Fracture Propagation in Anisotropic Rock During Drilling and Cutting

Transkript

1 Topics DOI: /geot Kurosch Thuro Nik Schormair Fracture Propagation in Anisotropic Rock During Drilling and Cutting Anisotropy plays a key role in fracture propagation models and rock fragmentation processes during rock drilling and cutting. Drilling tests with a percussive rock drill have been performed to examine the influence of anisotropy and inhomogeneity on fracture propagation in different rock types. Subsequently, thin sections of the bottom of the borehole were analysed to investigate the associated crack patterns. Based on these observations an attempt was made to simulate the drilling process in a specific rock material using the Particle Flow Code (PFC 2D ). Since the Particle Flow Code is based on a discontinuum mechanics approach, the rock sample is converted into an assembly of spheres, where the particles are able to interact with each other and fractures are able to propagate. Different rock cutting tools were assumed including button bits, disc cutters and roadheader bits to simulate the penetration of varying rock samples. The latter have been omitted in this paper due to space limitations. The aim of the numerical simulation of the drilling and cutting processes was to examine the crack patterns and correlate with existing foliation. In this paper preliminary results of the 2-dimensional PFC modelling are presented. Bruchausbreitung im anisotropen Gebirge beim Bohren und Schneiden Die Anisotropie besitzt eine Schlüsselrolle bei der Bruchausbreitung, sowohl im Modell als auch beim Zerkleinerungsprozess beim Bohren und Schneiden. Um die Charakteristik der Bruchausbreitung in Abhängigkeit von Anisotropie und Inhomogenität zu untersuchen, wurden In-situ-Bohrungen mit der üblichen auf der Baustelle vorhandenen Bohrausrüstung (Schlagbohrhammer) in Blöcken aus verschiedenen Gesteinstypen durchgeführt. Von den so gewonnen Proben aus der Bohrlochsohle wurden Dünnschliffe angefertigt und bezüglich des Rissmusters ausgewertet. Auf Grundlage dieser Untersuchungen wurde der Versuch unternommen, den Bohrprozess in verschiedenen virtuellen Gesteinstypen mit dem Particle Flow Code (PFC 2D Version 3.1) zu simulieren. Da der Particle Flow Code auf der Diskontinuumsmechanik basiert, wird ein Gestein als eine Anordnung verschieden großer Kugeln (oder Scheiben) nachgebildet, die miteinander wechselwirken und wo sich Brüche ausbreiten können. Um das Eindringen von verschiedenen Werkzeugen in diese virtuellen Gesteine zu simulieren, wurden Bohrkronenstifte, Diskenschneiden und Rundschaftmeißel generiert. Letztere wurden in diesem Artikel allerdings aus Platzgründen weggelassen. Ziel der numerischen Simulation war unter anderem die Untersuchung des Rissmusters und seine Korrelation mit der vorhandenen Schieferung. Letztere wird sehr häufig durch Glimmerminerale erzeugt, die einen mehr oder weniger großen Abstand das Gestein gerichtet durchziehen. In diesem Beitrag werden die ersten Ergebnisse der zweidimensionalen Modellierung mit dem Particle Flow Code gezeigt. 1 Introduction Geological structures such as foliation act as weakness planes in the rock and may have a significant impact on the fragmentation process beneath rock cutting tools. Extensive field and laboratory studies have been performed to examine drilling or cutting progress and its correlation with foliation orientation and other rock properties [1], [2], [3], [4], [5], [6]. This research indicates that the drilling velocity as well as the disc cutter penetration is best orthogonal to foliation or schistosity and poorest parallel to it. Between these angles, the drilling or cutting rates decrease significantly (Figure 1). Fig. 1. Correlation between orientation of foliation in phyllites and drilling progress (left) and penetration rate (right), respectively Bild 1. Korrelation zwischen Orientierung der Schieferung in Phylliten und Bohrgeschwindigkeit (links) bzw. TBM Penetration (rechts) Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin Geomechanik und Tunnelbau 1 (2008), Heft 1

2 Fig. 2. Procedure of drilling tests: Drilling process with a percussive rock drill (left), overcored and drilled rock sample (right) Bild 2. Durchführung der Versuchsbohrungen: Bohrungen mit einem Schlagbohrhammer (links), formatierte und überbohrte Bohrprobe (rechts) These field studies implied that significant potential existed for the investigation of induced microscopic fracture patterns beneath rock cutting tools in different rock types using numerical simulation. Using the Particle Flow Code PFC 2D [7], which is based on discontinuum mechanical properties, virtual rock types and drilling and cutting tests were modelled. 2 Rock fragmentation by drilling To investigate the rock fragmentation process in detail, onsite drilling tests have been performed with a common percussive rock drill using different rock types [8]. Special attention was given to the crack formation at the bottom of the borehole. The aim of the study was to detect and analyse the cracks, which were produced by the drilling process. In the first step, foliated (anisotropic) and isotropic rock samples (size approximately 0,4 m 0,4 m 0,3 m) were selected. Using an Atlas Copco COP kW rock drill [9] under on-site-conditions, approximately 10 cm deep boreholes were drilled into the samples. To examine the influence of anisotropy on the drilling process, the boreholes were drilled at varying angles to foliation (Figure 2). After the drilling process, the samples were filled with a two-component epoxy resin containing a yellow fluorescent dye (0.2 % of the quantity of resin) under vacuum conditions (Figure 3). A very low viscosity resin also guaranteed good infilling of the small fissures. The coloured dye in the resin was used to visualise the cracks under the microscope. A subsequent series of thin sections were produced with the intention of investigating specific areas of interest at the base of the borehole. The principal objective was to detect the crack patterns. Details on the results of this research are given in [10]. To get an impression of the crack patterns, a mica schist sample with inclined foliation is shown in Figure 4. Using thin sections, the microscopic crack patterns in different rock types were investigated. A correlation of the crack patterns between the spacing of the foliation and the grain size could be detected in all of the thin sections. In the very tightly foliated, fine-grained mica schist, the cracks develop along the mica layers as zones of weakness (Figure 4). Only rarely cracks are observed cutting across the mineral components (since there are no zones of Fig. 3. Granite sample with borehole bottom filled with resin and locality of thin section extraction Bild 3. Granitprobe mit Bohrlochsohle, gefüllt mit gefärbtem Kunstharz und Ort der Dünnschliffentnahme Fig. 4. Thin section of a mica schist sample. Drilling direction (here: vertical) is inclined to foliation. Cracks propagating parallel to foliation are marked. Bild 4. Dünnschliff einer Probe aus Glimmerschiefer. Bohrrichtung (hier vertikal) ist schiefwinklig zur Schieferung. Die sich in der Schieferung ausbreitenden Risse sind markiert. Geomechanik und Tunnelbau 1 (2008), Heft 1 9

3 clear, that in the parallel case, rock properties are the highest and drilling rates are low. In addition blasting conditions are often related to the drilling. So if the tunnel axis is parallel to the main foliation, drilling and blasting conditions are assumed to be very poor. Fig. 5. Schematic drawing of crack patterns slanting to foliation (see Figure 4). Macroscopic and microscopic cracks are marked. The borehole diameter is 40 mm. Bild 5. Schematische Skizze des Rissmusters schiefwinklig zur Bohrachse (vgl. Bild 4). Makro- und Mikrorisse sind markiert. Der Bohrlochdurchmesser ist 40 mm. weakness). At the bottom of the borehole a roof-shaped or stair-like structure in the crack patterns could be detected. Opening of the mica layers seems to be induced by the percussive process with the shearing process of the bit creating the breakout of the fragments. An angle of about 15 seems to be beneficial for the break out of the fragments. The crack patterns in different rock samples were observed for the cases normal, parallel and inclined to foliation and based on the characteristics of the macroscopic and microscopic crack patterns, schematic drawings were produced to illustrate the results of the investigations (Figure 5). Knowing that drilling performance is best normal to foliation and worst parallel, the observed macroscopic crack patterns support the following statements: When the direction of drilling is normal to the orientation of foliation, rock material is compressed normal but sheared parallel to it. Although cracks will develop radial to compression, the cracks parallel to the bottom of the borehole will be utilized in chipping. Usually in this case the highest drilling velocities are obtained because of the favourable schistosity orientation. Drilling is controlled by the shear strength of the foliated rock material. This causes large sized chips and a maximum drilling performance. If the drilling axis is oriented parallel to foliation, compression is similarly oriented parallel whereas the shear stress is normal to foliation. Less and smaller cracks (1 mm observed) develop for reasons of higher strength normal to the weakness planes. Drilling is controlled by the tensile strength parallel to the foliation producing small sized fragments and minimum drilling performance. Generally, drilling is controlled by the dip angle of foliation, forming medium sized fragments during the crushing process. Drilling performance is, by geometrical reasons, mainly a cosine function of the dip angle. It is The thin sections seem to indicate a relationship between the crack pattern and the direction and condition of the foliation. Fractures in widely foliated rock types sometimes propagate along the mica layers, but it is not compelling. The cracks develop parallel to the surface and use foliation only if the foliation runs along a surface-parallel crack. Predominantly they propagate across mineral components and the crack pattern resembles that observed in isotropic rock. In the mylonitic granite samples it can be clearly seen, that the spacing and the condition of the foliation is important. It can be imagined, that the smoother the surface of the foliation (clear mica layers) and the closer the foliation is, the more the cracks are aligned with the mica layers. This means, that the mica layers acting as zones of weakness were almost exclusively used in fracture development. This reflects, as suggested by [2] and [6], that the dip angle of anisotropy plays a key role in rock fragmentation. To date however, no precise statement can be made about the role of the foliation. To obtain a deeper understanding of crack propagation in anisotropic rock, a simulation of the drilling process has been conducted using the numerical code PFC 2D [7]. 2.1 Simulation of rock fragmentation Since the drilling investigations and subsequent thin section analyses only provided crack patterns, an attempt was made to simulate the drilling process and the rock material with a numerical code. The Particle Flow Code (PFC 2D, [7]) seemed to have all necessary features to perform this simulation, allowing the modelling of tools with different shapes as well as rock material with varying anisotropy and inhomogeneity. The PFC code is based on a discontinuum mechanics approach in which a sample can be composed of interacting particles. In PFC 2D, movements and interactions of loaded element assemblies are shown with two-dimensional balls. Through the randomized connection, arrangement and interaction of these elements, different physical systems can be simulated. The use of PFC 3D was not found to be necessary in this state of the project. 2.2 Procedure A common problem with PFC is that the material parameters in PFC do not correlate with the conventional rock properties used in continuum and discontinuum mechanical models. When designing the rock material, micromechanical parameters have to be defined such as bond strength. For calibration purposes, a virtual laboratory test, such as the unconfined compressive test, has to be performed to derive the unconfined compressive strength and other properties as macroscopic values. By varying the micromechanical parameters, reasonable rock properties can be gained. 10 Geomechanik und Tunnelbau 1 (2008), Heft 1

4 Using the PFC, it is possible to simulate micro- and macromechanical processes simultaneously. For example, on a loaded block both the micro-mechanical process of destruction and the macro-mechanical process of movements and cracks can be simulated at the same time. Therefore in the first step, virtual rock types had to be designed and tested in a biaxial load test to examine rock properties such as the unconfined compressive strength. 2.3 Virtual drilling tests The graph in Figure 6 shows that the stress-strain behaviour in the virtual biaxial load test corresponds with the diagram of the real unconfined compressive test. The typical hourglass failure structure in Figure 7 is the same as observed in a laboratory test. In the virtual sample, the number of micro cracks can be counted and it is possible to determine, if the cracks are developing from tension or compression. In the sample shown in Figure micro cracks have been generated. The black coloured cracks develop from tension, the white coloured cracks from compression. Most of the black coloured cracks propagate through the simulated mica layers. Up to now, four types of virtual anisotropic rock types ( samples ) were created which varied mainly in the ratio of weak and strong layers (equals the spacing of foliation). For the gneiss, the strong layers reflecting quartz and feldspar were notably thicker (20 mm) than the zones of weakness (2 mm mica layers, foliation ). As schist, a very tight foliated rock type was designed, where the weak Fig. 6. Stress-strain-curve of a virtual anisotropic material ( gneiss, see Figure 7) under uniaxial loading (UCS = 58 MPa) Bild 6. Spannungs-Dehnungskurve eines virtuellen anisotropen Materials ( Gneis ) unter einaxialer Belastung (s u = 58 MPa) Bauaufsicht und Vertragsmanagement bei den Hydroschildbaulosen H 3-4 und H 8 der Neuen Unterinntalbahn BERNARD Ingenieure ZT GmbH Bahnhofstr. 19 A-Hall in Tirol Telefon +43 (0)5223 / office@bernard-ing.com Hanauer Landstr D Frankfurt am Main Telefon +49 (0)69 / info.bgs-frankfurt@grontmij.de Dipl.-Ing. Bernd Gebauer Ingenieur GmbH Elsenheimerstraße 49 D München Telefon +49 (0)89 / bgm@bgebauer.de Geomechanik und Tunnelbau 1 (2008), Heft 1 11

5 Fig. 7. Virtual anisotropic rock sample ( gneiss ) with foliation (green) tested under uniaxial loading at failure with typical hourglass failure structures. The sample diameter is 10 cm; the assembly contains 13,000 particles. Bild 7. Virtuelles anisotropes Material Gneis mit Schieferung (grün), das unter einaxialer Belastung mit einer typischen Sanduhrstruktur versagt. Der Probendurchmesser ist 10 cm, die Anordnung enthält Partikel. layers had the same thickness as the strong layers (2 mm). The bond strength between particles was kept constant. After designing the rock materials and testing the unconfined compressive strengths, it was possible to simulate drilling tests. For the rotary percussive drilling tests different bits and buttons were also designed, e.g. conical, ballistic and spherical buttons (Figure 8). The hard metal buttons of the drilling bits used in underground construction are made of tungsten carbide and are crucial for the drilling performance. Simulation of the percussive component required implementation of a vertical movement; the simulation of the rotational component required implementation of a horizontal movement. In this way, it was possible to examine the developing crack patterns in the designed rock materials. The orientation of foliation and its influence was tested from 0 to 90 in steps of 10 resulting in 10 drilling tests per rock type. The rock type in Figure 9 (gneiss) shows the typical orientation of the forces induced by bit penetration. The forces are transmitted to the sample at the right side of the bit shape, because the movement of the bits is to the right. In the central area of the bit shearing of the dark coloured zone of weakness can be seen. Fig. 8. Hard metal bits for drilling tests, from left to right: conical, ballistic and spherical bit shapes Bild 8. Hartmetallstifte für die Bohrversuche, von links nach rechts: konische, ballistische und sphärische (runde) Stiftform Fig. 9. Anisotropic rock sample gneiss after the drilling test. Above: The forces from the bits are marked in black. Red balls represent zones of weakness (foliation). Below: Induced fracture pattern. Cracks caused by tension are marked in black; cracks caused by compression are marked yellow. Image width about 4 cm Bild 9. Anisotrope Gesteinsprobe Gneis nach dem Bohrversuch. Oben: Die durch die Stifte induzierten Kräfte sind schwarz eingetragen. Rote Kugeln repräsentieren die Schwächezonen (Schieferung). Unten: Induziertes Rissmuster. Die durch Zug erzeugten Risse sind schwarz markiert, die durch Druck verursachten gelb. Bildbreite etwa 4 cm The crack pattern illustrated in Figure 9 (below) corresponds to the forces shown above at the same time step. Most of the cracks in the zones of weakness are caused by tension; only a few cracks are caused by compression. In the area of high bond strength, only cracks caused by compression occur. Looking at the distribution of the cracks around the bits, a roof-shaped or stair-like structure of the crack patterns can be detected, which is traced by the red line. The biaxial compressive strength of this rock type is 48.1 MPa. In the entire sample 423 cracks were created by the indentation of the bits. All designed rock types were tested accordingly in different orientations to the dip angle of foliation which resulted in specific crack patterns. To demonstrate the differences between crack patterns, very tightly foliated schist tested under the same conditions is shown in Figure 10. The force distribution caused by the bits moving to the right looks similar to those of the rock type in Figure 9. But the crack patterns in Figure 10 differ significantly from the ones shown in Figure 9. Most of the cracks are caused by tension; fewer cracks are caused by compression. Examining the distribution of the cracks around the bits, a roof-shaped or stair-like structure of the crack patterns can be observed, which is marked by the line in Figure 10. The biaxial compressive strength of this rock type is 37.4 MPa. In the entire sample 807 cracks were created by the indentation of the bits. 12 Geomechanik und Tunnelbau 1 (2008), Heft 1

6 Fig. 10. Anisotropic rock type schist after the drilling test. Above: The forces from the bits are marked in black. Red balls are zones of weakness (foliation). Below: Induced fracture pattern. Cracks caused by tension marked in black, cracks caused by compression marked in yellow. Image width about 4 cm Bild 10. Anisotrope Gesteinsprobe Schiefer nach dem Bohrversuch. Oben: Die durch die Stifte induzierten Kräfte sind schwarz eingetragen. Rote Kugeln repräsentieren die Schwächezonen (Schieferung). Unten: Induziertes Rissmuster. Die durch Zug erzeugten Risse sind schwarz markiert, die durch Druck verursachten gelb. Bildbreite etwa 4 cm Fig. 11. Number of cracks after the drilling test plotted against the dip angle of foliation with respect to a vertical drilling direction. Gneiss relates to Figure 9, schist relates to Figure 10. Bild 11. Anzahl der Risse nach Durchführung des Bohrversuchs in Abhängigkeit der Orientierung der Schieferung bei vertikaler Bohrrichtung. Gneis bezieht sich auf Bild 9, Schiefer auf Bild 10. In this very tightly foliated rock type, the number of cracks produced by the bit indentation is nearly twice as high as in the rock type shown in the Figure 9. This implies, that in a tightly foliated rock type as in the schist (about 2 mm spacing), bonded with the same forces as a rock type with a wider spacing of the foliation as the displayed gneiss (about 12 mm), the crack formation and therefore the fragmentation in the mineral bonds are much better. To show the impact of anisotropy on the drilling tests, varying orientations of foliation were tested and the total number of cracks counted for each orientation angle. The dip angle of foliation refers to a vertical drilling direction. In the schist, the number of cracks produced is twice as high as in the gneiss (Figure 11). Unfortunately no correlation was found for the orientation of foliation. It seems that the microcracks do not correspond to the macrocracks that form the chips under the drilling bits. Furthermore the three different bit shapes (see Figure 8) were tested in the same rock type to analyse the influence of the bit geometry. In the gneiss, the ballistic bit shape created the highest number of cracks (Figure 12). 2.4 Virtual cutting tests Fig. 12. Number of cracks after the drilling test in gneiss plotted against the dip angle of foliation with respect to a vertical drilling direction using three types of bit shapes Bild 12. Anzahl der Risse nach Durchführung der Bohrversuche im Gneis in Abhängigkeit der Orientierung der Schieferung bei vertikaler Bohrrichtung unter Verwendung der drei verschiedenen Stiftformen An additional type of rock fragmentation encountered in tunnelling is cutting performed by a tunnel boring machine (TBM). Disc cutters were simulated as wedge indenters with a defined vertical load (penetration) into the rock samples. The cutting test involved 1,000 numerical time steps in the PFC 2D code. At 250 step intervals the penetrating forces induced by the disc cutters and the resulting crack patterns were analysed. In addition, symmetrical and asymmetrical wear of a disc cutter (Figure 13) was simulated to analyse the influence of tool wear during crack formation. In the tests, two discs at a specific distance from each other (based on tunnelling practice) were modelled to investigate the interference of the forces. When these forces exceed a critical value, chipping will occur. Geomechanik und Tunnelbau 1 (2008), Heft 1 13

7 Fig. 13. Disc cutters used for the indentation tests, from left to right: new disc cutter, symmetric and asymmetric wear of the cutter Bild 13. Diskenschneiden, die für die Schneidversuche verwendet wurden, von links nach rechts: neue Diske, symmetrischer und asymmetrischer Verschleiß des Schneidrings During the penetration process, the development of the forces by increasing indentation of the cutter can be observed. In Figure 14 the development of the forces is shown every 250 steps until the end of the test (1,000 steps). The overlap of the forces increases with increasing penetration. In Figure 14, picture 3 (750 steps) and 4 (1,000 steps), the overlapping forces form a large chip that can be released between the two disc cutters. In direct contact with the cutter, the bonds between the particles are completely broken. This corresponds with the zone of crushed and powdered rock underneath a real disc cutter. With increasing forces, the number of cracks in the assembly increases too. The number of cracks and the crack patterns depends on the rock type. Figure 15 shows the development of the crack patterns in the gneiss corresponding to Figure 14 for the same numerical time steps. In the zones of weakness most of the cracks are caused by tension; only a few cracks are created by compression. In the zones of high strength, cracks produced by compression are abundant. Directly under the disc cutters, where the highest forces are applied, cracks are induced mainly in the zones of weakness. The cracks form a radial corona around the disks as it can be observed at the tunnel face. The orientation of the cracks seems to be independent of the foliation. Most of the cracks are inclined or normal to foliation. The density of the crack pattern decreases with depth. The biaxial compressive strength of this rock type was determined to be 48.1 MPa. In the entire sample 1,605 cracks were created by the indentation of the disc cutters. To investigate the influence of anisotropy on the indenting forces and the resulting crack patterns, the designed rock types with different thickness of weakness zones ( foliation ) were tested in the same way. The distribution of the forces induced by the disc cutters in the schist of Figure 16 looks similar to the distribution in Figure 14. But in this rock type, the forces propagate much deeper into the rock. The overlap of the forces is not as clear as in the gneiss shown in Figure 14. It seems that this phenomenon is mainly caused by the tight foliation of the rock. The crack pattern in the schist of Figure 16 differs significantly from the crack pattern shown in Figure 10. The propagation of the cracks is closely connected with the forces. Zones of weakness and zones of high strength can- Fig. 14. Development of forces (black) induced by indenting disc cutters in gneiss. Red balls represent zones of weakness (foliation). From the top: 250, 500, 750 and 1,000 numerical time steps. Disk cutter distance about 10 cm Bild 14. Entwicklung der Kräfte (schwarz), die beim Eindringen der Diskenschneiden in einem Gneis induziert werden. Rote Kugeln repräsentieren Schwächezonen (Schieferung). Von oben nach unten: 250, 500, 750 und numerische Schritte. Abstand der Diskenmeißel etwa 10 cm Fig. 15. Development of cracks induced by indenting disc cutters in gneiss (see Figure 14). From the top: 250, 500, 750 and 1,000 numerical time steps. Cracks caused by compression marked in yellow, cracks caused by tension marked in black. Cutter distance about 10 cm Bild 15. Entwicklung der Risse, die beim Eindringen der Diskenschneiden in einem Gneis induziert werden (vgl. Bild 14). Von oben nach unten: 250, 500, 750 und numerische Schritte. Die durch Druck erzeugten Risse sind gelb markiert, die durch Zug verursachten schwarz. Abstand der Diskenmeißel etwa 10 cm 14 Geomechanik und Tunnelbau 1 (2008), Heft 1

8 Fig. 16. Disc cutter indentation in schist. Above: Development of induced forces. Red balls are zones of weakness (foliation). Below: Development of cracks. Cracks caused by compression marked in yellow, cracks caused by tension marked in black. Cutter distance about 10 cm, image width about 15 cm Bild 16. Eindringen von Diskenschneiden in einem Schiefer. Oben: Entwicklung der induzierten Kräfte. Rote Kugeln repräsentieren Schwächezonen (Schieferung). Unten: Entwicklung der induzierten Risse. Die durch Druck erzeugten Risse sind gelb markiert, die durch Zug verursachten schwarz. Abstand der Diskenmeißel etwa 10 cm not be distinguished as was possible in Figure 15. Furthermore most of the cracks occur directly under the cutting edge and crack density decreases with increasing distance from the disc cutters. The biaxial compressive strength of this rock type is 37.4 MPa. In the entire sample 3,801 cracks were created by the indentation of the disc cutters. The number of cracks observed in the schist is much higher than in the gneiss shown in Figure 15. This phenomenon was previously observed in the drilling tests within the same rock types. Also, there is an analogy in the decreasing density of the crack pattern with increasing depth. To show the impact of anisotropy on the cutting tests, different inclinations of foliation were tested and the total number of cracks counted for each orientation. In the schist the number of produced cracks is about twice as high as in the gneiss (Figure 17). Unfortunately no correlation was found for the orientation of foliation. It seems that the microcracks do not correspond to the macrocracks that form the chips under the disc cutters. In TBM tunnelling the wear of the disc cutters plays the economical key role. The two main wear types are the symmetric wear in the centre of the TBM cutter head and the asymmetric wear of the outer disc cutters. In Figure 18 symmetric worn disc cutters are cutting the schist. The forces propagating into the rock sample are distributed in a very close area around the discs only. The overlap of the forces between the disc traces is minor and the breaking of bonds around the disc cutters is limited. WKE - Brandschutzsysteme. Sichere Installationen bei extremen Anforderungen. Fig. 17. Number of cracks after the cutting test plotted against the dip angle of foliation with respect to a vertical indentation. Gneiss relates to Figure 14, schist relates to Figure 16 Bild 17. Anzahl der Risse nach Durchführung des Schneidversuchs in Abhängigkeit der Orientierung der Schieferung bezüglich der Eindringrichtung. Gneis bezieht sich auf Bild 14, Schiefer auf Bild 16 WKE Abzweigkästen Schutzart IP 66 Funktionserhalt E30 Abgesicherter Abzweig möglich Halogenfreie Duroplastgehäuse 0,5 bis 35² mm Praxisbewährt Extrem hohe Schlagfestigkeit Prüfzeugnis des MPA VDE zugelassen Kompletter Lieferumfang inklusive Zubehör Flexible Außenbefestigungsmöglichkeiten Referenz: Tunnel Stägjitschugge / Schweiz WKE-Hotline: / Günther Spelsberg GmbH + Co. KG Im Gewerbepark 1 D Schalksmühle Telefon: / Telefax: / info@spelsberg.de Geomechanik und Tunnelbau 1 (2008), Heft 1 15

9 Fig. 18. Development of forces (above) and cracks (below) by indenting disc cutters with symmetric wear in schist. Red balls are zones of weakness (foliation). Cracks caused by compression marked in yellow, cracks caused by tension marked in black. Cutter distance about 10 cm Bild 18. Entwicklung der Kräfte (oben) und Risse (unten), die beim Eindringen von symmetrisch abgenutzten Diskenschneiden in einem Schiefer induziert werden. Rote Kugeln repräsentieren Schwächezonen (Schieferung). Abstand der Diskenmeißel etwa 10 cm Fig. 19. Development of forces (above) and cracks (below) by indenting disc cutters with asymmetric wear in schist. Dark balls are zones of weakness (foliation). Cracks caused by compression marked in white, cracks caused by tension marked in black. Cutter distance 10 cm Bild 19. Entwicklung der Kräfte (oben) und Risse (unten), die beim Eindringen von asymmetrisch abgenutzten Diskenschneiden in einem Schiefer induziert werden. Rote Kugeln repräsentieren Schwächezonen (Schieferung). Die durch Druck erzeugten Risse sind gelb markiert, die durch Zug verursachten schwarz. Abstand der Diskenmeißel etwa 10 cm Fig 20. Number of cracks after a cutting test in schist according to the dip angle of foliation with respect to a vertical indentation and the disc cutter shape Bild 20. Anzahl der Risse nach Durchführung der Schneidversuche im Gneis in Abhängigkeit der Orientierung der Schieferung bezüglich der Eindringrichtung unter Verwendung der drei verschiedenen Diskenverschleißformen The crack pattern traces the obtained force distribution (see Figure 18). In contrast to a new disc, cracks are concentrated in a limited area around the cutters. The range of the cracks produced is only about half that compared to the one with a new disc. Also there are only few cracks developed between the cutter traces. Similar to the crack pattern of a new disc cutter, the orientation of foliation does not play a significant role and is not reflected in the distribution of the cracks. In the entire sample 2,316 cracks were created by the indentation of the disc cutters. During penetration with asymmetric worn disc cutters, two areas of the induced forces can be distinguished (Figure 19, above). The main area is located near the long side of the cutter; here the forces penetrate much deeper into the rock. The smaller area beneath the short edge results in a narrow range of the induced forces. In summary the forces propagate slightly deeper into the rock sample than in the case with asymmetric worn disc cutters. Again, the crack pattern traces the obtained force distribution (Figure 19, below). At the long side of the cutter more cracks are visible than along the short side. Also there are few cracks developed between the cutter traces. Similar to the crack pattern of a new disc cutter, the dip angle of foliation is not reflected in the crack distribution under the asymmetrically worn cutter since cracks are more or less oriented inclined or normal to foliation. In the entire sample 2,316 cracks were created by the indentation of the disc cutters. It is noteworthy and clear, that the wear status of the disk cutter is crucial to crack formation and fragmentation of the rock material (Figure 20). Nearly twice as many cracks are formed with a new and sharp disc cutter in contrast to a cutter with symmetrical or asymmetrical wear. In contrast, the difference between symmetrical or asymmetrical wear is negligible. 3 Conclusions Correlation between induced crack patterns in different rock types and foliation were successfully examined using PFC. It could be demonstrated, that fracture propagation is mainly influenced by the spacing of the foliation and 16 Geomechanik und Tunnelbau 1 (2008), Heft 1

10 therefore the thickness of zones of weakness. No clear correlation with the orientation of foliation could be determined in the microcracks. This may be due to the formation of large cracks that cannot be simulated by the numerical PFC model. Large (macroscopic) fragments could however be estimated by the surrounding forces and the breakage of bonds e.g. between disc cutter traces. Notably the different bit shapes and disc wear types had a significant impact on the crack pattern and the depth of the induced forces. Unfortunately due to code and calculation capacity limitations it is not possible at the moment to create a model of both rock material and tool interaction, which fully satisfies geological as well as geotechnical needs. References [6] Thuro, K. and Plinninger, R. J.: Hard rock tunnel boring, cutting, drilling and blasting: rock parameters for excavatability. In: Handley, M., Stacey, D. (Eds): Proc. 10 th ISRM Int. Congr., p Johannesburg, [7] Itasca: PFC 2D Particle Flow Code, Version 3.1. Itasca Consulting Group Inc., Minneapolis, [8] Schormair, N.: Rock fragmentation during rotary percussive drilling. Diploma Thesis. Technische Universität München, [9] Atlas Copco: Hydraulic Rock Drills COP 1838ME/MEX Technical specification. Örebro, [10] Schormair, N., Thuro, K. and Plinninger, R. J.: The influence of anisotropy on hard rock drilling and cutting. In: Culshaw, M., Reeves, H., Spink, T., Jefferson, I. (Eds): Proc. 10 th IAEG Int. Congr., Paper No London: The Geological Soc., [1] Spaun, G. and Thuro, K.: Untersuchungen zur Bohrbarkeit und Zähigkeit des Innsbrucker Quarzphyllits. Felsbau 12 (1994), p [2] Thuro, K. and Spaun, G.: Drillability in hard rock drill and blast tunnelling. Felsbau 14 (1996), p [3] Thuro, K.: Drillability prediction geological influences in hard rock drill and blast tunnelling. Geol. Rundsch. 86 (1997), p [4] Thuro, K.: Geologisch-felsmechanische Grundlagen der Gebirgslösung im Tunnelbau. Münchner Geologische Hefte, B18, Technische Universität München, [5] Thuro, K., Plinninger, R. J. and Spaun, G.: Drilling, blasting and cutting is it possible to quantify geological parameters of excavation? In: van Roy, J. L., Jermy C. A. (Eds.): Proc. 9 th IAEG Int. Congr., p Durban, o.-univ. Prof. Dr. rer. nat. habil. Kurosch Thuro Technische Universität München Lehrstuhl für Ingenieurgeologie Arcisstraße 21, München Germany thuro@tum.de Dipl.-Geol. Nik Schormair Maidl, B. et al. BUCHEMPFEHLUNG Hardrock Tunnel Boring Machines Dieser Band behandelt die Grundlagen der Tunnelbautechnologie: Bohrung, Tunnelvortrieb, Abtransport und Entsorgung des Abraums sowie Sicherungsmaßnahmen. Methoden zur Klassifikation von Gesteinen und technische Einzelheiten der Maschinen werden ebenso diskutiert wie juristische Aspekte. Zahlreiche Beispiele reflektieren den modernen Stand der Technik, illustrieren Anwendungen und veranschaulichen Problemfälle. Das erfahrene, erfolgreiche Autorenteam wendet sich gleichermaßen an erfahrene Tunnelbauer und interessierte Neueinsteiger. Ernst & Sohn Verlag für Architektur undtechnische Wissenschaften GmbH & Co. KG Für Bestellungen und Kundenservice: Verlag Wiley-VCH Boschstraße 12, Weinheim Telefon: +49(0) 6201 / Telefax: +49(0) 6201 / service@wiley-vch.de Fax-Antwort an +49 (0) Firma Name, Vorname Straße/Nr ca. 350 Seiten, ca. 255 Abb. Gebunden. In Englisch. 2 89, /sfr 142, ISBN: Hardrock Tunnel Boring Machines 89, 2 Land PLZ Ort Datum/Unterschrift UST-ID Nr. / VAT-ID No. Telefon * 2 Preise gelten ausschließlich für Deutschland. Irrtum und Änderungen vorbehalten _my Geomechanik und Tunnelbau 1 (2008), Heft 1 17