Tailings and Mine Waste Conference

Integrated hydrogeological and environmental restoration of landslides affecting a large asbestos mine… Oboni, Franco; Oboni, Cesar; Angelino, Claudio; Visconti, Bartolomeo Nov 30, 2011

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Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Integrated Hydrogeological and Environmental Restoration of Landslides Affecting a Large Asbestos Mine Dry Tailings Dump Franco Oboni Riskope International, Vancouver, Canada Cesar Oboni Riskope International, Vancouver, Canada Claudio Angelino Polithema srl, Turin, Italy Bartolomeo Visconti Polithema srl, Turin, Italy Abstract The Balangero asbestos open pit mine, located 35km NW of Torino (Italy), was the largest operation of this kind in Western Europe. The dry tailings were lifted by a conveyor belt from the mill and dumped over a natural slope with an approximate angle of 25 degrees, progressively reaching a maximum thickness estimated at 80 m. Mining operations ended at the beginning of the ‘70s. By the '80s the dump was deeply scarred by various local and large scale instabilities, to the point that houses located at the toe, on the opposite side of the valley, were evacuated. The award winning restoration project used a multidisciplinary approach including hydraulics, geotechnical, pedological and risk engineering to yield a well balanced and sustainable solution. This paper illustrates the Risk Based Decision Making (RBDM) process used through the feasibility, design and construction follow-up of the environmental restoration of the 60 Mm3 dry Balangero asbestos tailings dump. Introduction The Balangero asbestos open pit mine, located 35 km NW of Torino (Italy), was the largest operation of this kind in Western Europe. In 1918 it was foreseen that the mine would extract 26,000 m3 of rock per annum, but the actual tonnage exceeded the design value, with up to1.3 Mm3 of rock being extracted in 1961. In 1966 a new mill with a capacity of 25,000 tonnes (t) fibres per annum was installed. The dry tailings were lifted by a conveyor belt from the mill, then through a tunnel to the opposite side of a hill, and then dumped over a natural slope with an approximate angle of 25 degrees from the altitude of about 830 m above see level (a.s.l.) to the bottom of the valley at 580 m a.s.l.. As the dumping proceeded, a total surface area of approximately 250.000 m2 was progressively covered with tailings varying from a few meters to an estimated maximum of 60 to 80 m thickness. Mining operations ended at the beginning of the ‘70s. Risk Based Decision Making (RBDM) was used through the feasibility, design and construction follow-up of the environmental restoration of the 60Mm3 dry asbestos Balangero’s tailings dump (Oboni et Al., 1997, 1998, Bruce & Oboni, 2000). Risk Based Decision Making (RBDM) was used by the winning project at each and every step of the design.  Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 The project had several interesting problems related to environmental management, such as dusting and active instabilities. RBDM showed that using innovative and unusual solutions like, for example, an aerial tramway instead of classic hauling would allow a reduction in dusting and the project’s carbon footprint while bringing an income (selling electricity produced while braking the downhill loads) and lowering general human health risks. Overall, the integration of hydraulics, geotechnique, pedology and risk management led to a well balanced and sustainable project which will be turned into a “living museum” in the years to come. The problem By the '80s the dump was deeply scarred by deep seated instabilities, erosion gullies, mud slides etc., to the point that houses located at the toe, on the opposite side of the valley were evacuated. The site was furthermore recognized as one of the most serious environmental issues of Italy. In 1992 RSA, a public company formed by the Province of Torino, the Mountain Community of the Lanzo valleys, neighbouring communities and other key stakeholders was mandated by the regional government of Piedmont to organize an international design contest for the environmental rehabilitation of the dump. In 2000 RSA launched, on the basis of an existing preliminary design, a European design contest to find the most suitable solution for the environmental restoration and permanent stabilization of the dump slopes, orphaned since the end of the 80’s. The site was potentially critical in terms of large events including; mud flows, deep slides and wide spread dusting of asbestos fibres. Two communities with about 10.000 inhabitants were indeed under the influence of the dust plumes. To win the contest two basic objectives had to be met: a) limiting the volume of earth movements and b) reducing the overall costs of the restoration to comply with stringent financial limitations imposed by RSA’s budget. Alternatives assessment phase When engaging in the pre-feasibility of such a restoration project a wide array of design alternatives have to be carefully analyzed and compared. Risks can be used to discriminate, provided they are carefully evaluated at each specific phase of the project life (Oboni, 2006, Oboni et Al. 2001, Oboni & Oboni, 2004). Designs based on codes or recommendations may differ quite significantly from designs based on Risk Based Decision Making (RBDM). Differences may go as deep as choosing a different material hauling system, a different drainage pattern etc. Alternatives which are perfectly code-compliant and require the same investments and maintenance may expose the owner to totally different levels of risk all along their expected life. RBDM for reclamation projects requires robust and simple tools for choosing among alternatives at each and every step of a project life encompassing conceptual design, construction maintenance and then necessary performance monitoring and evaluations before reaching the end of its expected life. The method allowing this type of comparison was specifically developed for this project and then formalized at later date under the name CDA/ESM (F. Oboni, C. Oboni, 2007). CDA/ESM has since Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 been used extensively, on projects all over the world, to compare alternatives considering risks and uncertainties from cradle to grave. For example, the hauling of 250.000 m3 of excavated material had numerous possible alternatives including hauling trucks, cable car and even fluidized mud via a gravity pipe on the slope.  Their specific risk profile was, however, very different in terms of duration of works, air pollution, asbestos dusting, energy consumption (the cable car allowed for the production of energy while doing the job) and finally carbon footprint. Even the choice of the slope stabilization method can be conducted with the same methodology by optimizing all the key aspects involved in the final decision, which included: new geometry of the crest (unloading the crest by excavating three benches), number and gradient of the runoff system collection berms, vertical distance between berms along the slope and size of the berms. Design objectives Beyond the basic RSA’s goals, the restoration project had to consider numerous other global and sometimes competing objectives:  Minimize dusting during construction.  Limit air pollution, both from engines and from dusting related to traffic on unpaved roads inside the mine area, by reducing the number of hauling trucks.  Control the geotechnical stability of a large area, including several critically over steepened sections.  Control the global water runoff on the area and guide the collected water across very steep cross slopes.  Give strong and immediate support to new vegetation, as vegetating the slope was considered to be the best way to control erosion in such difficult conditions.  Implement specially designed and sophisticated systems of reforestation/vegetation planting on sterile soils.  Limit the use of concrete/steel or any other artificial material given the sensitive location of the site at the footsteps of the Alps, in a visible, densely inhabited area. Resulting design components Hauling One of the project’s major challenges was related to the amount of material, containing asbestos fibres to be excavated and disposed of within the old mine area, in order to unload the over steepened crest of the dump. The 4.5 km of dirt tracks between the top and the toe of the slope were a potential source of dusting and large carbon footprint, given the use of a fleet of small tonnage trucks. Thus, the use of trucks was ultimately discarded due to environmental risks (pollution from exhaust fumes and fibres dispersion from the excavated material) and the need to upgrade the existing tracks to roads (extra costs for ancillary temporary structures).  Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 CDA/ESM showed that the best overall results would be achieved by installing a temporary aerial tramway, designed with a single span of 960 m between the top and valley bottom terminal stations. The tramway was capable of unloading its bucket in any point along the track, at ground level (the tramway bucket could be lowered in any point of the trip to ground level), i.e. very efficiently limiting dusting. Furthermore, the excavated material was wetted at excavation time and remained wet during the full trip from the source to the final resting position to reduce fibre dispersion. The aerial tramway (Figure 1) was removed after only 1.5 years of operation, having very successfully and efficiently completed the difficult task.               Figure 1: The aerial tramway loading area Slope stability control procedures The basic principles of the slope stabilization design can be summarized as follows:  Unloading of the crest of the dump slope by digging three, 10 m wide benches and by storing the resulting material at the valley bottom in an 8 m high fill. The toe berm was also designed to protect nearby houses from possible residual mud slides in the over steepened eastern part of the slope (42°)  Cutting a series of eight, 2.5 m wide, “path-ways berms” across the slope, each about 600 m long. The selected design allowed building these berms with only lateral transfers (no longitudinal evacuation) of the material according to the design scheme depicted in Figure 2. This procedure dramatically reduced downhill hauling needs and minimized the dispersion of asbestos fibres in the air. The “path-ways” also created an easy access to the slope for maintenance monitoring activities. The stability of the “path-ways” was enhanced by incorporating a double system (upslope and downslope the berm) of 0.2 m diameter driven logs. The berms are also a main element of the runoff control system, since they collect water every 20 to 30 m across the slope, thus limiting erosion.  Building composite wood-earth structures to ensure the stability of the steepest parts of the slope, or create necessary working and maintenance platforms. Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Runoff control Deep gullies, up to 30 m deep, had formed in the upper part of the slope, where water concentration was higher. The remedial measures for surface water control included the following:  Overall control of runoff through a network of small wooden channels, 50 to 100 cm wide. The small dimensions were selected for ease of construction with small equipment on the slope and to assure a capillary system to maximize erosion control. The channels were located on the three top benches and are linked to a secondary network of canals located on the “path-ways”  Transfer of the collected water towards the toe of the slope using four main channels located along the slopes with the steepest gradient (as shown in Figure 3): again logs, stones, and natural materials were used to build these systems.               Figure 2: Path-ways berms design cross-section scheme  At the convergence of the four main channels, a unique main collector channel was required. The main channel was constructed with only stones and logs allowing the water to reach Fandaglia Creek. A decant basin, located at the very end of the collector channel, retains fine material and fibres before the water is released to the environment.  The release location constitutes the water quality control point.  Control of the underground water by installation of sub-horizontal drains drilled in critical areas along the slope.  Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011                 Figure 3: Main channels design longitudinal section Re-vegetation The reforestation/re-vegetation of the area was a major challenge within the project due to the scarcity of nutrient materials in the sterile soil of the dump. Good re-vegetation was a significant project objective and critical to overall dump stability.  The re- vegetation would enhance the following project components:  Erosion control.  Water absorption and runoff limitation.  Geotechnical stability of the surface layers of soil due to the mechanical stability given by the roots system.  Limitation of dusting of asbestos fibres.  Restart of an entire ecosystem not only in terms of vegetation, but also of a micro-flora and, in the future, of a fully developed natural ecosystem (deer are already back on the slope thanks to the new grass); and  Provide an important aesthetic value when local residents are looking to a newly vegetated green slope rather than to a grey dump of orphaned territory. Success was achieved by stimulating the natural re-vegetation, restarting the pedo-genetic process and accelerating the colonization of superior species by implanting pioneer species. The use of mycorryzae (special fungi pre-instilled in the root system of the newly planted vegetation), specialty hydro-seeding processes and autochthonous species have resulted in outstanding success as shown in Figure 4.  Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011  Figure 4: The re-vegetated slope The works The approval procedure of the design took a long time, given the high number of public administrations and subjects involved in authorizations and approval. The works started in 2003 and were completed in 2009 without any significant trouble. However, there were periods where work had to be suspended because of high winds and the cable car system was also badly damaged in one occasion with wind gusts recorded at 140 km/h. The aerial tramway system proved to be the ideal solution to the materials hauling problem and worked properly for the 1.5 years duration of the project.  As mentioned, the tramway also allowed the production of energy, which was sold to the Italian national power board, during the braking phases of downhill transfer of excavated material. Small, foreseeable and foreseen damages to the construction site due to violent summer storms and prolonged rainy period were recorded through the years and caused minor delays. Winter conditions, including work suspensions from December to March due to icing of the surface layers, impacted the construction site but only required shifting of certain operations to a later date. All the solutions expressly designed for this particular project ended up being “dynamically adapted” as the restoration progressed and experience of the site was gained by all the project’s stakeholders. Slight modifications to the initial designs allowed the design team to improve construction sequences and final performance. The reforestation process was intensely monitored and scrutinized.  By the end of the works, the first trees and shrubs, hydroseeding had already undergone 3 or 4 vegetative seasons and allowed a “real time” control of the success of the operation. In total the re-vegetation effort was quite intense and included: 450,000 m2 of hydro-seeding, 15,000 shrubs, 7,300 trees and 267,000 live cuttings. During the entire construction strict air quality monitoring campaigns were carried out both on the construction site and in the two neighbouring communities.  During violent wind storms work was stopped to avoid excessive (additional) dusting and the resulting potential public opinion scrutiny. Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Strict worker’s safety protocols were adopted including particular protective masks with a high filtration capacity and special protective working dresses. Furthermore workers were not allowed to leave the site without a proper changing of clothes and personal shower within a special decontamination unit. The overall budget for the project was 5.5 M €. Figure 5 shows the state of the slope after two seasons. Conclusions The use of natural construction materials available within easy reach from the construction site and the adoption of a hauling system via an aerial tramway to transport the excavated material facilitated the completion of difficult geotechnical works in a very sensitive area.  Objectives of the project included limiting pollution from heavy trucks and the dispersion of asbestos fibers in the built and inhabited environment. Risk Based Decision Making (RBDM) procedures at every step of the design process were used to develop the best engineering design in terms of technical results and budget limitations. The integration of geotechnique, hydraulics, pedology and risk management within the designers’ multidisciplinary group led to a well-balanced and environmentally sustainable project allowing the gradual recovery through natural processes of an otherwise highly compromised area.  Figure 5: The final slope, after two seasons References Angelino C., Visconti B., Curti R., Pastorino F., Parodi L., Oboni F., Oboni C., Il percorso virtuoso di una miniera, Quarry and construction (Italy), April 2010 Bruce, I. G., and Oboni. F., 2000 Tailings management Using Quantitative risk Assessment. In Tailings Dams 2000, Proceedings of the Association of State Dam Safety Officials, US Committee on Large dams, March 28- 30, 2000, Las Vegas, Nevada. P. 449 Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Oboni, F., et al. - A Risk Management Approach for Tailing Systems - Golden Jubilee Conference Canadian Geotechnical Society, Ottawa, 1997 (Canada) Oboni, F., I. Bruce, M. Aziz, K. Ferguson - A Risk Management Approach for Tailing Systems - Second International Conference on Environmental Management, Wollongong, 1998 Oboni, F., Geo-Environmental Risk: Assessment, Analysis, Management and Planning, SIGA 98 State of the Art Report, Sao Paolo, Brazil, 1998 Oboni F., C. Angelino, B. Visconti,Un database sui rischi quantitativi nella attività di coltivazione di miniere e cave, Rivista GEAM, n° 103, 2001 Oboni, C. Oboni, F., Risk Based Prioritization of Mitigative Funding (RBPMF) ©Oboni&Associates, 2003, Turin 2004 Oboni, F. , Risk Based Decision Making for the Design of Reclamation Projects, CLRA 2006 Reclamation and Remediation: Policy to Practice, 31st Annual Meeting and Conference, Ottawa, 2006 Oboni F., Oboni C., Improving Sustainability through Reasonable Risk and Crisis Management, ISBN 978-0- 9784462-0-8, 2007 www.oboni.net. 

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