Muñiz-Menéndez, M. (Laboratorio de Geotecnia, CEDEX) | Perucho-Martínez, A. (Laboratorio de Geotecnia, CEDEX) | Rodríguez-Peces, M. J. (Universidad Complutense de Madrid) | Cano-Linares, H. (Laboratorio de Geotecnia, CEDEX)
Interpretations of cavity expansion tests (pressuremeter, radial jack, etc.) are based (in most occasions) on the analysis of the deformability of a cylindrical cavity in a continuous, isotropic, and homogeneous medium. However, many rock masses show an anisotropic behaviour due to the presence of discontinuity planes of different origins. Cavity expansion tests in these media have been studied here with an empirical approach. Several tests have been simulated in a three-dimensional, anisotropic and discontinuous medium—using 3DEC by Itasca—and their deformation has been analyzed in order to establish the principal factors that control the behaviour of the rock mass in these situations. Based on this analysis, it has been developed a new method for the interpretation of the cavity expansion tests carried out in laminated rock masses, which allows estimating the principal deformation moduli of the rock mass (maximum and minimum). This method can be used for any dip of the discontinuity planes.
The deformation modulus of the rock mass is one of the most important parameters for a geotechnical engineering project. The determination of this parameter is an issue not completely solved, neither from a theoretical nor from a practical point of view [1, 2].
Most of the existing field testing methods for the measure of the deformability are expensive and hard to place. Among these methods, pressuremeter test is the easiest and the least expensive one.The interpretation of this test is based on the cavity expanding theory in a continuous and isotropic medium but, most of rock masses are anisotropic and discontinuous media due to the presence of bedding or joint planes. Because of this, the usual interpretation of pressuremeter tests must be questioned, reviewed and, probably, a new methodology must be proposed.
1.1. Theoretical background
As it has been mentioned, the habitual interpretation of pressuremeter tests supposes a homogeneous, continuous and isotropic medium with a radial symmetry. According to the cavity expanding theory, the pressuremeter modulus (EP) can be obtained from the analysis of the load-deformation diagram:
The stress-strain state of the rock mass in the neighbourhood of the mining galleries is absolutely critical considering their behaviour particularly load and deformation. The stress-strain state can be affected by numerous geological and mining factors i.e. depth, mechanical parameters of the rock mass, the presence of faults, previous mining operations in case of multiple seam extraction and by the influence of the current longwall mining process.
Considering this fact the advanced measurement plan was developed. The measurements were conducted in one longwall maingate of the Czech coal mines. The holistic investigation program consisted identification of initial rock mass stress tensor and monitoring of its changing during the approach of the longwall face. Two CCBO (modified overcoring) probes were installed to obtain pre-mining full stress tensor and afterwards three CCBM probes were installed to continuous monitoring stress state in rock mass ahead of advancing longwall. The triple height telltales and endoscopic methods were applied to monitor the behaviour of rockmass in close proximity of the gateroad. Hydraulic dynamometers and strain gauged rockbolts were used to determine the support load. Also the gateroad deformation observations were performed. The results of aforementioned measurements allowed to determine the potential relation between stress-strain rock mass state and behaviour of the investigated gateroad in coarse of longwall face approaching.
When applying a longwall extraction system in hard coal seams, it is essential to ensure that the gateroads are stable and of proper size during the whole mining process. In majority of European hard coal mining industry, there is a single gateroad system that prevails, consisting of steel arch yielding support. To ensure the proper maintenance of the roadway the mine managers have to efficiently cope with the rock mass movements of various intensity (e.g. roof sag, floor heave, and horizontal convergence) as a result of the extraction pressure impact .
A fast computational code is presented that is dedicated for the elastic analysis of three-dimensional excavations and cracks in rocks. The problem is solved on the boundaries that are discretized with a new triangular leaf constant displacement discontinuity element with one collocation point. The creation of the new triangular element was inspired from Mindlin’s special version of grade-2 or strain-gradient elasticity theory (second gradient of displacement, g2). This element is characterized by a much better measure of the average stress at the center of gravity of the triangular element compared to that of the classical elasticity element close to regions with stress or strain gradients (e.g. notches, cracks etc). In a verification stage, the accuracy of the computational algorithm for the pressurized penny-shaped and mixed-mode elliptical crack problems that have analytical solutions is demonstrated. More specifically, it is shown that the average error of the crack tip Stress Intensity Factor predicted by the gradient modified method for nine discretizations of varying density is around 3.5 % with a maximum error of 5 %, while the constant displacement discontinuity element displays errors varying around 14 %. Moreover, the new method preserves the simplicity and hence the high speed of the constant displacement discontinuity with only one collocation point per element, but it is far more efficient compared to it, especially close to the crack tips and corners of excavations where the displacement and stress gradients are highest.
It is almost certain that any planned underground excavation in the scale of 101 m or more will transect a fault or persistent joint. The problem in the design phase is to examine the effect of the faults on the behavior of the rock mass during excavation and then to optimize the design of the underground openings and pillars. In Geophysics there remains the reasonable trend to explain earthquake mechanisms by means of dislocation or fracture mechanics models. Also, in petroleum engineering as well as in Rock Mechanics, the hydraulic fracturing technique where a pressurized mode-I crack propagates from a shallow or deep borehole, is widely used for permeability enhancement and measurement of in situ stresses, respectively. There many more problems involving threedimensional excavations and fractures that should be attacked with computational methods capable to tackle in a formal and accurate manner crack tip or corner singularities.
During the operation of gas storage caverns in rock salt mass the internal pressure changes during filling and withdrawal phases. Additionally temperature variations occur versus operation time. During withdrawal phases the temperature decreases which can lead to stress states in tensile regions at the cavern wall. Because the tensile strength of rock salt is relatively low compared to its compressive strength it is likely that tensile stresses lead to discrete fractures orthogonal to the direction of the tensile stresses.
If fractures of this kind are created - whether vertical or horizontal - the gas will penetrate into the fracture at the relevant pressure and further extend the length of the fractures under certain circumstances. There are currently no theoretical approaches describing the manner in which the fractures might propagate into the not by temperature changes influenced rock salt mass during repeated cyclic pressure changes. This aspect is topic of prospective research.
Salt caverns cannot be entered but only explored by sonar measurements, with which it is not possible to detect tensile fractures at the cavern wall. Within this paper examples from mining configurations will be shown where temperature changes lead to tensile fractures in the surrounding rock salt. These fractures have been well mapped while the temperature development is well documented.
The paper deals with recalculations under consideration of different salt properties of the temperature distributions and the resulting stress state in the surrounding rock salt mass. The stress calculation results and the consequences for the dimensioning of natural gas caverns are going to be discussed and assessed.
The demands on the dimensioning of gas storage caverns in rock salt have changed considerably in recent years. The operating procedure of injection and withdrawal was influenced mostly seasonal. This meant a high gas inventory in the summer and therefore a high maximum internal cavern pressure, the gas withdrawal in autumn and winter and refilling the gas in the spring and summer, when gas was less needed [1, 2]. Within this phase relatively low operation rates were observed.
Kajzar, Vlastimil (Institute of Geonics of the CAS) | Pavelek, Zdenek (HBZS, a.s., Lihovarská, VSB-Technical University of Ostrava) | Konícek, Petr (Institute of Geonics of the CAS) | Kukutsch, Radovan (Institute of Geonics of the CAS)
Relatively accurate data regarding the temperature distribution in a rock mass can be obtained from measurements carried out in short boreholes drilled directly into mine workings. Such thermal field measurements were undertaken in selected mine workings in all active mines of the Ostrava-Karviná District (OKR), situated in the Czech part of the Upper Silesian Coal Basin. A temperature field survey of the rock mass at high depths was performed in the years 2011-2013, with a total of 204 valid in situ measurements of initial temperature in the carboniferous rock mass recorded, ranging from 27.0°C to 48.9°C. All measurements were registered in a newly created database.
During the project, many archival sources of individual temperature measurements made in the OKR were found, complementing and extending the existing knowledge base regarding the distribution of the temperature field.
A new spatial distribution model of primary temperature values for the carboniferous massif in the Czech part of the Upper Silesian Coal Basin was the final output of the presented project. The main purpose of this spatial modelling and data visualisation was to create an image of the area’s complex topological arrangement based on the available data. Following the generated thermal model, a 3D map of OKR temperature fields at around 1000 m depth was generated. In order to determine the temperature at a particular position in the thermal model, an executable plugin was created. It is also possible to import the final thermal model of the carboniferous massif in the Czech part of the Upper Silesian Coal Basin into commercial computational software for the determination of appropriate mine air conditions.
Mining activities involved in the extraction of energy resources in the Czech Republic are reaching ever more significant depths, a phenomenon that has led to increased consideration of mine worker safety, as well as the indirect safety of the general public. The project “Mine workings design and direction at depths of 800 metres and greater” focuses on the safe direction and operation of mine workings at depths exceeding 800 metres, due to the fact that at such depths, increased rock stresses and temperatures may lead to potentially dangerous anomalous stress phenomena or disturbances in ventilation and air quality. The latter in particular require specific procedures to be undertaken by the national Mines Rescue Service when dealing with such emergency situations.
Coal burst is a sudden and violent rock/coal failure that occurs in underground coal mines. It is considered to be a highly catastrophic phenomenon which can cause significant damage to mine workings and equipment as well as result in multiple fatalities. Throughout the history of underground pillar design, the relation between the post-peak behavior of pillars and stiffness of the surrounding strata has been extensively studied. These two concepts play an important role in determining the failure mode of the coal pillars and the amount of potential energy that can be converted to kinetic energy, which is the cause of coal burst. In this paper, the post-peak behavior of pillars and surrounding strata stiffness are reviewed and the criterion developed to investigate the instability of the pillar failures is explained. It is concluded that, as the pillar width to mining height (w/h) ratio increases, its post-peak modulus increases; and a pillar exhibits different failure modes for various w/h ratios.
Experiences in both hard rock and coal mines reveal that pillar failures are still one of the major hazards in underground mining and forecasting the mode of the pillar failure can be vital. Therefore, it is essential to fully understand the mechanics behind these failures in order to predict and control them. As suggested by Tincelin and Sinou , pillar failures can be classified into two categories:
i. Slow, progressive deterioration of the pillars that causes relatively delayed surface subsidence and even damage if the pillars fail,
ii. Sudden, violent collapse of pillar causing immediate surface damage and mostly associated with fatal accidents.
The first type of pillar failures is also named as controlled pillar failures which occur gradually and typically over long periods of time (i.e. pillar spalling). These pillar failures are also termed as creep and squeeze . Uncontrolled pillar failures, on the other hand, take place in a sudden and violent manner and fall into the second group of pillar failures as described above. Since the uncontrolled pillar failures occur rapidly and may not be preceded by any deterioration of the pillars, they cause significant health and safety risks (e.g. coal burst, entrapment, windblast etc.) .
Highwall mining involves driving a series of parallel entries with web pillars in between. These entries are driven by using a continuous miner with attached conveying system to extract locked up coal behind the highwall slope when the open cut coal mine reaches its ultimate limit. These entries are driven unmanned, unventilated and unsupported. Therefore, a detailed knowledge of structural features ahead of operations is essential in assessing the stability of these entries. Highwall mining operations can greatly benefit from accurate structural mapping of rock mass defects. The stability of these entries can be suitably assessed for any major roof failure by adopting discrete fracture network based structural modelling to characterize and delineate the regions of possible major roof failures to prevent damage to the conveyor and burial of the expensive continuous miner. In this paper, a generalized framework is described based on photogrammetric survey, digital mapping and discrete fracture network based structural modelling to characterise such failures for an Indian highwall mining operation. A sensitivity analysis is undertaken to demonstrate the significance of structure persistence in the geotechnical assessment. Such analysis would provide more insight into designing highwall mining layouts and in predicting possible impending highwall failures, and indirectly facilitates reducing machine downtime for better management of highwall mining operations.
Highwall mining operations offer the opportunity to improve coal extraction rates significantly provided geotechnical risks are managed properly. These risks include slope failure as well as roof collapse within the highwall entries induced by local failure, potentially resulting in loss of the continuous miner and conveying system. The Guidelines for Open pit Design  presents detailed recommendations for the management of uncertainty and risk in open cut slope design and the work described in this paper takes place within this framework. In this paper, we describe a framework that uses photogrammetric survey, digital mapping and analysis, stochastic structural modelling for the stability analysis of structurally controlled failures at an end user site in India. This study uses data previously acquired at an Indian coal mine site .
2. Site details
It is critical to obtain the rock strength along the wellbore to control drilling problems such as pipe sticking, tight hole, collapse and sand production. The purpose of this research is to predict the uniaxial compressive strength based on data of sonic travel time, formation porosity, density and penetration rate. For prediction of UCS, artificial neural networks were developed between UCS and input data resulting a practical correlation. In this research, a long well segment possessing complete and continuous data coverage has been analysed, and collected data of the wellbore are used to correlate data of the four mentioned input parameters of artificial neural networks with uniaxial compressive strength data as network targets. Selection of input parameters is based on a vast literature review in this area. Due to the fact that standard experimental test methods based on established standards require costly equipment and that the methods for sample preparation is difficult and time-consuming, indirect methods are more favourable. Using these methods, the UCS values are predicted in a simpler, faster and more economical way. In this study, it is concluded that artificial neural networks are a good predictor of rock strength, and can reduce drilling costs significantly. It is observed in this paper that UCS predicted values by neural networks are very close with lab and field data, which is concluded by analysis of network performance results including mean squared error and correlation coefficient. It is also concluded in this study that input parameters which are chosen in this study, have deep effects in UCS prediction studies, and should be considered in other scientific studies. Conclusions show that using artificial neural networks to predict UCS of formation rocks in petroleum fields around the world, would ease UCS estimation, optimize drilling plans and decrease costs.
A geomechanical model requires a great deal of input information including measurements of magnitude of vertical and minimum stresses, pore pressure, rock mechanics properties and drilling experiences, all oriented to determine the magnitude of maximum horizontal stress. To conduct a geomechanical reservoir characterization, it is essential to have the knowledge of the in-situ stress magnitudes and rock mechanical properties .
In this paper, a numerical investigation of uniaxial compressive tests on prismatic specimens with single cylindrical pre-existing cavities in brittle rock is performed. To investigate the rock fracture around cavities and to assess the potential of the numerical model to simulate this behaviour, published laboratorial physical models on granite are simulated numerically with a Bonded- Particles Model (BPM) by using a distinct elements code. The numerical models are presented and the calibration of the BPM micro-parameters is described. Then, the calibrated numerical models are used to investigate the potential of the BPMs to simulate the fracture initiation and propagation of the physical specimens. It is concluded that the laboratory and the numerical observations are in good agreement.
The bonded-particle model BPM  has extensively been used over the past decade to simulate the mechanical behaviour and fracture of rock under a variety of loading configurations. In the BPM, the intact rock is represented by a dense packing of rigid spheres (in 3D) or disks (in 2D) bonded together at their contacts. The model is implemented in the Particle Flow Code PFC . The BPM has been extended by Potyondy (2010)  to form the Grain Based Model (GBM) in order to simulate a rock grain structure of deformable, breakable or not, polygonal grains, cemented along their adjoining sides. The GBM has been successfully used to describe the crack initiation, crack damage and peak strength of Dionyssos marble specimens under uniaxial compression . However, more computational effort and extended calibration procedures are normally required to match the macroscopic rock behavior with the GBM. With the recent addition of the Flat-Joint contact logic in the BPM  particle interlocking and friction resistance at the contact are imposed, restricting the relative movement of particles, and thus attaining the advantages of simulating the rock structure.
In this study, the BPM is used to model the fracture initiation and damage around cylindrical openings in compression (e.g. [6-8]). Published laboratorial physical models on granite  are simulated numerically with the two-dimension Particle Flow Code PFC2D by using the flat-joint contact model.
During a uniaxial compression test on the aforementioned physical models, three (3) phenomena should be appeared. First, the primary fractures (denoted in Fig. 1 with red colour) initially start from the upper or lower hole’s boundary, extending upwards or downwards respectively. These fractures initiate due to the tensile stress concentration of the upper and lower regions of the hole’s boundary. Then, remote fractures (denoted in Fig. 1 with blue colour) away from the hole are formed on regions with high stress concentration, extending upwards and downwards. The angle of their formation and appearance is depended of the examined material’s characteristics. Finally, slabbing initiation (denoted in Fig. 1 with green colour) on the inner surface at the left and right hole’s boundary is observed, due to the high compressive stress concentration on these regions. The more the material is brittle, the more apparent are the V-shaped notches at the left and right of the hole.
Geotechnical design is evolving to adopt the limit state design (LSD) philosophy, also known as reliability-based design (RBD). This is evident by its inclusion in geotechnical design codes (e.g. Eurocode 7). Partial factors are often used in design codes to overcome the difficulty in performing probabilistic analysis suggested by the RBD. The increasing use of RBD suggests a need to investigate the applicability of design with partial factors for various rock engineering structures; this paper will investigate their application in the design of support for a rock wedge in an underground opening. The paper provides a critical overview of the design philosophy of RBD, the components necessary for its application, and the methods by which the probability of failure may be computed. In addition, it discusses how partial factors are calibrated from RBD and how code development can be subsequently performed. This is put into context with a design example for the support of a rock wedge.
Rock engineering design customarily uses deterministic methods with factors of safety. However, this is inappropriate as these methods do not appropriately account for the variable conditions prevalent in rock engineering. In addition, it is known that factors of safety do not always lead to safe designs .In the presence of variable conditions, probabilistic design - which asks “does the design satisfy a specified probability of failure given the observed variability?” - is known to be more appropriate. This has long been recognised by the structural engineering community, and fuelled the development of structural reliability - also known as reliability based design (RBD) or limit state design (LSD) - as a formal design philosophy that quantifies the probability of a structure behaving as intended. This includes ensuring stability and satisfying deformation limits, as well as any other requirements of the structure. Presently, this design approach is used globally in structural engineering, and is currently also being adopted by the geotechnical community [2, 3].