Wulf Schubert

A Study of the Failure Mechanism of Planar Non-Persistent Open Joints Using PFC2D

Particle flow code 2D (PFC2D) was adopted to simulate the shear behavior of rocklike material samples containing planar non-persistent joints. Direct shear loading was conducted to investigate the effect of joint separation on the failure behavior of rock bridges. Initially calibration of PFC was undertaken with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. Furthermore, validation of the simulated models were cross checked with the results of direct shear tests performed on non-persistent jointed physical models. Through numerical direct shear tests, the failure process was visually observed, and the failure patterns were found reasonably similar to the experimentally observed trends. The discrete element simulations demonstrated that the macro-scale shear zone resulted from the progressive failure of the tension-induced micro-cracks. The failure pattern was mostly influenced by joint separation, while the shear strength was linked to the failure pattern and failure mechanism. Furthermore, it was observed that the failure zone is relatively narrow and has a symmetrical pattern when rock bridges occupy a low percentage of the total shear surface. This may be due to the high stress interactions between the subsequent joints separated by a rock bridge. In contrast, when rock bridges are occupying sufficient area prohibiting the stress interactions to occur then the rupture of surface is more complex and turns into a shear zone. This zone was observed to be relatively thick with an unsymmetrical pattern. The shear strength of rock bridges is reduced by increasing the joint length as a result of increasing both the stress concentration at tip of the joints and the stress interaction between the joints.

Dealing with squeezing conditions in Alpine tunnels

SummaryIn fault zones, excessive deformation during and after tunnel excavation is frequently encountered. Shoterete in combination with grouted rock bolts has successfully been used in many applications to control the deformation process. The magnitude of deformation frequently exceeds the deformability of the shotcrete lining. In the past, this problem has been solved by dividing the shotcrete lining into “segments” and leaving gaps between the segments to accommodate deformation without damage to the lining. The need for minimizing deformation in an extremely heterogeneous fault zone at the Galgenbergtunnel (Austria) led to the development of low cost yielding elements, installed between the shotcrete “segments” and the use of a new type of re-groutable brock bolts. New techniques in evaluating the results of displacement monitoring have improved the short term prediction.

Is the OeGG guideline for geotechnical design compatible with the EC7?. Ist die ÖGG‐Richtlinie für geotechnische Planung kompatibel mit dem Eurocode 7?

In Oesterreich gibt es seit 2001 eine Richtlinie fuer die geotechnische Planung von Untertagebauten mit konventionellem Vortrieb, die 2008 aktualisiert wurde, sich aber weiterhin auf die Anwendung der Beobachtungsmethode konzentriert. Der Beitrag stellt nun die Anforderungen des EC7 als kuenftigem Regelwerk fuer die Planung und Berechnung von Tunneln den Regelungen der Richtlinie der Oesterreichischen Gesellschaft fuer Geomechanik (OEGG-Richtlinie) gegenueber. Der EC7 laesst die Anwendung der Beobachtungsmethode zu, wenn die fuer eine Bemessung des Tunnels erforderlichen Parameter nicht mit genuegender Sicherheit bestimmt werden koennen. Hinsichtlich der akzeptablen Grenzen des Verhaltens des untersuchten Tunnels sind EC7 und OEGG-Richtlinie inhaltlich kompatibel. Die Ermittlung der Bandbreite des Verhaltens von Tunneln ist im EC7 teilweise unklar formuliert, aber wie in der OEGG-Richtlinie ist die Durchfuehrung von Messungen auch im EC7 essentiell. In Oesterreich ist seit langer Zeit ein umfangreiches Messprogramm bei allen Tunnelvortrieben eine Selbstverstaendlichkeit und Teil des Sicherheitsmanagements. Zusammenfassend laesst sich feststellen, dass in der OEGG-Richtlinie einzelne Schritte zur Durchfuehrung der Beobachtungsmethode wesentlich detaillierter beschrieben sind als im EC7. Die OEGG-Richtlinie erfuellt alle Anforderungen des Eurocodes hinsichtlich der Beobachtungsmethode. Damit ist die Kompatibilitaet zwischen den beiden Regelwerken gegeben. ABSTRACT IN ENGLISH: The Austrian Society for Geomechanics has published a guideline for the geotechnical design for underground structures with conventional excavation in 2001, which was revised in 2008. The guideline clearly focuses on the observational approach due to the uncertainties in the ground model and ground parameters in the design stage. In this paper the requirements stipulated in the Eurocode regarding the observational method are compared to the respective formulations in the guideline. (A)

THE ROLE OF ON-SITE ENGINEERING IN UNDERGROUND PROJECTS

In the planning and design phases of underground structures, the information on geological setup, the rock mass structure and characteristics necessarily is incomplete and inaccurate. To allow for a safe and economical construction, a continuous updating of the ground model and an adjustment of the construction methods to the actual site conditions is required. For a smooth construction process, the conditions ahead of the face have to be predicted, and the ground surrounding the tunnel characterized. Based on this updated model the ground behaviour can be assessed, and the final layout of the construction determined. The expected interaction between ground and support (system behaviour) forms the basis for the monitoring program and the safety management plan, which includes warning and alarm criteria. As all decisions on site have to be made quickly, data acquisition, processing and analysis have to be well organized. Highly qualified and experienced geotechnical personnel, as well as appropriate site organization and contractual conditions are required to allow for a short reaction time to changing conditions. A number of tools and methods have been developed, which contribute to a more reliable assessment of rock mass structure and behaviour, which again enables a more precise determination of excavation and support methods. Digital stereo photos allow a precise evaluation of the rock mass structure, while advanced software for the evaluation of displacement monitoring data and prediction of displacements assists in predicting and controlling the performance of the underground structure. Up to date methods of monitoring data evaluation and interpretation will be demonstrated with the help of case histories. A key issue is the accurate prediction of the displacements in their development and final magnitude. Appropriate software can support engineers in assessing displacements and stresses of tunnel supports. The experience of on-site personnel in general is limited, and may not cover specific problems encountered on site. In the past experts had to be brought to the site and briefed on the conditions to solve such problems. This is slow and inefficient, as the level of information might be not sufficient or time consuming to upgrade. With the Internet nowadays an exchange of information is easy, allowing experts to give a profound advice, even if they are not on site.

Application of numerical simulation at the tunnel site

Since the tunnel engineer often has to respond quickly to unexpected ground conditions, rapid results from numerical simulations performed on site could serve as a tool, which assists important decisions. One object of the project was to evaluate the efficiency and applicability of 2D and 3D finite element methods (FEM) and boundary element methods (BEM) to simulate tunnel advance under different geotechnical conditions. Another task to be performed during this project was to make site data available to other projects within the JRI.

Tunnelling - the need for technological development and innovation

Tunnelling at a larger scale started in the 19th century with the development of the railway systems. In the beginning the practices used in mining for excavation and support were adopted also for the traffic tunnels. In contrast to tunnelling, in mining only a rather small portion of the underground structures is designed for permanent use. The rock mass conditions in mines are usually well known due to the long experience gained during the lifetime of a mine. This experience of excavation under the same or similar conditions explains also why in mining the layout of the excavations and the required support is mainly determined in an empirical way.

Improved site investigation Acquisition of geotechnical rock mass parameters based on 3D computer vision

Acquisition and evaluation of geotechnical data are integrated parts of subsurface and surface construction works. Geotechnical data serve as input for decision making processes during all phases of projects, ranging from feasibility studies to construction and maintenance. The present system of data acquisition, specifically applied during underground construction works, has a number of constraints. Sampling bias may be caused by the “human factor” of individual capabilities, inaccessibility of the rock exposure and time limitations. In most cases data are irrecoverable when excavation proceeds or support has to be applied. Data processing and evaluation is time consuming so that input data for numerical calculations cannot be provided on a daily basis. To overcome the listed shortcomings a digital stereoscopic colour imaging system has been developed which enables the evaluation of a large number of geotechnical data by interactive two and three dimensional image analysis. Among others the data can be used for the innovative modelling of the rock mass structure, for the provision of geometrical input data for numerical simulations performed on site as well as for a descriptive visualisation of complex structural conditions. The developed hardware and software components have been tested in different environments and on different rock mass types to investigate their general suitability and effectiveness. It was found that digital stereoscopic imaging and image evaluation are suitable for a cpmprehensive and reproducible documentation of the structural inventory of rock surfaces, and are most effective for acquisition of geotechnical data.