溪洛渡电站地下厂房爆破损伤范围及判据研究

通过现场爆破振动测试和数值模拟方法研究地下厂房爆破损伤范围及判据;在应用FLAC3D数值模拟软件中,损伤变量D被引入弹塑性本构模型,同时将简化的三角形荷载作为爆破冲击荷载施加在炮孔孔壁上,并根据岩石中有效应力确定爆破损伤范围。调整同时起爆炮孔数目,改变单段起爆药量,研究爆破损伤范围与单段药量的关系,结果表明:损伤深度和损伤水平半径均随单段药量的增加而增大,损伤范围随着炮孔深度增加而减小,爆破损伤最大水平半径出现在炮孔顶部面上。根据爆破近区质点速度衰减规律,并通过数据拟合得到对应于不同单段起爆药量的最大损伤水平半径和相应部位爆破质点临界损伤振动速度的关系,临界损伤振动速度可作为爆破损伤安全判据。研究成果有效地应用到实践中,对类似开挖爆破工程具有一定指导意义。     

广东岭澳核电站爆破开挖岩体损伤特征研究

为控制爆炸荷载作用下岩体的损伤范围,在岭澳核电站二期工程现场进行了爆前、爆后岩体声波测试,得到岩体的损伤范围。以此为基础,根据爆炸荷载作用下岩体损伤发展规律,基于概率形式定义损伤变量,利用有限元程序LS-DYNA和有限差分程序FLAC3D相结合的方法,对现场基岩爆破产生的岩体损伤范围进行数值模拟,并与现场岩体声波实验结果进行比较,确定爆炸荷载作用下岩体损伤门槛值Dcri=0.2,由此得到了岩体损伤范围随装药量的变化规律。结论认为,在柱状装药情况下,岩体损伤范围随装药量的增大而增大;爆炸荷载作用下的岩体损伤区深度小于损伤区半径,二者比例约为1∶3。     

Damage assessment of basaltic rock mass due to repeated blasting in a railway tunnelling project – A case study

The demand for tunnelling and underground space creation is rapidly growing due to the requirement of civil infrastructure projects and urbanisation. Blasting remains the most inexpensive method of underground excavations in hard rock. Unfortunately, there are no specific safety guidelines available for the blasted tunnels with regards to the threshold limits of vibrations caused by repeated blasting activity in the close proximity. This paper presents the results of a comprehensive study conducted to find out the effect of repeated blast loading on the damage experienced by jointed basaltic rock mass during tunnelling works. Conducting of multiple rounds of blasts for various civil excavations in a railway tunnel imparted repeated loading on rock mass of sidewall and roof of the tunnel. The blast induced damage was assessed by using vibration attenuation equations of charge weight scaling law and measured by borehole extensometers and borehole camera. Ground vibrations of each blasting round were also monitored by triaxial geophones installed near the borehole extensometers. The peak particle velocity (V_max) observations and plastic deformations from borehole extensometers were used to develop a site specific damage model. The study reveals that repeated dynamic loading imparted on the exposed tunnel from subsequent blasts, in the vicinity, resulted in rock mass damage at lesser vibration levels than the critical peak particle velocity (V_cr). It was found that, the repeated blast loading resulted in the near-field damage due to high frequency waves and far-field damage due to low frequency waves. The far field damage, after 45–50 occurrences of blast loading, was up to 55% of the near-field damage in basaltic rock mass. The findings of the study clearly indicate that the phenomena of repeated blasting with respect to number of cycles of loading should be taken into consideration for proper assessment of blast induced damage in underground excavations.     

Modelling of blast-induced damage in tunnels using a hybridfinite-discrete numerical approach

This paper presents the application of a hybrid finite-discrete element method to study blast-induceddamage in circular tunnels. An extensive database of field tests of underground explosions above tunnelsis used for calibrating and validating the proposed numerical method; the numerical results areshown to be in good agreement with published data for large-scale physical experiments. The method isthen used to investigate the influence of rock strength properties on tunnel durability to withstand blastloads. The presented analysis considers blast damage in tunnels excavated through relatively weak（sandstone） and strong （granite） rock materials. It was found that higher rock strength will increase thetunnel resistance to the load on one hand, but decrease attenuation on the other hand. Thus, undercertain conditions, results for weak and strong rock masses are similar.
2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.     

Numerical simulation of tensile damage and blast crater in brittle rock due to underground explosion

The prediction of blast crater in brittle rock due to an underground explosion has gained importance in recent years due to the great number of accidental events that affected engineering safety. This paper uses the Taylor–Chen–Kuszmaul (TCK) continuum damage model to analyze dynamic fracture behavior of rock in tension due to blast loading. The TCK damage model, together with an erosion algorithm, was implemented into the explicit FE code, LS-DYNA, as a constitutive augmentation. The damage pattern around the blasthole and the formation of blast crater near a free surface were subsequently simulated using the developed numerical tool. It is shown that the free surface is vitally responsible for the blast crater. Furthermore, the size and shape of the blast craters can be reasonably predicted if the erosion criterion of critical tensile damage is well calibrated. The effects of common charge modes on blast craters were also investigated numerically, and the mechanisms of the coupled, air-decoupled and water-decoupled charge mode are compared and presented.     

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

     

Analytical solutions for the stresses and deformations of deep tunnels in an elastic-brittle-plastic rock mass considering the damaged zone

The excavation impact (e.g. due to blasting, TBM drilling, etc.) induces an excavation damaged or disturbed zone around a tunnel. In this regard, in drill and blast method, the damage to the rock mass is more significant. In this zone, the stiffness and strength parameters of the surrounding rock mass are different. The real effect of a damage zone developed by an excavation impact around a tunnel, and its influence on the overall response of the tunnel is of interest to be quantified. In this paper, a fully analytical solution is proposed, for stresses and displacements around a tunnel, excavated in an elastic–brittle–plastic rock material compatible with linear Mohr–Coulomb criterion or a nonlinear Hoek–Brown failure criterion considering the effect of the damaged zone induced by the excavation impact. The initial stress state is assumed to be hydrostatic, and the damaged zone is assumed to have a cylindrical shape with varied parameters; thus, the problem is considered axial-symmetric. The proposed solution is used to explain the behavior of tunnels under different damage conditions. Illustrative examples are given to demonstrate the performance of the proposed method, and also to examine the effect of the damaged zone induced by the excavation impact. The results obtained by the proposed solution indicate that, the effects of the alteration of rock mass properties in the damaged zone may be considerable.     

The deformation modulus of rock masses — comparisons between in situ tests and indirect estimates

Three methods of in situ deformation modulus (Em) measurements of rock masses have been described, analysed and compared. The plate jacking (PJT) test, where the deformations are measured by extensometers in drill holes, gives generally the best results. A factor of 2.5 has been found between PJT and the Goodman jack test, and the plate loading test. From analyses of the results it has been pointed out that the damage from blasting of the test adit reduces the magnitude of test results with a factor between 2 and 4. The existing equations for indirect estimates of the rock mass deformation modulus from classification systems have been analysed and adjustment suggested. Taking into consideration the uncertainties connected to in situ deformation measurements caused by blast damage, test procedure and test method, a good characterization of the ground may give comparable, or possibly better Em values, using the RMi or the RMR system than the in situ tests. The RMR system gives, however, values that are too high for Em in massive rock.     

钻爆开挖条件下岩体临界破碎状态的损伤阈值统计研究

 研究岩体的临界破碎损伤阈值对于划分岩体的破碎状态和确定爆破岩体可开挖范围有重要意义。根据爆破作用过程和岩体破碎机制,分析钻爆条件下岩体的破碎状态,提出保留岩体与可开挖岩体的分界面处于一种临界破碎状态。基于岩体的声波特性和损伤理论,利用岩体损伤变量来表征岩体的破碎程度,并将岩体损伤分为初始损伤和爆破损伤。依托白鹤滩水电站工程项目,在坝肩爆破开挖区进行大量的声波测试试验,分析不同深度岩体的损伤状态,试验结果表明,爆后岩体临界破碎损伤变量在0.8左右。搜集国内外有关项目的声波实测数据,统计临界破碎状态岩体的损伤变量,并最终确定其损伤阈值为0.75～0.85。     

基于细观结构表征的岩石破裂热–力耦合模型及应用

 岩石热破裂的研究只有考虑各种矿物组分造成的岩石的非均匀性,才能更客观地反映岩石热破裂的本质。利用数字图像处理技术数字化表征岩石内部矿物颗粒的几何形态,充分考虑岩石真实的细观结构,结合细观损伤力学和热弹性理论,建立能更客观的分析岩石热–力耦合作用下破裂过程的数值模型。以花岗岩为例,运用数值模型研究花岗岩在温度和压缩荷载共同作用下的力学行为和破裂过程。研究结果表明,温度对岩石的力学性质和破裂演化过程影响显著,热破裂裂纹多发生在矿物颗粒边界处,并沿颗粒边界扩展,局部会形成闭合多边形,其热破裂演化过程与试验结果基本相符,从而验证了数值模型的合理性和有效性,该数值模型为细观尺度定量研究岩石热破裂提供一种新的方法。     

Blast-induced rock damage control in Fangchenggang nuclear power station,China

In the process of blasting excavation, stress wave propagation and gas expansion can basically induce damage to surrounding rocks, which is detrimental to rock mass integrity and engineering safety. In this case, evaluation and control of blast-induced effects are essential to the safety of nearby buildings and integrity of bedrock in blasting field. In Fangchenggang nuclear power station of China, the drill-and-blast method was employed for bedrock excavation. In order to reduce the blast-induced damage zone, the wave propagation and associated damage to rock mass should be carefully investigated. In this paper, the wave propagation regressively obtained from field monitoring data was presented based on empirical formula (e.g. Sadovsky formula). The relationship between the peak particle velocity (PPV) at a distance of 30 m away from the charge hole and charge per delay in blast design was derived. Meanwhile, the acoustic tests before and after blasting were conducted to determine the damage depth of rock mass. The charge per delay in blast design was then calibrated based on the blast-induced wave propagation regularity. The results showed that a satisfactory effect was achieved on blast-induced damage control of rock mass. This could be helpful to rock damage control in similar blasting projects.     

深部岩体强度准则

提出一种新的适用于深部岩体的强度准则,该准则考虑深部岩体的拉伸破坏,同时考虑深部岩体的剪胀和剪缩破坏,其破坏面可以是闭合的也可以是张开的,而且与RMR岩体地质力学分类指标建立联系.深部岩体准则中的参数均有明确的物理意义,可以很容易通过实验确定或通过RMR岩体地质力学分类确定.该准则不仅可考虑所有应力分量对材料破坏的影响,而且可反映深部岩体的受力特点.最后,将新准则与实验结果进行比较,从而验证了新准则的有效性.     

Assessment of excavation damaged zone using a micromechanics model

It is well known that acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located. The use of microseismic method has been shown as a valuable tool in a number of nuclear waste repository research programs to monitor the extent of the excavation damaged zone (EDZ), but most of the works are limited to a qualitative assessment.This paper presents a study on the quantification of the degree of damage, in terms of crack density calculated from the crack length, and the extent, in terms of crack density distribution, from microseismic event monitoring data. The approach builds on the finding that a realistic crack size corresponding to a microseismic event can be established by applying a tensile cracking model instead of the traditional shear model, commonly used in earthquake data analysis. It can be shown that brittle rock failure is the result of tensile crack initiation, propagation, accumulation, and interaction. Tensile stress can be generated in a confined rock with heterogeneous material properties. When a crack is formed by tensile cracking in this fashion, its orientation tends to become parallel to the direction of maximum compressive stress. A method is developed to take microseismic event monitoring data as input to determine the damage state and the extent of the EDZ by crack distribution. Based on the crack orientation and crack density information, the rock is modeled by a micro-mechanics based constitutive model which considers the anisotropic material properties. Numerical examples are presented using field monitoring data from a tunnel in granite to demonstrate how microseismicity can be quantitatively linked to dynamic rock mass properties.     

An evaluation of numerical approaches for S-wave component simulation in rock blasting

The shear wave(S-wave) component of the total blast vibration always plays an important role in damage to rock or adjacent structures.Numerical approach has been considered as an economical and effective tool in predicting blast vibration.However,S-wave has not yet attracted enough attention in previous numerical simulations.In this paper,three typical numerical models,i.e.the continuum-based elastic model,the continuum-based damage model,and the coupled smooth particle hydrodynamics(SPH)-finite element method(FEM) model,were first introduced and developed to simulate the blasting of a single cylindrical charge.Then,the numerical results from different models were evaluated based on a review on the generation mechanisms of S-wave during blasting.Finally,some suggestions on the selection of numerical approaches for simulating generation of the blast-induced S-wave were put forward.Results indicate that different numerical models produce different results of S-wave.The coupled numerical model was the best,for its outstanding capacity in producing S-wave component.It is suggested that the model that can describe the cracking,sliding or heaving of rock mass,and the movement of fragments near the borehole should be selected preferentially,and priority should be given to the material constitutive law that could record the nonlinear mechanical behavior of rock mass near the borehole.     

Assessing the effects of joint orientation and rock mass quality on fragmentation and overbreak in tunnel blasting

It is well known that rock mass properties exert considerable influence on the process of fragmentation. Observations in a model study and field study using conventional bulk blasting were compared in order to determine the effects of rock mass quality and joint orientation on fragmentation and profile control in tunnel blasting. It was observed in the study that blast results such as average fragment size and depth and cross-sectional area of broken zone were considerably influenced by the joint orientation. Accordingly, it is concluded that larger-capacity loading equipment and deeper blast holes are required informations with joint planes perpendicular to the tunnel axis. However, the number of blast holes should be greater in the case of joints parallel to the tunnel axis. Furthermore, the powder factor (kg / m³) is found to be directly related to the rock mass quality (Q). Optimization of pull and charge per round is required for efficient blasting in weak formations. The use of the contour blasting technique seems to be essential in poor and fair rock masses in order to minimize the overbreak, reduce the support cost, and improve the stability of the opening.     

Causes, impact and control of overbreak in underground excavations

Drill and blast system is used in hard rock excavation due to its economics and adaptability to changing rock mass conditions. Common question during mining and tunneling operations is ‘whether overbreak has been caused by blasting practice or poor rock mass quality’. Critical evaluation of the factors influencing blast damage is required to address such questions.In order to understand the mysterious nature of blast damage prediction and control, the field work involved the small scale blasting of physical models and the assessment of blast damage during drifting operations. The damage was measured by the Half cast factor, percentage overbreak and the Blast damage index. The influence of rock mass features, explosive characteristics and blast design parameters on overbreak has been examined in this study. A new approach for the judicious design of perimeter hole pattern and charge concentration has been proposed. Implications of blast damage have also been outlined in this paper.     

深埋隧洞爆破开挖损伤区检测及特性研究

 通过对锦屏二级辅助洞爆破开挖损伤区的检测和数值计算,比较岩体开挖动态过程(包括爆炸荷载和地应力高速释放)及静态过程(地应力重分布)所分别造成的损伤,探明地应力瞬态释放诱发岩体损伤的机制。检测结果表明,地应力的存在对深埋隧洞爆破开挖损伤区具有较大影响。隧洞轴线垂直于最大主应力时的钻爆开挖损伤明显要大于轴线平行于最大主应力时的情况;可以将开挖损伤区分为内损伤区和外损伤区,其中前者主要由爆炸荷载和地应力高速释放二者耦合作用引起,其主要特征是岩体声波速度显著降低;后者主要由应力重分布引起,特征是岩体声波速度缓慢降低。另外,辅助洞实测的岩体内损伤区深度显著大于外损伤区深度,且内损伤区在断面上的分布特性受到开挖二次应力场的影响,表明伴随爆破过程发生的地应力瞬态卸载效应是内损伤区形成的直接原因之一,声波检测检测和数值模拟计算均也证明了这一点。     

Physico-mechanical properties of granite after cyclic thermal shock

Understanding rock mechanical behaviors after thermal shock is critically important for practical engineering application.In this context,physico-mechanical properties of Beishan granite,Gansu Province,China after cyclic thermal shock were studied using digital image correlation(DIC),acoustic emission(AE) monitoring,and microscopic observation.The results show that the peak strength and elastic modulus decreased gradually with increase in thermal shock cycle.However,the above two parameters showed no further changes after 10 thermal shock cycles.The loading stress ratio(i.e.the ratio of the current loading stress level to the peak stress in this state) co rresponding to the occurrence of the uneven principal strain field and the local strain concentration zone on the surface of the granite specimen decreased with increase in thermal shock cycle.Three transformation forms of the standard deviation curves of the surface principal strain were found.For granite with fewer thermal shock cycles(e.g.no more than 2 cycles),the standard deviation curves exhibited approximately exponential growth in exponential form.With increase in thermal shock cycle,the S-shaped curve was dominant.After 10 thermal shock cycles,an approximate ladder-shaped curve was observed.It is displayed that AE activity was mainly concentrated around the peak strength zone of the granite specimen when the rock samples underwent fewer thermal shock cycles.With increase in thermal shock cycle,AE activity could occur at low loading stress levels.Microscopic observation further confirmed these scenarios,which showed that more microcracks were induced with increase in thermal shock cycle.The number of induced microcracks at the edge location of the granite specimen was significantly larger than that at the interior location.Finally,a continuum damage model was proposed to describe the damage evolution of the granite specimen after cyclic thermal shock during loading.     

田湾核电厂人工高边坡的三维数值模拟

在对岩体工程进行数值模拟分析时,岩体力学参数的确定是影响分析结果可靠性的关键步骤。由于岩体受结构面的控制,其变形和强度性质由岩块和结构面的性质共同决定。将岩块视为各向同性介质,其参数可由室内试验测得;将岩体中的结构面视为一种初始损伤,岩体变形参数通过岩块参数的损伤折减来确定。同时,利用Hoek-Brown岩体强度参数预测方程,结合RMR分类,通过弱化处理确定岩体的强度参数。将折减后的岩体变形参数和弱化处理后的强度参数用于田湾核电厂人工高边坡的三维数值模拟,对几种开挖方案进行分析对比,为最终设计提供了合理的挖坡角。     

岩体变形破坏过程的能量机制

 叙述岩体单元变形破坏过程中能量耗散与强度、能量释放与整体破坏等概念。在循环压缩载荷下,实测岩石的能量耗散及损伤,数据拟合表明,基于能量耗散分析建立的岩石损伤演化方程可以较好地描述岩石的损伤演化过程。在循环压缩载荷下同时实测不同加载速度及不同载荷水平下岩体内可释放应变能、耗散能、卸荷弹性模量及卸荷泊松比的变化规律,给出复杂应力条件下卸荷弹性模量的变化公式。基于可释放应变能建立岩体单元的整体破坏准则,该准则与大理岩的双压试验结果符合得比较好。对工程中常见的层状岩体,提出基于畸变能与广义体积膨胀势能而建立的层状岩体破坏准则,该准则与层状岩的双压试验也符合得比较好。