Caballo Rojo Mine Geologic Evaluation & Dewatering

Caballo Rojo Mine Geologic Evaluation & Dewatering
Caballo Rojo Mine Geologic Evaluation & Dewatering
Kennecott Corporation
Campbell County, Wyoming

The Caballo Rojo Mine is an active surface coal mine located in Campbell County, Wyoming. The mine was owned and operated by Kennecott Corporation and mined the Wyodak-Anderson Coal Seam in the Tertiary Wasatch-Fort Union Formations. The overburden bench consisted of saturated channel sands with interbeds and facies deposits of siltstones and shales. As mining depth increased away from the scoria outcrop, the saturated channel sands became more problematic and resulted in mass wasting and slumping of overburden into the mine pit and exposed coal seam. The geotechnical instability raised mine safety and operational concerns.

Wenck was retained to develop a dewatering program for the overburden to reestablish highwall stability. Wenck had previously completed a MODFLOW model of the Tertiary system underlying the mine. The initial data for the pit dewatering model were derived from Wenck’s three-dimensional MODFLOW model.  Wenck worked closely with the mine geologists to develop a drilling program to characterize the problematic overburden channel sands. Following completion of site specific aquifer testing and with the newly modeled overburden geology, Wenck developed a mathematical approach to determine dewatering well spacing to provide in-situ dewatering of the sands.

The model encompassed and area of approximately 1,044 square miles defined by a variably spaced grid. Vertical layers included in the model were the Wasatch Overburden and the Wyodak-Anderson Coal. Data from various sources were discretized to define the vertical layers, beginning potentiometric surfaces, aquifer characteristics, and drain conditions. Mine sequences of the four mines in the grid were included in the discretization process in order to determine the extent of open pits and backfill conditions at any given point in time. Aquifer characteristics obtained from long-term pump test data from each of the mines were included within the model.

The MODFLOW model was calibrated using the pre-mining potentiometric surface to verify that the data inputs were applicable. Upon calibration, the transient condition was validated using water surface data obtained from the Gillette Area Groundwater Monitoring Organization. Mining operations were simulated by varying the aquifer characteristics in the backfilled and open pit cells during the simulation period. Cells containing open pits were treated as drains with extremely high conductance. Impacts to the groundwater table were then projected for the term of permit and the life of mine. Cumulative impacts of the four mines were predicted as well as those of the Caballo Rojo mine alone.

A dewatering plan was developed and implemented which included dewatering rates, construction of new wells and established pumping frequency. Pumped water was delivered to Facilities Pond and ultimately incorporated into the Caballo Rojo dust control program. The dewatering program was submitted to and approved by the Wyoming Department of Environmental Quality, Land Quality Division, the Office of Surface Mining and the Wyoming State Engineer. Slope stability was achieved and issues with Mine Safety and Health Administration were addressed.

Major Project Features: 

  • Geologic Evaluation
  • Data Analysis
  • Aquifer Testing and Groundwater Modeling
  • Permit Document Preparation
  • Groundwater Modeling
  • Cumulative Impacts Assessment
  • Drawdown Predictions
  • Data Analysis

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