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In situ stress in a rock mass in Singapore is considered to be controlled by the forces related to the subduction of the Indo-Australian plate under the Sunda plate, as NE-SW to NNE-SSW oriented thrust faulting regime has been reported in the area. However, there are some cases of variability in orientations of maximum horizontal stresses, which may reflect the influence of local geology. Besides the plate tectonics, assessment of the in-situ rock stress should take into consideration of the influence of localized components such as geological structures, lithological boundaries, faults, etc. The paper discusses a variation in orientation of maximum horizontal stresses of Singapore in relation with local influence. The study involved compiling and assessing the in-situ rock stress dataset measured in the Bukit Timah Granite and the Jurong Formation, consisting of 29 mean horizontal stress orientations and 49 stress magnitudes of hydraulic fracturing test ranging in depths from 38.40 to 261.70 metres. The orientations of maximum horizontal stresses are interpreted with the hypothesized dextral fault arrays trending mostly NE-SW and ENE-WSW along with subsidiary (N)NW-(S)SE and N-S sets, and SW-dipping thrust system and NE-facing fold in Singapore. The strike orientation and related fracture system of hypothesized faults are also assessed with the results of paleostress multiple inversion analysis of fault slip data using the technique proposed by Sato (2006). In addition, orientation analysis of volcanic dyke was processed using the borehole televiewer image obtained from the boreholes of 200 metres depth in the Bukit Timah Granite to deepen the discussions on the paleostress and present-day stress field in Singapore.
The orientations of maximum horizontal stress in Singapore are generally NE-SW to NNE-SSW direction (Zhao et al., 2005; Meng et al., 2012; Winn and Ng, 2013). However, the orientations do present occasional variability, which varies in location (Kimura et al., 2016). Finding out the reasons causing the orientation variability would be helpful in the design and stability analysis of underground structure. This paper reports the results of the in-situ stress measurements in Singapore and discusses the effects of local-scale source of stress variation and its influence in relation to the previous works.
Sasaoka, T. (Kyushu University) | Urata, K. (Kyushu University) | Shimada, H. (Kyushu University) | Hamanaka, A. (Obayashi Corporation) | Hoshino, T. (Obayashi Corporation) | Hattori, T. (Obayashi Corporation) | Hatori, T. (Obayashi Corporation)
It is unavoidable to have cutter bit wear during shield tunnel excavation. In recent years, because of excavation in a variety of ground conditions, problems caused by excessive cutter bit wear sometimes occur during projects. Many studies on cutter bit wear have been done in past but a quantitative evaluation method for cutter bit wear has not been established. In particular, there is little research on the characteristics of bit wear in gravel ground. This paper discusses the effects of characteristics of gravel, gravel contents and binding material on bit wear based on the results of a series of laboratory tests in order to understand the mechanism of bit wear in gravel ground and develop a prediction method of bit wear under those conditions.
The shield method is now widely applied to the construction of tunnels for infrastructure. Because this method can be applied in various geological conditions and has small impacts on road traffic and the surrounding environment. This method is that the shield is pushed into the ground with cutting and maintaining the stability of the cutting face (Clark, 1987). The shield machine has a cutter head with bits and the ground is drivaged by rotating the cutter head and pushing it by thrust. Bit wear during cutting operation is inevitable, as shown in Fig. 1.
Nowadays, as a closed type shield machine has mainly been adopted and the conditions of tunnel construction have become various, the operating issues due to the bit wear often occurs and becomes serious. As the bit wear has an obvious impact on the construction cost and constraints, such as lowering of drivage efficiency, increasing the frequency of bit replacement, etc. (Shimada et al, 1989). Therefore, the prediction of cutter bit wear is very important in order to make adequate construction plans and calculate estimated cost. However, there is little research on the characteristics of bit wear in gravel ground and the prediction method of the bit wear both theoretically and quantitatively. The mechanism of bit wear in gravel ground seems to be very complicated compared with rock mass ground, as shown in Fig. 2.
From the results of previous research (Yamamoto et al, 2016), the bit wear when a gravel ground is excavated can be evaluated and predicted based on the abrasiveness of gravel itself. So, it can be expected that the characteristics of gravel itself and the gravel contents have an obvious impact on the characteristics of bit wear when the gravel ground is excavated by shield machine.
This paper focuses on some key design principles for rockbolting in underground rock excavation. The items discussed include underground loading conditions, the natural pressure arch around the underground opening, design methodologies, the responses of rockbolts under different loading conditions, the failure modes of rockbolts in fields, the determination of bolt length and spacing, determination of the factor of safety, and the compatibility between support elements. Rock support elements are loaded dynamically owing to the energy release during rock excavation and statically by the deadweight of potentially falling blocks afterward. In highly stressed rock masses, the dynamic load on the support elements is not a constant but correlated with the rock deformation. In this case, the released energy has to be taken into account in rock support design. There always exists a natural pressure arch in the surrounding rock of an underground opening after it is excavated. The methodology of rockbolting is associated with the position of the natural pressure arch with respect to the tunnel wall. Rockbolts should be long enough to reach the natural pressure arch when it is not far from the tunnel wall. However, an artificial pressure arch needs to be established in the fracture zone when the natural pressure arch is far from the tunnel wall. Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch. The factor of safety for a support system is determined by different parameters that are dependent on the loading condition. All support elements in a support system should be compatible in deformability.
Rockbolting is the most commonly method for rock support in underground works. Rockbolting design has been a trial and error business for a long time. In other words, it is mainly based on experiences. It is believed that the empirical rockbolting design will continue to dominate the rock support practice for a long time in the future because input data related to geology and the mechanical properties of the rock mass are always not completed in any rock engineering project. However, theories and new knowledge are helpful in guiding the rockbolting design, such as, in the aspects of the selection of rockbolt type, bolting pattern, bolt length and bolt spacing. Principles and methodologies for rockbolting design will be talked about in this paper, which include the concept of pressure arch, support principles under different rock conditions, the determination of bolt length and spacing, the factor of safety, the compatibility between support devices, etc.
The paper covers the construction and use of large caverns for temporary and permanent purposes on the example of the 27km long Semmering Base Tunnel in Austria. Semmering Base Tunnel is a twin tube, single-track railway tunnel with numerous cross passages and an underground emergency station with ventilation located approximately at the center of the tunnel system. The construction started in 2014 and is ongoing until 2026.
The emergency station, which is located at the toe of two 400 m deep shafts, requires the construction of large permanent caverns with dimensions in the range of 20 by 18 m. The available space in these caverns will also be used during construction for the placement of site installations underground in order to optimize the logistic procedures and avoid disruptions in supply and discharge via the shafts.
Intermediate access points are provided by 120 to 200 m deep shafts, which also require the construction of temporary caverns at the shaft bottom for site installations, material storage and transport purposes. In one case even shaft head caverns are carried out as the shafts start underground at the end of a 1.2 km long access tunnel.
For the construction in difficult geological conditions complex headings and special support measures using the SEM are applied. Advanced numerical 2D- and 3D-calculations were carried out to verify the adequacy of the designed solutions. The final configuration of the permanent and temporary caverns includes the installation of a drained, secondary lining or a complete backfill in case of the temporary structures.
The content of the paper covers the construction of temporary and permanent caverns at the example of an actual project currently carried out in Austria.
The boundary conditions requesting the construction of the caverns such as safety regulations, logistic purposes or overall schedule requirements are addressed.
The chosen solutions for the construction of the caverns as well as its configuration for the temporary and the final stage are presented.
2. Project overview
Semmering Base Tunnel is located app. 80 km south of Vienna in Austria and is part of the Baltic-Adriatic Railway Corridor, which runs between the Baltic Sea from Gdansk in Poland to the Adriatic Coast near Bologna in Italy (see Figure 1).
It is well know that the toppling failure of slopes often occurs in jointed rocks. It is obvious that the failure mechanism of toppling is not sliding, but may be the collapse of jointed rock masses associated with rotation of block movements. Therefore, a conventional approach for assessing the sliding of slopes cannot be applied because no particular sliding plane exists in toppling failures. To overcome this difficulty, the author proposes a back analysis method for determining the factor of safety for the toppling failures of slopes during/after their constructions by using measured displacements. Aiming at an easy application of the back analysis method to engineering practice, the author proposes a continuum model for simulating the mechanical behaviour of jointed rocks. The applicability of the method is demonstrated by physical model tests carried out in laboratory on jointed materials consisting of piling up of hundreds of aluminium bars. According to the proposed back analysis method, it is not necessary to assume the failure modes, either sliding or toppling, but it can identify whether sliding or toppling from measured displacements. In the proposed back analysis method both the critical shear strain and the anisotropic parameter play a major role. The critical shear strain can be determined by conventional laboratory experiments on an intact rock specimen with no worry about its scale effect, because the critical shear strain of large-scale rock masses is always greater than that of intact rocks, while the anisotropic parameters can be back-calculated from displacements measured on the ground surface by GPS so as to obtain a good agreement between the measured and calculated displacements. In this paper the proposed back analysis method is described together with the results of laboratory experiments.
In jointed rock masses, a toppling failure often occurs. It is obvious that the failure mechanism of toppling is not sliding, but may be the collapse of jointed rock masses associated with rotation of block movements. In order to assess the stability for toppling failure of jointed rocks, a discrete model such as Distinct Element Method (DEM) proposed by Cundall (1977) may be applicable, provided that all the joint systems of rock masses are known. However, it is almost impossible to explore all the joint systems of rock masses by field surveying, because of the complexity of joint systems. To overcome this difficulty, the author proposes a continuum model for simulating the toppling of jointed rocks, resulting that the factor of safety for toppling failure can be easily back-calculated from measured displacements. The applicability of the method is demonstrated by the laboratory experiments carried out on a jointed material consisting of piling up of hundreds of aluminium bars. According to the proposed back analysis method, it is not necessary to assume the failure modes, either sliding or toppling, but it can identify whether sliding or toppling from measured displacements.
Support design in tunnels is challenging if the uncertainty in surrounding rock mass is high. Traditional reliability based design approach accounts for this uncertainty by selecting lowest cost design whose reliability index exceeds a threshold value. This traditional design is highly sensitive to the variation in statistics of input parameters (noise parameters) and leads to over/under designed system if an over/under estimation of noise parameters is made. Accurate estimation of noise parameters requires testing of large number of rock samples in the laboratory, which is often difficult in many geotechnical projects. In this article, a new design approach - robust geotechnical design (RGD) methodology is applied for selection of tunnel support system which is least sensitive to noise parameters, cost effective and satisfies the safety requirements. Rock mass is assumed to follow elastic - perfectly plastic Mohr Coulomb constitutive model, representing weak rock mass. Two performance functions are used to evaluate reliability index of the support system which are based on maximum support capacity and tunnel wall convergence. A design is assumed to satisfy all safety requirements if reliability index obtained from each of these performance function exceeds a threshold value. Probability of failure (Pf) of a support design is evaluated for different possible values of noise parameters. The value of standard deviation (SD) of Pf is adopted to quantify the robustness of a design and finally a minimum distance algorithm is applied to obtain a most cost-effective design.
Selection of appropriate support for rock tunnel is of great importance for fulfilling stability and serviceability requirements of the tunnel. Traditional design approach involves deterministic analysis of the tunnel problem with single values rock mass parameters along with candidate design sets. The design with satisfies design code specific requirements and is economically cheaper is selected. This is basically a trial and error process. Existence of uncertainty in determining rock mass parameters is well stablished among practising engineers. Deterministic designs are generally not favoured when the expected amount of uncertainty is high. In such cases reliability-based design methodology is found useful. Several researchers have applied reliability-based design and stability assessment of rock tunnels (Li and Low, 2010). Reliability based methods treat input parameters as random variables and involves estimation of probability of failure of system (Pf). Several techniques such as First order reliability method (FORM), Second order reliability method (SORM), Monte Carlo simulations (MCS) and Point estimate methods (PEM) are adopted to estimate the Pf. Another approach of selecting design for a target reliability index of the tunnel problem is called reliability-based design optimisation and is applied by Lü et al. (2017). These approaches will be referred as traditional reliability-based methods in this paper.
There is a worldwide increase in tunnel excavation especially for infrastructure development which aims to improve the amenity of urban life. When construction of these tunnels is near urban dwellings and/or rockmass conditions dictate, mechanised tunnelling methods are often favoured. In Sydney, Australia, the geotechnical conditions suit the use of roadheaders for construction in massive sandstone of multi-lane vehicular tunnels with project costs measured in the hundreds if not billions of dollars. Despite their enormous costs, these projects are highly competitive with tight budgets and of relative short duration, consequently there is little incentive to invest in the development of computerised data collection and monitoring systems.
A recent project was undertaken involving a major road tunnelling project having multiple faces and construction sites where a system was developed comprising data collection, aggregation and analysis of a fleet of roadheaders with the aim of providing information to improve construction management. Over a 36-week period, data was collected from 22 roadheaders entailing 122,000 shift activities across 9200 shifts together with changes in geotechnical conditions. The data was combined into a single database that provided useful productivity metrics to the various project management teams and other stakeholders.
The project demonstrated the benefits of a largely automated, centralised data model that provided timely and reliable information and, eliminated a significant amount of data-entry work resulting in more productive time for site engineers. The value of such a system was realized when it was implemented in a subsequent construction project and used to provide reliable data in the planning and tendering of future projects. The system enabled roadheader productivity to be optimised in the project, by for example confirming the critical path in the roof support installation stage of the excavation cycle could be reduced in adopting split-face headings rather than full-face headings.
The use of large-scale data analytics to drive business improvement is increasingly being used across many industries as the technology used to collect and interrogate data becomes more accessible, and the commercial benefits of such information become apparent. For example, as the mining industry moves towards integrating robotics and automation into its processes, an unprecedented amount of data relating to every stage of a mining process is available to organisations to drive improvements in productivity, design and planning.
A quick, simple and quantitative method for the estimation of surface subsidence susceptibility in mined areas with a lack of detailed geological and geometrical information in underground is presented in this paper. In the method only gangway depth from the surface and the attitude (dip and dip direction) of main geological features are used as input data based on the degree of availability and reliability. Underground gangways are represented as a series of points instead of closed polygons for easy calculation. The core assumption in this method is that the susceptibility to subsidence within a unit area increases both as the depth of the gangway from the surface decreases and as the number of gangways below the unit area increases. In spite of the simplicity of the proposed method, it gave satisfactory results when applied to a virtual excavation model and a closed coal mine where subsidence occurred actually.
Several methods for predicting ground subsidence due to mining excavation such as the profile method and the influence function method have been proposed (Whittaker and Reddish, 1989; Sheory, 2000). The National Coal Board (1975) has presented a basic technique to determine the surface area affected by coal mining based on the height and width of mined areas and the angle of inclination of coal seams. All these methods were developed and verified for conditions involving horizontal coal seams and long wall mining, which are the common mining conditions in Europe. However, coal-associated geological structures in Korea are very complicated, and coal seams have various widths and irregular dip angles. Consequently, the slant chute block caving method has been widely used in Korea, and sinkhole type subsidence is more common than trough type. As a result, the conventional prediction methods must be adapted to the Korean geology and mining conditions, or new subsidence estimation methods must be developed.
The goal of this study is to develop a simple, general, quantitative and reliable method for identifying subsidence susceptibility of the closed or abandoned coal mines, which is proper to be employed in geologically complicated areas. The proposed method in this paper considers only gangway depths and attitude of geological features like dip and dip direction is an optional parameter, because these data are relatively easy to acquire and generally reliable.
2. Estimation of subsidence susceptibility
2.1 Basic assumption
The depth of gangways is selected as an input data of this study after surveying the availability and effectiveness of data because it is reliable and can be easily acquired. In fact, several researchers revealed that the magnitude (volume) and depth of excavation are the principal factors influencing on the subsidence (Whittaker and Reddish, 1989; Singh and Dhar, 1997; MIRECO, 2008).
The method proposed in this study is based on the fact that the excavation volume and shape (or distribution of coal seams) are closely related to the gangway distribution. Two basic assumptions considered in the method are that the susceptibility to subsidence within a unit surface area increases as the depth of a gangway from the surface decreases and the number of gangways below the unit area increases. The first assumption is based on the bulking of failed rock mass which can fill the excavation and prohibit the propagation of roof failure. The second assumption comes from the fact that the rock mass around the excavation is damaged due to blasting and induced stresses.
The susceptibility related to the depth of a gangway is quantified using a negative exponential equation based on the results of numerical analyses (Park et al., 2005) and statistical data of subsidence occurrences in Korean coal mines as shown in Fig. 1 (MIRECO, 2008). Park et al. investigated the influence of the depth and width of excavation and of the spacing and dip of discontinuity on ground subsidence using PFC2D capable of modeling the bulking effect and showed that the overburden remains undamaged as the mining depth increases. Fig. 1 shows that most of subsidence occurred within a depth of 100 m from the surface. The number of subsidence events decreases exponentially as the gangway depth increases.
The present work takes one deep mine repairing project of rectangular crossheading at fully-mechanized caving workface as the background. Based on both the theoretical analysis and experimental implement, the failure zone and evolution law of the surrounding rock around crossheading are achieved. The main influencing factors can be regarded as the magnitude and direction of initial rock stress, the current goaf status of the adjacent workface, mining stresses, rock properties, geological structure and groundwater, respectively. Large surrounding rock deformation, roof falling, rib spalling and asymmetric deformation are treated as the failure sign. The depressurization, reinforced support and yielding pressure supporting principles are proposed in the end, which the supporting scheme is applied along with the FLAC evaluation to guarantee the accuracy. The engineering application shows that the proposed supporting parameters are reasonable and effective.
With the development of deep mining (Liu, Q., et al., 2004), gas leakage, rock blasting, high mine pressure (Wan, Y., et al., 2006), big crossheading strain, hard crossheading repair and the increasing maintenance rate (Ren, J., et al., 2014), the stability and supporting technique of deep crossheading surrounding is currently an urgent issue (Wang, F., et al., 2016), which already causes the worldwide attention (Yang, X., et al., 2013).
Due to the stress concentration of rectangular crossheading excavation, rocks around the crossheading gain large deformation (ZHANG, X., et al., 2004), and may even triggered roof fall, floor heaving and rib spalling (Li, D., et al., 2009). These problems seriously restrict the excavation speed (Wang, M., et al., 2016), which directly impact the safe and efficient exploitation of mine construction (Zhang, L., et al., 2014). At the same time, the huge maintenance rate arise from improper supporting not only bring tremendous loss, but also get the whole mine in trouble, or even worse shut down (Fu, J., et al., 2004). It is one of the key problems for the deep mining development and safety excavation to solve the problem of rectangular crossheading supporting technique in deep coal seam. Therefore, it is necessary to analyze the failure properties of the surrounding rock around rectangular crossheading and put forward a reliable repairing support technique in deep thick coal seam, which is of great significance to realize the safety production of the coal mine.
2. Project background
The mine is located about in northwest of Binxian 20km away, which belongs to Xianyang city of Shaanxi province. The depth of the coal seam is 600m, the maximum thickness of coal seam is 26.2 meters, which owns an average thickness of 14.49 meters. As a deep thick mine, initial stress of the crossheading is high. Coal tends to possess the same characteristics of soft rock under high stress, which results in large deformation. Taking advantage of the fully mechanized sublevel caving method, the mining crossheading has obvious dynamic pressure along with the workface stoping, which aggravates the deformation and instability of the crossheading. The specific performance is large floor heave in crossheading mining. The deformation of transportation crossheading is also serious, which lead to the failure of supporting system for the initial roadway. The roof sinks, rib spalls, influencing zone expands, which seriously affects the normal safety production. The formation parameters are shown in Table 1.
Miyajima, Yasuyuki (Kajima Corporation) | Shirasagi, Suguru (Kajima Corporation) | Yamamoto, Takuji (Kajima Corporation) | Nishikawa, Koichi (Kajima Corporation) | Fukuda, Hiroyuki (Kajima Corporation)
The authors developed a system for predicting geological conditions not just of excavated areas but also ahead of the tunnel face by processing drilling data using Ordinary kriging, a geostatistical approach, and by visualizing conditions in real time. Using auto-controlled face drilling rigs, the authors confirmed the easy and rapid acquisition of drilling data from blast holes and rock bolt holes, along with three-dimensional coordinates.
The authors deployed the system in real time to decide whether pre-supports such as forepoling and facebolts would be necessary for a strongly sheared slate area at tunnel faces of the Shin-Kuzakai Tunnel (provisional name). The prediction provided information essential for decisions on construction methods. It also enabled safe streamlining of the process of deploying pre-supports. The authors also compared the applied support systems with P-wave velocity distributions of the rock mass derived from specific energy of drilling and rock mass classification distributions.
Choosing appropriate support systems and pre-supports are the key to ensuring quality, stability, and safety in tunnel construction projects. Typically, the tunnel face is observed during the excavation to evaluate geological conditions. The tunnel face is generally observed once a day, while excavations usually occur around four times a day. This means changes in geological conditions may be overlooked. In addition, as demonstrated by past incidents, the presence of a weak layer behind a tunnel face or a side wall may result in the collapse or significant deformation of a tunnel face or wall.
With respect to the geological conditions behind a side wall, hardness is determined empirically from drilling data for rock bolt holes. However, acquiring and analyzing this data requires a great deal of time and labor. Obtaining information on geological conditions that would allow real-time decisions on construction plans tends to pose extreme difficulties.
Using auto-controlled face drilling rigs whose use is increasingly common, we found a way to quickly and easily acquire drilling data from blast and rock bolt holes. We established a system that predicts geological conditions not just in excavated areas, but ahead of the tunnel face; this method visualizes conditions in real time by processing drilling data based on a geostatistical approach called Ordinary kriging. We applied this system to the construction site for the Shin-Kuzakai Tunnel (provisional name) on the Miyako-Morioka Cross Road and confirmed that the system provides essential information for construction plan decisions.