Engineering geological challenges related to shaft excavation - A case study of Nye Verma hydropower project
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In hard rock provinces like Norway, unlined pressure shafts and tunnels have proved to be the most economical option. However, selecting the right type of underground pressure conduit system and excavation method is still the one of the most challenging parts of the project development for engineers. Specifically, when it comes to shaft construction, careful evaluation including extensive engineering geological investigation should be done before finalizing the design, excavation methods and rock supports. Pressure shafts can be designed in various inclinations but this may influence the stability, choice of excavation methods and eventually the rock support requirements.This will affect the overall project cost.In this report, discussion on recent unlined pressure shaft design approaches, shaft excavation methods and stability issues regarding shafts excavation have been done. As a part of case study,evaluation of old and revised shaft design of Nye Verma hydropower project (22.5 MW) from Møre og Romsdal, Norway has also been done. Numerical modelling of the shaft usingRS2/Phase2 has been carried out in order to analyze the shaft stability and rock support requirements. The study showed that both of the shaft designs are safe against hydraulic splitting or fracturing although vertical shaft showed higher factors of safety for all design criteria. Several empirical relationships showed that the project area has average horizontal to vertical stress ratio of 2 to 2.5. This high horizontal stress ratio might have meaningful impact on the stability of the underground components such as pressure shaft. This high horizontal stress components resulted both the shaft options could be prone to high rock burst activity but it is more severe in case of vertical shaft. It was also found that the rock mass from project area is highly foliated granitic gneiss and it contains large amount of dark biotite minerals. This might be responsible for significant reduction of strength when test cores have axis perpendicular to shcistosity. Point load testing showed that the rock inthe excavation site has considerable strength anisotropy. The point load strength of rock when core axis is perpendicular to the schistosity is only 2 MPa which is one third of the strength when test core axis is parallel to the schistosity in diametral testing. Verma river valley itself could be the one and only weakness zone or discontinuity in the area. However, upon evaluation, it showed that this discontinuity is dipping with mild slope of 18◦away from the shaft construction area. This con-firms that inclined shaft as well as vertical shaft has no considerable effect of this zone of weaker rocks. This should be confirmed by core drilling in the project site. Since, each of the shaft type can be excavated using varieties of methods, both of them are equally possible from technical point of view. Each methods have their own advantages and disadvantages. For a specific project only some of these methods could be feasible but the final selection could be done after financial analysis of these possible methods at the end. Use of tunnel boring machine, full face drilling and raise boring with upward reaming methods are the most favorable methods to excavate small inclined shafts up to 45◦inclination. Whereas in case of vertical shafts of similar dimensions to that of Nye Verma, excavation using a shaft boring machine, full face drilling using drilling machine and raise boring with upward reaming are the most suitable methods. Alimak raising could be an alternative in both vertical and inclined shaft case but should only be chosen as the last option when all the above mentioned methods are not available or too expensive. On the other hand, drill and blast method such as Alimak raising may need more rock supports compared to other mechanical methods which should also be included in the analysis of choice of methods. Wire sawing could only be used to excavate shallow shafts with rectangular or square section and it is impractical to excavate circular shafts using this method. The choice of excavation methods could have high influence in rock support requirements. While using TMB in high rock burst area, proper rock support plan and preparation should be done beforehand as it could affect the manual works behind the cutterheads.