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

摘要

The objective of this thesis was to develop a comprehensive understanding of thegeotechnical nature of the proposed Cypress North Block Opencast Coalmine highwall inthe Mt William Range east of present mine operations at Stockton Opencast. Aninvestigation was undertaken to gather information on the rock material and rock massproperties of the Basement, Brunner Coal Measures, and Kaiata Mudstone stratigraphicunits that would make up the composition of the proposed highwall. The specific aims ofthe thesis were to identify the distribution of rock types and the locations and orientationsof mappable defects such as faults, joints, shears, and crush zones. The stratigraphic unitsare subdivided into their respective geotechnical units based on physical, and mechanicalintact rock material parameters.The basement lithologies comprised of interfingered layers of Greenland Groupmetasediments and intrusive Berlins Porphyry granite/granodiorite. These were dividedinto 3 geotechnical units where analyses of the rock parameters were determined. Theseunits (Berlins Porphyry, Greenland Group hornfels, & mixed basement) returned meanvalues 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), andmoderate UCS (78.8 -136.6MPa), tensile splitting strength (5.1 - 6.2 MPa), and cohesionvalues (6.38MPa).The Brunner Coal Measures are an alternating sedimentary sequence of massivesandstones, laminated sandstones, siltstones, mudstones, and coal that were divided into 5geotechnical units. Due to a lack of samples recovered from the two drillholes (DH 1694and DH 1717) that penetrated this layer limited results were returned. Testing wasconstrained to the coarse-medium grained lithology which showed high porosities(n=7.9%), and slake-durability index results (Id2=94.0% retained), and moderate frictionangles (33.2º), and dry densities (2411Kg/m³), and low strength characteristics with UCSintact 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 siltymudstone which a gradational contact with the BCM, this unit was therefore divided into 2geotechnical units. Due to the same constraints outlined above for the BCM testing wasconstrained 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 intactrock 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 WilliamRange adjacent to the Cypress North Block basin in an attempt to correlate the downholedata within the basement unit, as well as interpret discontinuity properties along theproposed highwall development. This was achieved by recording the rock mass propertiesand developing a kinematic analysis within the basement lithographies. The rock massproperties determined were; defect type, dip and dip direction, persistence, aperture, natureof infilling, defect roughness, and spacing.Joints are typically steeply dipping with mean joint set orientations in the northernregion of the ridge JS1 76°/041°, JS2 89°/261°, JS3 79°/118° (dominant set), JS447°/106°(where present), and JS5 85°/174°. Joint set in the southern section of thesurveyed area had mean orientations of JS1 78°/025°, JS2 70°/245°, JS3 84°/285°, JS443°/106°, and JS5 79°/161°.Structural domains were developed within the ridge crest using interpretation of thescanline survey and kinematic analysis to constrain the boundaries (along with physicaland mechanical properties),- with respect to both highwall orientation and the Mt WilliamFault. The fault is the major through going structure that is surmised to be the controllingfactor for defect formation propagation through the basement lithologies (and Tertiarysediments). These were further classified on the potential mode of failure after kinematicstability analysis was performed on the joints sets. Potential toppling failure on joints wasfound to be the dominant failure mode within the projected highwall orientation.