隧道围岩开挖及锚喷衬砌稳定可靠度计算

对隧道围岩开挖进行线弹性收敛分析,导出围岩径向位移计算公式;基于薄壁筒理论及锚杆端锚理论,确定隧道围岩锚喷衬砌支护抗力求解方法;进一步推演得到隧道围岩开挖及锚喷衬砌功能函数。针对功能函数表达式为高度非线性隐式导致常规SORM难以直接应用的问题,基于数值差分原理推导梯度矢量求解公式,以此置换常规SORM中梯度的解析求解方式,构建一种不受功能函数形式制约的隧道稳定可靠度直接计算途径。利用该方法对工程实例进行分析,与蒙特卡洛法计算结果进行对比,验证其精确性及广泛适用性,同时在标准独立正态空间内建议了具有普遍适用意义的步长系数取值v=0.01。     

软弱破碎围岩隧道锚喷钢架联合支护的复合拱理论及应用研究

初期支护以锚杆、喷层、钢筋网与钢拱架联合支护的方式在软弱围岩和不良地质的隧道建设中已经得到广泛运用,但目前对于这种联合支护的力学承载机制缺乏深入研究,特别是对于联合支护的承载能力及与围岩相互作用没有量化的解析式。根据围岩与支护相互作用特点,运用组合拱理论,针对隧道围岩中锚喷网和钢架联合支护的特点,建立由以系统锚杆为支护外拱和喷层与钢架的支护内层拱所共同构成的复合拱的力学模型;根据隧道围岩的弹塑性理论、衬砌的支护结构理论以及全长锚杆的锚固界面层模型,推求联合支护结构的承载力与围岩变形的相关解析式;并结合云南省麻昭高速公路打堡寨2#隧道,利用现场监测位移求解了内、外层拱各自承载能力,经由计算结果表明,以钢架和喷层组成的内层支护拱在软弱破碎围岩中起主要承载作用。     

利用差分法计算基于Spencer分析模式的边坡稳定可靠度

首先研究边坡稳定Spencer分析模式的过程和特点,归纳出安全系数的计算流程,发现Spencer模式下边坡稳定可靠度计算的难题在于安全系数是隐函数,导致偏导数无法计算,由此引进差分方法近似求解偏导数问题。其次依据复合函数求导方法,推导采用验算点方法求解Spencer模式下边坡稳定可靠度过程中各项偏导数的计算公式和可靠度指标的线性逼近循环迭代方法。基于上述研究,给出完整的验算点求解Spencer模式边坡稳定可靠度方法的7个分析步骤,以及每一步骤中的具体计算方法。最后采用该方法分析一个工程问题,并与蒙特卡洛模拟结果进行比较。结果表明,两者的计算结果十分接近;该方法精度可满足工程要求,同时工作量大大少于蒙特卡洛模拟方法,具有较好的工程应用价值。     

基于组合拱理论隧道锚喷支护稳定可靠度求解的一维积分方法

基于组合拱原理和剪切滑移破坏模型,导出隧道锚喷支护结构功能函数。该函数表现为高度非线性隐式特征,同时基本参数间存在相关性,导致其可靠度求解困难。首先,引入基于半解析的Nataf变换以消除基本参数相关性的影响,将原始随机变量导入到标准正态空间中。其次,根据统计矩点估计法原理,依靠基本随机变量统计特征导出功能函数统计特征的矩估计公式。最后,根据统计矩与其变量的关系,以功能函数的前四阶矩为约束,基于最大熵理论导出功能函数的概率密度函数。构建基于组合拱理论的隧道锚喷支护稳定可靠度求解的一维数值积分方法。通过数值算例验证其准确性,最后分析荆竹山公路隧道支护结构的稳定可靠度,展示该方法的应用前景。     

隧道围岩-喷射混凝土界面应力解析

根据喷射混凝土支护隧道围岩的界面力学特点,考虑喷层与围岩结合界面受力和变形协调关系,并结合围岩承载拱效应,建立了围岩喷层结构的复合曲梁共同承载模型,然后通过各微单元静力平衡推导复合曲梁的径向位移的控制微分方程,得到任意分布荷载作用下喷层与围岩界面应力以及喷层与围岩各自内力的解析式,可迅速获取喷层与围岩结合界面的力学状况,进而判断围岩稳定性与预测安全性,为隧道施工决策提供技术支撑。最后经隧道台阶法开挖的算例研究表明,喷层支护通过其与围岩的结合界面上传递应力使围岩内部形成压应力带,有利于围岩的稳定。     

锚杆支护煤巷稳定性可靠度分析

 有些锚杆支护的煤巷曾发生过严重的冒顶,事故原因很复杂。其中煤巷支护设计与分析存在的问题有两个方面：一是煤巷围岩的物理、力学参数存在不确定性;二是稳定性设计分析的力学模型很多,各自只能从一个侧面反映其力学机理。为了对安全系数进行补充,考虑设计中的随机因素,在分析这些不确定性因素的基础上,根据巷道地压理论中的散体模型,锚杆支护结构抗力原理,研究了煤巷锚杆支护围岩稳定可靠度分析中抗力和荷载的计算方法,分析了某一锚杆支护煤巷的稳定可靠度,并认为锚杆支护的煤巷破坏失效概率在10%左右是可以接受的。     

基于有限差分法的隧道新型支护结构稳定性分析

聚丙烯纤维混凝土具有良好的变形性能,可以很好地适应隧道施工过程中围岩应力释放而产生的围岩压力;混凝土湿喷技术的引入可有效地改善隧道内工作环境,提高混凝土喷射质量。将湿喷聚丙烯纤维混凝土和锚杆应用于隧道支护结构,可起到良好的经济效益和支护效果。为了确保隧道施工过程的安全性和稳定性,采用FLAC有限差分计算软件对该支护结构性能和隧道的稳定性进行了计算和分析。分析模型中考虑围岩材料的非线性特性,以及围岩–支护结构体系位移场、塑性区和锚杆轴力的分布特征。分析结果表明:这种支护结构可有效地降低拱顶下沉,底板上鼓,提高成洞空间;围岩和支护结构体系协同工作,极大地发挥围岩的自承载能力。     

鲁布革水电站地下厂房锚喷支护效果的研究

为了研究鲁布革水电站地下厂房围岩采用锚喷支护的效果,我们曾进行了相似材料模型试验研究和有限元计算。模型试验和有限元计算分别对有支护和无支护的洞室围岩应力分布;围岩位移;以及洞室失稳的破坏荷载、破坏范围和破坏形态进行了研究。分析研究结果表明,锚喷支护能够改善洞室围岩的受力条件,增加围岩整体变形刚度和提高围岩的承载能力。洞室进行锚喷支护后,围岩拉、压应力值都有所降低;围岩周边最大变形减少20％左右,洞室破坏荷载可提高50％左右。本文对洞室围岩、喷混凝土层及锚杆的破坏机制亦进行了讨论。     

隧道围岩与支护结构变形协调控制机理及工程应用

为解决高应力下隧道岩爆以及软岩大变形控制难题,本文从隧道围岩荷载与支护平衡、变形协调控制及围岩能量守恒三方面进一步分析地下工程平衡稳定方法的特点;根据不同围岩级别总结地下工程平衡稳定方法中隧道支护结构抗力与围岩变形特征曲线,并提出隧道围岩支护的施工建议;通过分析围岩与支护结构有效荷载传递规律与功能转换特征,揭示围岩平衡与支护结构变形协调控制机理,并结合高应变吸能层材料设计一种表征为“吸能让压 支承抗压”的一体化新型隧道围岩支护工艺。通过工程应用表明,基于新型支护工艺在岩爆、软岩大变形隧道中围岩压力降低28.09%,保障了隧道安全高效施工。     

隧道径向锚钉-斜交锚杆复合支护技术研究

基于长短锚杆组合支护理论,提出了一种隧道径向锚钉-斜交锚杆复合支护技术,该技术可提高支护结构的整体抗力,有利于缩小围岩塑性区半径,增加锚杆处于稳定围岩区的长度。为进一步研究锚钉和锚杆的组合长度,将隧道围岩划分为A、B和C三区,并把锚杆作用在围岩体内的剪应力简化为环向边界应力,运用厚壁圆筒弹塑性理论推导了锚钉和锚杆组合长度计算式。实例分析表明,0.8m(锚钉)和3.8m(锚杆)的组合长度可以使隧道塑性区半径降低28%,锚固盲区减小88.8%。可见,在隧道长短锚杆组合支护理论中,该计算方法对围岩锚固体长度参数设计有一定指导意义。     

Shear resistance contribution of support systems in double shear test

Rockbolt and surface support systems such as shotcrete and thin spray-on liners (TSLs) are widely used as underground support elements to resist the convergence and maintain the stability of excavations. In order to evaluate the bearing capacity of combined reinforced rockbolt and surface support systems in preventing sliding along discontinuities, double shear tests (DST) was carried out using fully grouted rockbolts installed in three separate blocks. These blocks were covered with a 5 mm layer of TSL followed by a 50 mm layer of shotcrete. Two rockbolts were installed at an inclined angle of 45°, and 20 kN lateral constraining force was applied to clamp together the three blocks. Three different support combinations were tested: 50 mm shotcrete only, 5 mm TSL only, and combined shotcrete and TSL, with and without rockbolts. It was confirmed that the shotcrete plays a mechanical role in resisting the shear load, and TSLs increase the bond strength between shotcrete and substrate replicating the side wall of an excavation. The contribution of rockbolt and surface support system in resisting joint movement was also compared. The failure mechanism of rock substrate, rockbolt and surface support system was also analysed.     

Principles and methods of rock support for rockburst control

This paper presents the principles of rock support for rockburst control and three rockburst support systems used in deep metal mines.Before the principles of rock support are presented,rock fracture related to strain burst is first discussed with the help of photos taken on site,and the energy sources and transformations during bursting are illustrated through conceptual models.Surface parallel extension fracture usually occurs in the ejected and surrounding rocks in a strain burst event,while the ejected rock in a fault-slip rockburst is often already pre-fractured before the event.There must be excessive release energy available for rock ejection.The excessive release energy comes from both the ejected rock itself and the surrounding rock.To prevent rock ejection in a rockburst,the support system must be able to dissipate the excessive release energy.All support devices in a support system for rockburst control must be able to dissipate energy,be firmly linked,and be compatible in deformability.A support system for rockburst control comprises surface-retaining devices and yield rockbolts as well as yield cablebolts when needed.Laying mesh on the top of shotcrete liner is a good practice to enhance the surfaceretaining capacity of the support system.Energy-absorbing yield rockbolts dissipate energy either by stretching of the bolt shank or by sliding of the inner anchor in the borehole.Mesh,mesh strap and shotcrete are the surface-retaining devices widely used in the current rock support systems.The three types of rock support used for rockburst control at present are soft support system using Split Set bolts,hybrid support system using rebar and two-point anchored yield bolts,and entirely yieldable support system using strong yield bolts.     

Impact of fire on tunnels in Hawkesbury sandstone

The majority of tunnels in Sydney, Australia are within near-saturated Hawkesbury sandstone. Crown support in these tunnels typically comprises permanent rockbolts, and shotcrete ranging in thickness from about 75 mm to 250 mm. Sidewalls are mostly exposed sandstone with occasional rockbolts, and, in places, a thin shotcrete skin for surface protection.Rock will quickly be exposed to high temperature in a tunnel fire where no, or a thin layer of shotcrete exists. Rock with thicker shotcrete may also be exposed where spalling of the shotcrete occurs.The phenomenon of spalling in fire has been widely researched for concrete and, to a lesser extent, shotcrete [e.g. Tatnall, P.C. 2002. Shotcrete in fires: effects of fibers on explosive spalling. Shotcrete, 10–12]. It is assessed as being primarily associated with steam pressures produced by evaporation of water in pores [Hertz, K.D. 2002. Limits of spalling of fire-exposed concrete. Fire Safety Journal 38, 103–116].This paper assesses, by means of laboratory and field tests, how exposed sandstone is likely to respond in a tunnel fire.It is concluded that substantial explosive spalling will occur early and at relatively low temperatures. This spalling could create dangerous conditions for rescue, and escaping personnel. However, beyond the zone of spalled sandstone there would be only minor structural impact on the rock mass.     

Cavern roof stability—mechanism of arching and stabilization by rockbolting

For stabilizing of rock cavern roof arches, tensioned rockbolts should be used with caution since they may have a negative influence on the stability of the arch. A proper design of rockbolts should therefore be based on a clear understanding of both the features of rockbolt reinforcement and the mechanism of the rock roof arch structure. For this purpose the mechanism of a rock cavern roof arch is studied and effects of different types of rockbolts are evaluated. Numerical modeling of rockbolt support of the Xiaolangdi powerhouse cavern as well as application of arching theory are carried out with the objective to study the effects of fully grouted rockbolts and tensioned cable anchors in reinforcing the cavern roof and walls, and on the forming of the roof arch.     

On the selection of the rock support system for the powerhouse area of the Fljótsdalur hydroelectric project

The rock support system for the powerhouse and transformer hall area of the Fljótsdalur Hydroelectric Project has been provisionally selected based, on one hand, on the boundary element method suggested by Hoek and Brown (1980); and, on the other hand, on finite element analysis, carried out with a program called STAUB (EWI Engineers and Consultants 1991). The analyses indicate that a combination of tendon cables, rockbolts and shotcrete will ensure the necessary redistribution of stresses and stability of the surface rock. Preliminary predictions of displacements are presented.     

Principles of rockbolting design

This article introduces the principles of underground rockbolting design.The items discussed include underground loading conditions,natural pressure zone around an underground opening,design methodologies,selection of rockbolt types,determination of bolt length and spacing,factor of safety,and compatibility between support elements.Different types of rockbolting used in engineering practise are also presented.The traditional principle of selecting strong rockbolts is valid only in conditions of low in situ stresses in the rock mass.Energy-absorbing rockbolts are preferred in the case of high in situ stresses.A natural pressure arch is formed in the rock at a certain distance behind the tunnel wall.Rockbolts should be long enough to reach the natural pressure arch when the failure zone is small.The bolt length should be at least 1 m beyond the failure zone.In the case of a vast failure zone,tightly spaced short rockbolts are installed to establish an artificial pressure arch within the failure zone and long cables are anchored on the natural pressure arch.In this case,the rockbolts are usually less than 3 m long in mine drifts,but can be up to 7 m in large-scale rock caverns.Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch.In addition to the factor of safety,the maximum allowable displacement in the tunnel and the ultimate displacement capacity of rockbolts must be also taken into account in the design.Finally,rockbolts should be compatible with other support elements in the same support system in terms of displacement and energy absorption capacities.     

Modeling the effect of water, excavation sequence and rock reinforcement with discontinuous deformation analysis

A powerful numerical method that can be used for modeling rock-structure interaction is the discontinuous deformation analysis (DDA) method developed by Shi in 1988. In this method, rock masses are treated as systems of finite and deformable blocks. Large rock mass deformations and block movements are allowed. Although various extensions of the DDA method have been proposed in the literature, the method is not capable of modeling water-block interaction, sequential loading or unloading and rock reinforcement; three features that are needed when modeling surface or underground excavation in fractured rock. This paper presents three new extensions to the DDA method. The extensions consist of hydro-mechanical coupling between rock blocks and steady water flow in fractures, sequential loading or unloading, and rock reinforcement by rockbolts, shotcrete or concrete lining. Examples of application of the DDA method with the new extensions are presented. Simulations of the underground excavation of the ‘Unju Tunnel’ in Korea were carried out to evaluate the influence of fracture flow, excavation sequence and reinforcement on the tunnel stability. The results of the present study indicate that fracture flow and improper selection of excavation sequence could have a destabilizing effect on the tunnel stability. On the other hand, reinforcement by rockbolts and shotcrete can stabilize the tunnel. It is found that, in general, the DDA program with the three new extensions can now be used as a practical tool in the design of underground structures. In particular, phases of construction (excavation, reinforcement) can now be simulated more realistically. However, the method is limited to solving two-dimensional problems.     

Performance of tunnel support under large deformation static and dynamic loading

Tunnels under high stresses and deep mining conditions are often subjected to large static and dynamic deformations. It is usually not practically possible to contain the energy involved by means of stronger support. Instead, the support must yield and, in yielding, absorb energy. With yielding support of suitable deformation capacity, it should be possible to contain very severe static and dynamic deformations. Research into the performance of tunnel support has recently been carried out in three areas: (1) the behaviour of retainment support elements such as rockbolts under dynamic loading. This showed that specially designed yielding rockbolts could absorb more than 50 kJ each in yielding some 0,5 m, without showing any damage, (2) the energy absorbing capacity of mesh and fibre reinforced shotcrete, based on the results of static testing of shotcrete panels, and (3) the behaviour of containment support elements, such as wire mesh, wire rope lacing and reinforced shotcrete under dynamic loading. This has shown that the practical capacity of wire mesh alone is about 15 kJ/m², but that this capacity is more than doubled when wire rope lacing is added.     

Analytical approach for the design of active grouted rockbolts in tunnel stability based on convergence-confinement method

Assuming that grouted rockbolts increase internal pressure within a broken rock mass, a new procedure for computation of ground response curves for a tunnel reinforced with active grouted rockbolts is presented, while the effect of distance of bolted section to tunnel face has been also considered. This analytical solution for a circular underground excavation under hydrostatic stress field, and with consideration of a non-linear strength criterion for rock mass and on the basis of two material behavior models has been developed. In this work, the equation of the ground response curve for a tunnel which has been reinforced with passive grouted rockbolts is also derived. The proposed model allows one to take, the effect of the distance of the bolted section to the tunnel face, the effect of increasing rockbolts spacing, the influence of increasing pretension load in calculating of the ground response curve, and the effect of increasing the cross-section area of rockbolts, into account. The results show that decreasing rockbolts spacing increase the support system stiffness rather than preloading of them.