Rockfall Modelling Parameters and the Control Barrier at Stockton Mine, New Zealand

Solid Energy New Zealand plans to mine a 6 to 10m thick coal seam below the Mt. Augustus and Mt. Fredrick ridgeline at Stockton Mine near Westport, NZ. The coal is covered by up to 30m of overburden, which requires removal to access 4 million tones of high quality coal. However, the Coal Mining Lease boundary (CML) is located just below the basal coal measures and the neighbouring land is owned by the Department of Conservation (DoC). In addition, the neighbouring DoC estate is Powelliphanta Augustus snail habitat. It is necessary to remove the overburden without releasing any material above natural discharge levels onto the DoC land. In order to control the rockfall risk at the site, the largest design-build rockfall protection project in the southern hemisphere was constructed using a high-capacity dynamic rockfall barrier installed along the length of the ridgeline. During the design phase of the project, it was evident that current methods to determine the coefficient of restitution (normal and tangential) are subjectively based on the designer's judgement. Currently, there is limited quantitative information available for the determination of dampening coefficients (restitution coefficients) for use in rockfall computer simulation programs. Accurate parameters are necessary for the design and dimensioning of rockfall protection structures. This project investigates an objective method to calculate these parameters for use in rockfall modelling based on field measurements of the slope. The first stage of the project is a review of current rockfall simulation programs and rockfall mechanics. This is followed by a review of the design of the rockfall protection measures installed at Stockton Mine. The site is revisited and detailed investigations are performed to further classify the slope conditions and observe current ridgeline mining methodology and effectiveness. Included in this are detailed geotechnical investigations of the slope (soil and rock) materials. The majority of the slopes below the ridgeline mining are heavily vegetated. This project investigates the interrelation of rockfall and vegetation. A series of laboratory tests are conducted using rock and soil samples from the ridgeline-mining project. Overburden samples were cut into spheres and cubes to investigate the influence of shape and rockfall trajectory. A rockfall simulation device was fabricated to drop samples of various shapes onto rock slabs and soil beds. The drop test trajectories were filmed using high-speed video recordings and used for rebound calculations. The purpose of these tests was to observe the effect of impact angle (slope angle) and shape on the coefficients of restitution. Also investigated was the influence of soil moisture and density on rockfall impacts. Observations from the field investigations and laboratory experiments were then used to calibrate the original rockfall design parameters at the site. This included comparing several common commercially available rockfall simulation programs for trajectory analysis. Recent rockfall events that have occurred during the ridgeline mining were compared to original estimates of volume and block-size to actual rockfall events (both natural and mining-induced)