damaged by dynamic disturbances from various directions. To investigate the influence of blasting load directions on the stability of a tunnel with a pre-crack nearby, blasting tests were conducted on the physical models of an external crack around a tunnel (ECT) in this study. Failure modes of the tunnels were analysed based on stress wave theory. The Riedel–Hiermaier–Thoma (RHT) material model was employed to perform the numerical simulations on ECT models. Stress distribution around the tunnels and final failure patterns of the tunnels were characterised. The results show that, under blasting loads, the pre-crack propagates and then new cracks initiates on the incident side of the tunnel. These cracks extend towards each other and eventually coalesce. Blasting load directions significantly influence the ultimate failure mode of the tunnel in the fractured rock masses. The new cracks on the shadow side of the tunnel appear at different positions when the blasting stress waves come from various directions. The results are meaningful to the analysis of tunnel stability and optimisation of the tunnel support scheme.     

Numerical investigation of arching mechanism to underground excavation in jointed rock mass

In this paper, the discontinuous deformation analysis (DDA) is applied to numerically investigate the formation of pressure arch and the stabilization mechanism of rock bolts for underground excavation in laminated rock mass. A global pressure arch with elevated stress is observed sustaining the weight of the overburden and providing the natural shield over the opening. Below the pressure arch is a loosened zone, where a group of local voussoir beam arches are formed to transfer the weight of each layer to the abutments. Detailed parametric studies regarding the in-situ stress ratio, joint friction angle, overburden depth and dip angle of strata are conducted and their influences on the formation of global pressure arch and local voussoir beam arches are identified and discussed. Based on the observed phenomena, the corresponding reinforcement scheme is proposed and the effectiveness and stabilization mechanism are examined.     

Anisotropic characteristics of jointed rock mass: A case study at Shirengou iron ore mine in China

Rock mass is characterized by the existence of distributed joints whose properties and geometry strongly affect the mechanical behavior of jointed rock masses. A finite element model considering the anisotropic characteristics of fractured rock mass was proposed which could deal with a wide variety of joint distribution in rock mass and then applied in Shirengou iron ore mine in Tangshan, China. First, the scale effects and anisotropy were investigated by using multi-scale discrete fracture network models under uniaxial compression tests. Then, the principal direction of elasticity was found and used in the constitutive law of the equivalent continuum model. Finally, the deformation and failure behavior were studied and verified through site-specific microseismic data. It is found that the stress and damage zone are influenced by joint orientation. This proposed model can efficiently study the effects of rock joints on rock mass behavior and thus contribute to a more reasonable explanation on the dominant effect of the joint sets on deformation and failure of rock mass.     

Numerical simulation of Brazilian disk rock failure under static and dynamic loading

A numerical simulator based on RFPA (Rock Failure Process Analysis) is used to study the deformation and failure process of a Brazilian disk of heterogeneous rock when subjected to static and dynamic loading conditions. In this simulator, the heterogeneity of rock is considered by assuming that the material properties of elements conform to a Weibull distribution; an elastic damage-based law that considers the strain-rate dependency is used to describe the constitutive law at mesoscopic scale; and a finite element program is employed as a basic stress analysis tool. The simulator is firstly validated by simulating the dynamic spalling of a homogeneous rock bar and by comparing with the theoretical and experimental results. Then, the failure process of a Brazilian disk of rock subjected to static and dynamic loading is numerically simulated, and the numerical results are compared with the available experimental results. Particular attention is given to the typical failure patterns of the rock disk when the incident compressive stress waves with different amplitudes are applied. The numerical simulation also identifies the failure mechanisms of rock during dynamic failure processes that are closely related to the propagation of the stress wave.     

The impact of the physical model selection and rock mass stratification on the results of numerical calculations of the state of rock mass deformation around the roadways

At present, the basic methods used for designing and evaluating the stability of mine workings are numerical models. The finite element method is the most popular method for engineering purposes. However successful calculations depend not only on the proper selection of geomechanical properties of rocks but mainly on the proper selection of a physical model describing the behavior of the rock mass and a selection of the correct failure criterion. The best way of verifying results of the calculations is to carry out investigation in the field, then.This article shows how the choice of a numerical model affects the size of the calculated damage zone around the working. To that end, numerical calculations considering elastic and elastic–plastic models were performed for six roadways. The rock mass was further differentiated in terms of its stratification and approach to mechanical properties of the rock mass. The results of these calculations were compared with measurements of mine convergence and the damage zone range in the roof. Such measurements were carried out at hard coal mine roadways.     

考虑围岩性质劣化的深埋软弱隧道破坏机理数值模拟研究

介绍劣化损伤本构模型在FLAC3D中的二次开发流程以及本构模型中软弱围岩力学参数的选取原则。然后利用FLAC3D劣化损伤本构模型对高速铁路客运专线双线深埋隧道(Ⅳ、Ⅴ级别围岩)损伤破坏机理进行数值模拟研究,分析围岩应力场特征从而确定压力拱边界,在此基础上说明深埋隧道围岩受力分区特点;接着对埋深、侧压力系数、围岩级别对受力分区的影响进行数值模拟研究。研究结果表明:FLAC3D劣化损伤模型可以描述深埋隧道开挖损伤区域的损伤程度,揭示深埋隧道破坏机理即深埋洞室围岩稳定性丧失的区域集中在沿最小主应力方向的"楔形"区内;深埋隧道开挖后受力分区:由隧道洞壁往外依次为劣化损伤严重区-压力拱拱体-原岩应力区;埋深、侧压力系数、围岩级别对隧道围岩受力分区范围有很大的影响。     

Influences of the valley morphology and rock mass strength on tunnel convergence: With a case study of Khimti 1 headrace tunnel in Nepal

Underground structures are constructed at the bottom of the valley sides for various purposes and for different reasons. Hydropower projects and transport tunnels are some of the examples of such structures. In this paper, literatures on topographical effects on the in situ stresses in valley and fjord sides are reviewed. An attempt is made to correlate stress anisotropy problems with the valley side topography by using Phase² numerical modelling. Based on an underground construction case study, fifteen in situ stress measurements and the Phase² analysis, stress induced problems have been found to be influenced by the valley morphology. This influence can be monitored by the convergence measurement and by the stress measurement. In addition to the overburden height, the total valley height and the slope need to be considered in the assessment of the stress induced problem. The second aspect dealt with is the influence of the rock strength on the tunnel convergence. In the Khimti 1 headrace tunnel and 66 cases from 15 countries, it has been observed that the tunnel convergence is larger in the weaker rocks than in the stronger rocks though they may have similar Q-values. Rock type such as gneiss or phyllite (corresponding to the rock mass strength) is not considered in the Q-system but it has influence on the convergence that takes place in underground works. Thus, it also needs to be considered in the assessment of potential convergence of an underground structure.     

Two-scale modeling of jointed rock masses

Numerical modeling of the mechanical behavior of jointed rock masses is a difficult task as the discontinuities not only require special modeling consideration, but also require different treatments depending upon the scale of problem involved. A framework is proposed here that is based upon a meshed based partition of unity method, also known as the numerical manifold method. Specifically, the discontinuities posed by joints or faults are divided into two groups. The primary discontinuity set is the one with a wider spacing and dominates the kinematics of the system. It is modeled explicitly as physical discontinuities. The secondary discontinuity set, characterized by high density and small spacing, is modeled as an equivalent continuum by incorporating joint compliance and strength characteristics. Following the presentation of the formulation, two examples are also provided at the end to highlight the ease with which the proposed approach may be used in mechanical analysis of a complex jointed rock system.     

Critical strain and squeezing of rock mass in tunnels

The squeezing of tunnels is a common phenomenon in poor rock masses under high in situ stress conditions. The critical strain parameter is an indicator that allows the degree of squeezing potential to be quantified. It is defined as the strain level on the tunnel periphery beyond which instability and squeezing problems are likely to occur. Presently, in the literature, the value of critical strain is generally taken as 1%. It is shown in this study that the critical strain is an anisotropic property and that it depends on the properties of the intact rock and the joints in the rock mass. A correlation of critical strain with the uniaxial compressive strength, tangent modulus of intact rock and the field modulus of the jointed mass is suggested in this paper. It is also suggested that the modulus of deformation being anisotropic in nature should be obtained from field tests. In absence of field tests, use of a classification approach is recommended, and, expressions are suggested for critical strain in terms of rock mass quality Q. A rational classification based on squeezing index (SI) is proposed to identify and quantify the squeezing potential in tunnels. Applicability of the approach is demonstrated through application to 30 case histories from the field.     

Design of large cuttings in jointed rock

The article presents a stability study on a very high cutting in a granitic gneiss, as part of the construction of a bypass in the French Pyrenees. The preliminary study highlighted questions as to the most appropriate method of designing the rock cuts, as such methods are not standardized in France. A temporary stop in the roadworks allowed a second study to be carried out based on a statistical analysis of the discontinuities of the site and better-developed modeling tools, including Resoblok. A comparative analysis of these methods and their results in terms of bolt sizing is presented, taking into account the difficulty of use of each method.      

直剪条件下节理岩体的变形和强度特征研究

通过建立单一闭合水平节理岩体的数值模型，运用数值模拟软件对节理岩体模型在直剪试验条件下节理的剪切变形和强度特征进行模拟，并对模拟结果做了分析。     

节理岩体各向异性力学研究综述

对节理岩体各向异性力学研究的现状与进展进行了系统分析和阐述,分别探讨了节理本身的各项异性力学性质,岩石介质和节理综合(即整个节理岩体)的各向异性力学性质,并对节理岩体本构模型、各向异性力学参数和屈服准则研究进行了较为详细的描述和比较,以指导实践。     

Reinforcement of rock mass with cross-flaws using rock bolt

Rock bolting is one of the most effective and economical means of rock mass reinforcement. Existing studies of rock bolt reinforcement are mostly focused on rock masses without flaw, with a single flaw, or with parallel flaws. However in rock masses, cracks or flaws usually exist in the form of cross-flaws. In order to understand the impact of cross-flaws on rock bolt reinforcement and to further explore the differences of bolt reinforcement between rock mass with cross-flaws and rock mass with a single flaw, reinforced analog specimens with cross-flaws and with a single flaw were tested under uniaxial compressive condition. The experimental results show that the uniaxial compressive strength of the reinforced rock mass with cross-flaws in this research is higher than that of reinforced rock mass with a single flaw. This observation can be explained by the difference in the failure modes of reinforced specimens: the reinforced rock masses with a single flaw fail due to the formation of a shear crack while reinforced rock masses with cross-flaws fail as a result of a tensile fracture or interaction between tensile fracture and shear fracture.     

A methodology for evaluation and classification of rock mass quality on tunnel engineering

In this paper a new methodology for evaluation and classification of rock mass quality that can be applied to rock tunneling is presented. An evaluation model based on combing the analytic hierarchy process (AHP) and the fuzzy Delphi method (FDM) for assessing the rock mass rating is the main procedure. This research treats rock mass classification as a group decision problem, and applies the fuzzy logic theory as the criterion to calculate the weighting of factors. The main advantage of this procedure is that it can effectively change the weighting of each rating parameter with the variation of geological conditions. The proposed method was evaluated and applied to the actual cases that are the two tunnels along the Second Northern Highway around Taipei area in Taiwan, namely Mu-Zha and Hsin-Tien tunnels. It was found that the determined results were in a good agreement with the original data assessed by the RMR. Results of the analyses show that it can be provided a more quantitative measure of rock mass and hence minimize judgmental bias. The proposed method should be more feasible for future tunnel construction and for suggestions of tunnel support design in the geological area of Taiwan.     

Numerical simulation of the rock fragmentation process induced by indenters

Rock fragmentation processes induced by single and double indenters were examined by a numerical method. The simulated results reproduce the progressive process of rock fragmentation in indentation. Rock deforms elastically at the initial loading stage. Then tensile cracks are initiated around the two corners of the truncated indenter and propagate in the well-known conical Hertzian manner. The rocks immediately under the indenter are in a highly tri-axial stress state, and some of them fail in the ductile cataclastic mode with the stress satisfying the ductile failure surface of the double elliptic strength criterion. With the tensile cone cracks and ductile cataclastic failure releasing the confining pressure, the rocks under the indenter are compressed into failure and the crushed zone gradually comes into being. With increasing loading displacement, the re-compaction behaviour of the crushed zone occurs. Side cracks initiated from the crushed zone or bifurcated from cone cracks are driven by tensile stress associated with the crushed zone to propagate in a curvilinear path and finally intersect with the free surface to form chips. It is pointed out that the curvilinear path is caused by heterogeneity. The simulated force-penetration curve is in fact the indication of the propagation of cracks, the crushing of microstructural grains and the formation of chips. It is found that the confining pressure has an important influence on the indentation results. With decreasing confining pressure, there is a decrease in the indentation strength and a change in the rock failure process from the formation of rock chips to a vertically axially splitting failure. The simulated fragmentation process in the double indenter test reproduces the side cracks, which are induced by two indenters, propagate, interact and finally coalesce, chipping the rock between the indenters. The line spacing is an important factor that affects the fragmentation efficiency in multiple indenter tests. It is pointed out that simultaneous loading with multiple indenters with an appropriate line spacing seems to provide a possibility of forming larger rock chips, controlling the direction of subsurface cracks and consuming a minimum total specific energy. According to the simulated results, it is believed that the numerical simulation method will contribute to an improved knowledge of rock fragmentation in indentation, which will in turn help to enhance mining and drilling efficiency through the improved design of mining tools and equipment.     

节理岩体变形分析与计算模型探讨

主要分析了岩体模型的简化,提出了均质节理岩体的本构模型,并在简单讨论了非连续变形数值分析方法的同时,讨论和分析了非连续变形计算力学模型的基本原理,它可以很精确地预测岩土工程中的变形和接触力分布及其整体稳定安全系数,并论证了它的合理性。     

超大断面隧道软弱围岩卸荷渐进破坏特性研究

以深圳市东部过境高速公路连接线工程为背景,针对谷对岭“Y”形喇叭口大断面分岔隧道,通过室内地质力学模型试验和数值模拟等手段,对大断面隧道围岩的渐进性破坏过程、岩体内部变形和应力变化规律进行了研究。研究结果表明:软弱隧道围岩的破坏是始自拱腰以下的岩体,而后自拱腰向上继续扩展成拱,为此必须要对拱腰以下岩体施作锁脚锚杆,从而制止岩体的初始剪切破坏;当采取左右导洞分块开挖时,后开挖导洞会引起既有洞室围岩的破坏,因此需要对导洞之间的隔墙进行加固;拱顶上方0.95B（B为隧道跨度）范围内的岩体变形受到隧洞开挖影响,但最终塌落成拱的高度为0.55B;隧道开挖后,拱顶上方岩体应力升高区主要集中在0.4B~0.95B的范围内。