Determining the Mechanisms of Subsidence at a Dewatered Carlin Trend Underground Mine using Numerical Modeling Methods
Subsidence can be produced by multiple types of subsurface ground behavior, and is a potential concern in a mining environment due to the impacts this behavior can have on underground excavations. Subsidence was detected by InSAR methods in the region of the Leeville underground mining complex between 2004 and 2015. The Leeville operations are located on the Carlin Trend in northern Nevada, include extraction of multiple tabular orebodies, and primarily utilize longhole stoping methods with backfill. The measured subsidence forms an irregular shaped pattern that is much broader than the underground footprint of mining, with a trough that experiences higher displacements located over a mined-out area. The subsidence mechanism was investigated because it was unclear whether the subsidence was caused by mine dewatering or underground mining. Geologic, groundwater, and extraction data were reviewed to form hypotheses of the causes and controls of subsidence, which were then tested in a 3-D geomechanical model. A method to numerically simulate subsidence due to the coupled effects of underground mining and dewatering was developed, which involved starting with very simple elastic models, testing each mechanism or control incrementally, and eventually building in the appropriate mechanisms and controls into the model. The InSAR data was used as the main calibration dataset. The main findings of this thesis include: subsidence in the Leeville area is likely the result of two mechanisms, mining-induced deformations and compaction in the deep carbonate aquifer; geologic factors, such as low permeability features and hydrothermally altered/fractured zones, form controls on the subsidence pattern; and that yielding of the rock mass is likely producing higher strain magnitudes over stoping. This thesis has implications for understanding ground behavior and mine design at the Leeville operations and other underground Carlin Trend mines.