Rock mechanical problems and optimization for the long and deep diversion tunnels at Jinping Ⅱ hydropower station

According to site-specific environments such as high water pressures,high in-situ stresses and strong rockbursts,the design scheme of the long and deep diversion tunnels at Jinping II hydropower station was optimized to ensure construction safety.New drainage tunnels were considered.Furthermore,lining structures and grouting pressures were modified during the excavation of tunnels.The construction scheme was updated dynamically based on the complex geological conditions.For instances,the diversion tunnels w...     

Management of rock bursts during excavation of the deep tunnels in Jinping II Hydropower Station

Rock mass breakage induced by high in situ stresses during excavation of deep tunnels is one of the key problems in the construction of the Jinping II Hydropower Station. It is not only a threat to the safety of workers, but also seriously affects the construction process. This paper discusses where the rock bursts occur, how they occur and how to reduce their effect.     

Practices on rockburst prevention and control in headrace tunnels of Jinping II hydropower station

Rockburst problems induced by high in-situ stresses were prominent during construction of the headrace tunnels of Jinping II hydropower station. The rockbursts occurred in various forms, and it is necessary to take pertinent measures for integrated prevention and control of rockbursts. In view of the rockburst characteristics during tunnel construction of Jinping II hydropower station, the engineering geological conditions were presented, and the features, mechanisms and forms of rockbursts observed during construction were analyzed in detail. A large number of scientific researches, experiments and applications were conducted. Multiple measures were adopted to prevent and control rockbursts, including the prediction and early warning measures, stress relief by blasting in advance, optimized blasting design and optimized tunnel support in the tunnel sections prone to strong rockbursts. The effectiveness of these prevention and control measures was evaluated. Experiences have been accumulated through a great number of helpful explorations and practices for rockburst prevention and control. A comprehensive rockburst prevention and control system has been gradually established.     

Evaluation of deformation response for adjacent tunnels due to soil unloading in excavation engineering

A major challenge in the design and construction of soil excavation and foundation pit engineering in urban areas is the protection of adjacent underground structures, such as existing tunnels. Excavation-induced soil unloading can adversely affect and even damage the tunnels in the vicinity. A simplified analytical approach is presented to analyze the deformation response for adjacent tunnels due to excavation-induced soil unloading in excavation engineering. Firstly, the green soil unloading stress due to adjacent excavation is estimated at the existing tunnel location. Secondly, the deformation response of the tunnel subjected to green soil unloading stress is calculated by the Galerkin’s method, which can be used to obtain the finite element equation converted from the differential equation. The accuracy of the proposed method is verified by comparisons with 3D finite element numerical simulation, centrifuge model tests provide by Kusakabe et al. (1985) and measured data in situ. Finally, the parametric analysis for deformation influence factors of the existing tunnel, including the tunnel buried depth, the distance from the excavation site, the soil geo-characters and the outer diameter of the tunnel, is presented to demonstrate the performance of the proposed method. This proposed method may provide certain basis to make protective measures of existing tunnels influenced by excavation engineering and enables a quick estimate of the deformation behavior of excavation-induced adjacent tunnels, resulting in savings in time and costs.     

Analysis of stress evolution characteristics during TBM excavation in deep buried tunnels

Bulletin of Engineering Geology and the Environment - Full-face tunnel boring machines (TBMs) are widely used in tunnel engineering projects as an efficient, safe and economical tunneling method....     

Rockmass damage development following two extremely intense rockbursts in deep tunnels at Jinping II hydropower station, southwestern China

Two extremely intense rockbursts took place during the excavation of the drainage tunnel and headrace tunnel No. 4 at the Jinping II hydropower station in southwestern China. The geological conditions, damage, and failure of the surrounding rock mass at the sections where the rockbursts occurred were surveyed and are described here with a focus on the development of the damage to the rock mass and the mechanism by which it occurred. The numerical method based on the Cohesion Weakening and Friction Strengthening model and various indices, including the failure approaching index and the local energy release rate, were adapted for these purposes. The results shown here may help us understand the generation and process of development of rockbursts and evaluate the degree of failure in the rockmass. This information may be valuable in the study and design of rockburst-resistant measures in deep tunnels.     

黏土中隧道开挖引起的地表及地表以下土体长期 沉降计算方法

基于 Mair 提出的隧道开挖引起的土体瞬时沉降的理论以及进行的有限元数值模拟结果,推出地表及地表以下不同深度土体长期沉降的计算方法。首先模拟了 Ong 的离心机试验以验证数值模型的合理性,然后对隧道开挖引起的不同深度处土体长期沉降问题进行了模拟研究,建立了长期和瞬时情况下地层损失率之间的关系,并推导出长期情况下各深度处沉降槽宽度系数和最大沉降值的计算式,从而可确定沉降槽的形状。经与现场实测资料和数值模拟结果对比,验证了方法的合理性,可作为实际工程的借鉴。     

基坑开挖引起下卧既有电力隧道变形的控制技术研究

基坑工程对周围管线的影响是基坑工程环境土工问题的重要研究内容。上海市轨道交通9号线二期杨高中路站新建出入口上跨于Φ3000 mm的电力隧道正上方,为了预防基坑开挖对下方电力隧道可能带来的过大变形,采用了MJS工法进行坑内外土体加固,分区分块开挖,并借助于有限元分析对基坑开挖造成的隧道变形进行了预测,同时在隧道内安装静力水准仪、表面式测缝计和远程通讯设备开展结构变形实时监测,基于监测数据实现反馈施工,通过这一系列手段实现了对电力隧道结构在上方基坑开挖情况下的变形控制,确保了电力隧道结构的安全。对监测数据的分析表明：利用MJS工法进行土体加固变形控制具有良好的效果,隧道结构变形实时监测系统能及时有效地反映隧道结构状态,为反馈施工提供决策依据。     

Transient characters of energy changes induced by blasting excavation of deep-buried tunnels

As energy release plays an important role in engineering disasters such as rockburst, seismicity induced by deep rock mass excavation via drill and blast, energy changes caused by excavation have been studied for a long time. However, previous studies ignored the time factor and took the unloading of in-situ stress on the excavation boundary as a quasi-static process. In this paper, energy changes induced by quasi-static unloading of in-situ stress (QSUIS) and transient release of in-situ stress (TRIS) were analyzed for the case of circular excavation under the condition of in-situ hydrostatic stress. Results show that, different from energy changes caused by QSUIS, the dynamic adjustment of strain energy induced by TRIS which first decreases and then increases, is a transient process. With the propagation of the unloading stress wave from the excavation boundary to the far surrounding rock masses, energy is transmitted by the way of radial stress doing work from the far surrounding rock masses to the near ones which causes the transient aggregation of strain energy. Comparison based on the effects of energy changes on the damage range indicates that higher aggregation degree of strain energy causes larger damage range induced by TRIS than that induced by QSUIS. In addition, a practical application in Jinping II hydropower project was presented as a verification.     

Rockburst response in hard rock owing to excavation unloading of twin tunnels at great depth

This study aims to investigate the rockburst characteristics of hard rock during the successive excavation unloading of twin circular tunnels subjected to high active stresses. The entire evolution process of the rockburst phenomena around the tunnels is reproduced. The numerical results indicate that the unloading rates, burial depths, and presence of structural planes between the twin tunnels play important roles in the occurrence and damage degrees of rockbursts. The failure intensity and dynamic responses are aggregated with the increase of the unloading rate of the subsequent adjacent tunnel. The rockburst damage degree is exacerbated with increasing buried depth, and the rock response of the twin tunnels becomes more sensitive to the dynamic disturbance (as compared to a single tunnel at a great depth). The presence of a structural plane between the twin tunnels has both favourable and unfavourable effects on the stability of the surrounding rock. When the structural plane is parallel to the maximum tangential stress, the dynamic disturbance from the adjacent tunnel can be attenuated by the structural plane or rock joints via reflection and scattering, thus reducing the dynamic response between the twin tunnels. However, for those structural planes oblique to the maximum tangential stress, a violent rockburst is more prone to be induced, owing to the integrated response to shearing and sliding along the structural plane, and slabbing from the excavation unloading process. It is also found that the effect of the structural plane on the rockburst response is largely dependent on the burial depth.

     

Time-dependent damage constitutive model for the marble in the Jinping II hydropower station in China

To accurately describe the damage creep properties of marble, especially during the acceleration creep phase, creep testing was performed on the marble in the Jinping II hydropower station in China. Based on the experimental results, a time-dependent damage constitutive model was proposed in terms of fractional calculus theory and damage variables to describe these time-dependent damage characteristics. The time-dependent constitutive equations were derived for constant loading levels below and above the marble’s long-term strength. The robustness and parameter sensitivity of the proposed model were analysed by utilizing data of creep testing. The results of the analysis showed that the proposed model could describe not only the attenuation and steady-state creep of marble but also the acceleration creep characteristics and negative exponential attenuation law of the yield strength over time when the constant loading was above the long-term strength. These are crucial to failure prevention during rock engineering construction and operating periods.     

水电工程引水隧洞施工安全管理分析

安全管理是一个非常受重视的概念,工程施工中如何做好安全管理必须慎之又慎。引水隧洞是引水式水电站中较为重要的建筑物之一。通常在水电站引水隧洞洞口前设有一道拦水坝,用以保证隧洞施工的安全。结合在小型水电站施工中的实践经验,文章探讨了在引水隧洞施工中如何开展施工安全管理;同时针对如何确保工程顺利、人身财产安全问题提供了解决方案。     

Damage evolution mechanisms of rock in deep tunnels induced by cut blasting

The method of cut blasting is widely used in tunnelling excavation, which concerns the success in subsequent stop blasting and smooth blasting. Rock masses in deep tunnels are subjected to high in-situ stress, and the mechanisms of damage evolution of rock mass in deep tunnels induced by cut blasting are not well studied. In this paper, a tension and compression-shear damage model is developed, and then is implemented into the commercial software LS-DYNA as a user-defined material model. To validate the newly developed model, the comparison between numerical results and an existing field test is conducted. The effects of free-surface boundary conditions, in-situ stress and the coefficient of lateral pressure on cut blasting are considered in depth. Numerical results indicate that the superstition of stress wave and the reflected tension waves from free surfaces contribute to the damage interconnection near cut holes. The high in-situ stress has the resistance on the radially oriented pressure and the damage extension around cut holes. The coefficients of lateral pressure influence the extending direction of the tensile damage zone.     

锦屏一级水电站开挖边坡安全监测设计及应用

针对锦屏一级水电站岩石高边坡的工程实际,在边坡布设了多套位移计、锚杆应力计和锚索测力计,进行了边坡的安全监测。监测数据表明：边坡的锚固效果良好,边坡的变形得到了有效的控制。     

Protection of cement-soil reinforced regions for adjacent running tunnels during pit excavation

In pit excavation,cement is introduced into ground by deep mixing method to form an improved soil raft below final formation level to diminish deflection of retaining wall and effect on surrounding structure.Owning to complicated site conditions and improper workmanship,there are always some regions left untreated in the embedded improved soil raft.In this work,Several schemes of cement-soil mixed piles arrangement are modeled in order to discuss the effect of different cement-soil reinforced regions on protection for adjacent running tunnels.Finite element results show that:when lateral regions above tunnels are not enhanced by cement-soil mixed piles,effect of enlarging vertical enhanced regions around tunnels on diminishing lateral displacement of tunnel is really small;enhancing the lateral regions next to retaining wall is more effective in reducing the deflection of tunnel and retaining wall;uplifting of tunnel under the middle pit mainly depends on lateral reinforced regions and lateral displacements of retaining wall;as cement-soil mixed piles near retaining wall in east pit are removed during east pit excavation,effect of cement-soil mixed piles in east pit on reducing the final wall deflection can be neglected;upward shaft resistances are exerted along left side of diaphragm wall during excavation,which helps to reduce the wall deflection;positive effect of single-head cement-soil mixed piles in east pit is to decreasing the uplifting of soil inside east pit.Double-head cement-soil mixed piles arranged in"T"shape decrease the effect of east pit excavation on tunnels under middle pit apparently.     

Unloading deformation during layered excavation for the underground powerhouse of Jinping I Hydropower Station, southwest China

The paper discusses the strong asymmetric deformation and failure of the rock surfaces which have taken place in the 40–75 MPa marble during the excavation for the underground powerhouse and transformer chamber of the Jinping I Hydropower Station, southwest China. The deformation and damage of the rock mass closely relate to the magnitude of the geo-stresses, boundary conditions of the underground caverns, rock properties and the method and sequence of excavation. Ground stress test results show that the maximum principal stress reaches 35.7 MPa in the slope, and the average σ₁ intersects with the axis of underground powerhouse at an angle of 28°. Strong displacements had taken place at the sidewalls of the caverns and significant instantaneous uplift deformation of the floor occurred, particularly when the first layer was excavated.     

Cases of instability caused by weakness zones in Norwegian tunnels

Cave-in and rock fall are rare occurrences in Norwegian tunnelling today. Despite all precautions that are taken to prevent such incidents from happening, a few serious cases have occurred during the last few years. In this paper, one recent case of cave-in during tunnelling (the Atlanterhav tunnel) and two cases of rock fall/cave-in after completion of the project (the Oslofjord and Hanekleiv tunnels) are discussed. The main focus is on discussion and comparison of the rock engineering aspects of these incidents, and on the lessons that may be learnt from them. It is concluded that the presence of swelling clay (smectite) has been a main cause of instability, although low internal friction may be a destabilizing factor at least as important as swelling, and that the most difficult situation is a combination of very poor quality material and high water pressure in faults/weakness zones.     

既有地下结构受下穿施工影响的力学响应与安全控制研究

北京地区下穿工程中,新建隧道断面及施工方法种类众多、地层条件复杂、既有结构形式多样,有极其复杂的组合关系,目前尚无对既有结构力学响应的系统分析。通过北京地区13个下穿工程案例,总结了新建隧道结构形式及施工措施,明确既有地下结构变形特点,采用两阶段法分析及预测了各因素影响下既有地下结构的力学响应。研究表明:(1)新建隧道包括市政管道、地铁区间及车站,常见的施工方法有多导洞法、台阶法,洞桩托换法和中洞法,新建隧道开挖面积与施工方法有较明确的对应关系。(2)既有地下结构实测最大沉降概率分布符合数学期望4.89,方差16.4的正态分布。其中,新建市政管道、地铁区间和地铁车站下穿施工引起的既有地下结构平均最大沉降分别为2.56,3.82,11.07 mm。(3)根据新旧隧道空间位置关系的不同,穿越工程可分为7种组合,其中,既有地下结构力学响应有V,U,W 3种模式。(4)严格控制新建隧道开挖面积,不留或少留间隔土,尽可能选择W型穿越模式,以此减小对既有地下结构的扰动。     

开挖诱发隧道围岩变形的红外热像试验研究

采用相似材料模拟试验方法,借助红外热像仪等红外设备,对岩爆诱发的深埋隧道围岩变形破坏规律进行试验研究。配制强度较高的脆性相似材料,模型采用三轴向约束,通过单轴加载,模拟三维初始地应力状态。试验采用先加载后开挖方式,对开挖过程中深埋隧道围岩变形破坏情况进行红外热像分析,研究考虑开挖速度和支护条件等因素影响的岩爆诱发过程中深埋隧道围岩变形破坏时温度变化规律,试验结果表明：开挖速度越快,模型表面温度变化越大,隧道围岩弹性能积聚越剧烈,围岩岩爆倾向越高;提高支护强度对降低深埋隧道围岩的岩爆倾向具有一定的作用;隧道围岩表面热辐射温度的快速升高引起的红外异常可作为发生岩爆前期征兆。试验揭示深埋隧道变形破坏与温度场变化之间的对应关系,为深埋隧道的合理开挖和安全支护提供参考依据。     

中国锦屏地下实验室二期工程安全原位综合监测与分析

 中国锦屏地下实验室是目前世界上埋深最大的实验室。以中国锦屏地下实验室二期(CJPL–II)工程为研究对象,通过变形、开裂、弹性波、扰动应力、微震、声发射、三维激光扫描、岩体结构面遥测、爆破振动测试等原位综合监测手段,实时获取深部地下实验室开挖全过程的岩体响应及其演化特性,实现对深部岩体微观到宏观多尺度破裂、隧洞表面到岩体深部变形的综合监测。研究工程场址的地质辨识方法,给出深部复杂地质结构条件下的地质分区,结合真三轴、结构面实验和地应力测试,揭示各实验室开挖过程中不同的变形特征和破坏模式,实时捕获了深部工程灾害变形破裂的前兆信息,有效预警了岩爆和塌方工程灾害,确保了实验室施工全过程的工程安全。研究成果和基础数据为中国锦屏地下实验室安全建设运行、探讨深部岩体力学与工程科学难题均具有十分重要的意义。