The geotechnical factors controlling the design of the open pit slopes of the Pasminco Century Mine near Lawn Hill, Queensland are presented below. Pasminco Century is a stratiform shale hosted zinc deposit in the Lawn Hill Formation which is overlain in part by the Thorntonia Limestone Formation. The deposit is located in a structurally complex regime that consists of a synclinal basin truncated by easterly striking reverse faults through its centre and northern and southern limits and by an unconformity on its eastern and western margins. Deposit geometry and slope stability is further complicated by small scale folding, a pattern of northeast and southeast striking subvertical faults and northeast dipping thrust faults. Fault emplaced blocks of Lawn Hill Formation occur in the limestone overburden. A staged mining approach has been adopted with the excavation of final pit walls around the deposit margins and interim pit walls along the internal boundaries between adjacent stages. Deposit geometry, major discontinuities and groundwater pressure controls the design of the interim and final pit walls. Two and three dimensional limit equilibrium stability analyses were conducted using both deterministic and probabilistic approaches to determine appropriate pit wall design criteria and to select a suitable excavation sequence and schedule. Interim western walls in the northern ore block are extremely sensitive to wall location, inclination and excavation schedule due to deposit geometry and low residual shear strength along tuffaceous clay beds.
Geotechnical assessment for design of the Pasminco Century Mine open pit slopes commenced in 1991 with the introduction of a basic geotechnical logging system and the establishment of a database for all cored boreholes drilled on the deposit. The level of geotechnical data collection and input into pit design increased as the deposit investigation advanced into detailed feasibility studies.