Sakatti ore is mainly located at a depth of more than 500 metres in the corner of the Viiankiaapa Natura 2000 site. In the project, the starting point for planning was the early minimisation of negative environmental impacts and the net positive impact (NPI) of combined impacts. After careful assessment, the solution for implementation was chosen, where the concentration plant is located outside the protected area on hard ground in a commercial forest and access to the underground mine is arranged by a group of tunnels covering a total distance of more than five kilometres.
An illustration of the basic functioning of the double-shielded tunnel boring machine and technical solutions for leakage water management. Pre-injection is used to seal the rock from the front of the machine.
The solution is unusual, as in underground mines, the connection to the ground surface is usually arranged by a spiral inclined tunnel or a shaft and lift. As no above-ground structures are introduced to the protected area, everything that enters or leaves the mine – ore, waste rock, drainage water, mine backfilling material, persons, machinery, propulsion and also air – are transported using the tunnel.
The tunnel must meet two important criteria. The amount of water seeping into the mine shall be controlled and the construction phase of the tunnel should be short. These conditions may be implemented using a tunnel boring machine.
What is TBM?
TBM (tunnel boring machine) is the constantly running “production line” of a finished rock space. It is an alternative technology to the drilling-blasting method. TBMs typically consist of a rotating cutting wheel in front, called a cutter head, followed by a main bearing, a thrust system, a system to remove excavated material (muck), and an operational module to build a finished space.
In a TBM, the breaking of the crystalline bedrock occurs in such a way that the blades located on the cutter head are pressed against the surface of the rock using high surface pressure. In this case, due to the local tensile tension peaks between the cutters, the stone flakes as chips from the rock surface to be extracted. The strength properties of the rock influence the penetration rates [metres/day] achievable using the TBM device.
Although the tunnel boring machine has not really been used in Finland, it is a well-known and tested method for tunnel construction in Central Europe and the Nordic countries. Anglo American has experience in using TBM in three different mines: The Woodsmith Project in North East England, the copper mine in Los Bronces in Chile, and the Mogalakwena platinum mine in South Africa.
The Anglo American Woodsmith Project in North East England has nearly 30 kilometres of mined tunnel ready. The tunnel is lined with concrete elements.
What benefits does TBM bring in Sakatti?
In the Sakatti Mining Project, the advantages of TBM compared to the traditional drilling and blasting method are better groundwater impact management and high speed of progression. At Sakatti, pre-injection will be used to control the water, in which the rock is sealed using a sealing device located at the front of the machine. In addition, pre-cast concrete sealing and support elements will be used in hydrologically significant tunnel sections. Using these, a coherent, non-water-conductive structure is built on the walls, vault and bottom of the tunnel, which limits the quantity of water leaking into the tunnel.
Another important advantage of TBM is the speed of tunnel completion. The drilling and blasting method requires several simultaneous drifts for long sections of tunnels. In addition, the work consists of numerous phases, where separate equipment, personnel, and subcontractors are used. TBM excavation makes the completed tunnel similar to a production line, so it achieves better progress compared to the more conventional drift-making. Sakatti aims to achieve an average daily progression of 20 metres.
The variation in the rock quality of Sakatti, such as fracturing and the characteristics of rock types, favours TBM excavation. In TBM excavations, fracturing is particularly useful in the excavation of very hard and abrasive rock types. The rock mass must be “good enough” so that the work does not slow down due to extensive additional reinforcements, but not “too good”, which means that the drilling machine’s progress in the rock slows down due to the low penetration rate. The circular shape of the tunnel is the most optimal tunnel profile for the distribution of rock stress changes caused by the excavation.
An overview of the TBM tunnel route from the Kuusivaara industrial area to ore. A continuous core sample dataset of the tunnel route is available for planning, using which the identification and locating of rock types with different geotechnical characteristics is done.
Careful planning is important
TBM is a set of devices designed to perform a specific task in a specific environment. For planning purposes, it is important to know the characteristics of the rock and to define the technical objectives for the tightness of the space and the progress of the tunnelling work.
The design of the inclined tunnels at Sakatti is based on detailed rock studies and detailed information about the geological, hydrogeological and geotechnical conditions of the area. The design is based on a continuous set of samples obtained by drilling throughout the tunnel and comprehensive hydrological data collected, for example, using pumping tests and water flow sensors installed in boreholes.
As part of the environmental impact assessment for the project, a numerical groundwater flow model has been created based on extensive measurement data, which describes the impacts of underground mining and the impact routes to the hydrogeological conditions of the ground surface. The numerical groundwater flow model is used to determine the regional sealing targets for the inclined tunnel, i.e. the maximum allowable inlet of groundwater per 100 metres [litres/minute/100 metres]. Based on the characteristics and technical objectives of the rock, it has been determined that Sakatti’s tunnelling will be carried out using so-called double-shielded drilling equipment without continuous installation of supporting elements. The installation is carried out according to plan and when necessary, in which case the element will act as a rock support and promote waterproofing. TBM represents the key operating principle of Sakatti. The best technological solutions are always sought for the management of environmental impacts.
The original article was published in the Materia magazine in January 2024.