Design parameters for the Engebø open pit: With focus on slope stability
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The objective of this thesis was to examine the geotechnical conditions at the Engebøfjell in Naustdal Municipality, Sogn og Fjordane, Norway with regard to a proposed open pit mine there. Another aim was to develop design parameters for the mine, i.e. bench angles, inter-ramp slope angles and overall slope angles. Finally, the economical ramifications of changes in the design parameters were examined.Geotechnical field mapping campaigns were conducted according to described standards and the results were processed by methods such as stereographic analysis in Rocsciences Dips 6.0. In addition, previously drilled cores were re-examined to extract geotechnical information, such as Rock Quality Designation (RQD). Using gathered data, the pit area was divided into geotechnical domains, and a kinematic analysis was performed for each domain. The analyses were carried out in Dips 6.0 as well. Empirical methods were applied to find supplementary estimates of overall slope angle and rock mass conditions. Rock Mass Ratings (RMR) were calculated for each of the domains. The Barton-Bandis failure criterion was used in conjunction with tilt tests to estimate the shear strength of the discontinuities in the rock mass. Scale corrections were applied to the Joint Roughness Coefficient (JRC) and the Joint Compressive Strength (JCS).It is concluded that three separate joint sets exist in the rock mass and that their dip-directions are: West (steep), North (steep) and SSW (low incline). The discontinuities have tight apertures with a moderate spacing and are commonly fresh and clean, however some staining was observed. The persistence of the discontinuities was concluded to lie in the interval 3-10 m. Results from the rock mass analysis indicated good rock mass quality. The following scale corrected results stem from the field mapping: JRC is approximately 7 and JCS=[56,115]MPa(depending on lithology). Basic friction angles were found to lie in the region [25deg,32deg], which implicated joint friction angles of [46deg},54deg}].Based on the kinematic analysis it is concluded that maximum stable bench face angles can be in the range of [60deg,80deg],while inter-ramp angles are [47deg,63deg]and overall slope angles are [45deg,59deg]. Different stability problems were expected to occur in different regions of the pit: the northern part was exposed to toppling, the south-western to planar failure and the south-eastern to both wedge and planar failure.The large scale stability of the pit has been discussed and it was concluded that large scale structures could affect the stability of the pit, and since these are likely to be parallel to the discontinuity sets further efforts should be directed towards uncovering such features. Drilling cores perpendicular to the expected orientations was recommended. It is noted that, should such features be encountered, the frictional properties of each one of them must be estimated.It has been shown that the benefits of steepening an open pit wall are significant for the project as a whole. Recommendations to couple sensitivity analyses of pit design parameters with the geotechnical investigations were made. In this way, the money that is set aside for geotechnical work will be put to the best possible use.