HCItasca Denver, Inc.

DEPRESSURISING AN UNDERGROUND ORE BODY AT THE MCARTHUR RIVER MINE IN NORTHERN SASKATCHEWAN, CANADA

Pre-mining depressurising of a deep ore body at the McArthur River mine in northern Saskatchewan ((1) below) was considered to decrease the risk associated with mining near 5 MPa water pressure and increasing the amount of ore that can be extracted. Currently, ground freezing is used to isolate the ore from this high pressure groundwater. However, this methodology only enables ore to be extracted from the lower portion of the Zone 4 ore body ((2) below) without additional freezing and utilizing the boxhole boring method. The potential of ground failure due to the high water pressure and low rock strength currently presents a challenge to extraction of a significant portion of the ore in the upper part of the Zone 4 ore body ((2) below).

(1) Location of McArthur River mine

Based on a limited amount of field data, a three-dimensional finite element groundwater flow model using MINEDW was developed to predict the amount of water that would have to be pumped from surface wells or extracted with underground drainholes to reduce the pore pressure of the upper part of Zone 4 ore body from 5 MPa to about 1 to 2 MPa and the associated magnitude and extent of drawdown that would propagate to the surface over the life of the operation and its impact on surface-water resources. In addition, the results from the preliminary model were used to design a prototype depressurising well and a 30-day pumping test. Because of the predicted large volume of water (about 2,000 m3/hr) that would have to be discharged, another still on-going investigation has been conducted to assess the effectiveness of localized depressurisation in improving mine tunnel stability in conjunction with ground freezing and cementaceous grouting

(2) Cross section of stratigraphy and ore body