Heavy haul in the high north
24 Jul 2008 | Carolyn Fitzpatrick
CANADA: Within a few years, Baffin Island in the Arctic archipelago will see construction of the world's most northerly railway, currently being designed by Canarail. The harsh environment poses daunting challenges in terms of design, construction and operation.
Canarail project manager Carolyn Fitzpatrick is a civil engineer with 25 years of experience providing railway design expertise in North and South America, Africa, the Middle East and Asia.
Construction of the world's most northerly railway is on course to get underway in 2010, subject to regulatory approval. Baffinland Iron Mines Corp is developing an iron ore mining operation in the northern part of Baffin Island with direct shipping to serve European markets, where five steel producers have provided letters of intent to take 40% of the initial annual production.
In 1962 five high-grade iron ore deposits were discovered in the Mary River area, but they remained dormant until the market value for the mineral reached a high enough level to support their exploitation. Baffinland, a publicly-traded Canadian company, has now established a base camp at the site, and is in the process of extracting a sample 250 000 tonne shipment for testing in Europe.
The Mary River project is expected to produce 18 million tonnes annually for 20 years or more. Based on average prices of US$67/tonne for lump ore and US$55/tonne for fines, the project should generate a pre-tax rate of return of around 20%; payback period would be just 3·7 years. Pre-tax cash flow over the life of the mine is forecast at US$18·1bn, generating a return of US$11·2bn after tax.
The railway project
Although Milne Inlet, 100 km north of the Mary River mine site, offered the closest access to the sea, Steensby Inlet, 149 km to the southeast, has been selected as the preferred location for the port. The extra rail distance was justified by a longer ice-free period, which allows a 12-month shipping season.
In many ways the Baffinland project is neither complex nor exceptional. The ore is sufficiently high grade that no processing is required before shipment, so the mining operation is essentially one of large-scale quarry and crush. The product will be transported by rail to the port for loading year-round onto dedicated ice-strengthened ore vessels.
The single track railway is designed to haul 18 million tonnes of raw iron ore a year, well within the limits of what is being done elsewhere. What makes this project unusual is its geographic location - its isolation, the climate, and the permafrost. These factors, along with the logistics of delivering construction materials, will strongly influence design and operation.
It is essential that the railway is completed on schedule to avoid delay to other aspects of the project. Con-struction is expected to commence in the summer of 2010, with the first commercial shipment of ore at full production rates in the summer of 2014.
Regular shipping without ice-reinforced vessels is only available for six to eight weeks each summer, therefore all of the equipment and materials required to build and operate the railway must be delivered during four or five extremely short windows.
The Arctic environment
Baffin Island lies entirely above the tree line, most of it north of the Arctic Circle. The average annual temperature is -15°C, but -34°C is typical in February, with the minimum temperature sometimes plunging to -54°C.
The high latitude of 71°N gives three months of continuous summer daylight and a three-month winter of relative darkness, when the sun sets on November 18 and doesn't rise again until January 23. Only 20 of the 66 sunless days have 8 h or more of civil twilight, light enough to work safely outside without supplementary lighting, and at the winter solstice there are only 5 h. Therefore, it is planned that part of the open pit mining operation will shut down for a month around Christmas.
All of Baffin Island is underlain with continuous permafrost. At the Mary River site, the permafrost is approximately 400 m thick with a stable temperature of -10°C. An active upper layer of 2 m or more undergoes seasonal thawing and freezing but liquid water cannot flow beneath that depth, resulting in a poorly-drained environment.
The alignment
The presence of ice and the prospect of global warming make it difficult to build on permafrost terrain. Except for rock and clean gravels or sand, most materials heave or become unstable under the seasonal freezing and thawing. Clays and silts are very sensitive to thawing and, when combined with high water content, cannot bear human weight so they must be kept frozen or removed. Creep settlement of embankments built on ice-rich material is a big concern.
These conditions create challenges in constructing a subgrade that can bear heavy axleloads, and provide good drainage to limit water incursion and erosion. It is important to preserve the natural state of the frozen ground as much as possible for foundation stability and alignment integrity. Even slight alterations or damage to the permafrost can cause significant changes in soil behaviour, resulting in major maintenance problems.
The railway design has been driven by the need to avoid thaw-sensitive soils requiring protective embankments wherever possible, and to minimise cuts into glacio-fluvial sand and gravel terraces that often hide remnant glacial ice. These constraints, combined with the requirement to minimise the number of stream crossings and lake encroachments, mean the alignment cannot always follow the most direct route.
From the mine, the line will run eastward across a long series of sand and gravel terraces deeply cut by drainage channels from mountains to the north. It then angles south and requires a bridge of 245 m to cross a river in an area of significantly ice-rich soils. The route continues for about 30 km across a smooth plateau sloping gradually up towards the southeast. The area is mainly bedrock at or near the surface, with some deposits of ice-rich sands. Drainage varies from sheet flow to well-established watercourses.
At the south end of the plateau the route enters a valley and follows a tributary to a lake, which it crosses via a 432 m bridge at a natural constriction in the valley. It continues along the lake shore for about 14 km, through an area requiring tunnels and substantial benching in bedrock for the subgrade.
The line will follow the lake for another 13 km, across well-established sand and gravel terraces and benches. It then works its way southeast, avoiding several rocky hills and the waterlogged ground around small lakes, until it reaches Steensby Inlet.
Roadbed
To upgrade the bearing capacity of roadbeds constructed on thaw-sensitive soils, options include consolidation during the thaw season, construction of an embankment to keep the soil frozen, and removal of weak soil. Based on experience in Alaska, northern Canada, and Tibet, the most effective approach is usually a protective embankment that keeps the subsoil frozen. These can be constructed with ventilation to allow winds and breezes to cool the subsoil layer, either by using coarse rock fill or embedding ducts in the embankment.
Cuts in the active layer of the permafrost must be avoided, particularly on soils that are unstable when they thaw. Where such cuts are unavoidable, a protective layer of coarsely-crushed rock will be required across the entire cut surface. Properly designed culverts will also keep water and ice from the toe of the embankment and limit its exposure to freeze/thaw movements of the soil.
Bridges, tunnels and culverts
Given the size of rivers to be crossed, the presence of permafrost and volumes of water flow, five major bridges with a combined length of 1 400 m are planned, along with some shorter simple span bridges. Span lengths will depend on soil conditions, types of material best suited for the Arctic climate, environmental restrictions, and construction methods.
Since the mine will operate for at least 20 years, the recommended design life for the bridges is 50 years. The detailed design process will assess the potential deterioration of materials in an Arctic environment, including the durability of concrete and corrosion of metals; the performance of sealant, waterproofing, coating and other forms of protection; and the long-term performance of foundations in permafrost.
Standard Arctic foundation construction techniques, such as embedding piles in bedrock or the use of ad-freeze piles, have been assumed. Special consideration, especially for foundations, will also be given to the potential effects of global warming, which could increase the depth of the active layer of permafrost.
All the construction materials for the railway will have to be brought in, first by ship to Baffin Island, and then overland using temporary access roads. This requirement, along with the need to minimise manual work on site, will influence the design of the bridge spans. The severe northern climate dictates that pre-assembly be completed as much as possible in the south before the bridges are shipped.
Minimising manual work on site will help to keep costs down, given the expense of housing, food and transport, as well as the provision of extra work gangs to cover home leave. It is also an efficiency issue, as heavy clothing is required for most of the year. Heavy gloves make detailed manual work like bolting material together more time-consuming than it would be further south.
Two tunnels will have to be excavated for the railway, one of 800 m and the other of 250 m. As the permafrost extends to much greater depths than these tunnels, sections of them will have to be insulated to prevent the ground from thawing and re-freezing, as this could cause instability and rock failures.
More than 200 culverts will also be required. Protective methods will be used to minimise erosion, and to avoid ice jams at the inlets and outlets. For environmental reasons, clean stones will be installed in culverts to recreate fish habitats.
Track
Premium steel rails will not be used, because the material has an increased potential to fracture at very low temperatures. Regular carbon steel is preferred, with a very high premium on the cleanliness of the steel. For this project, a low-alloy rail with standard strength and a Brinell hardness in the range of 300 would be most appropriate.
Standard timber sleepers have been considered for the base design through the feasibility study; however, due to the extreme sensitivity of the Arctic environment, preservative treatments are likely to be limited to air drying only. Both steel and concrete sleepers remain options, pending research regarding their behaviour in the extreme cold.
Operations
As working conditions in the Arctic are severe, a conservative approach has been adopted when planning the railway operations. This includes parameters such as planned shutdowns, train lengths and speeds, and axle-loads. Rail operations will be halted if temperatures fall below -49°C, and also during a four to five week period around Christmas when many of the open pit mining activities are expected to shut down. An assumption of 300 operational days a year has been used for developing the railway operating plan.
The main line will be designed for a maximum speed of 75 km/h, but the initial operating speed is not expected to exceed 60 km/h. Temporary speed restrictions may be required over parts of the route during the warmest period between mid-June and late August.
The line is being designed to handle 32·4 tonne axleloads, but trains will initially be limited to 30 tonnes per axle due to the extreme nature of the Arctic environment. As more experience is acquired, axleloads may be increased.
To transport the 18 million tonnes of ore a year, an average of six trains will be needed on each of the 300 operating days. The line will also carry some mixed general freight traffic to supply the mining operation, including containers of ammonium nitrate amounting to 1 000 TEUs and 43 million litres of diesel fuel per year. A staff train will also operate between Steensby and the airfield at the mine site three times a week.
Rolling stock
Several types of locomotive have proven effective in conditions similar to but not quite as extreme as those found on Baffin Island. Both the EMD SD70 and GE Dash-9 have a good reputation for northern service, although AC locomotives have a better history of reliability in extreme cold.
Typical problems in northern operations are caused by a combination of snow ingestion and high humidity with cold temperatures, rather than extreme cold alone. Baffin Island's relatively low precipitation and humidity in the winter should obviate these problems most of the time. Locomotives operating in northern regions need specialised logic for the control of cooling systems, the provisions for which are already known.
One type of locomotive will be used for all operations, in order to reduce variations in technical expertise, spare parts and training.
No particular problems are foreseen in the acquisition of suitable ore cars although the specifications for rotary couplers will require special care, and the development and testing of a prototype in extreme cold conditions has been recommended. The ore cars will not need to be covered to prevent contamination of the ore during transportation. They will be top-loaded in motion, and unloaded by rotary dumping. Given the low annual precipitation, snow or ice accumulation in the open cars should not be a problem.
Next steps
Canarail completed its feasibility study in February 2008, and submitted its design brief in March. We are now working on the detailed design phase, which is targeted for completion by the end of 2009.
Pending regulatory approvals, Baffinland expects to initiate construction of the mine, the port and the railway in the summer of 2010. Mining production will begin in 2013 and the first commercial delivery of iron ore is expected to take place in the summer of 2014.
Including the mine, the railway, deep-water port facilities, an airport and accommodation for 500 employees, but excluding the cost of ships, the capital cost of the project is expected to be US$4·1bn. It is an ambitious project that may grow to 30 million tonnes per year, almost equal to Canada's entire iron ore production in 2007. In the process, Canarail will have played a pioneering role in designing the world's most northerly heavy haul railway.
CAPTION: The 245 m Raven River bridge will be built in an area of ice-rich soils. Since ice creeps under load, the design must accommodate differential settlement between bridge and embankment foundations.
CAPTION: Mary River will be the world's most northerly heavy haul railway. Although some mines lie north of Mary River, none have railways.
CAPTION: The mine site at Mary River, looking from the south.
CAPTION: The line will run alongside this lake on a ledge cut into the rock, although two tunnels - one 800 m in length, the other 250 m - will be required.
CAPTION: Cockburn Crossing - the most complex bridge location.
CAPTION: The route of 149 km crosses three zones: the southern third mostly on bedrock, the middle section requiring design foundations for permafrost, and the northern 20 km requiring serious engineering to work around significant ice features.
CAPTION: The port site at Steensby Inlet, looking west from the mainland across the islands which will house the stockpiles and the deep-water dock for the ore carriers.
bron:
http://www.railwaygazette.com/news_view ... north.html
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