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.     

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.     

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.     

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.     

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.     

上海世博500kV地下变电站圆形深基坑逆作法变形与受力特性实测分析

上海世博500kV地下变电站是直径为130m的深埋圆筒形全地下结构,采用全逆作法施工。针对基坑工程特点,基坑开挖期间建立了全过程、全方位的监测系统,完整地记录了施工过程中围护结构的变形与内力和周边环境的变形。监测数据表明：围护结构的侧向变形明显小于相同开挖深度的其它基坑工程,地下连续墙呈现以水平受力为主的圆形结构特点,体现了圆形基坑环向受力的特点和利于变形控制的优点。该工程各项监测数据均在允许值范围之内,验证了相关设计参数的合理性和理论研究的正确性,为软土地区类似深基坑工程的设计和施工提供参考和借鉴。     

Pilot bore excavation with TBM for the design and construction of larger tunnels

The author reviews the use of the tunnel boring machine (TBM) for pilot bore excavation in Italy, and summarizes the main functions of the TBM pilot bore with respect to large tunnel design and construction. Cost comparisons with traditional excavation methods are included.     

山西中部引黄工程小断面隧洞快速掘进施工方法

结合山西中部引黄工程的水文地质条件,分析了小断面隧洞快速掘进的施工难点及关键因素,并从施工部署、加宽段距离、工序衔接等方面,阐述了具体的施工方法,总结了施工安全、质量保障措施,实现了小断面隧洞的快速施工。     

Deformation mechanism and stability evaluation for the left abutment slope of Jinping I hydropower station

There are very complicated engineering geological conditions in the left abutment slope of the Jinping I hydropower station with the stress-release opening of fractures along existing faults and where lamprophyre veins are present. Prior to the excavation for the dam, the slope generally consisted of a thick marble mass between fault f₅ and the free face. As a consequence of the removal of some of the marble to create the abutment, the stability of the slope would be reduced. The paper reports the stability assessments undertaken, based on engineering geological analysis and block theory and the results of the analysis of key rock masses under four conditions: natural, heavy-rain, earthquake and heavy-rain plus earthquake.     

Damage mechanism of tunnels in the high-content salt rock stratum

Bulletin of Engineering Geology and the Environment - The high-content salt rock stratum is a highly water-soluble, corrosive, and expansive stratum, and it is extremely difficult to construct...     

围垦工程对大桥桩基变形和应力影响的分析

采用有限元分析了围垦工程对大桥桩基应力和变形的影响,进行了4种工况的计算,计算结果表明,4种工况中桩基的变形都不大,但南移250 m方案中桩的轴向力最大,轴力为5.23 MPa,位于-63.3 m高程.     

水电站引水洞钻爆法中掏槽眼技术要点

介绍了水电站引水洞钻爆法施工中掏槽眼布设技术要点,详细的阐述了引水洞不同地质情况下掏槽眼形式的选择方式,以及由此所取得的技术经济效益,并对引水洞钻爆法施工中掏槽眼布设作了较全面的总结,指出其对钻爆效果和掘进效率起着关键性作用。