On a land where the water table is high and the gold concentration rich, Echo Bay Ontario Ltd. has determined to mine precious minerals from the ground.

To extract these minerals, while protecting neighboring bodies of water and preventing inflows of groundwater to the mine excavation, a frozen earth barrier will be constructed around the perimeter of the proposed Aquarius open pit gold mine in Timmins, Ontario.  The frozen barrier, created by converting the groundwater to ice, has been determined to be the most environmentally compatible technique to prevent massive groundwater inflows into the mine excavation.  The installation of this barrier leaves the groundwater table and surrounding lake levels virtually unaffected.

Layne Christensen Company's Elgin Exploration Ltd., out of Calgary, Alberta, has been awarded the contract by Echo Bay Ontario, Ltd. to design, install and maintain the frozen barrier.

This construction technique, referred to as Artificial Ground Freezing, has been in existence for over 100 years and is used extensively to provide groundwater control and earth support for excavations in the mining and underground construction industry. Layne Christensen Company is the largest ground freezing and water well drilling company in the world, with headquarters in Kansas City, Missouri and offices worldwide.

The frozen barrier is created by drilling and installing 76 mm. diameter pipes on 1.5 meter centers around the 3.5 kilometer perimeter of the mine. Approximately 2000 pipes will be installed to depths of 45 to 125 meters using conventional water-well drilling techniques. Once it has been confirmed that the barrier have been installed, they will be connected to an enclosed piping system that encircles the entire perimeter of the mine. This system will deliver a brine, refrigerated to –20'C, to each of the individual freeze pipes.

The brine will be cooled using 8 large 1500 hp compressors located in two buildings at opposite ends of the mine. This cooling system, referred to as the Poetsch process, is non-explosive, non-flammable and non-toxic. The system is identical to those used for food processing and ice rink refrigeration, although substantially larger. Once the system is initiated, it will take approximately 20 weeks to form the frozen barrier.

Complete formation of the barrier will be evaluated with a comprehensive system of electronic ground temperature and groundwater monitoring equipment. Individual sensors will be connected to a series of computers and telephone lines to measure, record, and plot the data on a continuous basis. Once it has been confirmed that the barrier is in place, the groundwater from within the pit will be pumped down with a series of high-production wells. During this pumping, groundwater levels outside the pit area will be measured every two seconds to verify that there is minimal influence to the surrounding water table. Once the pumping is complete, excavation of the pit will begin.

The ground freezing system has been designed to be in operation for eight years. During this time, monitoring of temperatures and groundwater will remain in effect 24 hours a day, 7 days a week.

Ground freezing was selected over conventional dewatering systems primarily since it has the fewest effects on the surrounding water table. Most mines of this type rely on an extensive system of pumping groundwater out of the pit as the excavation proceeds to bedrock. That pumping system remains in effect during the entire life of the project. Due to the sandy nature of the soils near the Aquarius mine, such pumping would result in significant depression of the groundwater and lake water levels for a considerable distance in the surrounding area .

Layne Christensen Company and its subsidiary, Elgin Exploration Ltd., have installed ground freezing systems throughout the world. All engineering, design and monitoring will be conducted by Layne Christensen Company's Milwaukee office. The drilling and pipe installation processes will be conducted by the Elgin Exploration Company.

Engineering Design

The design of the frozen earth barrier was governed by the thermal properties of the underlying soils and related response to the freezing system. The subsurface soils were categorized into three individual strata and the underlying bedrock. A general characterization can be summarized by two sandy zones separated by a 20-meter clay zone approximately 10 meters deep. Depths to bedrock vary from 45 to 125 meters across the site, with an average depth of 3 meters to the groundwater level.

While the rate of formation of the frozen barrier is limited by the relatively low thermal conductivity of the clay zone, the higher thermal conductivity of the sands and bedrock establish the required refrigeration loading.

Layne evaluated the thermal requirements of this project using a time-dependent finite element model to determine the pipe spacing between adjacent freeze pipes as well as the required refrigeration load and duration of the freezing process.

The above factors were then evaluated with electrical power requirements to optimize a technically appropriate design that was cost effective and met the schedule requirements of mine construction. The final design established a total of approximately 2000 freeze pipes spaced around the perimeter of the shaft. These pipes will be installed using mud-rotary drilling techniques.

Following the installation of each individual freeze pipe, it will be pressuretested to verify system integrity and surveyed with gyroscopic equipment to verify alignment.

The refrigeration and coolant distribution system are designed to deliver 5000 tons of refrigeration at -20'C. Two permanent refrigeration buildings will be situated on opposite sides of the mine. Each building will have four 1500 hp electrically powered compressors.

These compressors are computer operated and equipped with an extensive monitoring system. The estimated refrigeration time is 20 weeks, assuming that the silty clay layer will be the last to freeze. Unlike conventional frozen earth walls that provide structural support for excavations, this wall is only a groundwater barrier. Therefore, once the soil is frozen at the midpoint between all the pipes, the testing operation will commence.

The formation of the frozen earth wall is carfully monitored through an extensive data acquisition system. One hundred individual ground temperature monitoring pipes will be installed at planned locations around the perimeter. Approximately 1000 temperature monitoring devices will be placed within the coolant system to verify proper flow and balancing of the refrigerated coolant.

This system provides real-time information which can be closely monitored from remote locations in the United States and Canada

Once the barrier has been installed and formed, several wells within the pit area will be started to dewater the proposed excavation. After completion of the dewatering, excavation of the overburden material will begin. The excavation will be benched away from the perimeter to ensure slope stability.

The anticipated duration of the project is eight years. During this time, daily monitoring and evaluation of temperature, groundwater and the refrigeration system will be maintained.