隧道围岩大变形研究分析
due to the influence of natural geological conditions, collapse accident often bring enormous damage to the tunnel construction in the large section tunnel excavation of road (or railroad) construction.engineering equipment and the safety of personnel are seriously threatened by the collapse of tunnel which has the character of high incidence and high risk.
1、primary influence factors and types of tunnel collapse 1.1 The main factors of tunnel collapse There are many influence factors in tunnel collapse, generally can be divided into external and internal factors, internal factors are mainly refer to the engineering geological and hydrogeological conditions, external factors are mainly refer to improper design and construction.1) unfavorable geological conditions unfavorable geological mainly includes the weathered metamorphic rock, fractured rock ma and rock pile area, fault, karst cave, landslide, mudslide, swelling strata, etc.when the tunnel encountered to those geological condition,some slight negligence may lead to the collapse of tunnel.2) underground water Groundwater is one of the important factors influencing the stability of surrounding rock.it not only reduces the strength of rock which has high softening coefficient, and also reduces the shear strength of structure plane.3) geostre geostre mainly includes the bias, plasticity, landslides and high ground stre area, etc.,there are many instances of collapse happened in the entrance zone of the tunnel were caused by bias and landslide,extrusion destruction of tunnel is often caused by plastic preure,and rock burst often occured in high ground stre area, etc.4) improper design and construction.At present, \"New Austrian Tunnelling Method\" is used to design and construction in most of the tunnel, However, the deviation understanding of the concept and connotation of the principle led to many failure in engineering.collapse of tunnel is induced by many factors and the mechanism is of more complex, therefore, improper treatment measures for the complex geological during the construction will easily cause the occurrence of collapse disasters.1.2 analysis of collapse types there are many types of partitioning method, generally based on the cause of collapse, form, scale, mechanism, etc.it mainly divides into the following five kinds of forms.1）Partial collapse：Mainly happened in large blocky rock ma, the rock ma is cutted by structure surface and formed into different shapes of unstable structure.After cavern excavation, the friction direction of unstable structure surface slide and collapse occured.2）Arch collapse：Typically occur in layered rock ma or broken rock ma, it has two kinds, one kind is in the range of the pit, only occur in the arch department; Another kind is expanding arch collapse, including wall collapse.3）Deformed collapse：collapse induced by special geological conditions (caves, sinkhole, etc.), shallow buried and bias.4）swelling rock tunnel collapse：Due to high inflation preure acts on the shotcrete support; The expansion of the rock body reduces the friction and shear strength; High water preure accumulation。
5）Large deformation of tunnel the rock burst：surrounding rock large deformation problem of deep buried,long-large tunnels and underground engineering under high geostre condition has become a problem in underground engineering and become a major iue to be solved.
2、deformation analysis for surrounding rock of tunnels With the rapid development of tunnel and underground engineering, the characteristic of its long-scale-deep are increasingly obvious, and it is prone to geological disasters，such as large deformation of surrounding rock in certain geological and environmental geological conditions.Based on a lot of literature retrieval, it is obviously that surrounding rock large deformation is a kind of great hazardous geology casualty of tunnel and underground engineering.2.1 current research of large deformation of Tunnel surrounding rock though a lot of domestic and foreign scholars have done a lot of exploration efforts, due to the large deformation theory research is not mature, as well as the complexity of on-site geological rock ma condition, the current research work is mainly manifested in the following aspects.1）The concept for large deformation there is no form a consistent and clear definition About the large deformation of surrounding rock, Some scholars according to whether the deformation of surrounding rock is within the reserved deformation of primary support to define it.Also some scholars think it shouldn\'t be defined by the absolute value of the deformation which is the external performance of large deformation problem.It should be defined by the eence that the surrounding rock occurs shear deformation，deflection，rupture and failure under the shear stre, and causes the rock ma squash the underground cavity.2) analysis of the soft rock so far，Rock engineering community has been unable toreach a consensus on the concept of soft rock, the definition of soft rock is more than a dozen.the definition of soft rock of ISRM (international society for rock mechanics, 1990，1993) is refers to the uniaxial compreive strength of rock was 0.5 ~ 25 mpa; In 1984 coal mine term discuion (kunming)， soft rock is defined as \"low strength, high porosity ,poor cementation and affected by structural plane cutting and weathering or contains a lot of expansive clay minerals and loose, soft rock (body)\"; And some scholars also divide it into geological soft rock and engineering soft rock and so on.3)
People generally divide it into two types base on the formation mechanism of the surrounding rock large deformation.The first type is caused when redistribution stre exceeds surrounding rock\'s strength after tunnel excavation, which makes the surrounding rock be plasticity.the other type is caused when certain minerals in the rock react with water and inflation.swelling mineral and water are significant for the expansion deformation.2.2 type and mechanism of Large deformation of tunnel surrounding rock Tunnel surrounding rock large deformation can be defined as: a kind of plastic deformation and failure with progreive and obvious time effect of tunnel and underground engineering surrounding rock, it is different from brittle failure of rock burst and distinguish it from collapse and slide, etc, which are caused by the rock\'s structure plane of surrounding rock loose zone.1) the type of Large deformation of tunnel surrounding rock According to the analysis of typical examples and the research on mechanism of large deformation, claification on the large deformation was carried out base on different controlled conditions: controlled by lithology of surrounding rock , controlled by the structure of surrounding rock and affected by artificial excavation disturbance.在公路(或铁路)隧道大断面开挖施工中,由于自然地质条件、施工等的影响,常会发生塌方事故,给隧道施工带来极大危害。塌方以其高发性、高危性严重威胁着工程设备和人员安全。本报告从塌方的类型、塌方的影响因素进行了总结，并对隧道中的大变形进行了一定的介绍。
1、隧道塌方类型及塌方主要影响因素 1.1隧道塌方主要影响因素 影响隧道塌方的因素很多，大体上可分为内因和外因，内因主要是指工程地质和水文地质条件,外因主要是指设计和施工不当。
1）不良地质条件
不良地质主要包括风化变质岩体、裂隙发育岩体、崩塌岩堆地区、断层带、溶洞、滑坡、泥石流、膨胀性地层等,当隧道从这些岩体中通过时,如稍有疏忽,就可能发生大塌方。
2）地下水
地下水是影响围岩稳定的重要因素之一。这不但能使岩石强度降低，特别是软化系数较大的岩石,而且使结构面的抗剪强度减小。对于断层,地下水影响更大,一般张性断层是储水结构,压性断层带中断层糜棱岩是隔水的,而另一侧的破碎带为含水的,当揭穿断层后,便时常发生突发性涌水导致塌方。
3）地压
地压主要包括偏压、塑性地压、滑坡及高地应力区等,偏压和滑坡在隧道洞口段造成的塌方实例很多,塑性地压引起隧道挤出性破坏也时有发生,高地应力区的完整坚硬岩体常发生岩爆等。
4）设计和施工不当
目前,大多数隧道都号称采用“新奥法”设计和施工,由于对“新奥法”概念和原则的内涵理解有偏差,所以在许多工程中遭到失败。
由于塌方诱发因素多、机理复杂，因此，在施工中针对复杂地质时若处理措施不当，将极易造成塌方灾害的发生。
1.2塌方类型分析
塌方类型的划分方法有多种，一般根据塌方的原因、形式、规模、机理、发生的部位等进行划分，主要分为以下五种形态：
1）局部塌方：主要发生在大块状岩体中，由于岩体被结构面切割后构成不同形状的不稳定结构体。洞室开挖后，不稳定结构面的摩擦力向洞内滑移而发生塌方。
2）拱形塌方：一般发生在层状岩体或破碎块岩体中，它有两类，一类是在坑跨范围内，仅出现在拱部；另一类是包括侧壁崩塌在内的扩大的拱形塌方。
3）异形塌方：由于特殊地质条件（溶洞、陷穴等），浅埋、偏压隧道等原因产生的塌方。 4）膨胀岩隧道塌方：由于高膨胀压力作用在喷混凝土支护上；岩土体的膨胀原因使摩擦和剪切强度损失，造成很大的岩石材料重力荷载；高水压力积聚等造成隧道塌方。
5）大变形的隧道和岩爆：高地应力状态下的深埋、长大隧道及地下工程的围岩大变形问题已经成为困扰地下工程界的难题和尚待进行深人研究的重大课题之一。
2、隧道围岩大变形分析
随着隧道工程以及地下工程的迅猛发展,其长、大、深的特点日趋明显,而在一定的围岩地质和环境地质条件下则往往易于发生围岩大变形等地质灾害。根据大量文献检索结果显示,隧道工程围岩大变形已是困扰地下工程界的一个重大问题。
2.1隧道围岩大变形研究现状
国内外学者已经做过不少的探索工作,但由于大变形理论的研究不尽成熟,加之现场地质岩体状况复杂性,目前的研究工作主要表现在以下方面。
1）大变形的定义
关于围岩大变形,目前还没有形成一致的和明确的定义。有的学者提出根据围岩变形是否超出初期支护的预留变形量来定义大变形。也有的学者认为,不能从变形量的绝对值大小来定义大变形问题,具有显著的变形值是大变形问题的外在表现,其本质是由剪应力产生的岩体的剪切变形发生错动、断裂分离破坏,岩体将向地下空洞方向产生压挤推变形来定义大变形。
2）关于软岩的分析
至今岩石工程学界仍未就软岩的概念达成共识，软岩的定义有十几种之多。ISRM(国际岩石力学学会1990,1993)定义的软岩是指单轴抗压强度为0.5～25MPa的一类岩石；1984年煤矿矿压名词讨论会(昆明)将软岩定义为“强度低、空隙大、胶结程度差、受结构面切割及风化影响或含有大量易膨胀粘土矿物的松、散、软、弱岩层(体)”；还有的学者将软岩划分为地质软岩和工程软岩两类等等。
3）大变形机制
人们一般按形成机制将围岩大变形分为两类:一是开挖形成的应力重分布超过围岩强度而发生塑性化。二是岩石中的某些矿物和水反应而发生膨胀。水及某些(膨胀性)矿物的存在,对于膨胀变形是必须的。
2.2、隧道围岩大变形的类型与机制
隧道围岩大变形可以定义为:隧道及地下工程围岩的一种具有累进性和明显时间效应的塑性变形破坏,它既区别于岩爆运动脆性破坏,又区别于围岩松动圈中受限于一定结构面控制的坍塌、滑动等破坏。
1）隧道围岩大变形的类型分析
根据对围岩大变形典型实例的分析和对大变形机制研究,可以按照不同的受控条件对大变形进行类型划分:受围岩岩性控制的大变形,受围岩结构构造控制的大变形和受人工采掘扰动影响的大变形三大类型
2）large deformation mechanism of Tunnel surrounding rock 2）隧道围岩大变形机制研究
围岩的变形破坏首先取决于围岩性质,其中包括围岩体的岩性、结构条件,其次受到围岩的环境条件即地应力的大小、地下水的发育分布状况的影响,同时也与围岩的支护条件密切相关。
一般而言，隧道变形将经历初始变形、等速变形及加速变形3个阶段。在隧道开挖初期，由于围岩变形刚开始，岩体内应力场和位移场处于线性变化阶段，处于力学平衡状态，这时围岩处于初始变形阶段。随着开挖过程的进行、围岩地应力释放、应力状态重新分布，引起某些区域局部变形、微裂隙产生、应力增加，使得岩体内部微裂隙不断增加和扩展。这时围岩处于等速变形阶段。随着围岩变形的增长，裂隙进一步扩展、连通、丛集，甚至局部贯通，围岩发生较大的变形，变形速率持续增加，直至整体失稳坍塌，这时围岩处于加速变形阶段。若局部变形发展过程中，围岩系统受到外界干扰（如：降雨、人工影响、振动等），则将促使其变形和破坏过程的加速发展。
3、隧道围岩大变形的预测
在大变形的预测方面,目前国内外尚无实用的预测大变形的方法、日本学者viladkar(1995)提出利用地层Q值的“临界埋深法”,当实际埋深大于临界埋深时,围岩应视之为挤压性围岩,具有发生大变形的条件。但此种方法与开挖断面及支护不发生关系,与支护变形量的大小也无内在联系,当实际埋深大于临界埋深时可能发生的支护变形值有多大难以确定。
大变形的预测另一方法是,对于一个具有锚喷支护的洞室,采用弹塑性理论解析公式计算其在地应力作用下的最终洞壁位移,然后确定位移与地应力及岩体强度三者的关系(绘出曲线图形),如已知地应力及岩体强度支护位移值及大变形的等级可从图中查得。
4、隧道围岩大变形的支护对策
交通隧道的支护包括初期支护和二次衬砌。对于软岩隧道,支护的首要任务是如何遏制初期支护的大变形。随着人们对大变形现象认识不断深化,提出了各种各样的支护措施和手段,这些支护措施基本可以分为以下三大类
1）刚性支护
这种支护措施的核心是通过加大支护结构的强度和刚度来抵抗巨大的围岩压力，这种支护措施无论从技术上还是从经济上,都是欠合理的,现在已经较少采用。
2)可缩支护
这种支护的理论依据是允许围岩发生适度的变形,以降低作用于结构的支护压力,从而减少超挖量并降低支护强度。
可缩支护系统在国内外煤矿巷道支护方面已经取得一些成功的经验,但该方法只允许适度变形，若超出支护体系的允许变形范围，这类支护结构一般很难抵挡围岩的巨大压力，该方法在交通隧道支护中的应用效果并不理想。
3）锚、注一体化围岩加固--支护系统
锚喷衬砌作为隧道初期支护的主要手段,也构成了永久衬砌的重要组成部分。喷锚衬砌是一种加固围岩、充分利用围岩自承能力的一种支护衬砌形式,并且快速及时,可以节约劳动力及成本,作为复合式衬砌的初期支护在工程实际应用中日益受到重视。
5、隧道工程围岩大变形实例
首例严重的交通隧道软弱围岩大变形应该是1906年竣工的长19.8km的辛普伦Ⅰ线隧道。此后,日本的惠那山(Enasan)公路隧道、奥地利的陶恩(Tauern)隧道、阿尔贝格(Arlberg)隧道等都是典型的隧道围岩大变形灾害工程事例。中国的青藏线4.0km长的关角隧道、宝中线3.136km长的木寨岭隧道及1.904km长的堡子梁隧道、南昆线上的穿越煤系地层的家竹箐铁路隧道、在建的国道317线鹧鸪山公路隧道(4.442km),以及铁山隧道(2.099km)等工程均出现了不同形式和程度的围岩大变形情况，给工程建设造成极大的困难。
木寨岭隧道是国道212线连接西南和西北的重要通道之一，长1710m，东西引线总长9010m。隧道位于高海拔地区，不仅埋深较大(272.28m)，而且工程地质条件极为复杂。自开工以来，隧道发生强烈变形和破坏，严重困扰着施工安全并影响工期。 5.1木寨岭隧道工程地质环境
木寨岭隧道为深埋大跨度公路隧道，由于其罕见的复杂地质条件，断层发育，岩体破碎软弱、地下水丰富，地应力高，隧道成洞条件及自稳能力极差，围岩变形和隧道破坏严重。 5.2木寨岭隧道变形破坏特征 1）围岩变形特征
隧道开挖后，围岩变形非常强烈，其表现为变形量大、初期变形快且变形速率大、变形持续时间长、空间分布不均匀和不对称、隧道某些地段重复性变形等特征。 2）隧道破坏特征 木寨岭隧道洞身主要为含炭页岩，岩体软弱破碎，褶皱发育，围岩自稳能力差。开挖过程中局部经常出现掉块和塌方现象。变形破坏最严重的区域为各断层破碎带。其主要的破坏特征有初期支护变形破坏、掌子面变形破坏、仰拱变形破坏和二次衬砌的变形破坏。 5.3木寨岭隧道围岩大变形机理分析
木寨岭隧道围岩大变形是在岩性（(膨胀性岩石）、地下水和地应力场综合作用下，因开挖卸荷，围岩发生塑性流动和膨胀所致。隧道围岩压力同时包括形变围岩压力和膨胀围岩压力，此即为该隧道大变形和严重破坏的原因和本质。 5.4大变形处治实践
通过正确认识围岩变形破坏机制，根据围岩动态演化规律，从塑性变形和围岩膨胀两方面，通过封闭工作面、锚注支护与自进式锚杆和联合支护体系，快速“强行”支护，确保了大变形软岩段返修成功，同时可为后续段的施工提供参考。
6、结语
进入21世纪,地下空间的开发利用将以新的深度和广度发展,交通隧道、水工隧道及其它地下工程“长、大、深、群”的特点将更加显著。深埋长大隧道工程软岩大变形问题的进一步深人研究将是今后地下工程界的一个重大课题。对大变形机理的正确认识和合理描述、围岩大变形预测、锚、注、喷一体化(锚、注为核心)围岩加固--支护技术的进一步完善、优化,是今后隧道围岩大变形问题讨论和研究的重点。
