1.0 Introduction

For mining operations that are conducted below the water table, there are two important water-related problems that mine operators could face: groundwater inflow into an underground excavation or an open pit if the country rock is relatively permeable or, if the rock is impermeable, pore pressure affecting the stability of open-pit highwalls or underground excavation. For economic and safety purposes, it is important to be able to predict the nature and magnitude of these potential problems so that appropriate dewatering or depressurizing systems may be installed.

Numerical groundwater flow models are now routinely used to predict inflows and pore-pressure distributions for both open-pit and underground mines and to help design mine dewatering systems. Although adequate for addressing broad issues such as the impact of mining operations on regional water resources, available groundwater numerical codes are limited in their ability to quantify the more detailed problems of the phreatic surface in highwalls and inflows to both open pits and underground openings. These limitations arise primarily from the following:

  • How the flow domain is subdivided into finite, geometric pieces—the discretiza­tion used in a numerical model—could strongly affect the predictions of inflow and the shape of the phreatic surface near an excavation. Not only must discrete features, such as faults and contacts between different hydrogeologic materials, be included in the discretization, but predicting the essentially radial flow toward an excavation is more accurately performed by using small, approximately logarithmic mesh spacing. Finite-difference codes have an inherent limitation relative to finite-element discretization when applied to any problem that is hydrogeologically or geometrically complex.

  • The seepage face—the surface of the highwall of an open pit through which lateral flow occurs—is usually not properly estimated in most commonly used finite-difference codes. The height of the seepage face affects both the amount of lateral inflow and the height of the water table behind the highwall. A poor estimate of the height of the seepage face can introduce significant errors to the predicted inflows and pore pressures.

  • For slope-stability analysis, it is critical that the output of the pore pressure can be readily used in the geomechanical model. The pore pressure data from the groundwater flow model that are not compatible with the data requirements of the geomechanical model will delay the integration between these two models.

To overcome limitations such as those described above, Itasca Denver, Inc. (Itasca) developed MINEDW, a three-dimensional (3-D), finite-element groundwater flow code. The core of the code is based on algorithms previously developed by Durbin and Berenbrock [I] for the United States Geological Survey (USGS) code FEMFLOW3D. As of early 2018, MINEDW has been used successfully at more than 50 mines located throughout the world and in diverse hydrogeologic and climatic conditions. The code has been in use for approximately 30 years, and its predictions have been validated by field data collected over many years.

Since Itasca first commercially released MINEDW in 2012, Itasca has continued to improve the functionality of MINEDW. The current version, MINEDW 3.05, represents over 20 years of development and is commercially available.

Itasca would like to acknowledge the contributions from the past and current employees of Itasca and its predecessor, Hydrologic Consulting, Inc. Among them, Mr. Timothy Durbin and Dr. Lee Atkinson were instrumental in the inception and early development of MINEDW.