Artificial ground freezing for shafts and tunnels related to subway, water and sewer projects have been the focus of many articles and conference topics in recent years. Few readers are aware however, that this technique to provide temporary earth support and ground water control originated in the mining industry in the late 1800s. The first documented project was a coal mine in Wales. Another unknown fact is the magnitude of ground freezing projects on mining projects. This article reviews past mine projects throughout the world where ground freezing was essential for successful shaft sinking and surface drift construction. It mentions other innovative mine projects where again, it was necessary to adopt ground freezing to enable the projects to be undertaken and three of these are discussed in more detail. Projects of the magnitude in mining works require specialized design and analysis techniques that are reviewed. The construction methods and quality assurance programs are emphasized. The projects discussed show how innovative methods set records for projects on a very large scale.
Although patented by H. Poetsch in Germany in 1883, artificial ground freezing (AGF) was first adopted for mine shaft construction in South Wales in 1862 providing temporary ground support and ground water ingress control during sinking. Since then, the process has been applied globally in the mining field for deep shaft and surface drift construction. Many innovative projects have only been successful with the adoption of AGF; exemplifying the importance and substantial value of AGF in the mining world over many years.
UK coal mine shafts. The German freezing system was introduced into England about the year 1900 and successfully employed in several shaft sinkings (Neelands 1926). An English company, known as the Shaft Freezing Company with headquarters at Selby, Yorkshire, was formed to exploit the process. In 1912, Shaft Freezing Company was contracted to bore the freezing holes and carry out the freezing, sinking and lining of two 6.7-m (22-ft) ID shafts to be sunk to 500 m (1,640 ft) depth at Thorne Colliery, South Yorkshire. The freeze holes were drilled, cased and the inner inlet tubes installed, but activities ended due to World War I. The German operatives, unsuccessful in returning to Germany, were interned. New mine projects at the time were also halted for the term of the war to concentrate the producing pits and war munition factories. After the Armistice, in November 1918, the freezing process was abandoned due to difficulties reinstating the contract with the German company. The work was completed using the cementation process for ground stability and water ingress control.
Table 1 lists the deep mine shafts constructed in the UK between 1947 and 1960 (Wild and Forrest, 1981). The location and freezing depths are shown with the maximum freeze depth indicated as 268 m (880 ft).
During 1968-1974 two 5.486-m (18-ft) ID shafts were sunk to 1,150 m (3,770 ft) at Boulby, North Yorkshire, for the Cleveland Potash mine (Cleasby et al 1975). Presently they are the deepest shafts in the UK.
Two methods were used to overcome the highpressure saline water in the Sherwood Sandstone. The grouting and tubbng method was adopted for the No. 2 shaft and the freezing and steel lining method was used in the No. 1 shaft (Fig. 2). For the freezing method, the drilling of the freeze holes from the surface to a depth of 1,000 m (3,280 ft) and the deflection of these holes into positions later to be intersected by an underground freeze chamber at 590 m (1,935 ft) was successfully achieved.
Between 1977 and 1986, six shafts were sunk by Cementation Mining Ltd. for the Selby Coalfield Project in North Yorkshire, UK. These were at Wistow, Riccall and North Selby. Thyssen Mining (GB) Ltd sunk the other four at Stillingfleet and Whitemoor.
In the No. 1 shaft at Wistow, the freeze depth was 273 m (895 ft) to provide ground water ingress control through the Bunter Sandstone and Lower Magnesian Limestone plus ground stability control through the Basal Sands. In all the other cases, the freeze depths ranged from 148 m (485 ft) at the Wistow No. 2 shaft site to the deepest at 305 m (1,000 ft) in the case of Whitemoor to provide ground water ingress control through the Bunter Sandstone.
Canadian Potash Mine shafts. Of the 21 shafts sunk for the Saskatchewan potash industry since the 1950s, five early ones had major water inflow problems, and one had to be abandoned during the sinking process. To address the problem, ground freezing was adopted for the construction of subsequent shafts. Table 2 lists a number of these shafts. The greatest obstacle shaft sinkers faced was a succession of water-bearing formations, as many as 10 in some areas, all the way from the glacial till near the surface to the Dawson Bay dolomites just above the salts of the Prairie Evaporite Formation. Of these waterbearing formations, the one to prove the most difficult was the Blairmore (Fig. 3). It ranges in thickness from 60 m (197 ft) to 150 m (492 ft) and occurs at a depth from 375 m (1,230 ft) to 440 m (1,444 ft) in the Esterhazy area and from 520 m (1,706 ft) to 640 m (2,100 ft) west of Saskatoon. It consists of unconsolidated water-bearing sand, clay, shale and silt under pressures of up to 6.5 MPa.
Prior to 1963, the established freeze hole drilling technique was either a modified percussion method developed early in Europe or the standard rotary method with whipstocks. Both had severe limitations, even at shallow depths. A new technique could greatly improve the drilling efficiency, especially since depths in excess of 610 m (2,000 ft) were expected in the potash fields in Western Canada. In 1963 Precision Drilling Co. Ltd, together with Eastman Oil Well Survey Co. of Canada, undertook the drilling of a hole at Esterhazy, Saskatchewan, for International Minerals Corp. to test the feasibility of turbo-drilling in conjunction with rotary drilling for deep freeze hole application (Adamson and Storey, 1969). The overriding criterion in the test hole specification was the limited tolerance specified for deviation. A target of 305 mm (1 ft) radius over the entire length of the hole was chosen as the deviation limit for the exercise. The test hole was completed to a depth of 467.3 m (1,533 ft) within the target area, except for two short sections falling outside the 305 mm (1 ft) radius limit. Surveying of the hole was carried out at 9.1 m (30 ft) intervals with the Eastman single-shot magnetic equipment.
German coal mine shafts. The importance of ground freezing for shaft sinking in the West German coal mining industry is demonstrated by the 10 shafts constructed between 1980 and 1990 (Fig. 5). German ground conditions consist of unstable sands, silts and clays down to depths of around 600 m (1,969 ft) in some cases. They require a specially designed “sliding” lining system as temporary ground support to accommodate freeze wall deformation before the permanent lining can be installed upwards from the bottom (Fig. 4). The concrete blocks with chipboards (squeeze packs) allow the large freeze wall deformations to be carried while the inner lining is constructed.
Chinese coal mine shafts. Zhang et al. 2012, report that more than 600 shafts have been sunk in China using ground freezing for temporary support. The thickest sinking through alluvium was 587 m (1,926 ft) with a freezing depth of 800 m (2,625 ft).
Mine surface drifts
Selby Gascoigne Wood mine surface drift. The drift was driven through Basal Sands, known to be weakly cemented, and when water is allowed to flow through them, the sands also flow. To provide the necessary ground support and ground water ingress prevention, a single line of vertical freeze holes was drilled and kicked off alternatively to form a tent of frozen ground over the drift (Fig 6). At the time, the method adopted was believed to be the first of its kind in the world.
Innovative mine projects
Apart from the shaft sinking and surface drift construction projects, several innovative mine projects have been considered or successfully completed only with the help of artificial ground freezing:
- Underground Oil Platform, Alaska – Anaconda Minerals 1984. The concept was to recover heavy oil from the shallow deposits on the North Slope of Alaska by using artificial ground freezing to enhance the 609.6 m (2,000 ft) of permafrost. Shafts could then be sunk to a depth of 1,219 m (4,000 ft) to enable the oil to be extracted by drilling from an underground mine environment. The project did not go ahead because of the drop in oil price at the time and it became uneconomical to proceed.
- Aquarius Gold Mine – Timmins, Ontario, Canada.
- Underground heating of oil shale, Colorado Basin, Shell MIT Project.
- Ground stability and water ingress prevention for open cast mining of Oil Shale – Fort Hills, Alberta, Canada. In 2015 Suncor/TOTAL/ Teck were in the process of developing an open pit mine at Fort Hills for the extraction of oil sands. To prevent the ingress of ground water into the excavation, a cut-off barrier was being considered using ground freezing. This was a large-scale project involving many kilometres of freeze wall.
- Cameco Cigar Lake Uranium Mine, northern Saskatchewan, Canada – frozen ore body. Ground freezing from the surfacer has been used to stabilize the ore body at depth to facilitate retrieval of the ore by drilling from underground tunnels.
- Crown pillar excavation project, Quebec, Canada – Noranda.
Three of these projects are described in more detail in the following sections.
Aquarius Gold Mine – Timmins, Ontario. While many large-scale AGF projects have been conceived, the first field implementation (but not completed) was the Aquarius Gold Mine in 1996. The Aquarius property was originally owned by Asarco and started as an underground mine in the 1970s. High ground water inflows required Asarco to abandon the mine and it was sold to Echo Bay Mines. In 1996, Echo Bay proceeded with plans to mine the gold from a large open pit with a conventional approach using high-capacity dewatering wells around the 4-km (2.5-mile) perimeter. Hydrogeological studies indicated that this massive dewatering program had the potential for depleting the water in several small lakes at a provincial park adjacent to the project, as well as several residential wells.
A frozen earth barrier was proposed and installed around the 4-km (2.5-mile) perimeter. The ground freezing system had 2,335 individual 8.9-cm (3.5-in.) diameter freeze pipes into the underlying bedrock. The spacing between pipes varied depending on the depth to the underlying bedrock to compensate for deviation during drilling. In some locations, the bedrock was as shallow as 42 m (140 ft) but could be as deep as 153 m (505 ft) at locations on the east side of the project. Pipes at the shallower depths were spaced approximately 2 m (6.5 ft) apart, while deeper ones were spaced at 1 m (3.2 ft). Since the pipes deviation during drilling increased with depth, the shallower pipes would have less deviation and could be placed further apart at the ground surface.
The refrigeration system was based on two permanent buildings at the north and south ends of the frozen barrier. Each building had five 900-hp compressors for a combined capacity of 4.5 kt (5,000 st) of refrigeration. The large compressors used ammonia as the primary refrigeration gas that cooled the circulating calcium chloride brine. The circulating coolant system was a major engineering challenge. Each freeze pipe required a minimum of 20 gpm of the refrigerated calcium chloride brine. To ensure a balanced flow, it was necessary to have a supply, return and reverse return (balancing) distribution manifold.
As the installation of the ground freezing system was nearing completion, gold prices fell to below $300/ oz (U.S.). The freezing system was completed and tested and put into a standby mode. For four consecutive years the system was started up and tested. During that time, the gold price remained too low to justify the expense of operating the ground freezing system and mining the ore. It was eventually abandoned. While never fully operational, the Aquarius ground freezing system provided sufficient data to confirm that ground freezing systems could be installed on large scale projects.
Underground heating of oil shale – Shell MIT project. The Mahogany Isolation Project (MIT) was a pilot test conducted near Meeker, CO to evaluate the effectiveness of a frozen soil barrier used with high temperature heating of oil shale. Shell’s process used in situ heating of the oil shale that converts the kerogen to shale oil. Heating probes were installed into boreholes and warmed to approximately 662 °F (350 °C). This heating would result in the conversion of the shale to oil. After this conversion, the oil would be pumped to the surface. In the early stages of the testing, it was observed that the ground water present within sand seams in the shale would cool the probes preventing them from reaching the required temperature. Additionally, toxic by-products and gases would form requiring the isolation of the process from the ground water.
The concept of creating a frozen earth barrier around multiple probes was considered as a method to both prevent the inflow of ground water and isolate the toxic by-products until remediated. A pilot test was conducted to evaluate the effectiveness of a frozen earth barrier. The initial pilot test is shown in the drawings.
There were 18 freeze pipes, two temperaturemonitoring pipes and seven ground water instrumentation borings, drilled to depths of approximately 381 m (1,250 ft). Two heating wells and one oil extraction well were installed in the interior of the frozen cell. Freezing was completed using a total of 400 t (450 st) of refrigeration.
The remoteness of the site added considerable logistical issues for a ground freezing operation. Diesel powered generators were used to provide the 1,500 kW power required. The mobile refrigeration plants had water-cooled condensers requiring water to be delivered to the site daily.
The freezing process was longer than originally anticipated due to a geothermal gradient that had not been previously discovered.
Crown Pillar Excavation project – Noranda. The Quemont Mine in Rouyn-Noranda, Quebec, was completed and closed several decades ago. A crown pillar remained in place and was known to contain approximately 11,000 m3 of zinc. The deposit was located 24 to 37 m (78 to 121 ft) below water-bearing unconsolidated mine tailings and very soft clay. Mining from the surface had been considered for several years; however, excavation support was the limiting factor, both technically and economically.
After evaluating several open-cut options with very narrow slopes and potential dewatering, the concept of creating one large excavation was considered. The concept called for a large frozen earth wall to provide temporary earth support and ground water control.
The final design had a 61-m (200-ft) diameter circular excavation to a depth of 30 m (98 ft). Laboratory tests indicated that the clay material had a very high-water content and was susceptible to creep deformation when frozen. To compensate for the long-term creep potential, a 10-m (33-ft) thick frozen earth wall was designed. Additionally, the excavation time was limited to 120 days.
The freezing operation started in September and was specifically coordinated so that excavation would begin in early January when temperatures were known to be well below freezing.
Excavation proceeded from January through March. As the ambient air temperatures started warming, sloughing of the south wall was observed in an area that was exposed to direct sunlight. Large concrete blankets were hung form the surface to protect the face of the frozen earth wall. While they helped somewhat, ambient temperatures continued to increase as mining operations continued. During the early part of April, a severe thunderstorm occurred, and lightning damaged the transformer for the refrigeration plants. It was decided to begin backfilling and terminate the project with a minimal quantity of ore left in the excavation.
The review of previous mine shaft and surface drift construction on a worldwide basis has demonstrated how important the use of ground freezing has been in enabling the projects to be attempted and completed successfully. Innovation has also been a critical element in many mining projects, as demonstrated by the projects which have been reviewed. Without the use of ground freezing, none of the work described could have been achieved.
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Cleasby, J. V., Pearse, G. E., Grieves, M. and Thorburn, G. 1975. Shaft-sinking at Boulby mine, Cleveland Potash Ltd. Transactions/Section A (Mining Industry), Vol. 84, January, Institution of Mining and Metallurgy. pp. A7-A28.
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Klein, J. 1989. Shaft sinking by ground freezing in the coal-mining industry in the Federal Republic of Germany. Shaft Engineering Conference. 5-7 June. Institution of Mining and Metallurgy.
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Zhang, Y., Liu, Z. and Duan, B. 2012. Achievements of China’s coal mine construction in recent years. 3rd International Conference on Shaft Design and Construction. April. London. Tunnels and Tunnelling International. pp. 127-130