Acoustic emission monitoring of rockbursts during TBM-excavated headrace tunneling at Jinping Ⅱ hydropower station

To better understand the mechanical properties of marble at Jinping II hydropower station, this paper examines the changes of brittle rocks in excavation damaged zones(EDZs) before and after excavation of tunnel with the tunnel boring machine(TBM). The paper attempts to employ the acoustic emission(AE) to study the AE characteristics and distribution of rockburst before and after TBM-excavated tunnel. It is known that the headrace tunnel #2, excavated by the drill-and-blast(D&B) method, is ahead of the headrace tunnel #3 that is excavated by TBM method. The experimental sub-tunnel #2–1, about 2000 m in depth and 13 m in diameter, between the two tunnels is scheduled. In the experimental sub-tunnel #2–1, a large number of experimental boreholes are arranged, and AE sensors are installed within 10 m apart from the wall of the headrace tunnel #3. By tracking the microseismic signals in rocks, the location, frequency, quantity, scope and intensity of the microseismic signals are basically identifed. It is observed that the AE signals mainly occur within 5 m around the rock wall, basically lasting for one day before tunnel excavation and a week after excavation. Monitoring results indicate that the rockburst signals are closely related to rock stress adjustment. The rock structure has a rapid self-adjustment capacity before and after a certain period of time during tunneling. The variations of rock stresses would last for a long time before reaching a fnal steady state. Based on this, the site-specifc support parameters for the deep tunnels can be accordingly optimized.     

Evaluation of high local groundwater inflow to a rock tunnel by characterization of geological features

High local groundwater flow into rock tunnels is a technical and environmental problem for underground constructions. Geological features such as fault zones, open fractures and dykes can be the source of very high local inflows. However, prediction of possible groundwater inflow from these features using analytical and numerical tools often failed due to given hydrogeological assumptions and simplification of these heterogeneous media. In fact, the characteristics of geological features are not often detected by normal exploratory methods and missed from geological models. In order to identify factors that affect inflows from the geological features, a detailed investigation has been carried out at a tunnel excavated to supply potable water to the city of Semnan in Iran. The tunnel passes through formations of Cambrian age with thin and thickly laminated limestone, sandstone, shale and siltstone, which thrust on tuff Formation of Tertiary age by a main thrust fault. It was found out that:

(1)

High local inflows (from 20 up to 750 l/s) come from five geological features and much of tunnel is dry or has less inflow.

(2)

Maximum inflow occurs in an open fracture with amount of 750 l/s.

(3)

Faults have different hydraulic effect; they may act as conduits, barriers or combined conduit-barrier systems.

(4)

Dykes are generally barriers along which a large volume of water can be stored.

(5)

Evaluation of high local groundwater inflow to rock tunnels based on characterization of geological features is more reliable compared to available analytical and empirical estimation.

     

Analyses of the grouting results for a section of the APSE tunnel at Äspö Hard Rock Laboratory

The grouting results for a tunnel at a depth of 450 m in crystalline rock at Äspö HRL were studied. The aims were to investigate whether the methodology used resulted in a successful grouting design and producing a sufficiently dry tunnel, and whether grout penetration and inflow into the finished tunnel corresponded to the predictions. An analysis was made of data from an original cored borehole, drilled before the tunnel was constructed and mapped thoroughly with regard to fractures and transmissivities. The predicted inflow into the tunnel was calculated and found to be four times higher than the measured inflow. The latter was 5 l/min along the 70 m tunnel, considered to be a good result at the current depth. New cored control boreholes were drilled along a section of the tunnel. The inflow positions and quantities in these holes, and the positions of grout found in the corresponding cores, were compared with the data from the original borehole. It was found that at the predicted positions of larger fractures, grout was observed and there was no inflow, showing that these had been successfully sealed. At the predicted positions of small fractures, no grout was visible in the cores, and small inflows showed that the grout had not sealed these fractures. The results indicated that cement-based grout successfully sealed fractures down to a hydraulic aperture of about 50 μm but not below 30 μm. This concurs with the initial design aimed at sealing fractures larger than 50 μm.     

Parameters regulating groundwater inflows into hard rock tunnels—a statistical study of the Bolmen tunnel in Southern Sweden

Inflow of groundwater into tunnels has always been a major technical and environmental problem for underground constructions. Predictions of groundwater inflow using analytical and numerical tools have often failed due to generalization and simplification of important parameters, especially in heterogeneous media such as crystalline rocks. In order to identify those parameters that regulate inflows, a systematic statistical analysis has been carried out at a tunnel constructed in the hard crystalline rocks of southern Sweden. The parameters included topographically, technically and geologically important variables in the hard crystalline rocks as well as in the overburden. The study revealed that many factors related to the rock quality, as well as to the overburden—such as the number of fractures, the thickness of the overburden, the soil type and the amount of pregrouting—regulated the leakage. It seems that a clear difference exists between the parameters that regulate the major and the minor leakage. The minor leakage is associated more with the drainage of the rock mass, while the major leakage is clearly associated with different parameters in the overburden. Unless the overburden and the rock mass are considered as an interrelated system, predictions of groundwater inflows are likely to fail.     

Evaluation of common TBM performance prediction models based on field data from the second lot of Zagros water conveyance tunnel (ZWCT2)

Zagros water conveyance tunnel (ZWCT) is a 49 km tunnel designed for conveying 70 m³/s water from Sirvan River southward to Dashte Zahab plain in western Iran. This long tunnel has been divided in 3 Lots namely 1A, 1B, 2. By November 2014, about 22 km of the Lot 2 (with a total length of 26 km) has been excavated by two double shield TBMs from two southern and northern portals. The bored section of tunnel passed through different geological units of 3 main formations of Zagros mountain ranges which mainly consist of weak to moderately strong argillaceous-carbonate sedimentary rocks. In this paper, the operating and as-built geological data collected during construction phase of the Lot 2 of ZWCT project was used to compare the calculated machine performance by empirical methods such as the Hassanpour et al. (2011), Q_TBM, NTNU, Palmstrom, and theoretical model of Colorado School of Mines or CSM. The predicted penetration rates were then compared with the observed field performance of the machine and the variations of predicted rates were examined by statistical analysis. The results showed that the site-specific model, which was based on TBM performance in similar formations can provide estimates closer to actual machine performance.     

A case study on rock damage prediction and control method for underground tunnels subjected to adjacent excavation blasting

The effects of tunnel blast excavation on the surrounding rock mass and the lining systems of adjacent existing tunnels are comprehensively studied for the Damaoshan highway tunnel project as a case study. The damage of the surrounding rock and the lining system under different blast loads are analyzed by field tests and numerical simulations. It is observed that the rock damage extent around the tunnels linearly increases with the peak particle velocity (PPV) of the existing tunnel. A feasible PPV-based damage control method is then proposed for different portions of the tunnels. For the Damaoshan tunnel project, a PPV threshold of 0.22 m/s in the adjacent existing tunnel is prescribed to limit the damage extent to approximately 1.6 m at the tunnel exit and entrance portions. Furthermore, the PPV criteria for the other portions are also determined accordingly. It is also shown that no failure occurs in the linings or at the rock–lining interfaces if the PPV is less than 0.30 m/s. The control method and the threshold PPV proposed in this study have been successfully applied to restrict blast-induced damage during the new tunnel excavation of the Damaoshan tunnel project.     

Effect of adverse geological condition on TBM operation in Ghomroud tunnel conveyance project

With the planned length of 36 km, Ghomroud tunnel is one of the longest tunnels under construction in central Iran. About half or 18 km of this tunnel was excavated by a double shield TBM. Several adverse geological conditions encountered, consisting of ground squeezing and face collapse, hindering TBM performance, and caused several TBM stoppages and jamming. This paper presents the impact of ground conditions on machine performance based on the information obtained from field observations and geotechnical site investigations. As built geological conditions are described while the method and results of tunnel convergence measurements and their impacts on tunneling operation is examined. Based on the detail study of the available geological information and tunnel convergence measurements, it was evident that the existence of weak structures in rock mass resulted in high rate of the convergence, which was the dominant factor in the TBM jamming. Since it was not possible to make observation and measurements of geological parameters when working in a lined tunnel built by a shielded machine, an attempt was made to correlate TBM operational parameters and ground convergence. The preliminary result of the analysis has indicated a good correlation among machine’s operational parameters and tunnel convergence. If the system is fully developed, these parameters can be used as an indicator of the potential for high rates of convergence. An early warning on ground convergence is essential for taking precautionary measures to avoid TBM from getting jammed by squeezing ground.     

Design and construction of river crossing tunnel beneath Tachia River,Taiwan

Due to the impact of the 1999 Chi-Chi earthquake (ML = 7.3) and the following typhoon induced heavy rainfalls and floods, the tailrace tunnel of the Kukuan Hydropower Plant was severely blocked and must be realigned and rebuilt. The new tailrace tunnel is 1991 m long with a 140 m section passing underneath the Tachia River, where the shallowest rock cover is 3.5 m. In view of the common phenomenon that weak zone developed along river channel and debris accumulated on river bed, the ground improvement from the surface to the tunnel is difficult. Therefore, a construction shaft, a water sealing pre-grouting plan and a special supporting system were designed to prevent the potential water inflow when tunneling underneath the Tachia River. In the construction phase, adequate excavation cross sections, support elements, auxiliary treatments, and water sealing grouting methods were selected to overcome the difficult ground condition encountered. The experience learned from this successful case can be a valuable reference for the design and construction of similar river crossing tunnels in the future.     

裂隙岩体深埋长隧洞断裂控水模型及突、涌水量多因素综合预测

 由于深埋长隧洞围岩的力学、水力学特性明显区别于浅埋隧洞,且深部岩体不易勘探,因此深埋隧洞突、涌水量预测难度较大。针对高地应力、高水力梯度和高地温等复杂的深部地质环境,结合南水北调西线工程,提出了以断裂控水模型为基础的多因素综合评判方法。该方法是以大型断裂带为主要地下水通道的宏观地质模型作为基础;再分别研究对突涌水有影响的各种深部地质环境因素的影响规律;最后对宏观模型估算的突、涌水量进行综合修正。该方法为评价深埋长隧洞突涌水和隧洞施工与防护提供一种新的思路。     

A review of the Namntall Tunnel project with regard to grouting performance

The 6 km tunnel under the Namntall hill is a part of the Botniabanan railway project in northern Sweden. The tunnels were excavated by means of drilling and blasting and with, for Scandinavian conditions, a normal grouting routine. The grouting is performed to reduce water ingress into the tunnel to the level defined in the contract. When the water ingress requirements proved difficult to meet, it was obvious that the geological and the hydrogeological conditions in the tunnel would dictate the work processes. A distinctive change in rock conditions influenced both grouting performance and seepage into the tunnel. The rock conditions and the grouting were quantified throughout the project and these are summarized in this paper. It can be concluded that the strongest correlation is between the water ingress, the hydrogeological conditions and the density of the zones and the conditions in and around these zones. The paper suggests a different approach to hydrogeological prognosis and the grouting process, such as distribution of grouting classes, increased mixer capacities and regular use of two grouting rounds.     

Numerical simulation of rock deformation for support design in tunnel intersection area

Passenger and vehicle adits and ventilation shafts are commonly used for emergency access and ventilation in long tunnels. Increasing support load and additional tunnel deformation may endanger tunnel stability during construction in the intersection of the access and main tunnels. To understand the mechanical behavior of rock masses in the intersection area, 75 cases of 3D numerical analysis were conducted. These analyses were conducted under various tunneling conditions including rock strength, rock mass rating, overburden depth, and intersection angle. Following analysis results, a criterion for assessing the effect of intersection angles on tunnel behavior has been established, and three categories of support design suggestions for different geological conditions have also been proposed.     

Mechanical behaviour of scaled-down unsupported tunnel walls in hard rock under high stress

A large number of scaled-down tunnel experiments were undertaken to investigate the response of unsupported walls to an increased stress field. The experiments were undertaken in 200 mm diameter tunnels that were drilled into intact rock blocks of sandstone and granite ranging in strength from moderately strong to very strong. The tunnels were loaded by a servo-controlled, 450 tonne capacity INSTRON compression testing machine. As the ratio of intact rock strength to induced stress decreased, the unsupported tunnel walls became increasingly unstable. Critical ratios of compressive strength to induced stress were determined for critical instability stages such as tunnel spalling and also pillar crushing adjacent to the tunnels. The physical models have been simulated using three-dimensional finite element modelling. The values of the critical ratios correlate well with underground observations of full scale tunnels with similar Uniaxial Compressive Strength materials. Dynamic ejection velocities similar to those calculated from back analysis of actual failures have been determined. In addition, the seismic responses prior and during key failure stages have been established as a function of the increased loadings.     

Rock quality classification and stability evaluation of undersea deposit based on M-IRMR

The mine improvement of rock mass rating (M-IRMR) evaluation method was put forward based on the existing theory of the rock quality classification and the stability evaluation in the undersea deposit of Sanshandao Gold Mine, China. The M-IRMR evaluation method includes 9 evaluation indexes which are rock compressive strength, rock quality index RQD, joint spacing, joint state, groundwater state, joint direction, ground stress, blasting vibration, and exposed area, respectively. During the evaluation process, according to the special features of the undersea deposit, the rock mass rating (RMR) method was used as the foundation, and the four geological parameters (i.e., rock compressive strength, rock quality index, joint spacing, and ground stress) were modified. The computing methods of the two engineering factors (blasting vibration and exposed area) were presented. The M-IRMR rock quality classification and stability evaluation method was applied in the level of −420 to −690 m in the undersea deposit of Sanshandao Gold Mine and the classification results were consistent with the actual situations, which can provide a scientific basis for choosing the suitable mining method and stope support system of the undersea deposit.     

Review and perspective of expressway tunnels in Taiwan,China

Since the construction of the first expressway in the 1970s, the total length of expressways in Taiwan has increased to over 1 000 km, of which 40 km are aligned with tunnels. These twin-tube tunnels, which have two or three lanes in each tube, are characterized by large cross-sections. Due to the complicated topography and heterogeneous geological conditions of Taiwan, tunnel construction has encountered many difficulties. Thus, many advanced excavation methods were developed during tunnel construction. To satisfy the concurrent requirements of safety, economy and efficiency, new construction methods and techniques should be developed or introduced. Moreover, environmental protection and ecological conservation must be paid increasing attention to the goal of substantial development.     

Time-dependent behavior of tunnel lining in weak rock mass based on displacement back analysis method

Weak rock mass behavior is an important and challenging consideration during construction and utilization of a tunnel. Tunnel surrounding ground deformation in weak rocks causes to gradual development of loading on the support system and threats the opening stability. In this research, time-dependent behavior of Shibli twin tunnels was investigated using laboratory testing, monitoring data, and finite difference numerical simulation approaches. The host rock of Shibli tunnels are mainly composed of gray to black Shale, Marl and calcareous Shale. Geological maps and reports demonstrate a heavily jointed condition in the host rock through two orogenic phases. The experiment was organized in following order to understand the behavior of the rock mass around the tunnels. At first, triaxial creep test were conducted on intact rock specimens. Then, the time-dependent behavior of the tunnel host rock was numerically simulated considering Burger-creep visco-plastic model (CVISC). Finally, displacement based direct back analysis using univariate optimization algorithm was applied. Also, the properties of the CVISC model and initial stress ratio were estimated. Numerical modeling was verified by its comparison with tunnel displacement monitoring results. The creep behavior of the rock mass was predicted during tunnel service life based on back analysis results. Results show that thrust force, bending moment, and the resulting axial stresses will gradually increase at the spring line of the final lining. After 55 years of tunnel utilization the compressive strengths of lining concrete will not be stable against the induced-stresses by thrust force and bending moment, thus the tunnel inspection and rehabilitation are recommended.     

Estimation of deformation and stiffness of fractures close to tunnels using data from single-hole hydraulic testing and grouting

Sealing of tunnels in fractured rocks is commonly performed by pre- or post-excavation grouting. The grouting boreholes are frequently drilled close to the tunnel wall, an area where rock stresses can be low and fractures can more easily open up during grout pressurization. In this paper we suggest that data from hydraulic testing and grouting can be used to identify grout-induced fracture opening, to estimate fracture stiffness of such fractures, and to evaluate its impact on the grout performance. A conceptual model and a method are presented for estimating fracture stiffness. The method is demonstrated using grouting data from four pre-excavation grouting boreholes at a shallow tunnel (50 m) in Nygård, Sweden, and two post-excavation grouting boreholes at a deep tunnel (450 m) in Äspö HRL, Sweden. The estimated stiffness of intersecting fractures for the boreholes at the shallow Nygård tunnel are low (2–5 GPa/m) and in agreement with literature data from field experiments at other fractured rock sites. Higher stiffness was obtained for the deeper tunnel boreholes at Äspö which is reasonable considering that generally higher rock stresses are expected at greater depths. Our method of identifying and evaluating the properties and impact of deforming fractures might be most applicable when grouting takes place in boreholes adjacent to the tunnel wall, where local stresses might be low and where deforming (opening) fractures may take most of the grout.     

Characterisation and engineering properties of tectonically undisturbed but lithologically varied sedimentary rock masses

Tectonically undisturbed sedimentary rocks deposited in a quiescent shallow marine environment often include a sequence of strata that may present significant lithological variety at the scale of an engineering structure. Such rock masses exhibit engineering properties that are significantly different from tectonically disturbed rock masses of similar composition. For example, molasse consists of a series of tectonically undisturbed sediments of sandstones, conglomerates, siltstones and marls, produced by the erosion of mountain ranges after the final phase of an orogeny. They behave quite differently from flysch which has the same composition but which was tectonically disturbed during the orogeny. The molasses behave as continuous rock masses when they are confined at depth and the bedding planes do not appear as clearly defined discontinuity surfaces. Close to the surface the layering of the formations is discernible and only then similarities may exist with the structure of some types of flysch. Therefore extreme care is necessary in the use of geotechnical classification systems for the selection of design parameters, in order to avoid penalizing the rock mass unnecessarily. A discussion on the use of the geological strength index (GSI) for the characterisation of such rock masses is presented. Two GSI charts are proposed for estimating the mechanical properties of these masses: one mainly for tunnels; and the second for surface excavations. An example is given to illustrate the process of tunnel design in molassic rocks.     

Excavation-induced hydraulic conductivity reduction around a tunnel – Part 1: Guideline for estimate of ground water inflow rate

Field observations indicate that current engineering practice does not consistently estimate ground water flows into unlined rock excavated tunnels due to various factors that analytical solutions do not take into account. These factors include significant geological features, groundwater drawdown, inadequate estimates of hydraulic conductivity from packer tests, and stress-induced rock-mass permeability reduction in the vicinity of tunnel (lining-like zone). A key variable that is not properly accommodated in current practice, is the hydro-mechanical interaction within the joints in the surrounding rock mass. The significance of this variable is discussed in the 1st part of the paper which presents an analytical solution assessing ground water inflow rate into a tunnel using a mathematical derivation that takes into account the excavation-induced rock permeability reduction in the vicinity of a tunnel based on hydro-mechanical coupling effect. In the 2nd part of the paper, results from numerical analysis are presented to verify the proposed analytical solution for estimating ground water inflow rate into a tunnel. Further studies are currently underway to identify other key variables and their impact on the behavior of unlined tunnels and hydrological flow regime in the surrounding fractured rock mass using a distinct element method program which can fully consider hydro-mechanical coupled behavior of joints.     

The effect of principal stress orientation on tunnel stability

In order to investigate the effect of principal stress orientation on the stability of regular tunnels and cracked tunnels, experiments by using square specimens with a centralized small tunnel were conducted, and the corresponding numerical study as well as photoelastic study were implemented. Two kinds of materials, cement mortar and sandstone, were used to make tunnel models, and three types of tunnel models were studied, i.e. (1) regular tunnel models loaded by different orientation’s principal stresses, (2) tunnel models with various orientation’s radial cracks in the spandrel under compression, and (3) tunnel models with a fixed radial crack loaded by various orientation’s principal stresses. In the numerical study, the stress intensity factors of the radial cracks were calculated, and the results agree well with the test results. For regular tunnels, when the angle θ between the major principal stress and the tunnel symmetrical axis is 45°, the corresponding tunnel is the most unfavorable; for tunnels with a radial crack in the spandrel, when the angle β between the crack and the tunnel wall is 135°, the corresponding tunnel is the most unfavorable; for tunnels with a β = 130° radial crack, when θ = 0° or θ = 70°, the compressive strengths of the tunnel models are comparatively low, whereas when θ = 90°, it is the highest.     

深埋硬岩隧洞围岩的破坏模式分类与调控策略

 根据锦屏II级水电站地下隧洞的地质条件、现场试验、数值分析和支护设计与施工全过程,建立深埋硬岩隧洞围岩的破坏模式分类方法。该分类方法依据支护要求和控制因素将深埋硬岩隧洞的破坏现象分为3个大类、9种典型破坏模式,分析各种破坏模式的发生机制、表现形态以及调控策略,重点讨论岩爆的机制以及相应的调控策略。应用该分类方法对锦屏II级水电站地下隧洞围岩的破坏模式进行识别、机制分析,相应的调控措施也得到采纳。工程实践表明,该分类方法全面、实用,为现场设计、地质和施工工程师等在深埋硬岩条件下进行隧洞开挖和支护设计优化提供具体指导方针。