Passive Vertical Drains for one of the largest Copper Mines in the World1
Special foundation engineering elsewhere:
Passive Vertical Drains for one of the largest Copper Mines in the World
Dipl.-Ing.HolgerItzeck
BAUER Resources Canada Ltd., Edmonton, Canada
Dipl.-Ing.Verena Schreiner
BAUER Resources Canada Ltd., Edmonton, Canada
1Summary
In the vicinity of Kamloops in the Canadian province of British Columbia, the mining company TECK Resources Limited operates one of the largest copper ore mines in the world. During operations for extending the life of the mine, problems pertaining to the stability of the mine slopes were encountered. For the "Big Bear Pushback", special measures were required to ensure the stability of the pit walls. BAUER was commissioned to drill 27 passive drainage wells, viz. "Passive Vertical Drains". For this purpose, bores with a diameter of 1,200mm were drilled down to a depth of around115m and subsequently filled with a filter material specially made for this purpose. The objective was to drain water from the landslide-prone slopes and to discharge it into a deeper, water-conducting layer. The holes drilled are amongst the deepest holes ever drilled using the kelly-drilling method and could be drilled with the equipment available on site only after some adjustment were made by using a little trick. This presentation deals with the basic concepts of slope stabilization and the special challenges of such types of boreholes. Furthermore, the authors attempt to offer an insight into mining in Canada.
2Copper mining in Canada
After iron and aluminium, copper is one of the most widely used metals and constitutes an important economic factor for producers and consumers. Copper is primarily used in the electrical industry, in communication technology and in construction.
Fig. 1: World copper production 2010[16]
Although in some countries, a remarkably large amount of copper is obtained from recycling processes – around 45% in Germany – worldwide around 15.9 million tonnes (2010) of copper are extracted and processed in mining operations. Chile is by far the largest producer of copper ore, followed by the Peru, China and a number of other countries, which contribute quite a considerable amount to the world production. Copper ore is thus a frequently occurring material worldwide (see Fig. 1). Over the years, Canada's ranking in this list has varied depending on the source. In any case, with an annual production of 525,000tonnes, Canada is one of the 10 largest producers of copper in the world.
After a slump caused by the financial crisis of 2008, copper prices once again reached new heights owing to the boom in the demand for raw materials worldwide at the end of 2010 (Fig.2) and has only been going down slightly since. Copper mining has thus once again become a lucrative option.
Fig. 2: Copper prices in the last 5 years [7]
3Economic considerations in open pit mining
3.1Strip Ratio
The "Strip ratio", i.e. the mass ratio of the extracted ore to the mass of the waste rock be removed and displaced in order to extract the ore, plays an important role in determining the economic viability of an open-pit mining operation. The general angle of the slope and the installation and execution of berms are the determining factors (see [3]). Figure 3 shows an example of the effect of the strip ratio – in this case, for a coal mine – on the economics of a mining operation. Depending on the value of the extracted material, the rates – of coal in British Columbia and iron ore in Quebec to uranium ore in Saskatchewan – vary significantly from 1:3 to 1:100 (see 6th HLS). For economic reasons, the objective was to optimize the construction of the mining slopes in such a manner that while ensuring the stability, a steepest possible slope can be created and a favourable strip ratio can be achieved.
Fig. 3: Impact of Stripratio on profitability (as per[3])
The biggest cost factor for an open pit mine are generally the vehicle fleet and the facilities for processing and treating the extracted material. Therefore, uniform utilization of the equipment and facilities has high priority. Continuous operation is an important prerequisite for the economic use of the multimillion dollar vehicles, crushing plants and ore treatment facilities.
Fig. 4: Mining operation: Constant strip ratio (as per[3])
Any unnecessary interim storage or additionally needed movement of enormous masses of mining waste or ore has significant economic consequences. Also, supply contracts with buyers on the world market are very often long term and based mostly on continuous supply. Fig.4 shows a simplified mining plan and Fig.5 the corresponding production rate and transport vehicle utilization for the intended uniform mining operation. Needless to say, such plans are liable to change depending on fluctuations in demand in the markets, discontinuities in deposits or other influences, all of which cannot be accurately predicted at the beginning of operations. This especially includes factors arising from geological or hydrogeological conditions for the operation of an open pit mine.
Fig. 5: Uniform utilized capacity for a constant stripping ratio
4Slope Stability
A number of calculation models are used to determine the stability of a slope. These are based almost exclusively on 2D and 3D observations and comparisons of active, pushing forces and passive, holding forces along theoretically given slip surfaces which follow circular paths or spirals. The resulting values are set in relation with each other and a safety factor is determined. When using this procedure it is clear to all parties involved that the in-situ conditions cannot be reproduced 100% by a model. Many factors such as rock stresses, cracks and crevices, groundwater fluctuations, accumulation of surface water and other climatic influences are difficult to estimate, but have a significant impact on the slope stability. In addition, the use of heavy equipment and the often very intensive use of explosives in daily mining operations can lead to slope failure. In British Columbia, there are additional considerations pertaining to earthquake events.
Fig. 6: Effect of water pressure on movement of the slope (Smreka iron mine) [9]
In practice, therefore, slopes formed on this basis are generally monitored through continuous geodetic measurements at least. In the Highland Valley mine, around 500(!) measuring points are continuously monitored. Complex evaluation systems can immediately help detect unusual movements. The number of measuring points might initially seem excessive, but is to be considered appropriate in light of the scale of open pit mining operations. Because of the diameter and depth of the pits, the accuracy and reliability of the measurement data obtained during on-going mining operation are not always completely unambiguous and reliable from the beginning. Owing to a continuous reduction in potential sources of error, however, this monitoring has meanwhile become the basis for control activities in the field of slopes (see [11]). These results are complemented by further measures and facilities for observing processes in slope-areas that are several hundred meters high. Extensometers are used to check for cracks and crevices, while piezometers are used to monitor the groundwater situation.
There is a good reason as to why water pressure monitoring has come to assume special significance. According to experts, an astonishingly high number of mine-slope failures can be attributed to the effects of water. Figure 6 shows how an increase in pore water pressure can quickly lead to slope movements that are difficult to control. In approximately 40% of the cases evaluated by the Mandzic[9] method, water-related risks play a decisive role in the failure of slopes. Conversely, a reduction in water level or water pressure in a slope can improve safety to a significant extent. This is also what the engineers in charge of the Highland Valley Mine do in order to stabilize the east-side side mine slope.
5The Highland Valley Copper Mine
5.1Type of copper deposit
Copper deposits in the Highland Valley Copper mine are of the porphyry type. Porphyry deposits typically possess low quantities of copper, ranging from 0.4 to 1.0%, but owing to their large volume, they are the major sources of copper in the world. These deposits usually also contain small amounts of other metals such as molybdenum, gold or silver. In the Highland Valley Copper Mine, in addition to copper, molybdenum is also extracted and treated.
The estimated copper ore deposit in the Highland Valley mine (as of December 2011, [10]) is 673 million tonnes, with an average copper content of 0.2% and a molybdenum component of 0.008%. These estimations are not final; they are regularly updated on the basis of results from exploration works.
5.2History
The history of the Highland Valley Copper mine dates back to 1962, with the start of the mining operations in the Bethlehem Copper Mine. Until 1982, copper was extracted from three open pit mines there. The Lornex ore deposit was discovered in 1963 and in 1970, stripping of the overburden in order to reach the ore deposit was started. In 1972, the Lornex ore was smelted for the first time in the copper processing plant. Copper ore deposits in the valley were detected in 1964 and stripping of the waste rock began in 1982. The Highmont Operating Corporation commenced copper mining operations in 1979, and operations continued until the mine was closed down in October 1984. In January 1989, the copper processing facilities of Highmont Mill were added to Lornex Mill's existing operations. In 1986, the Lornex Mining Corporation and Cominco Ltd. came together to form the Highland Valley Copper partnership; in 1988, the Highmont Operation Corporation joined the partnership. [4] Today, the Lornex and the Valley open pits are still in operation. With a depth of 800 m and a diameter of around 3 km, the "Valley Pit" (see Fig.7) is the largest open-pit mine in Canada and one of the largest open-pit mines in the world.
Fig.7: Valley Pit, 2010
5.3Production numbers
In 2010, a total of 42,488 thousand tonnes of copper ore with an average copper content of 0.27% was extracted from the Valley and Lornex pits. At a recovery rate of 86.3%, approx. 98.5 thousand tonnes of copper was produced and marketed. In addition to copper, 3,130 tonnes of molybdenum was extracted. [15]
Despite the relatively low grades of copper and the temporary reduction in extracted volumes owing to stability problems with the mining slopes, good returns could be obtained because of the high price of copper.
5.4Copper ore extraction
Copper ore is extracted by means of the so-called 'truck and shovel' method. After the ore body is uncovered underneath the overburden, it is loosened by blasting; the crushed rock is loaded by means of electric shovel excavators onto haul trucks that can transport up to 250tonnes of ore in a single load. The trucks transport the ore to one of the semi-mobile crushers, which crushes the rock into small pieces with a maximum particle size of 165mm, and the crushed ore is transported to the processing plant over a 2km long conveyor belt system.
5.5Copper ore processing
The pre-crushed copper ore is transported from the crusher to the processing plant, where it is pulverized to a particle size approximately equal to that of fine sand. The pulverized ore is transported to the flotation tank. Copper and molybdenum are separated from the rock matrix here. In another flotation process, the copper concentrate is separated from the molybdenum concentrate. The concentrates are filtered, dried and then prepared for shipping. The tailings are transported through pipes to the tailings pond of the mine.
6Expansion of the Valley Pit
In 2007, a "Mine Life Extension" was announced by Teck. The mine life was to be extended from 2013 (originally) to 2019, during which a further 247 million tonnes of copper ore are expected to be extracted. This goal was to be realised through expansion of the Valley open-pit. In order to be able to begin mining in 2009, a substantial additional investment in equipment was necessary. [14]
Fig.8: Slope failure in the Valley Pit in June 2009
During the expansion of the Valley Pit, the eastern slope slid in June 2009 (see Fig.8). During the expansion of the open-pit mine, a basin of glacial and fluvial deposits – the so-called Highland Valley – surrounding the ore body was increasingly exposed (see Fig.9). Around 200m of water-bearing sediments were present above a layer of lacustrine clay (layer 10B). Due to the water problem mentioned in Section4, the stability of the slope decreased with the progress in excavation, ultimately leading to slope failure.
Fig. 9: Cross-section of the Valley Pit and the neighbouring Highland Valley
Subsequently, a number of immediate measures were taken. First, copper mining in the critical areas was stopped so as to not further endanger the slope stability. To reduce the load on the slope, removal of the overburden on the eastern periphery of the Valley Pit was commenced. A part of the removed material was moved to the foot of the slope to give further support. For a long-term restoration of slope stability, however, additional measures were necessary.
Fig. 10: Eastern slope of the Valley Pit before and after the dewatering program
A major part of load reduction on the eastern slope was achieved through a dewatering program involving a system of dewatering wells [5]. In addition to a number of conventional drainage wells from which groundwater is to be pumped, a large number of passive dewatering wells (Passive Vertical Drains) were constructed (see. [15]). Figure10 provides an overview of the complete program for depressurization of the eastern slope.
7Passive Vertical Drains as part of the dewatering program
BAUER Resources Canada Ltd. was commissioned to drill 27 passive vertical drains. These PVD wells should collect water from the upper layers and discharge it into the so called basal aquifer, a deep aquifer (schematic diagram see Figure11).
Fig. 11: Principle of a Passive Vertical Drain
7.1Technical description
The specified diameter of the wells was 1.2m. The final depth is between 80 and 115m below ground level. The target was to drill each well at least 15m into the "basal aquifer". The wells were drilled into the soil of the Highland Valley from two levels, 1070 and 1055m above sea level. The following geological formations were encountered while drilling: The top 40-55 meters of drilling is in zone 10A, a silt with fine sand and traces of organic material, clearly noticeable by its dark colour. Beneath it is a layer of lacustrine clay (10 B), partially silty, with an approximate thickness of 15m. Below the clay is a 20m thick layer of silty sand (10C). The basal-aquifer, sandy gravel with a quite varying composition was expected to be found at around 75-90m below surface. The wells had to be driven sufficiently deep into this layer to make sure the water from the upper aquifer could be discharged.
The depth of this aquifer varies considerably, in some areas itwas not encountered until 100m drilling depth. This was the reason for the new record depth for the BG36 in kelly drilling mode to be achieved in the course of the project.
Each borehole was supported by a double walled casing down to a depth of 30m. At greater depths, the borehole stability was ensured using a slurry fluid consisting of water and polymers (see section 7.2.2).
Fig. 12: Principle of the drilling procedure
7.2Scope of work and challenges
7.2.1Drilling depth
The deepest of the holes drilled was 114.5m below top of the casing, i.e. about 113m below ground level –one of the deepest holes ever drilled with a BAUER BG using the kelly drilling method.
7.2.1.1Kelly bars
Basically, there are two types of kelly bars; the friction kelly and the lockablekelly. Both are telescopic, the difference being that in a lockable kelly, the individual segments are force-fitted together by means of a sophisticated system consisting of lock bars, drive keys, grooves and locking pockets (see Fig.13). Thus, the entire crowdforce of the machine, around 35tonnes in case of the BG36, can be activated as pressure for the drilling tool. As against that, in the case of the friction kelly, there is only the dead weight of the kelly bar (here around 16tons) and the weight of the drilling tool. The power transmission between the individual segments, takes place, as the name suggests, only through friction. Thereby, the crowd force can hardly be transferred onto the drilling tool. In simple words, you can imagine such a type of kelly bar as an upside down car antenna of an older design. In hard, dense soils it is difficult to achieve a good drilling progress using the friction kelly. However, these kelly bars are easier for the rig operator to handle as they don’t have to be locked/unlocked which, especially for longer bars, requires attention and experience. Friction kellys have a lower dead weight and - at the same maximum drilling depth - a slightly shorter overall length. This is important if the operating limits of the drilling rigs are reached, and the available mast length and the maximum capacity of the main winch become critical factors.
Fig. 13: Structure of a lockablekelly bar [1]
For the Highland Valley Copper Project, an 85m long friction kelly as well as a 40m and a 72m lockable kelly were used. Always, for the first 40m, the short kelly bar was used. Thus, the tool could be lifted over the casing (which was sticking out more than 5m above ground level at times, without any difficulty. Particularly in the10B layer, which mainly consists of very stiff clay, no significant drilling rate could be achieved using the friction kelly. The performance in the basal aquifer, which is made of relatively densely packed and gravelly sand, was not satisfactorytoo. Therefore, for greater depths than 40m, the 72m lockable kelly was used. To reach the required depths of up to 115m, kelly extensions had to be used after having reached the maximum depths of the kelly bars. It turned out that the drilling rate achieved using the 72m lockable kelly in combination with an extension was better than the drilling rate achieved using the 85m friction kelly. More on this in the following section.