Shao, Hua (Federal Institute for Geosciences and Natural Resources (BGR)) | Hesser, Jürgen (Federal Institute for Geosciences and Natural Resources (BGR)) | Paul, Benjamin (Federal Institute for Geosciences and Natural Resources (BGR))
Local hydrocarbon occurrence in tight salt rock has been intensively investigated during the site exploration for the disposal of high-level radioactive waste. Special in-situ measurement equipment has been engineered to quantify the amount of gaseous and liquid hydrocarbons and to monitor pressure development with high resolution. Based on the data analysis it can be concluded that such kind of occurrences are bounded locally in the tight rock and is only then mobile when local channeling with high permeability is created due to stress redistribution by drilling or excavation.
Fluids including both liquid and gas phases can be found within the crystal structure or along grain boundaries in all types of sedimentary rock and were formed by accumulating and reacting of different mineral and/or organic particles under pressure and temperature conditions during the genesis. These small inclusions range in size of several micrometers and are usually invisible in detail without microscopic studies. However, these fluids usually dispersed in a very low amount can form local accumulations in a rock volume up to some cubic metres. In an underground facility in salt rock, hydrocarbon occurrences have been found during the site exploration for the disposal of high-level radioactive waste in north Germany.
The initial hydrocarbons in undisturbed rock are situated under petrostatic pressure which is much higher than hydrostatic pressure in deep underground. Because of the very low rock permeability, migration of such fluids is almost impossible even under the high pressure-gradient condition. Fluid release from the crystal structure will take place if the stress state changes. In case of drilling or excavation, stress will be redistributed with the result of a deviatoric stress state. If fluid pressure is higher than the minimal stress, dilatancy-controlled fluid migration occurs (Xu et al. 2013). This results to the generation of micro-fissures between crystal structures with an increased permeability.
With regard to the long-term performance of a potential repository, it is important to characterise the distribution, amount and interconnectivity of the fluid inclusions. It is also important to determine the permeability of micro-fissures and to characterise the hydraulic properties after the fluid release.
The effect of rock anisotropy caused by one and consecutively by the combination of two planes of weakness on the loading of a circular tunnel is investigated using numerical analysis. Finite element method is employed. The rock mass discontinuities are simulated separately with joint elements. The effect of multiple jointed rock mass is examined by adding a second joint set at specific orientations. Parametric analyses are carried out for the joint friction angle, for the stress field, in combination with varying joint orientation, joint density and joint stiffness. The analysis results are used to identify the extent of the slip zones around the opening and to compare them to existing theoretical solutions. The variation of displacements along the periphery of the tunnel is also compared to existing solutions and the additional loading of the tunnel caused by the slip of the joint planes is presented and commented.
Stratified rock masses are often encountered in underground works such as tunnels and mining. Their stratification is associated with bedding planes developed during sedimentation or at a later stage, during metamorphosis. When the stratification is at the scale of the underground construction, the behavior of the system is governed by the mechanical properties of the bedding planes, rather than the properties of the rock mass itself.
Wittke (1990) showed that the overstressed zones around a tunnel depend on the anisotropy of the rock mass, Amadei & Pan (1992) that the gravity-induced horizontal stresses depend on several parameters such as the type, degree and orientation of the rock anisotropy with respect to the ground surface. According to Gerrard (1977), intact rock anisotropy is stress dependent with a decrease in anisotropy associated with an increase in confinement. The stress dependency of rock anisotropy implies that linear elasticity may be of limited value when describing the deformability of anisotropic rocks and that it should be replaced by non-linear elasticity or more complex constitutive behavior if permanent deformation occurs. Acceptable predictions of rock behavior can still be achieved assuming linear anisotropic elasticity as long as the selected rock properties are determined in a stress range comparable to what is expected in situ. Being able to account for the directional character of anisotropic rocks instead of assuming them isotropic is certainly a step in the right direction, Amadei (1996).
An observational approach to tunnel design and construction is commonly employed in order to assess excavation driven displacements and to verify the design of temporary support systems utilized to control and minimize surface deformation. The umbrella arch is such a support system, composed of longitudinal support members, which provide stability to both the working face and region ahead of the excavation. However, a distinct lack of knowledge exists in terms of the ground-support interaction and performance beyond the working face. Within this context an application of a novel distributed optical strain sensing technique in combination with umbrella arch support members is presented as a supplemental tool to conventional monitoring techniques of the observational approach with the capability of “seeing” past the working face.
During the design and construction of tunnels in weak ground, a temporary support regime is often considered which is utilized in order to provide provisional support until the final liner is installed. Such support systems are primarily designed contingent upon the anticipated ground conditions as well as the project specified requirements and limitations. Considering the example of shallow urban tunnelling, the control and mitigation of surface settlements will be of primary concern. The assortment and arrangement of potential support systems installed in order to abide by such project demands involve but are not limited to: umbrella arch support (i.e. forepoles, spiles), ground freezing, face and radial bolting, steel sets, shotcrete, inverts, etc. (Figure 1). An observational tunnelling method, as provided by the Austrian Society for Geomechanics (2010), provides a design rational that integrates the surrounding rock into the overall support structure (i.e. the supporting formations will themselves be a part of the supporting structure as the rock is able to support itself to a certain degree) (Romero 2002). By permitting a controlled deformation of the ground mass (i.e. a limited strain of approximately 1%) stresses are provided with an opportunity to be partly released, becoming less stiff, and ultimately allowing a less cost (and time) intensive support system to be implemented (Kontogianni & Stiros 2002). However, this deformation based, observational tunnelling approach requires that the mechanisms of the ground and ground-support interaction be well understood in terms of the three-dimensional (3D) tunnel effects / considerations. Furthermore, a monitoring program capable of capturing such behaviour must be implemented as to verify or falsify the assumptions made during the design stage (Schubert 2008).
The German Federal Railway Authority (EBA) has issued a new approval for an alternative slope drapery system. Experts now have the option to choose between alternative, approved concepts to fit their solution to the project requirements individually. With the new EBA approval, a manufacturer emerges, who is able to cover the entire range of protective rockfall protection measures. The new Austrian standard ONR 24810 has increased the measures for rockfall protection, compared to ETAG 027. The competent experts of the Austrian Railways ÖBB, subsequently have developed their own criteria catalogue, based on the ONR, which now include requirements for handling, sustainability and performance. In contrast to one single standard product solution, that defines just one netting type for the entire field of slope protection measures, a modular netting system offers potential savings. The proposed contribution compares the different requirements in Germany and Austria, and responds to the full range of rockfall protection measures, including an advanced corrosion protection in accordance to DIN EN 10233-3.
Introduction and Overview
In the last years the German Railway Authority (EBA) requires approval for an alternative slope drapery system. In summer 2014 the German Authority issued an approval document for the Maccaferri Steelgrid HR. For the first time experts involved in the assessment, planning, procurement and construction have an option to choose among approved systems, and provide sound engineering solutions to meet the project requirements.
Along with the EBA approval, manufacturers can cover a broad range of rock fall protective measures. They include modular steel wire meshes with multiple grades of strength (35-250 kN/m), rock fall barriers with energy classes 500-8,600 kJ (ETAG 027 and CE certified), debris flow barriers and rock fall protection embankments, made with modular steel wire mesh reinforcing elements. In Austria, with the release of the new Ö-Norm ONR 24810:2013, requirements have become more comprehensive than just complying with the ETAG 027 crash test in rock fall protection measures. Further to ONR 24810, the Austrian Railways ÖBB later developed their own criteria, which includes requirements for handling and sustainability as well, in addition to the performance requirements. Another aspect deemed important for sustainability is in fact the durability against corrosive effects in the long-term performance of the protection measures.
Although the issue of corrosion resistance found an interest in Germany for gabion applications (and salt spray test for more than 3000 hrs are mandatory requirements), in rock fall applications Authorities are still not remarkably sensitive to this aspect, as opposed to Scandinavian countries where, due to the location along the coast of a considerable part of the infrastructure network, slope drapery systems require the highest standards for durability.
Recent developments in organic coatings on the steel wire allow today a full protection of the rock fall systems meshes with a remarkable increase in the life expectancy. Even after 6000 hours of test exposure no corrosion is detected, as confirmed by accredited German Institutes. Moreover, the realease of DIN EN 10223-3, Annex A, provides an assumed working life for various environment conditions C2-CX, in relation to the coating type, and in the case of polymer coated steel wire assumes a life expectancy of 120 years.
A sound knowledge of the in situ stress field is a necessary prerequisite for the analysis of long-term safety of a repository for spent nuclear fuel. The assumptions on the rock stress field have major impact on the analysis of the repository integrity, e.g. excavation damage zone, spalling and susceptibility to earthquakes. The geological structures, stress measurements and evidences that lead to the stress model for the Forsmark site are discussed in this study. The physical limits for possible stress states are determined by means of geomechanical considerations. This allows for the structural settings to be taken into account, and leads to an evidence-coherent stress model. The analyses in this study lead to an alternative stress model for the Forsmark site. The similarities and differences in the stress modelling assumptions by SKB and in this study are discussed and the implications concerning the repository layout and long-term behaviour are highlighted.
One of the most important aspects in each geomechanical analysis is the appropriate understanding of the stress field, i.e. the in situ stresses including the pore pressure with their spatial and temporal variation. The stresses define the mechanical performance of the rock, the behaviour of fractures, fracture networks and faults. The virgin rock stresses also influence the hydraulic behaviour of the system. Any geomechanical or geohydraulic model used is generally bound directly or indirectly to the assumptions about the stress field. Hence, the knowledge of the in situ stress field, the pore pressure and their evolution over time is a necessary prerequisite for the analysis of the long term safety of a repository for spent nuclear fuel.
Substantial errors in the estimate of the initial stress field will influence the majority of mechanical interpretations of the repository performance, including safety during construction and operation, spalling during the thermal phase, fracturing in periods of increased fluid pressures during and at the end of glaciation cycles, and the impact of earthquakes on the existing faults and fractures.
This contribution consolidates all stress field information available about the Forsmark area and discusses its consistency. Based on that discussion an alternative stress model for the Forsmark area is derived and compared to existing models.
When dealing with tunnels in difficult ground conditions, the knowledge of the rock mass parameters is of utmost importance for the selection of appropriate excavation methods and support measures. Sections with a high content of fault material or cataclasites form the most challenging stretches during tunneling, and a proper geomechanical characterization is imperative. However, investigating the overall properties is currently a challenging task, originating from difficulties in sample acquisition, sample preparation and laboratory testing. In order to gain insight into the overall mechanical properties of bimrocks an extensive laboratory program was carried out. Artificial bimrocks were fabricated for both direct shear tests and large oedometer tests, covering a wide range of possible block proportions and block orientations. The laboratory tests were accompanied by in-situ tests, allowing the identification of differences between the small- and large-scale tests and the determination of upscaling factors. A straightforward evaluation method is presented, highlighting the effect of block orientation and block proportion on the shear behavior, shear strength and deformation behavior of bimrocks.
Tectonic faults are usually composed of lens-shaped, relatively competent rock blocks surrounded by finely grained cataclastic material (e.g. Medley 2001 and Riedmüller et al. 2001). Hence, their properties are highly anisotropic and depend on the degree of the regularity of the block orientation, the total volumetric amount of the competent lenses as well as the properties of the matrix.
To study the principle mechanical properties of fault material an extensive laboratory program was conducted on both artificial block-in-matrix rocks and real fault material (Pilgerstorfer 2014). For the examination of the mechanical behavior direct shear tests were performed, allowing investigation of the behavior of bimrocks exposed to large strains. Another important issue is the knowledge about the stress dependency of the deformation properties, especially for TBM-advances in weak rock masses. The amount of displacements, which are expected, and the risk for a shield-TBM getting stuck should be known a priori.
Key management decisions, whether they be in civil, mining or reservoir engineering are often underpinned by multi-parametric rock mechanics requirements. Simulation Aided Engineering (SAE) assists in streamlining this decision making process, by removing the assumptions and bias that are associated in many empirical and conventional methodologies. SAE highlights future issues, by using adopting a unified approach to rock mass simulations, developing a more streamlined solution.
Application of the science of rock mechanics has evolved greatly since its emergence in the 1950s. Initially, engineers would conceptualize and hypothesize the rock mass behavior using available data and experience to develop basic analytical and empirical relationships for the purpose of forecasting. The development of computers resulted in alternative methods of analysis being available. Generally, these computing methods evolved around specific disciplines. The next step in this evolution of analysis techniques is the application of multi-parametric, interdisciplinary, physics based simulations. Simulation Aided Engineering (SAE) is the concept of a computer based design simulation across multiple disciplines that results in an improved engineering and design process. This concept is already applied in many other engineering disciplines.
Improvements in the computational capacity and modelling techniques have reduced many previous limitations on the geometry, structures or complexity of the mining sequence that makes up the simulation. Simulations are now limited, most often, by the scale and resolution of input data available for the analysis.
This paper explains the SAE process where physical observations, measured properties and interpreted parameters are compiled into a simulation of the rock mass. Additionally, recent improvements and examples of its application are discussed.
Hydraulic fracturing operations are necessary to extract hydrocarbons and geothermal energy from tight reservoirs or hot dry rock. Particularly challenging is the design of hydraulic fracturing in anisotropic rock. A Devonian Slate was characterized for its rock mechanical properties in order to provide a sound database for numerical modeling of fracture initiation and propagation. Each parameter showed strong anisotropy. This paper summarizes anisotropic features of the most relevant rock mechanical parameters and demonstrates their effects with two examples from laboratory hydraulic fracturing tests.
By nature, most rock masses are not isotropic due to either intact rock properties and / or discontinuities. The texture and the petrographic content of slate form a highly anisotropic rock. Slate is a fine-grained metamorphic rock and is formed from shales by low-grade regional metamorphism. Because of this process, slate shows an expressed cleavage. This feature allows splitting slate into very slim plates when correctly executed. Slate was used as an electrical insulator, roof tiles or serves now as a possible host rock for gas.
There exists some information of rock mechanical data on slate, however, the focus was mainly placed on its anisotropic triaxial strength (Donath 1964, Ramamurthy 1993, Kwasniewski 1993). Alam et al. (2008) reported about elastic properties and fracture toughness of a slate. Haimson & Avasthi (1975) published results from hydro-frac laboratory experiments. One reason for the rather sparse database may be the difficulty in preparing lab specimen according to the ISRM suggested methods (Ulusay & Hudson 2007). The cleavage in the slate separates easily under drilling, sawing and finishing operations and samples disintegrate into slim discs not fit for testing. With the slate at hand, we observed approximately 60% of vain specimen preparation work. The use of non-waterbased drilling and sawing fluid was necessary to prepare proper specimen from large blocks obtained from a roof slate mine in Central Germany.
Heise, Christoph (Institute for Production Technologies and Logistics (IPL)) | Schwarte, Stefan (Institute for Production Technologies and Logistics (IPL)) | Böhm, Stefan (Institute for Production Technologies and Logistics (IPL))
Ultrasonic-assisted technologies have enhanced the existing limits of conventional cutting processes of brittle metal-based materials. Superimposing the infeed with an adapted ultrasonic vibration enables the reduction of the machining forces. This, in turn, leads to an diminished tool wear and longer tool life. Furthermore, an increase in removal rates can be achieved. This paper presents cutting tests on granite, which have been performed to evaluate the applicability of the ultrasonic assistance for the machining of stone. Contrary to the state of the art, this study focused on carbide metal and polycrystalline diamond cutting tools with geometrically defined cutting edges. The ultrasonic frequency was maintained at 20 kHz. Different oscillation amplitudes up to 25 µm were applied to reduce process forces in comparison to the conventional machining. The observation and, subsequently, measurement of the wear behavior of the tools according to the process parameters were carried out by using a stereomicroscope and a 3D measurement system.
The pressure to be innovative puts especially high demands on the cutting process with regard to production costs and resource efficiency. Hence, the early recognition of potential for improvement and it's implementation is of significant importance. High strength materials require innovative process and machining concepts that are in principle energy and resource efficient (Treppe 2011).
In addition to the continuous development of materials, increasing quality demands and changing economic conditions require a steady advancement of the cutting processes. If an optimization is insufficient or pushes a previously used cutting process to its limit, hybrid manufacturing technologies offer an important approach for enhancement.
In the metal processing industry, machine tools are therefore increasingly supplemented by additional manufacturing technologies. Furthermore, different hybrid-based concepts are investigated. While laser-assisted cutting causes a heating of the workpiece and therefore a strength reduction in the structure, the mechanism of ultrasonic-assisted cutting is based on an optimized chip-formation and the correlating reduction of friction and heat. Consequently, the ultrasonic assistance diminishes the necessitated cutting energy and hence enables an efficient cutting process (Neugebauer et al. 2009 and Zäh & Löhe 2012).
An FEM-based numerical testing system of the split Hopkinson pressure bar (SHPB) is established to study the dynamic behaviors of rocks. Effects of parameters including striker length, striker shape, impact velocity, rock specimen size and bar size on the testing results are studied, followed by explanations of the physical mechanism lying behind. The simulation results show that the duration of platform section of the incident waveform increases with the striker length. A constant strain rate can be achieved if the special octagon-shape striker is adopted due to the slowly arising time of the incident wave, which gains an advantage over the traditional rectangular strikers. The dynamic increase factor of rock specimens varies with different impact velocities, attributed to the strain rate effect. The specimen size also has great effects on rock dynamic strength, due to the size effect. What is more, the bar size has very few influences on the modelling results if its diameter is greater than rock’s, while the rock dynamic strength decreases significantly when the bar diameter is smaller than rock’s.
Rock engineering problems are often subject to dynamic loading, such as explosion, impact, and seismic events. The dynamic behaviors of rock material significantly influence the stability and safety of rock engineering. Consequently, it is most significant and essential to understand the mechanisms of rock material when it is suffering from the dynamic loading.
The Kolsky bar, which is firstly driven by Kolsky (Kolsky 1949, 1963), and is also widely called the split Hopkinson bar, can generate an immediate deformation of the specimen under a high strain rate level in the range of 102 to 104 s-1. It is widely adopted in studying the dynamic characteristics of brittle materials, including rock (Christensen et al. 1972; Frew et al. 2002, 2001; Li et al. 2000), ceramics(Ravichandran & Subhash 1994) and concrete (Zhang et al. 2009, Li et al. 2009, Bischoff & Perry 1991, Lu & Li 2011). However, there is no standard or standard-like procedure in the design of split Hopkinson bar apparatus. In recent years, a large number of researchers modified the split Hopkinson bar apparatus for different research purposes under different experimental environments. For example, different bar diameters of bars are designed for testing different materials. SHPB with a bar size of 74 mm was designed to determine the dynamic compressive strength enhancement of concrete (Zhang et al. 2009). Frew et al. (2001) used a SHPB of 12.7 mm bar diameter to study the dynamic behaviors of rock. For pulse shaping methods, many researchers (Lok et al. 2002, Zhou et al. 2011, Li et al. 2000) adopted a conical strikers to modify the shape of incident wave in order to achieve the stress equilibrium and constant rate deformation in the specimen during the SHPB tests. And many other researchers would like to use the paper (Wu & Gorham 1997), Plexiglas (Togami et al. 1996), polymer (Chen et al. 1999), copper (Frew et al. 2002, 2001), stainless steel (Frantz et al. 1984) or other materials sandwiched between the striker and incident bar to obtain a slowly arising incident wave. Thus, the measured SHPB results from different researchers exhibited large difference even for the same material, due to the different designs of the SHPB apparatuses.