N. Vlachopoulos

Full Scale Testing of Geosynthetic Reinforced Walls

The paper is focussed on the preliminary results of a program of full scale tests of geosynthetic reinforced retaining walls carried out by the Geotechnical Research Group at the Royal Military College of Canada (RMCC). The long term research program involves the construction of 10 walls. The paper describes the test program, some instrumentation details and presents selected results from the first four walls completed to date. Three of the walls were constructed with a column of dry -stacked modular concrete units and one nominal identical wall was constructed with a wrapped-face. All of the structures were surcharge loaded to stress levels well in excess of working load conditions. The data gathered from this program has been useful to identify important performance features of the reinforced soil structures and to identify possible sources of conservatism in current methods of analysis for geosynthetic reinforced soil structures in North America. The high quality data will also prove useful for calibration of numerical models.

Appropriate Uses and Practical Limitations of 2D Numerical Analysis of Tunnels and Tunnel Support Response

In spite of the gradual development of three-dimensional analysis packages utilizing finite element models or finite difference algorithms for stress–strain calculations, two-dimensional (2D) analysis is still used as the primary engineering tool for practical analysis of tunnel behavior and tunnel support performance for design—particularly at the preliminary stage of a project. The applicability of 2D finite element analysis or analytical convergence confinement solutions to staged support installation depend on the application of an assumed or validated longitudinal displacement profile. Convergence in commonly applied 2D staged models is controlled by boundary displacement or internal pressure relaxation. While there have been developments to improve this methodology, this often assumes independence between the ground reaction curve and the support resistance, independence between the longitudinal displacement profile to support application, and the assumption that non-isotropic stresses and non-circular geometries can be handled in the same way as circular tunnels in isotropic conditions. This paper examines the validity of these assumptions and the error inherent in these extensions to 2D tunnel analysis. Anisotropic stresses and lagged (staged) excavation present a particular problem. Practical solutions are proposed for support longitudinal displacement LDPs in simplified conditions.

Integration of Lidar-Based Structural Input and Discrete Fracture Network Generation for Underground Applications

In this study the authors present an approach of establishing and validating discrete fracture networks (DFNs) for underground projects using LiDAR (Light Detection and Ranging) as the source data. With the use of LiDAR in geotechnical and geological engineering becoming increasingly popular, it is necessary to establish the interactive application of this technology with other tools. Such a tool is the generation of DFNs and their integration into the geomechanical design, with a specific focus on underground projects such as tunnels, caverns, repositories etc. This paper attempts to show an approach in which LiDAR data from the Brockville Tunnel, located in Ontario, Canada, is used as the source for the determination of input parameters of DFN modelling based on manual and automatic mapping techniques. Having determined a representative set of input parameters, a deterministic DFN model is created in order to calibrate other modelling parameters associated with the generation process, leading to the creation of multiple DFN models. By employing the representative elementary volume (REV) concept, these models are used in order to examine the effect of the different joint sets on the estimated REV, and to introduce an approach of determining the required number of DFN realizations and the size of the DFN models.

The Effect of Jointing in Massive Highly Interlocked Rockmasses Under High Stresses by Using a FDEM Approach

In deep underground mines and deep infrastructure tunnels, spalling and strain bursting are among the most common failure mechanisms observed and reported in massive rockmasses under high stresses. Therefore, the need to be able to estimate such conditions and counter them with an economic and effective design is rising. Part of the common practice is the use of computer packages involving numerical methods based on continuum approaches. However, the failure mechanisms involved and the rockmass response observed are often difficult to capture by employing such methods and usually discontinuum approaches are better suited for this task. Additionally, discrete structures observed within the rockmass in situ, such as joints and other discontinuities, can be explicitly incorporated into the numerical model in order to investigate their effect on the overall rockmass response during an underground excavation and adjust the design if necessary. In this study, the presence of joints and their effect on the response of a hard rockmass under a high stress regime during an excavation is examined by employing a FDEM (Finite-Discrete Element Method) approach. The setup of the numerical model is based on the URL (Underground Research Laboratory) Test Tunnel located in Pinawa, Manitoba, Canada and a Discrete Fracture Network (DFN) model is implemented in order to simulate the impact of joints on brittle failure. Numerical results show that the presence of joints increases the intensity and evolution of the damage during the excavation.

Improvement to the Convergence-Confinement Method: Inclusion of Support Installation Proximity and Stiffness

The convergence-confinement method (CCM) is a method that has been introduced in tunnel construction that considers the ground response to the advancing tunnel face and the interaction with installed support. One limitation of the CCM is due to the numerically or empirically driven nature of the longitudinal displacement profile and the incomplete consideration of the longitudinal arching effect that occurs during tunnelling operations as part of the face effect. In this paper, the authors address the issue associated with when the CCM is used within squeezing ground conditions at depth. Based on numerical analysis, the authors have proposed a methodology and solution to improving the CCM in order to allow for more accurate results for squeezing ground conditions for three different excavation cases involving various excavation-support increments and distances from the face to the supported front. The tunnelling methods of consideration include: tunnel boring machine, mechanical (conventional), and drill and blast.

Detection of Rock Discontinuity Traces Using Terrestrial LiDAR Data and Space-Frequency Transforms

Part of the rockmass assessment and its application in numerical modelling, within the geotechnical engineering field, is acquiring information such as discontinuity number, density, intensity, size etc., which can be obtained by mapping fracture traces on exposed rockmass surfaces and processing of the recorded field data. Moving past from traditional mapping techniques in the field, fracture traces can be extracted from terrestrial light detection and ranging (LiDAR) point-clouds or LiDAR-derived surface models. However, similarly to field mapping, the extraction of fracture-traces is often done manually. This is an arduous and timely task in most cases. The automatic-detection of such traces is an emerging topic in geotechnical engineering; however, existing methods focus solely on the spatial domain. Space-frequency representations are ideal for detecting singularities due to their localization in space and frequency. Furthermore, they allow multiscale analysis, which is important for isolating LiDAR-data noise and weak traces in lower scales. In this study, three space-frequency transforms are evaluated, namely, (1) wavelet, (2) contourlet, and (3) shearlet. In addition, the well-known methods of Sobel, Prewitt, and Canny for edge detection are used for comparison purposes. The performance of the different edge-detection methods is tested using data collected from the Brockville Tunnel in Ontario, Canada. Numerical and visual assessment show that contourlets and shearlets achieve the highest agreement with manually-extracted traces that are used for validation. The two methods, along with minimal user interaction, can be used in order to increase the efficiency of rockmass mapping and geometric modelling in stability assessment of tunnels, mines, slopes, and related applications.

The Numerical Simulation of Hard Rocks for Tunnelling Purposes at Great Depths: A Comparison between the Hybrid FDEM Method and Continuous Techniques

Tunnelling processes lead to stress changes surrounding an underground opening resulting in the disturbance and potential damage of the surrounding ground. Especially, when it comes to hard rocks at great depths, the rockmass is more likely to respond in a brittle manner during the excavation. Continuum numerical modelling and discontinuum techniques have been employed in order to capture the complex nature of fracture initiation and propagation at low-confinement conditions surrounding an underground opening. In the present study, the hybrid finite-discrete element method (FDEM) is used and compared to techniques using the finite element method (FEM), in order to investigate the efficiency of these methods in simulating brittle fracturing. The numerical models are calibrated based on data and observations from the Underground Research Laboratory (URL) Test Tunnel, located in Manitoba, Canada. Following the comparison of these models, additional analyses are performed by integrating discrete fracture network (DFN) geometries in order to examine the effect of the explicit simulation of joints in brittle rockmasses. The results show that in both cases, the FDEM method is more capable of capturing the highly damaged zone (HDZ) and the excavation damaged zone (EDZ) compared to results of continuum numerical techniques in such excavations.

Application of Reliability Methods to Tunnel Lining Design in Weak Heterogeneous Rockmasses

Tunnel design in weak, heterogeneous materials such as flysch poses a variety of engineering challenges. The complex depositional and tectonic history of these materials leads to significant in situ variability in rockmass behaviour. Additionally, the alterations of sandstone and pelitic layers make rockmass characterization using traditional methods difficult. As a result, significant uncertainty exists in the ground response for a tunnel through such materials. Reliability-based methods can be used to better understand the impact this uncertainty has on convergence and tunnel lining performance. By assessing the impact of input uncertainty on ground response, the probability of failure can be evaluated for a given limit state. A quantitative risk approach can then be used to select the optimum design option on the basis of both safety and cost. This paper explores this issue further and presents a reliability-based, quantitative risk approach for the design of the Driskos tunnel along the Egnatia Odos highway in northern Greece.

Performance of Forepole Support Elements Used in Tunnelling Within Weak Rock Masses

The mechanics associated with forepoling structures explicitly, has never been fully investigated in order to determine the associated support mechanics when installed in isolation and/or in groups as an umbrella arch. Further, numerically, these support structures cannot be modelled using the commonly used, industry standard, two-dimensional (2D) numerical software packages. In numerical software using three-dimensional (3D) codes, these support elements are commonly standardized using pile or rockbolt simplified noded elements that do not truly describe their behaviour when subjected to the true 3D stress conditions that result at face or near the face due to the tunnel or mining excavation process. As such, a deficiency exists with regards to prediction of the interaction of umbrella arch support systems with forepoles element for tunnelling practices within weak rock masses. Methods have been developed in order to predict the behaviour of radial support systems to include temporary support elements such as rock bolts, steel sets, and liners such as the convergence-confinement method. However, these tools do not have the ability to capture the influence of support systems installed longitudinally at the face of tunnel such as forepole, and core reinforcement elements. In an attempt to improve tunnel design strategies, this paper will focus on the mechanical response of the application of the forepole element as part of the umbrella arch method installed in deep and shallow excavations; As well, other issues associated (influences) with the use of forepoles are also highlighted and discussed.