Krishna Kanta Panthi

Evaluation of rock bursting phenomena in a tunnel in the Himalayas

Tunnels passing beneath deep rock cover (overburden) are subject to instabilities caused by induced rock stresses. In a relatively unjointed massive rock mass the instability is associated with rock spalling/rock bursting, while if the rock mass is weak, schistose, sheared and deformed, squeezing is more likely. This paper reports a study which reviews stress-induced instability along the Parbati II headrace tunnel, evaluates the rock mechanical properties directly linked to the stress-induced instabilities and back-calculates the magnitude of the in situ stress state using finite element numerical modeling. An attempt is made to evaluate the magnitude of the tectonic horizontal stress component and to estimate the rock burst depth-impact. It is emphasized that more cases of tunnel damage should be studied to verify the applicability of the proposed equations and to establish the approximate range of the horizontal tectonic stress component along the Himalayan chain.RésuméLes tunnels sous forte couverture rocheuse sont sujets à des instabilités résultant des contraintes induites à grande profondeur. Dans un massif rocheux de roche dure et relativement peu fracturée, l’instabilité se traduit par des écaillages et de la décompression violente. Dans un massif rocheux de roche tendre, schisteuse ou en zones de cisaillement, l’instabilité se traduit par de la forte convergence. L’article présente une revue des instabilités résultant des contraintes induites observées le long de la galerie d’amenée de Parbati II. Il évalue les propriétés mécaniques directement liées à ces instabilités et, par rétro-analyse, calcule l’état de contrainte in situ à partir d’une simulation numérique par éléments finis. Une tentative est faite pour évaluer l’intensité de la composante horizontale des contraintes d’origine tectonique et pour estimer la profondeur critique d’apparition des phénomènes de décompression violente. Il est souligné que davantage de dommages en tunnel devraient être étudiés pour vérifier l’applicabilité des équations proposées et pour établir les plages de variation de la composante horizontale des contraintes d’origine tectonique le long de la chaîne himalayenne.

Development of a 3D structural model of a mine by consolidating different data sources

Joints and faults are inherent parts of the rock mass. In the vast majority of mining slopes, discontinuity structures play an important role in slope stability and may trigger a slope failure. The most important step in understanding the slope failure mechanism is to have a reliable model, which shows how all the discontinuity sets are constituted in the rock mass and how they interact with each other. However, building a fracture model is not a straightforward process, since it needs to combine discontinuity information from a variety of sources, such as detailed slope mapping, borehole logging data and remote sensing technologies. Hence, this manuscript attempts to develop a comprehensive structural model of the complete mine area in an open pit, which is the biggest in Norway with respect to its depth and area of coverage. The manuscript demonstrates on how it is possible to consolidate information from different sources in order to identify typical orientation of the detailed fractures that are associated with the main structural lineaments. The process involves analysis of different sources of data in order to correlate this information into useful evidence about the orientation of the fracture systems in terms of dip and dip direction. Further, the mine is divided into different structural domains and a 3D structural model is developed. As an end result, the domains are kinematically tested with respect to different types of failure modes in both overall slope and bench slope scale of the mine for both a hanging wall and foot wall. It is highlighted here that the results presented in this manuscript are the part of the research project called “Decisive Parameters for Open Pit Slopes (DePOPS)”.

Assessment of the effect of stress anisotropy on tunnel deformation in the Kaligandaki project in the Nepal Himalaya

Most of the analytical approaches that are available for assessing plastic deformation in tunnels that pass through weak, schistose, and foliated rock masses assume an isostatic stress state. However, in situ stresses are seldom isostatic in a tunnel passing through a varying rock overburden. The work reported here analyzed the effect of stress anisotropy on the magnitude of plastic deformation in the Kaligandaki headrace tunnel in the Nepal Himalaya, where extensive deformation monitoring plans were implemented during tunnel excavation. Recorded tunnel deformation, mapped geological information, lab-tested rock mechanical properties, and an approach reported by Hoek and Marinos (Tunn Tunn Int 32(11):45–51, 2000) were used to estimate rock mass parameters. The convergence confinement method (Carranza-Torres and Fairhurst, Tunn Undergr Space Technol 15(2):187–213, 2000) was used to assess the effective support pressure for the known tunnel deformations of 77 tunnel sections assuming an isostatic stress state. Numerical modeling was carried out to assess the effect of stress anisotropy on tunnel deformation. The analysis indicated that CCM overestimates the magnitude of tunnel deformation. This may be explained by the fact that CCM applies for a circular tunnel in the isostatic stress state, which is seldom the case. Actual measured deformations were calibrated using numerical modeling to develop equations that may be used to estimate the plastic deformation of tunnels that are subjected to stress anisotropy. However, it should be emphasized that the proposed equations are based on the data records for a single tunnel, so further validation will be needed using data records of other well-monitored tunnel projects.

Analysis of the plastic deformation behavior of schist and schistose mica gneiss at Khimti headrace tunnel, Nepal

The Khimti headrace tunnel experienced instability related to tunnel deformations in several tunnel stretches through mica schist and schistose mica gneiss rock mass. The monitored convergence pattern indicated that the tunnel deformation continued for a long period of time after excavation. This paper aims to analyze plastic deformation behavior of the selected four headrace tunnel sections with considerable deformation where extensive deformation monitoring was conducted. The Convergence Law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech 24(3):145–154, 1987a) for estimating long-term time-dependent deformation, and the convergence confinement method proposed by Carranza-Torres and Fairhurst (Tunn Undergr Space Technol 15(2):187–213, 2000) for estimating tunnel convergences have been used as analytical tools for the assessment. The achieved results are compared, and similarities and differences are discussed. In addition, the results are also verified by three-dimensional (3D) numerical modeling. The achieved results indicated a fairly good match between measured, back-calculated and numerically modeled results. However, one should note that such calculations are highly sensitive to the accuracy and reliability of the estimated input parameters.

Himalayan rock mass and possibility of limiting concrete lined pressure tunnel length in hydropower projects in the Himalaya

In recent past, the development activity in hydropower sector got momentum in the Himalayan region. However, the investment cost is considered considerably higher than in other part of the world, and there is a strong need to look for alternative solutions that reduces the investment cost. Civil engineering part of the costs is one of the major areas where cost reductions could be made by involving todays construction technology. As it is well known, the use of underground space, such as waterway tunnels, shafts, and underground caverns, are inevitable in the development of hydropower projects in the Himalaya. Finding innovative solutions in underground construction is therefore a key issue where cost reductions can be achieved to improve financial viability. It is highlighted here that the traditionally used fully concrete lined waterway system has proven to be costly solutions; therefore, innovative solutions are needed to reduce the fully concrete lined length of the pressure tunnels. However, applied solutions must guarantee long-term stability and sustainability, cost-effectiveness, and construction time savings. This paper presents some design considerations that could be used to reduce fully concrete lined tunnel length for low to medium pressure waterway tunnels. Adoption of such an approach will help reduce overall construction costs and times. However, one should make sure that the prevailing rock mass conditions and applied tunnel rock support consisting of sprayed concrete and systematic bolting are enough to secure long-term stability and safety of the waterway system.

Estimating Tunnel Strain in the Weak and Schistose Rock Mass Influenced by Stress Anisotropy: An Evaluation Based on Three Tunnel Cases from Nepal

Total plastic deformation in tunnels passing through weak and schistose rock mass consists of both time-independent and time-dependent deformations. The extent of this total deformation is heavily influenced by the rock mass deformability properties and in situ stress condition prevailing in the area. If in situ stress is not isotropic, the deformation magnitude is not only different along the longitudinal alignment but also along the periphery of the tunnel wall. This manuscript first evaluates the long-term plastic deformation records of three tunnel projects from the Nepal Himalaya and identifies interlink between the time-independent and time-dependent deformations using the convergence law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech 24(3):145–154, 1987a, Int J Rock Mech Min Sci Geomech 24(3):155–164, 1987b). Secondly, the manuscript attempts to establish a correlation between plastic deformations (tunnel strain) and rock mass deformable properties, support pressure and in situ stress conditions. Finally, patterns of time-independent and time-dependent plastic deformations are also evaluated and discussed. The long-term plastic deformation records of 24 tunnel sections representing four different rock types of three different headrace tunnel cases from Nepal Himalaya are extensively used in this endeavor. The authors believe that the proposed findings will be a step further in analysis of plastic deformations in tunnels passing through weak and schistose rock mass and along the anisotropic stress conditions.

Slope stability assessment of an open pit mine using three-dimensional rock mass modeling

Rock mass classification systems are commonly used to evaluate the likelihood of instability in mining environments. Most frequently used classification systems are the rock mass rating (RMR), geological strength index (GSI), and the Q-system. These methods are used widespread among geotechnical engineers as one practical way to assess quality of the rock mass. In most hard rock open pit mines, bench faces with no clear discontinuities present one or two joint sets that may dominate on the failure mechanism. To address this, slope mass rating (SMR), which is a modified version of the RMR system of rock mass classification, can be used. The benefit is that SMR takes into consideration the influence of joint orientation on the classification method itself. Hence, the main aim of this paper is: first, to develop a three-dimensional RMR model of an open pit mine under active operation based on the extensive field mapping carried out, and then produce an SMR surface model that is derived using current topography, modeled RMR values and jointing conditions prevailing in each structural domain. The final goal is to create an SMR susceptibility map of the mine area for the dominant topographic condition and the main structural domains present in the open pit, and to present a methodology that may be easily replicated at any given hard rock open pit mine. The authors emphasize that the use of an SMR model is a very helpful tool in evaluating the areas of the mine that are most vulnerable to potential slope instability in different periods of operation of the mine.

Analysis on the Dynamics of Burst Debris Flood at the Inclined Pressure-Shaft of Svandalsflona Hydropower Project, Norway

Long-term stability of the waterway system of the hydropower plants is crucial and should not be underestimated. The compromise may result in severe economic consequences related to revenue loss caused by the plant closedown for needed repair, extra resources and time required for repair work, and third party loss related to industries and societies at large. In addition, possible contractual disputes between the clients and the contractors may arise in some occasions. Serious accidents may happen during repair and construction work with loss of life, since engineering geological environment (conditions) in the rock mass changed once under water for long period. This article focuses on one of the recent shaft collapse that happened in Norway in 2008. The article discusses and analyses the dynamics of burst debris flood that took place on 9 May 2009, while removing the slide rock mass deposited in the 45° inclined shaft of the Svandalsflona hydropower plant located at the Southern Norway. Careful review on the geological conditions inside the shaft, evaluation on the course of events, investigations on the inspections and inspections reports, assessment on the temperature and precipitation conditions have been carried out to come to the conclusion on what might have triggered the sudden burst flood.