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英文原文The optimal support intensity for coal mine roadway tunnels in soft rocksC. Wang*Mining Engineering Program, Western Australian School of Mines, PMB 22, Kalgoorlie WA6430, Australia1. IntroductionThe essence of underground roadway support is to provide the surrounding rocks of an underground roadway with assistance to help them achieve stress and strain equilibrium and ultimately stability of deformation.The approaches to this goal are either to reinforce the rock mass by rock bolting or injection(internal rock stabilization) or to provide the surrounding rocks with a support resistance with a magnitude being described as the support intensity (external rock stabilization).When an underground roadway is located in soft rocks which are too soft to be reinforced by bolting and/or unsuitable for rock injection because of restraints imposed by either the rock mass impermeability or rock mass deterioration when water is encountered, external rock support, such as steel sets, therefore becomes the only option for the stability control of the roadway. Under this circumstance, the support intensity means a support force acting per unit surface area of the surrounding rocks of the roadway. In soft rock engineering practice, the design of a support pattern for a roadway in underground coal mining is normally based on rules of thumb. In most cases, heavy support measures are adopted to secure a successful roadway.Fig. 1(a) demonstrates the excellent condition of a sub-level roadway within soft rocks at an underground coal mine in north China, where an excessive capital cost was applied for the achievement of roadway stability. In some cases, such as a service roadway driven in soft rocks at the same mine (Fig. 1(b), insufficient support intensity was specified as a result of a lack of relevant experience and design codes. Consequently, failure of the roadway stability was inevitable and an extra cost was incurred when the subsequent roadway repair or rehabilitation was undertaken.The critical issue in both cases lies in the determination of an optimal support intensity which is the function of the geometry and dimension of a roadway and its geotechnical conditions including rock mass properties, stress conditions and hydrological status.Physical modelling using simulated materials based on the theory of similarity provides a direct perceptional methodology for mining geomechanics study 1-6.Using simulated materials of the same composition to construct a roadway and its soft surrounding rocks, applying a certain magnitude of simulated support intensity to the surface of a roadway under simulated stress conditions, the three-dimensional physical modelling method depicted in this Note emonstrates a quantitative solution for strategic design of roadway support concerned with soft rocks. A relation between the support intensity and deformation of the surrounding rocks of a roadway has been established after a series of simulation tests had been conducted. A discussion on the optimal support intensity for a roadway in soft rocks is also given. Fig. 1. Examples of successful and unsuccessful support of underground roadways within soft rocks: (a) Good condition of a sublevel roadway, (b) Unsuccessful support of a service roadway.2. Features of the three-dimensional physical modellingA physical modelling study of the interaction between support intensity and roadway deformation was carried out using the three dimension physical modelling system (see Fig. 2) at the Central Laboratory of Rock Mechanics and Ground Control, China University of Mining and Technology. Features of this system are described in the following sub-sections. Fig. 2.Three-dimensional loaded physical modelling system at the Central Laboratory of Rock Mechanics and Ground Control, China University of Mining and Technology.2.1. Size of the physical modelThe effective size of a physical model is 1000 mm wide, 1000 mm high and 200 mm thick.2.2. Three dimensional active loading capabilitySix flatjacks are used to apply loads to the six sides of the physical model in the form of a rectangular prism. Each flatjack was designed to cover the full area of one of the six sides and be capable of applying a pressure of up to 10 MPa on to the surface of the simulated rock mass. This means that the flatjacks are capable of applying an active load of up to 1000 tonnes and 200 tonnes simultaneously on the front and back facets, the top and bottom, and the two side facets of a model, respectively.2.3. Long-term continuous loading capabilityA high-pressure, nitrogen-operated, hydraulic pressure stabilising unit was employed to maintain a consistent magnitude of load applied to the model so that the physical modelling test is able to last continuously for weeks, months or even years without interruption. This feature ensures that the study of the long-term rheological behaviour of soft rocks can be carried out.3. Physical modelling testsPhysical modelling of an underground roadway/ tunnel within soft rocks with a hydrostatic stress condition was carried out. The same simulated materials were repeatedly used six times to construct six physical models. Each roadway model was provided with a different magnitude of support intensity.3.1. Geotechnical conditions for the prototype and the modelling scaleA specified underground roadway within soft rocks was assumed to be the prototype for the modelling study. Detailed geotechnical conditions of the roadway and its surrounding rocks are:circular roadway with a diameter (D) of 4.5 m and cross-sectional area of 16 m2; UCS (Rc ) of the surrounding rock was 20 MPa; bulk density of the surrounding rock was 2500 kg/m3;depth of the roadway location was 500 m below surface;rock mass stress (s0 ) was 12.5 MPa in all directions;support intensity(pa) to be applied to the roadway was 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 MPa, respectively.The geotechnical modelling scale (Cl ) determined was 1 : 25. The bulk density (gm ) of the simulated rock mass materials was 1600 kg/m3.Therefore, all the related simulation constants are:similarity constant for bulk density: Cg 1600/2500=0.64;similarity constant for strength: Cs ClCg 0:256; similarity constant for load: CF CgC1 4:096 105 ;similarity constant for time: Ct C l:5 0:2: Geotechnical conditions of the simulated rock mass and roadway were derived from those of the prototype rock mass as presented below:strength of the simulated rock mass: Rm=RcCs=0.512;diameter of the simulated roadway: Dm=DCl=180 mm;load intensity on the facets of the model: pm=s0Cs=0.32 MPa;Simulated support intensity: pam=paCs=0.00256, 0.00516, 0.00768, 0.01024, 0.0128 and 0.01536 MPa; respectively.3.2. Realization of support intensity in physical modellingDue to the restraints of the small dimensions of the model roadway on the simulation of support structure, the support pattern and structure were unable to be simulated. Instead, an equivalent support intensity was simulated and applied to the surface of the surroundingrock of the model roadway. A Static Water Support and Deformation Measurement System (SWSDMS) was designed specially. Fig. 3 illustrates the SWSDMS being installed in the model roadway. The mechanism of SWSDMS is to use 4 separate water capsules to apply a support intensity to the surface of the roadway roof, two side walls and floor. Four rubber tubes, each of which was linked to a water capsule and filled with water, were used to generate a water pressure at the capsule/rock interface and measure it through the water level reading. A certain constant simulated support intensity was achieved by applying a certain height of static water pressure. A change to support intensity could be made by changing the water height in the rubber tube. The volume change of each of the four water capsules was measured at the due time by collecting and weighing the water overflow. The volume of water coming from each of the four water capsules was used to calculate the radial deformation of roadway surrounding rock, i.e., roof subsidence, wall-to-wall closure and floor heave. The proposed simulated support intensities, i.e., Pam 0:00256, 0.00516, 0.00768, 0.01024, 0.0128 and 0.01536 MPa, were achieved by adjusting the static water level to 256, 516, 768, 1024, 1280 and 1536 mm high, respectively.Fig. 3. Static Water Support and Deformation Measurement System (SWSDMS) being accommodated in a roadway model in the real 3-D loaded physical modelling system. 3.3. Construction of physical modelThe compositions and properties of materials to be used for the construction of physical models were studied prior to the physical model construction. Given the significant rheological deformation of roadways excavated in soft rock, sand and paraffin wax were chosen for the simulated soft rock. The properties of a series of sand/paraffin wax mixtures were studied in laboratory and are presented in Table 1. Table 1 Compositions and properties of sand/paraffin wax mixturesAccording to the geotechnical conditions of the prototype rock mass and the model scale, a mixture of sand/paraffin wax of 100 : 3 was selected to construct the rock mass model. The procedures involved in the model construction include cold mixing of the sand and paraffin wax, oven heating the sand/wax mixture and constructing the physical model using the hot sand/wax mixture.3.4. Process of physical modelling The real process of an underground roadway excavation, support installation and deformation of the surrounding rocks with time was simulated in the laboratory physical modelling. After the model had cooled down, prestressing the model, excavation of the roadway under pressure, installation of the SWSDMS device and measurement of the roadway deformation were carried out step by step. The whole process of modelling was strictly conducted according to the time similarity constant. Each physical modelling step lasted for 10-25 days in the laboratory, which were equivalent to a real time period of 50-125 days approximately.4. Relations between support intensity and roadway deformation Comparable results of the six physical modelling tests conducted with the identical materials and geotechnical conditions revealed the significance of the support intensity in underground roadway/tunnel support.4.1. Effect of support intensity on the deformation characteristics of a roadwayThe deformation characteristics of an identical roadway with different support intensity is graphically presented in Fig. 4(a) and (b). It can be seen that the influence of support intensity on the deformation characteristics is significant. With a support intensity of 0.1 MPa, the roadway experienced a large eformation for a period of 118 days after the roadway excavation and the provision of support intensity. During this period, an average of 828 mm deformation was accumulated. Following this period, the wall-to-wall closure and roof-to-floor convergence stayed steady at a level of 4.4 mm/day. By contrast, when a support intensity of 0.6 MPa was provided to the identical roadway, its post-excavation deformation merely lasted for 36 days with an accumulative closure/convergence of 40 mm, followed by a rheological deformation of 0.08 mm/day, which was continuously reducing with time. The comparison shows that the deformation magnitude of the latter was only 4.8% that of the former.A negative exponential relation between the deformation rate and support intensity can also be deduced from the curve of deformation rate vs. support intensity presented in Fig. 5 and be mathematically expressed as: v 0:023pa2:4 :where v is the rheological deformation rate of the surrounding rock of a roadway in mm/day, pa is the support intensity in MPa provided to the surrounding rock.Fig. 4. Deformation of roadway with a series of support intensities:(a) Deformation of roadway with time, (b) Deformation rate of roadway with time.Fig.5 Relations between rheological deformation rate and support intensity of a roadway in soft rocks.4.2. Optimal support intensity for a roadway in soft rocksRequirements on the control of roadway deformation depend on the usage and service life of the roadway. It is known that a zero deformation rate is impossible practically to target in supporting a roadway in soft rocks. A wise approach is to exercise a design principle that the roadway deformation is allowed to take place to a degree within an acceptable limit. Physical modelling results indicated that an increase of support intensity from 0.1 to 0.5 MPa can markedly reduce the deformation rate of the surrounding rocks. A further increase of support intensity from 0.5 to 0.6 MPa, however, did not bring about as much reduction of deformation rate as that created by the support intensity increase of from 0.1 to 0.2 MPa or from 0.3 to 0.4 MPa. This means that a reasonable range of support intensity exists and an increase of support intensity can be rewarded with a significant reduction of roadway deformation if the actual support intensity is within this range.Further increases of support intensity can only cause less reduction of roadway deformation. Therefore, if both technical and economical considerations are taken into account, a support intensity of from 0.3 to 0.5 MPa would be appropriate for most temporary tunnels such as roadways in underground coal mining. With this support intensity, the rheological deformation rate of the surrounding rocks can be controlled within a range of from 0.1 to 0.4 mm/day, with which an ordinary temporary roadway can be maintained safely for years to one decade.5. Conclusions The three-dimensional physical modelling method provides a conceptual approach to quantitative designof roadway support associated with soft rocks. With lack of knowledge of the constitutive relations, especially for the rheological mechanisms, in rock engineering practice, the modelling results could serve as a foundation on which a scientific design of underground roadway/tunnel support is developed, particularly when a large amount of rock mass deformation is concerned. The experimental study conducted with a series of support intensities revealed that a reasonable support intensity exists. Its value depends on the geotechnical and geometric conditions of the underground roadway/tunnel concerned and the requirements applied by the roadway/tunnel safe use specifications and the roadway/tunnel service life span. The results indicate that a support intensity of 0.3 to 0.5 MPa can securely control the closure rate for the conditions tested within a magnitude of 0.1 to 0.4 mm/day for a medium size underground roadway/tunnel driven in soft rocks of around 20 MPa at a depth of about 500 m below surface.AcknowledgementsThe author would like to thank his colleagues at the Central Laboratory of Rock Mechanics and Ground Control, China University of Mining and Technology, for their generous assistance and help in the physical modelling study.References1 Internal Research Report. Study on the technology of large deformation control for roadways within soft rocks. China University of Mining and Technology, 1995 in Chinese. 2 Wang C. Study on the supporting mechanism and technology for roadways in soft rocks. PhD thesis, China University of Mining and Technology, 1995 in Chinese.3 Internal reference (1993). Properties of simulated materials for physical geomechanical modelling. The Central Laboratory of Rock Mechanics and Ground Control, China University of Mining and Technology in Chinese.4 Lin Y. Simulated materials and simulation for physical modelling. Publishing House of China Metallurgy Industry, Beijing, China, 1986 in Chinese.5 Durove J, Hatala J, Maras M, Hroncova E. Supports design based on physical modelling. Proceedings of the International Conference of Geotechnical Engineering of Hard Soils Soft Rocks. Rotterdam: Balkema, 1993.6 Singh R, Singh TN. Investigation into the behaviour of a support system and roof strata during sub-level caving of a thick coal seam. Int J Geotech Geol. Engng. 1999;17:21-35. 中文译文煤矿软岩巷道支护强度优化C. Wang采矿工程专业，西澳矿业学校，港口及航运局22卡尔古利WA6430，澳大利亚1引言地下巷道支护的实质是给巷道围岩提供支撑以实现应力应变平衡，并最终使变形稳定。为达到这一目标，需通过锚杆支护加固岩体或注浆（内部岩石稳定）或为围岩提供被描述为支撑强度的具有有一定数量级的支撑阻力（外部岩石稳定）。当地下巷道处于松软岩石中，岩石过于松软以致锚杆加固或不适合注浆加固。这是因为遇到水时岩体渗透性或岩体恶化施加的限制。因此，外部岩石支护如钢棚支护，成为了巷道稳定控制的唯一选择。在这种情况下，支护强度是指单位巷道围岩表面积的支撑力。在软岩工程实践中，地下煤矿巷道支护模式设计通常是基于经验法则。在大多数情况下，采用支护强度大的支护措施，确保巷道稳定。图1（a）展示了在中国北方一煤矿为实现巷道稳定投入过多资金成本的煤矿井下软岩分段巷道的良好条件。在某些情况下，例如在同一煤矿软岩中开掘的服务巷道（如图1（b），支撑力不足被指定为缺乏相关经验和设计规范所致。因此，巷道失稳是必然的。在随后进行巷道维修或重建时，又需支出额外的费用。这两种情况的关键问题在于最佳的支护强度，与巷道的断面形状和岩土工程条件，包括岩性，应力条件和水文状况呈函数关系。基于相似理论的相似材料的物理模拟为矿山地质力学研究提供了直接感知的方法。1-6利用组成相同的相似材料来模拟巷道及周围软岩，模拟应力条件下施加一定的支护强度到巷道表面。在这份说明中描述的三维实体建模方法，展示了软岩巷道支护战略设计方面定量计算的方案。通过一系列相似实验的结果，支护强度和巷道围岩变形间的关系建立。关于软岩巷道最佳支护强度的讨论也由此展开。图1 地下软岩巷道支护成功和失败的例子：a分段巷道的良好条件 b服务巷道支护失效2.三维实体模型的特征在中国矿业大学岩土力学与地面控制中心实验室进行的关于支护强度和巷道围岩变形间关系的物理模拟研究采用了三维实体模型系统（见图2）。该系统的特征描述如下：图2 中国矿业大学岩土力学与地面控制中心实验室三维加载实体模型系统2.1实体模型尺寸物理模型的有效尺寸为1000毫米宽，1000毫米高，200毫米厚。2.2三维实时加载能力六个千斤顶用于向长方体形式的物理模型的六个面加载。六个千斤顶设计能够各自覆盖一个面，并能够向模拟岩石表面施加10MPa的压力。这意味着千斤顶能够同时在前后上下左右六个面动态施加1000 t到2000 t的力。2.3长期连续加载能力高压氮气操作的液压稳定单元是用来保持相同负载应用到模型上，使物理模型试验能够持续数周，数月甚至数年连续无间断。此功能确保了软岩长期流变行为研究的进行。3物理模型测试地下软岩巷道或隧道的物理模拟在静水条件下进行，同样的模拟材料重复使用六次来兴建六个物理模型。对每个巷道模型提供不同程度的支护强度。3.1原型和模型比例的岩土工程条件为进行模拟研究，假定一个指定的软岩巷道为原型。巷道和围岩详细的岩土工程条件有：圆形巷道，直径4.5 m，截面积16 m2；围岩单向抗压强度为20 MPa；岩石体积密度为2500 kg/m3；巷道位于地面以下500 m；岩石各向压力为12.5 MPa；巷道支护强度分别为：0.1,0.2,0.3,0.4,0.5,0.6 Mpa。岩土模拟比例定为1:25。模拟岩体材料的容重（gm）为1600 kg/m3，因此，所有相关模拟常数为：容重相似不变：Cg 1600/2500=0.64;强度相似不变：Cs ClCg 0:256;负载相似不变 CF CgC1 4:096 105 ;时间相似不变 Ct C l:5 0:2: 模拟岩体和巷道的地质条件依据如下所示的原岩：模拟岩体强度 Rm=RcCs=0.512;模拟巷道直径: Dm=DCl=180 mm;模型各面加载强度 pm=s0Cs=0.32 MPa;模拟支护强度: pam=paCs=0.00256, 0.00516, 0.00768, 0.01024, 0.0128 0.01536 MPa; 3.2物理模型支护强度的实现由于小尺寸模拟巷道在支护结构上的限制，支护模式和结构不能被模拟。相反，相同的支护强度被模拟并施加到模拟巷道围岩。专门设计了一种静水支撑和变形测量系统（SWSDMS）。图3说明了SWSDMS被安装在模型巷道。SWSDMS的机制是用4个单独的水胶囊向巷道顶板，两帮和底板的表面提供支护强度。连接胶囊的并充满水的四个橡胶管用在水胶囊和岩石界面生成水压，并通过读取水位来测量水压大小。图3静水支撑和变形测量系统（SWSDMS）被安置在真实三维物理模拟加载系统下的巷道模型施加一定的静水压高度可以获得某一数值的模拟支护强度，通过改变橡胶管水的高度来实现模拟支护强度的变化。每个水胶囊的体积变化可以通过在适当时候收集并测量溢出水量来获得。来自每个水胶囊的水的体积用来计算巷道围岩的径向变形，即顶板下沉，两帮移近和底板臌起。通过调节静水位至256, 516, 768, 1024, 1280, 1536 mm 高度来实现建议的支护强度 0:00256, 0.00516, 0.00768, 0.01024, 0.0128, 0.01536 MPa。3.3物理模型的构建用于构建物理模型的材料组成和性质的研究优先于物理模型的构建。鉴于软岩巷道出现的显著流变，沙子和石蜡被用于模拟软岩。在研究实验室得出沙子石蜡混合物的一系列特性，列于表1。表1沙子石蜡混合物的组成和性质配比（质量）沙子：石蜡单轴抗压强度（MPa）试样1试样2试样3平均100:20.0330.0300.0290.307100:30.05540.0530.0530.0538100:40.08640.08420.08520.0853100:50.100.1070.1120.106100:60.1280.13040.1240.1275100:70.13860.13800.14240.1397根据岩体的原型和模型比例的岩土工程条件，选用配比为100:3的沙子石蜡混合物构造岩体模型。模型的建设所涉及的程序包括冷混合沙子和石蜡，烘箱加热沙子石蜡混合物，使用热沙子石蜡混合物构建物理模型。3.4物理模拟过程地下巷道掘进，支护安装和围岩随时间变形的真实过程是在实验室物理模型中模拟的。模型冷却后，预加应力到模型上，带压掘进巷道，安装SWSDMS设备，测量巷道围岩变形。建模的全过程严格按照时间相似常数进行，每个物理建模步骤在实验室持续10-25天，相当于约50-125天的真实时间。4支护强度和巷道变形的关系比较相同材料和岩土条件下进行的六个物理模型实验结果表明，支护强度在地下巷道或隧道支护中的重要性。4.1支护强度对巷道变形特征的影响相同巷道不同支护强度下的巷道变形特性以图的形式展现在图4（a）和（b）。可以看出，支护强度对巷道变性特性的影响很大。在0.1 MPa的支护强度下，巷道开掘完成并提供支护强度后118天，巷道经历了大的变形。在此期间，累计变形828 mm。此后，两帮收缩和顶底板收敛稳定在4.4 mm/d 的水平。与此相反，当提供给同一巷道0.6 MPa的支护强度时，开挖后变形仅仅持续了36天，累计收敛40 mm，紧接着是0.08 mm/d的流变，且随时间不断减少。比较结果显示后者的变形程度仅仅是前者的4.8%。变形速率和支护强度的负指数关系可以从图5中所示变形速率和支护强度曲线推导出来，数学表达为：v 0:023， pa 2.4 :其中v指巷道围岩变形速率，mm/day；pa指提供给围岩的支护强度，MPa。图4 一系列支护强度下的巷道变形(a) 巷道变形随时间的变化 (b)巷道变形速率随时间的变化图5 软岩巷道流变速率和支护强度的关系4.2软岩巷道支护强度优化对巷道变形控制的要求取决于巷道用途和服务年限。众所周知，支护软岩巷道达到零变形速率是几乎不可能的。明智的做法是行使此种设计原则，在允许范围内巷道发生一定程度的变形。物理模拟结果表明：支护强度从0.1增加到0.5 MPa，可以显著减少围岩变形速率。支护
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