Single phase water flow through rock fractures

Flow analysis plays a major role in various geotechnical applications, and the understanding of flow mechanisms is essential for the development of a hydro-mechanical flow model suitable for underground excavations in rock. Discrete flow analysis through discontinuities is reviewed including empirical and analytical flow models. The influence of external loading on joint deformation and single-phase flow show that the surface roughness and aperture size are the prime factors influencing flow rate. Nevertheless, the idealization of natural fractures as smooth parallel plate joints is still followed in many numerical models, because of the simplicity of the cubic law when applied to fracture networks. A numerical study of water flow through a network of joints employing Universal Distinct Element Code (UDEC) is used to quantify the effects of joint orientation and external stress acting on idealized joints.It is found that, for the same joint spacing, the flow rate into an excavation depends on the boundary block size (A_b) relative to the excavation size (A_e). The inflow becomes excessive if A_b/A_e is less than 4, but becomes very small if A_b/A_e exceeds 8.     

Analytical techniques for the estimation of mine water inflow

Summary This paper outlines various analytical techniques for mine water inflow estimation incorporating several refinements such as finite boundary conditions, linear, fracture and turbulent flow conditions to pumping wells and underground excavations. These modifications enable practical mining to be simulated and therefore, permit more accurate predictions of mine inflow quantities. Application of these techniques are given together with the scope of the application.     

The impact of tunneling construction on the hydrogeological environment of “Tseng-Wen Reservoir Transbasin Diversion Project” in Taiwan

Groundwater flow and the associated surface water flow are potential negative factors on underground tunnels. Early detection of environmental impacts on water resources is of significant importance to planning, design and construction of tunnel projects, as early detection can minimize accidents and project delays during construction. The groundwater modeling software package Groundwater Modeling System (GMS), which supports the groundwater numerical codes MODFLOW and FEMWATER, was utilized to determine the impact of tunneling excavation on the hydrogeological environment in a regional area around the tunnel and a local hot springs area, at the “Tseng-Wen Reservoir Transbasin Diversion Project”, in Taiwan. A hydrogeological conceptual model was first developed to simplify structures related to the site topography, geology and geological structure. The MODFLOW code was then applied to simulate groundwater flow pattern for the hydrogeological conceptual model in the tunnel area. The automated parameter estimation method was applied to calibrate groundwater level fluctuation and hydrogeological parameters in the region. Calibration of the model demonstrated that errors between simulated and monitored results are smaller than allowable errors. The study also observed that tunneling excavation caused groundwater to flow toward the tunnel. No obvious changes in the groundwater flow field due to tunnel construction were observed far away in the surrounding regions. Furthermore, the FEMWATER code for solving 3-D groundwater flow problems, in which hydrogeological characteristics are integrated into a geographic information system (GIS), is applied to evaluate the impact of tunnel construction on an adjacent hot spring. Simulation results indicated that the groundwater drawdown rate is less than the groundwater recharge rate, and the change to the groundwater table after tunnel construction was insignificant for the hot spring area. Finally, the groundwater flow obtained via the GMS indicated that the hydrogeological conceptual model can estimate the possible quantity of tunnel inflow and the impact of tunnel construction on the regional and local groundwater resources regime of the transbasin diversion project.     

基于裂隙渗流效应的水封油库涌水量预测分析

准确预测洞室涌水量和库区渗流场的空间分布特征是确保地下水封油库施工期安全性和运行期经济性的基础性工作之一。为真实地反映地下水封油库围岩随机分布裂隙的渗流效应对涌水量预测和渗流场空间分布特性的影响,提出了一种基于嵌入裂隙单元模型（EFE模型）的裂隙岩体渗流分析方法,并采用该方法对湛江水封油库的三维渗流场进行了计算分析,通过对比工程实测资料,验证了所提出方法的可靠性,进而预测了实例工程在运行期的涌水量。计算结果表明,EFE模型能够较好地模拟裂隙对洞室岩体局部渗流场的影响,反映库区渗流场及洞室涌水量空间分布不均匀的特性。研究成果可为水封洞库渗控措施的精准设计以及运行期污水处理设施配套设计工作提供参考。     

Prediction of dilation and permeability changes in rock salt

A numerical procedure has been developed for predicting dilation (porosity) and gas permeability changes in rock salt. The hierarchical single-surface constitutive model of Desai and co-workers is used a finite element program to calculate the state of stress and strain surrounding excavations in rock salt. The elastoplastic constitutive model accounts for strain hardening, a non-associative volumetric response and stress-path-dependent behaviour. The calculated stress and strain fields are used in a flow model based on the equivalent channel concept to predict permeability. Parameters for both the mechanical and permeability models are developed from laboratory test results. Two field experiments adjacent to underground excavations are modelled. The extent of the dilated rock zone around the excavation is predicted well, but the magnitude of the porosity and gas permeability is underpredicted very near the excavations. This discrepancy is attributed to model parameters derived from loading-only laboratory tests, whereas significant unloading occurs in the field. The shape of the yield surface was found to be an important factor in dilation and permeability predictions. Similar stress, strain and permeability fields were obtained with different model types (plane strain or axisymmetric) and initial stress states, and with instantaneous and progressive excavation.     

Effects of flow dimension in faulted or fractured rock on natural reductions of inflow during excavation: a case study of the Horonobe Underground Research Laboratory site,Japan

Major inflows of groundwater can occur during excavation in faulted or fractured rock masses, even if pre-excavation grouting is applied; postexcavation grouting may then be required to reduce these inflows. However, the diffusion equation for fluid pressure suggests that inflows may reduce naturally by 50–90% or more within days or weeks when the dimension of the flow in faults or fractures that feed the inflow (flow dimension) is close to 1, but inflow reduction is minimal when the flow dimension is close to 3. Therefore, if the flow dimension is close to 1, the natural reduction in inflow may obviate countermeasures. Nevertheless, this natural reduction being dependent on flow dimension is seldom considered explicitly when planning excavations or countermeasures. To verify the applicability of the relationship between natural changes of inflow and the flow dimension, this study measured changes in inflow at six locations at the Horonobe site, Japan, where major inflows occurred during excavations of tunnels or shafts in faulted or fractured siliceous rocks. The flow dimension at each location was assessed using pre-excavation packer tests in surface-based boreholes. The results confirm that changes in the inflows during the days and weeks immediately after their commencement depended consistently on the assessed flow dimension. Natural reductions in inflow during excavation are predictable based on the flow dimension, which can be estimated using pre-excavation borehole investigations or from the initial changes in inflow during the first several days. This approach may be helpful for improving the efficiency of excavations.

     

Effect of highly pervious geological features on ground-water flow into a tunnel

Current practice for estimating water inflow rate relies mostly on analytical solutions which assume a homogeneous, isotropic porous medium around a tunnel. Field measurements indicate that current engineering practice does not consistently make adequate estimate of ground-water flow into a tunnel during excavation due to various factors that analytical solutions do not properly take into account. Among the various factors affecting ground-water flow, the significance of a highly pervious feature located near the tunnel is discussed in this research. The highly pervious feature, which is located near an underground opening and connected to a large source of water, can provide a path for relatively high-head water to the joints intersecting the opening. This paper describes the influence of a highly pervious feature on the ground-water flow regime around a tunnel and the change of inflow rate as the tunnel approaches a highly pervious feature.     

Prediction of water inflow to mechanized tunnels during tunnel-boring-machine advance using numerical simulation

An accurate estimate of the groundwater inflow to a tunnel is one of the most challenging but essential tasks in tunnel design and construction. Most of the numerical or analytical solutions that have been developed ignore tunnel seepage conditions, material properties and hydraulic-head changes along the tunnel route during the excavation process, leading to inaccurate prediction of inflow rates. A method is introduced that uses MODFLOW code of GMS software to predict inflow rate as the tunnel boring machine (TBM) gradually advances. In this method, the tunnel boundary condition is conceptualized and defined using Drain package, which is simulated by dividing the drilling process into a series of successive intervals based on the tunnel excavation rates. In addition, the drain elevations are specified as the respective tunnel elevations, and the conductance parameters are assigned to intervals, depending on the TBM type and the tunnel seepage condition. The Qomroud water conveyance tunnel, located in Lorestan province of Iran, is 36 km in length. Since the Qomroud tunnel involved groundwater inrush during excavating, it is considered as a good case study to evaluate the presented method. The groundwater inflow to this tunnel during the TBM advance is simulated using the proposed method and the predicted rates are compared with observed rates. The results show that the presented method can satisfactorily predict the inflow rates as the TBM advances.     

Numerical analysis of the failure process around a circular opening in rock

Damage and fracture propagation around underground excavations are important issues in rock engineering. The analysis of quasi-brittle materials can be performed using constitutive laws based upon damage mechanics. The finite element code RFPA2D (Rock Failure Process Analysis) based on damage mechanics was used to simulate a loading-type failure process around an underground excavation (model tunnel) in brittle rock. One of the features of RFPA2D is the capability of modeling heterogeneous materials. In the current model, the effect of the homogeneous index (m) of rock on the failure modes of a model tunnel in rock was studied. In addition, by recording the number of damaged elements and the associated amount of energy released, RFPA2D is able to simulate acoustic activities around circular openings in rock. The results of a numerical simulation of a model tunnel were in very good agreement with the experimental test using the acoustic emission technique. Finally, the influence of the lateral confining pressure on the failure mechanism of the rock around the model tunnel was also investigated by numerical simulations.     

Tunnelling and Hydrogeological Issues: A Short Review of the Current State of the Art

Since the 1960s, there has been an increasing interest in the understanding of the hydraulic flow inside a hard rock mass, since water inflow into deep tunnels constitute a hazard, in addition to being an important factor in controlling the advancement of excavation. The characterisation of fluid flow through hard rock masses is still one of the most challenging problems faced by geologists and engineers. A rock mass is characterised by networks of discrete and ubiquitous discontinuities that strongly affect its hydraulic properties, but detailed knowledge of the discontinuity properties allows for the evaluation of the hydraulic flow in the rock mass affected by the excavation of a tunnel. A geostructural field survey is fundamental in order to correctly define the discontinuity types, settings and networks. Numerous approaches have been proposed to estimate the water inflow based on empirical relations supported by field experience and case studies, as well as analytical solutions. Often, however, these approaches are not easily applicable in standard practice and in complex scenarios. The most appropriate approach to characterising the hydraulic flow of the rock mass and to predicting in the most effective way the expected water inflow during the excavation of a tunnel is based on a detailed geological model and geostructural analysis as described in this paper.     

基于高分辨率示踪技术的岩溶隧道涌水来源识别及含水介质研究

利用高分辨率示踪技术探讨了重庆三泉隧道突水来源,并对含水介质进行了刻画,结果表明:（1）隧道涌水段受长滩地下河影响,而麦阴槽落水洞为长滩地下河的补给来源之一;（2）试验段岩溶含水介质通畅,地下水流速快,为典型紊流态;地下河管道结构不均匀,发育两条过水通道,为主管道并联支管道,无溶潭和地下湖发育;利用Qtracer2软件,计算得到地下河几何参数及水文地质参数:地下管道储水体积为1 148.4 m3,表面积为1.30×106 m2,平均直径为1.37 m,长度为780 m;摩擦系数为0.51,舍伍德数为1 055.1,施密特数为1 140,水力深度为1.08 m,分子扩散边界层厚度为1.3 mm;（3）因试验时间短、试验为小流量期及存在其他地下河出口等条件制约,示踪剂天来宝的回收率较低,上湾洼地与隧道涌水段连通关系有待进一步研究;（4）由于三泉隧道涌水点与地表水具有直接水力联系,且涌水量大,建议引走地表洼地水源、填埋隧道上方麦阴槽落水洞或在隧道下方新建泄水洞排水。      

Influence of rock mass properties on tunnel inflow using hydromechanical numerical study

One of the primary geotechnical problems encountered during tunnel construction involves the inflow of groundwater into the tunnel. Heavy inflows make tunnel construction difficult and result in higher costs and delays in construction period. Therefore, it is essential to estimate the volume and rate of water inflow that is likely to appear in the tunnel. In this research, water inflow to the tunnel was calculated using numerical hydromechanical analysis. Effect of rock mass properties including fracture characteristics (normal and shear stiffness, hydraulic aperture, dilation angle, and fracture nonlinear behavior) on inflow was studied using a two-dimensional distinct element method. Results show that fracture properties play important role in inflow to the tunnel and must be considered in prediction of inflow to the tunnel. Based on numerical analysis results, inflow of groundwater into the tunnel increases with the increasing of normal and shear stiffness, dilation angle, and hydraulic aperture of rock mass fractures. The measured inflow with considering nonlinear fracture behavior was more than the calculated inflow with linear constitutive behavior.     

Underground excavation shape optimization considering material nonlinearities

The shape of an underground opening is a major factor influencing the stability of the underground excavation. Obtaining an optimized shape is significant in civil and mining engineering applications for increasing stability and reducing costs. This paper presents an updated method for finding the optimal shape of an underground excavation using the latest bi-directional evolutionary structural optimization (BESO) techniques considering material nonlinearities. Recent development in the BESO applications to underground excavation is discussed through illustrated examples. Details are given of the BESO method used to finding the optimal shape based on the global stiffness. The methodology of the stiffness based optimization techniques is described with examples of applications to underground excavation. Applications to underground void structures, such as cavern and tunnel, as well as underground solid structure such mine pillar are demonstrated. It is concluded that stiffness based optimization techniques are applicable to underground excavations and practical shape of an excavation can be generated as a result of the BESO application.     

莲花隧道软弱围岩大变形预测方法适用性评价研究

评估地质条件以及开挖风险是地下空间设计和施工中的重要阶段之一。在选择合适的开挖方法和支护系统时,识别和估计岩体的变形潜在趋势非常重要。在本研究中,对萍莲高速莲花隧道中的岩体变形潜在趋势进行了研究。首先介绍了莲花隧道的工程地质环境、隧道围岩工程特性、岩体质量评价以及现场实测的隧道变形情况,然后运用经验法和半经验-半理论法预测莲花隧道变形趋势,并将预测结果与实际变形监测情况进行对比。结果表明,莲花隧道的大变形主要受松散破碎、遇水膨胀的软弱围岩和丰富的地下与地表水以及断层破碎带与构造活动的影响。对于莲花隧道而言,现行大变形预测方法普遍存在一定问题：经验法主要依据岩体质量分级结果进行变形趋势预测,具有一定的主观性,其预测结果与实际变形情况存在一定偏差;半经验-半理论方法中运用围岩强度与应力关系进行预测效果较好,但基于围岩强度应力比的Jethwa法、Hoek法与ISRM法预测结果偏保守,且针对无变形段的预测效果较差。综上所述,在工程勘察与设计阶段,应客观、细致地进行围岩质量分级、地应力量测、地下水与地质构造勘探等工作,结合经验法、半经验-半理论方法对大变形趋势和变形量值进行预测,综合研判隧洞沿线...     

基于DDA方法一种流-固耦合模型的建立及裂隙体渗流场分析和应用

以非连续变形分析方法（DDA）为基础并采用稳态流体计算方法将二者结合进行裂隙岩体流-固耦合分析。利用DDA方法生成裂隙岩体模型,在此基础上采用矩阵搜索等方法形成新的裂隙水通网络模型。采用稳态迭代算法和立方定律求得裂隙水压力,并把裂隙水压力作为线载荷施加到块体边界,在DDA算法中每个迭代步完成后更新裂隙开度和水压值,与DDA算法结合研究裂隙水与块体之间相互作用关系。利用以上裂隙岩体流-固耦合计算方法研究了某水封油库开挖和运行过程洞室围岩流量和密封性,为该工程预测水封效果提供了有益的主要依据,也是国内首次采用DDA方法做大型工程的流-固耦合模型分析。     

Experimental investigation of fracture propagation and inrush characteristics in tunnel construction

It is well known that water inrush during excavation is one of the greatest challenges in modern underground engineering. However, the fracture propagation and inrush characteristics induced by excavation and high-pressure water are poorly understood due to the lack of an appropriate experimental apparatus and an online and real-time monitoring approach. Accordingly, a model test system for the simulation of water inrush during excavation and water injection was developed. Acoustic emission (AE) monitoring during excavation and injection was used to investigate the fracture propagation and water-inrush channel formation in the host rock. Three distinct stages were observed in the AEs over time and were related to the fracture propagation during excavation and injection, namely fracture initiation, fracture extension, and unstable fracture growth (fracture network). The AE results exhibited an increase in AE activities and changes in the AE spatial correlation during the excavation and during the increase in injection pressure. A comparison of photographs of the water-inrush locations and the mechanical characteristics obtained from the AE test verified the proposed method. The results provide valuable insights and a suitable method for the investigation of the mechanism of water inrush in underground engineering.

     

地下综合管廊穿越活动地裂缝变形与内力的响应分析

由于场地条件限制,西安城市地下综合管廊建设需以不同角度穿越地裂缝,必将受到地裂缝活动作用的影响。建立地下综合管廊穿越活动地裂缝的三维有限元模型,模拟了地下综合管廊穿越地裂缝时,设置了5 cm、10 cm、15 cm、20 cm、25 cm和30 cm 6种不同沉降量工况以及正交、斜角60°与斜角30° 3种穿越角度。研究结果表明,地下综合管廊的变形与轴向应力随地裂缝活动量的增加而不断增大,管廊的水平轴向位移、竖向位移以及轴向应力均集中在地裂缝两侧一定范围内,管廊上表面轴向应力峰值大于下表面。管廊与地裂缝交角越小,管廊的水平轴向位移越小,竖向位移的影响范围越大,管廊水平轴向应力越大,实际工程中应避免管廊不设缝小角度穿越活动性强的地裂缝。研究成果对土体与地下综合管廊之间的相互作用机理分析以及对防灾减灾工作有一定的参考价值。     

Three Dimensional Numerical Analysis of Underground Bifurcated Tunnel

As far as the bifurcated tunnel of underground engineering is concerned, it is usually used in the water conveyance system. Due to the complexity of underground rock masses and concrete lining, researches on mechanical characteristic and stability of the bifurcation tunnel have attracted more and more attention in the geotechnical field. In order to understand bifurcated tunnel in detail, three-dimensional (3D) numerical method is applied to solve the above key subjects by simulating a practical project. Furthermore, sub-model technology is applied to analyze the intersection position, corresponding deformation and stress results in the practical condition. Meanwhile, 3D excavation and support calculation under four conditions have been simulated based on 3D self-compiled code and Ansys software. In addition, the paper plays emphasis on the stress, displacement analysis considering different stress releasing ratio instead of rheological analysis, and the results corresponding with the fact indicate the feasibility of 3D elasto–visco–plastic code.