Engineering Geology and Geotechnical Investigation of Highwall Stability of the Proposed Cypress Opencast Mine Mt William Fault Zone, Cypress North Block Upper Waimangaroa

• Main Author: Pehi, Te Ngere Russell
• Language: en
• Published: University of Canterbury. Geological Sciences 2010
• Online Access: http://hdl.handle.net/10092/3910

The objective of this thesis was to develop a comprehensive understanding of the geotechnical nature of the proposed Cypress North Block Opencast Coalmine highwall in the Mt William Range east of present mine operations at Stockton Opencast. An investigation was undertaken to gather information on the rock material and rock mass properties of the Basement, Brunner Coal Measures, and Kaiata Mudstone stratigraphic units that would make up the composition of the proposed highwall. The specific aims of the thesis were to identify the distribution of rock types and the locations and orientations of mappable defects such as faults, joints, shears, and crush zones. The stratigraphic units are subdivided into their respective geotechnical units based on physical, and mechanical intact rock material parameters. The basement lithologies comprised of interfingered layers of Greenland Group metasediments and intrusive Berlins Porphyry granite/granodiorite. These were divided into 3 geotechnical units where analyses of the rock parameters were determined. These units (Berlins Porphyry, Greenland Group hornfels, & mixed basement) returned mean values of low porosity (n= 0.8-2.3%), and slake durability index results (Id2 =99.0-99.6% retained), and high friction angles (40.6-44.5º), dry densities (2657-2666Kg/m³), and moderate UCS (78.8 -136.6MPa), tensile splitting strength (5.1 - 6.2 MPa), and cohesion values (6.38MPa). The Brunner Coal Measures are an alternating sedimentary sequence of massive sandstones, laminated sandstones, siltstones, mudstones, and coal that were divided into 5 geotechnical units. Due to a lack of samples recovered from the two drillholes (DH 1694 and DH 1717) that penetrated this layer limited results were returned. Testing was constrained to the coarse-medium grained lithology which showed high porosities (n=7.9%), and slake-durability index results (Id2=94.0% retained), and moderate friction angles (33.2º), and dry densities (2411Kg/m³), and low strength characteristics with UCS intact rock strength (15.3MPa), tensile splitting strength (1.32MPa), and cohesion (2.1MPa). The Kaiata Mudstone is a marine sedimentary layer comprised of a massive silty mudstone which a gradational contact with the BCM, this unit was therefore divided into 2 geotechnical units. Due to the same constraints outlined above for the BCM testing was constrained to the massive silty mudstone lithology which showed the highest porosities (n=9.9%), and greatest variability in slake-durability index results (Id2=34.2-94.5% retained), and the lowest friction angles (18.6º), dry densities (2.377t/m³), and UCS intact rock strength (9.9MPa), as well as low tensile splitting strength (1.47MPa), and cohesion (3.0 MPa). Scanline survey traverses were conducted along exposed areas of the Mt William Range adjacent to the Cypress North Block basin in an attempt to correlate the downhole data within the basement unit, as well as interpret discontinuity properties along the proposed highwall development. This was achieved by recording the rock mass properties and developing a kinematic analysis within the basement lithographies. The rock mass properties determined were; defect type, dip and dip direction, persistence, aperture, nature of infilling, defect roughness, and spacing. Joints are typically steeply dipping with mean joint set orientations in the northern region of the ridge JS1 76°/041°, JS2 89°/261°, JS3 79°/118° (dominant set), JS4 47°/106°(where present), and JS5 85°/174°. Joint set in the southern section of the surveyed area had mean orientations of JS1 78°/025°, JS2 70°/245°, JS3 84°/285°, JS4 43°/106°, and JS5 79°/161°. Structural domains were developed within the ridge crest using interpretation of the scanline survey and kinematic analysis to constrain the boundaries (along with physical and mechanical properties),- with respect to both highwall orientation and the Mt William Fault. The fault is the major through going structure that is surmised to be the controlling factor for defect formation propagation through the basement lithologies (and Tertiary sediments). These were further classified on the potential mode of failure after kinematic stability analysis was performed on the joints sets. Potential toppling failure on joints was found to be the dominant failure mode within the projected highwall orientation.