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ABSTRACT: Degradation of engineering structures is an inevitable process. Maintenance provides ways to fulfill the requirements on safety and functionality under financial constraints. A large proportion of tunnels in Taiwan deteriorate within 20-30 years after construction because of active plate movement and increasing extreme climate events. In some special cases, tunnel degradation persist for a long time until adequate rehabilitation were held according to the deteriorating mechanism. Thus, accurate structural health monitoring, proper interpretation on monitoring data, accurate performance prediction and timely rehabilitation are the fundamentals to tunnel maintenance. Focusing on a methodology to clarify the interaction between degradation features of tunnels, this research monitored a tunnel for over four years. In successive processing and comparison on monitored tunnel displacements and lining cracks, the turning point of tunnel degradation can be identified.
Although some underground structures survived over 12,000 years (Kusch & Kusch 2009), there are areas around the world where tunnels degrade in a rate much higher than the average (Aydan 2015, Chiu et al. 2014, Konagai et al. 2009, Lee & Wang 2015, Sandrone & Labiouse 2011, Wang 2010, Wang & Chen 2001, Wu et al. 2014). For example, a large proportion of the tunnels in Taiwan and Japan deform and crack after construction complete for only several years (JSCE 2003, Chiu et al. in press). Among the cases, there are some tunnels deteriorate so severely that it continues after rehabilitated for several times. Thus, it is essential to understand the time-dependent behavior of operational tunnel cases before we achieve a true life-cycle based tunnel maintenance.
Abnormal phenomena in tunnels, also known as tunnel anomalies or pathologies, are possibly to be induced by changing environment and/or deteriorating engineering structures. The often mysterious cause induced tunnel deformation then proceed to produce cracks on tunnel lining. Therefore, the most straightforward methodology of effective tunnel maintenance is to establish the relationships between anomaly cause, tunnel deformation and lining cracks.
Colombero, C. (Università degli Studi di Torino) | Comina, C. (Università degli Studi di Torino) | Vinciguerra, S. (Università degli Studi di Torino) | Jongmans, D. (Université Grenoble Alpes) | Baillet, L. (Université Grenoble Alpes) | Helmstetter, A. (Université Grenoble Alpes) | Larose, E. (Université Grenoble Alpes) | Valentin, J. (Université Grenoble Alpes)
ABSTRACT: Passive seismic monitoring data acquired at the potentially unstable granitic cliff of Madonna del Sasso (NW Italy) are presented in this work. The spectral content of seismic noise systematically highlighted clear energy peaks on the unstable sector, interpreted as resonant frequencies of the investigated volume. Ground motion at these frequencies was found to be controlled by the main fractures observed at the site and numerical modelling is currently under development for geomechanical and modal analysis of the site. Both spectral analysis and cross-correlation of seismic noise showed seasonal reversible variations related to temperature fluctuations. No irreversible changes, resulting from serious damage processes, were detected during the monitored period. Classification and location of microseismic events had also been attempted but resulted challenging due to the complex structural and morphological setting of the cliff.
An appropriate monitoring of unstable rock masses may provide a better knowledge of the active processes and help to forecast their evolution to failure. Classical monitoring mainly involve both remote-sensing techniques and in-situ geotechnical measurements. Although all these methods are largely applied and successfully tested on landslides, they are not suitable to early forecast sudden rapid slides or localized collapses of rocks. Passive seismic monitoring can be potentially helpful for this purpose.
Detection, classification and localization of microseismic events within the prone-to-fall rock mass can provide information about the incipient failure of internal rock bridges (Spillman et al. 2007, Lacroix and Helmstetter 2011). Acceleration to failure can be detected from an increasing microseismic event rate. The latter can be compared with meteorological data to understand the external factors controlling stability (Amitrano et al. 2010, Helmstetter & Garambois 2009).
On the other hand, ambient vibration surveys have been recently applied to potentially-unstable rock slopes, with different geological settings and volumes, showing the capability of spectral analysis of seismic noise to detect both reversible and irreversible modifications within the rock masses. Literature studies, mostly carried out in the Alpine context (Moore et al. 2011, Burjánek et al. 2012, Bottelin et al. 2013), systematically highlighted clear energetic peaks at specific frequencies on the unstable sectors, which were interpreted as resonant frequencies of the investigated volumes. Horizontal ground motion at the fundamental frequency was moreover found to be orthogonal to the main fractures observed at the sites and consequently parallel to the potential direction of collapse. Each unstable compartment showed seasonal reversible variations of the resonant frequencies related to temperature fluctuations. In some cases, also irreversible variations were detected. Lévy et al. (2010) recorded a significant drop (> 1 Hz) in the lowest resonant frequency two weeks before the effective collapse of a limestone column.
ABSTRACT: Modern satellite technologies, i.e., GPS (Global Positioning System) and SAR (Synthetic Aperture Radar), have begun to be used for monitoring deformation over extensive areas in the field of Rock Engineering. SAR is an attractive tool which does not require any devices on the ground. However, the improvement of its monitoring accuracy is a key issue for practical applications. In this paper, a simple multi-temporal analysis is proposed for this purpose. In order to verify the procedure, the monitoring of the subsidence in a city in Indonesia is shown as an example. The results are then compared to displacements measured by GPS to confirm the validity. A map of the subsidence over the large area has been generated, and it is clearly seen that the trends in subsidence depend on the ground conditions.
Monitoring is an important task for assessing the stability of structures and for confirming the validity of the design. It is also important for predicting risks and managing safe operations. Many methods have been established to conduct this task, such as geotechnical monitoring instruments, survey methods and artificial satellite technologies. GPS is one of the useful methods for continuously monitoring displacements over an extensive area with high accuracy (Shimizu et al. 2014). However, GPS is only capable of measuring the displacement of points which have been installed with a sensor. Thus, if monitoring is to be conducted over a large area, such as a city, a mountain, a coastal region, etc., a huge number of GPS sensors will be required.
On the other hand, SAR is powerful technology for mapping the Earth’s topography. In particular, Differential Interferometry SAR (DInSAR) is a useful technique for observing the deformation of the ground surface (Hanssen 2002). DInSAR has already been applied to monitor the ground subsidence in large areas, the slope deformation in mines and landslide behavior, etc. (Raucoules et al. 2007, Hartwig et al. 2013, Akbarimehr et al. 2103), but the procedure still needs to be improved in order to obtain reliable results.
In this paper, a simple procedure is proposed to obtain accurate results for the long-term monitoring of subsidence. It is a method of the multi-temporal analysis of DInSAR, which consists of many pairs of selected SAR data with a short period and a small perpendicular baseline. The method is applied to monitor the subsidence over the city of Semarang in Indonesia. The results are then compared with displacements measured by GPS to confirm the validity of the method. A map of the subsidence over the large area has been generated, and it is clearly seen that the trends in subsidence depend on the ground conditions. Moreover, the hyperbolic method is also applied to the results of Multi-Temporal DInSAR in order to smooth the data and to predict future subsidence.
ABSTRACT: The thorough and accurate measurement of rock mass deformation or convergence is often limited to the use of a small number of discrete point to point measurements. These measurements are often not representative of how a rock mass is responding to the excavation of material over a larger domain. Advances in LiDAR (Light Detection and Ranging) technology and data processing techniques have developed portable, and accurate devices for three-dimensional mapping of excavations in GPS (Global Positioning System) deprived environments. This technology has the ability to generate large, representative spatially continuous data sets showing excavation convergence. These data sets can be especially useful for the calibration and forecasting of excavation convergence in non-linear, three-dimensional, strain softening, discontinuum finite-element models (DFEM). This paper discusses the findings from an ongoing study into the use of this new measurement technique and the resulting data sets for the calibration of numerical models and the importance of incorporating sufficient structural resolution in the models to match the observed convergence with sufficient accuracy.
Numerical modelling techniques in rock mechanics have advanced significantly in recent times. There are no longer limitations requiring a rock mass to be assumed as 2-dimensional, linear-elastic and homogeneous. Non-linear, 3-dimensional, strain-softening, discontinuum Finite-Element simulations, with multiphysics coupling can now accurately represent a rock mass at many different length scales.
With these sophisticated numerical simulations, it is common now to be limited in the model construction phase by a lack of representative scale structural data, and representative observational data during the calibration phase. The inability to to incorporate realistic structures on the correct length scale due to a lack of data can be overcome through the use of discrete fracture networks where no actual data can be acquired. However, these kind of models must be calibrated in order to allow a realistic representation of the rock mass response.
Commonplace in-situ measured and observational datasets include stress measurements, seismicity records, damage mapping, extensometers or point closure measurements. Stress measurements and seismic records provide a valuable insight into the rock mass state and reaction, however the representative strain response of the rock mass has been very difficult to measure. Laboratory testing can provide some insight, however post-peak behavior is rarely considered, and size limitations restrict the upscaling to a rock mass representation.
ABSTRACT: Subsidence engineering comprehension is crucial if planners and designers are to develop effective measures to prevent, minimize and or remedy subsidence effects. Subsidence and its components, tilt and strain, have disastrous effects on structures, human life and mining operations economics among other effects if not adequately tamed. This paper utilizes survey monitoring pre and post mining field observations to evaluate coal mining induced surface subsidence. Several issues concerning subsidence due to underground mining are discussed presenting an opportunity to share insights on this subject, which affect underground coal mines. Some of the issues covered include subsidence due to different types of mining, characteristics of subsidence, prediction of subsidence, monitoring and management of subsidence, effects of subsidence and control of mining-induced subsidence effects. The survey monitoring data was processed by the author and the results are presented in this paper in form graphical presentations derived from tables of calculations. The graphs of subsidence profile, tilt profile and major and minor principal strains profiles are also provided. The interpretations and comments about any anomalies are also provided thereof.
A clear understanding of subsidence and its mechanisms is crucial if one is to design an underground mining operation that presents minimal effects of subsidence. If not appropriately designed for, subsidence can cause disastrous effects on linear, tower, block and agricultural structures, dealing a severe blow on the economics of the mining operation. Technical approaches and statutes have been developed in order to contain subsidence to a sustainable level and hence afford a profitable business venture. This paper uses survey monitoring data to evaluate surface subsidence due to underground coal mining. The survey monitored data consist of two lines before and after mining. The observations lines were along the long axis of the panel. Each line had 65 points and the x, y and z readings were recorded before and after mining situations. The edges of the evaluated panel occurred at approximately 70 m and 250 m, see Figure 1. The mining depth is 127 m, mining height 4 m and panel width is 130m. The mining method used is pillar extraction. Graphs illustrating the Major InducedTilt and the Principal Strains are presented in the paper and any noticed anomalies in the subsidence are commented on. Some of the issues discussed in this paper include subsidence due to different types of mining, characteristics of subsidence, prediction of subsidence, monitoring and management of subsidence, effects of subsidence and control of mining-induced subsidence effects.
ABSTRACT: Safe, cost-effective tunnel construction is possible when the ground is stabilized in a reasonable pace. To ensure that an optimized support system is used, detailed in-situ geological data is needed. Currently, one of the main measures to address this issue is monitoring the ground and support components (i.e. in SEM or NATM systems). However, improving ground support requires reliable data. The most common support system used in underground spaces is rock-bolt, its drillhole can be used for probing and obtaining geological data, e.g. rock strength. This paper will introduce a recently developed probe (Rock Strength Borehole Probe) designed to estimate the rock strength by scratching the borehole wall. The probe uses a micro-controller to record the data from various sensors onto a mini-SD card. RSBP applications and design considerations will be discussed in this paper, along with the results of preliminary laboratory and field tests.
Assessment of rock mass properties is an essential part of analyzing stability of any surface or underground structure in rock. Availability of geological information including condition and frequency of discontinuities as well as rock strength is the crucial component of such analysis. One of the important parameters in evaluating rock mass properties is the intact rock strength. This parameter is usually measured by testing the core samples, obtained from exploration borings or from coring outcrop boulders. These core specimens are subsequently tested in rock mechanic laboratories. The test results offer limited information about the rock at the few locations along the borings despite all the efforts, time, and costs allocated for it. Moreover, the results may not necessarily be representative of the behavior of the rock in the field since the test cannot provide assessment for the impact of in-situ condition of the ground. The ideal solution is to be able to estimate the rock properties in-situ, preferably inside exploration borings or other drilled holes in an underground space. Therefore, the objective of the current study was to develop a method for evaluation of rock strength in various type and size boreholes to assess in-situ rock strength.
ABSTRACT: The Compact Conical Borehole Overcoring (CCBO) method originally developed in Japan to determine stress in rock has been widely used by the Institute of Geonics, Czech Academy of Sciences. Only one borehole is required to install the CCBO instrument, measure strains and determine the full stress tensor in rock mass. To determine the pre-mining stress around the circular tunnel in undisturbed hard rock, the measurement requires a borehole of certain length to install the CCBO cell in location where the excavation does not affect the stress field. To reduce the drilling and installation costs, the Institute is developing new theoretical solutions for a short borehole strain measurement method enabling the CCBO cell installation closer to the circular tunnel. The proposed semi-analytical approach uses the short borehole strain measurement results, theoretical calculations of stress distribution in the elastic medium around the circular tunnel and numerical modelling, to back analyze the probable pre-mining stress tensor. This approach enables determination of pre-mining stress obtained from the short borehole strain measurements in excavation-induced zone around underground opening. Discussed here are the theoretical calculation methods, numerical modelling results and a practical example of the pre-mining stress determination from the short borehole strain measurements using the CCBO instrument.
1 STRESS DETERMINATION
Compact Conical Borehole Overcoring (CCBO) is a method used for stress tensor determination in rock mass. Overcoring methods are used worldwide, and like all methods of in situ stress determination, these methods also have their limits. The correct determination of the stress based on measured strains should include the characterisation of the rock mass with respect to its anisotropy (Amadei 1996, Hakala et al. 2003, Hakala 2006, Kang 2000, Zang & Stephanson 2010) to describe the most possible, true relationship between stress and strain.
The principle of the method is the stress relief of the rock core, which is formed by overcoring of the installed probe. The stress relief due to overcoring is manifested by deformation response of the rock core, which is monitored by strain gauges. The full stress tensor is then determined from strains measured in specified directions.
The measurement should be made in unfractured rock. The confidence of the strain measurement is enhanced by camera inspection in a borehole in order to avoid the probe installation that is influenced by a local anisotropy and some structural inhomogeneity. So the surroundings of the measurement probe can be considered as a homogeneous, elastically responding rock core without any cracks and discontinuities.
One of the research objectives of the Institute of Geonics, Czech Academy of Sciences, is the stress state of the rock mass and its influencing factors such as the stress strain relationship determination for the anisotropic rock and evaluation of the general error of the CCBO method.
ABSTRACT: The authors involved with Cappadocia region started through an international joint research project supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 09044154). The project involved the studies on (a) short- and long-term mechanical characteristics of host rocks, (b) the assessment of in-situ stresses, (c) the investigation of instabilities and their extent, (d) short- and long-term stability analyses of the most famous underground city called Derinkuyu, and (e) long-term surveys on climatic conditions in some of the selected historical and modern underground rock structures. This study was extended to another historical site, namely, Zelve and modern structures such as Azimli underground storage complex and Avanos underground museum. In this paper, the authors describe geomechanical investigations and pioneering monitoring attempts performed in the Cappadocia Region.
The involvement of the authors with Cappadocia region started through an international joint research project supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Aydan et al. 1999). The project involved the studies on (a) short- and long-term mechanical characteristics of host rocks, (b) the assessment of in-situ stresses, (c) the investigation of instabilities and their extent, (d) short- and long-term stability analyses of the most famous underground city called Derinkuyu, and (e) long-term surveys on climatic conditions in some of the selected historical and modern underground rock structures. This study was extended to another historical site, namely, Zelve and modern structures such as Azimli underground storage complex and Avanos underground museum (Fig. 1).
A multi-parameter monitoring system consisting of parameters such as acoustic emission (AE) counts, electric potential, crack displacement, climatic conditions such as temperature, humidity, air pressure and rainfall was developed and utilized at Derinkuyu Underground City, Zelve semi-underground (cliff) settlements, Azimli underground storage complex and Avanos underground congress center of which construction has not been completed. The power source of the system is entirely based on the batteries and it can be used in any place and the life of batteries last more than 1 year. Among all parameters, Derinkuyu Underground City was the first application of AE monitoring system based on batteries and it has become one of powerful items of monitoring for short and long-term response and stability of rock engineering structures.
ABSTRACT: Ok Tedi Copper-Gold mine in the Western Province of Papua New Guinea receives in excess of 10,000 mm annual rainfall. Surface water runoff generated during high intensity rain events initiates localized erosion on benches and bench faces on a daily basis. In regions of friable and highly fractured rock, the erosion rapidly undercuts geological structures; enabling large instabilities to occur. When uncontrolled, the problem progressively worsens leading to the development of large chasms along major geological structures on pit slopes. Chasms up to 450 m high and 250 m wide have developed at Ok Tedi through progressive landslides and slope instabilities involving several hundred-thousand tonnes of debris. Over the last decade, several remedial projects involving improving surface water management, slope depressurization and targeted ground support campaigns have been undertaken to reduce chasm enlargements. Slope deformation monitoring systems and hazard awareness measures are used to manage the risk to personnel operating in the pit.
The chasms that have developed on the pit slopes of Ok Tedi openp it, Papua New Guinea, are not due to singleevent rock wedge slides from slope faces. Rather, these chasms are the end product of the enlargement of small initial voids, such as a localized bench failures, by progressive attrition of the rock mass around the starter void (Bar et al. 2014).
In an ideal world, all chasms could be prevented from developing on mine pit slopes. However, mining could not proceed unless the activity was economically profitable, and as such the orebody to waste rock strip ratios are minimized as much as possible. Since mine slopes are relatively short-term excavations (often 5-25 years) and not long term-civil engineering structures (50-100+ years) in rock, lower geotechnical factors of safety (FOS) are acceptable in mining (FOS ~ 1.2 to 1.3) than in civil engineering (1.5 to 2.0). Economic viability is also tied into project life, where options for risk mitigation are often financially assessed on the basis of project age and projected life.
Zhou, Z. (China University of Petroleum-Beijing) | Zhang, G. Q. (China University of Petroleum-Beijing) | Zhou, D. (China University of Petroleum-Beijing) | Wang, Y. (China University of Petroleum-Beijing) | Dong, H. (China University of Petroleum-Beijing)
ABSTRACT: During multi-stage fracturing operation for tight reservoirs with cluster perforation, a main fracture will initiate from each perforation cluster position. By adjusting spacing of fractures to change the interference intensity between multiple fractures, fracture network can be formed to achieve better results effect. Based on the plane strain theory of semi-infinite fracture, the characteristics of hydraulic fracture-induced stress are analyzed; according to effect that the superposition of induced stress can effectively reduce the horizontal stress difference (σH-σh), formation of complex branched fractures network becomes easier. In this paper, optimization of spacing of fractures—spacing of perforation clusters—is made for stimulation under two different kinds of circumstances, so that the guidance for parameters of the perforation clusters technology can be obtained.
At present, the shale gas industry has developed rapidly worldwide. No matter in china or out of china, one of the commonest exploitation methods for shale gas is multi-stage fracturing technology with clustering perforation in horizontal wells. The operation separates the horizontal well section into several fracturing segments with bridge plugs and each segment contains 2-5 perforation clusters. During the process of hydraulic fracturing, fractures will initiate simultaneously and extend from each perforation cluster, as shown in Figure 1. Mutual interference among different fractures will promote the formation of more complicated fractures network, namely, volume fracturing. For ultra-low permeability and tight shale reservoirs, volume fracturing possesses larger flow area than conventional fracturing, which can achieve better effect of reservoir stimulation (Jaeger 1959).
During the fracturing operation, the opening of each fracture will squeeze the surrounding formation to generate induced stress. Induced stress itself brought about by multiple fractures will yet superimpose to influence the extension direction of each fracture itself, so “stress shadow” effect will be formed (Zoback 2007, Roussel & Manchanda 2012, Manchanda et al. 2012, Cheng 2012).
Currently, scholars from home and abroad have done some research on the ambient induced stress field around a single hydraulic fracture. While research on superimposed effect of induced stress among fractures is not enough, and practical calculation method of fracture spacing optimization design is not provided either. Based on the analysis of induced stress field in a single fracture, this paper further illustrates the stress inversion phenomenon existed around the fracture. In addition, according to the characteristic that induced stress field can reduce the horizontal stress difference of shale reservoirs, this paper optimizes the designed value of perforation cluster spacing and proposes relative practical design method. Theoretical foundation for further efficient development of shale gas can be provided.