Our La Plata Mine in New Mexico, USA, began producing coal in 1986. The coal was hauled to the San Juan Mine, where it was stockpiled for supply to the San Juan Generating Station. Coal production ceased in December 2002 and the coal haul was completed three months later, enabling full-time reclamation of the site to commence. In 2001, it had been determined that the reclamation program faced challenges that required more than the conventional practices. An innovative approach was initiated, involving the computer-generation of topographical designs that simulate natural landforms and create a landscape similar to that which naturally would have formed over time. Reclamation using these principles is now well under way, with positive results.
The main reason for initiating a more comprehensive approach to reclamation at La Plata is that the terrain is steeply sloping and highly erosional. The landscape is dominated by mesas with their steep rock walls topped by flat sandstone caps. Some of these caps, which control headcutting from moving upstream, were removed when the mine was constructed. Without them, the terrain will erode upstream and eventually create a landform quite different to the pre-mine landscape.
There were other challenges. For example, the conventional approach is to develop terraced slopes with rock chutes for draining water down the slopes. If this approach were used as part of the reclamation process, long-term maintenance would be required. Terraces would not provide the desired topographic diversity, and minimal precipitation and high evaporation rates would limit the amount of available water.
Additionally, during mining, the overburden rock was broken. As the broken overburden occupies a greater volume than the unmined rock, it all could not fit in the mined-out pit. The excess had to be reclaimed as a permanent out-of-pit spoil dump, which was developed with approval from the State of New Mexico authorities. Existence of the dump provided another challenge to the reclamation program.
It was considered essential that, as well as reclaiming the landscape, the final landforms must harmonise with the surrounding natural terrain and provide habitat for wildlife, in particular the deer that are abundant in the area.
This is extremely important for the people who live in La Plata and for those who use the La Plata valley for recreation such as hunting and hiking.
Fluvial geomorphic principles
Fluvial geomorphic principles involve recontouring of reclaimed surfaces and simulating natural stream configurations to create a landscape similar to that which naturally would have formed over time. Landform shaping and grading plans are based on computer-generated topographical designs. In addition, the principles are used to provide slopes and drainage channels that are stable over the long term.
Application of the principles takes into account the creation of landforms together with the processes by which those landforms would be created naturally over time. Soil, slope and weather are all considered. The objective is to create functional landforms that blend with the surrounding natural terrain and are stable, ensuring long-term, maintenance-free reclamation.
To shape the land according to the computer-generated topographical designs, global positioning system (GPS) technologies are utilised in the earthmoving equipment. The GPS units provide real-time data, allowing the bulldozer operators to monitor their work in the field. This improves productivity and efficiency in materials handling and helps accomplish the required surface configuration designs.
The application of fluvial geomorphic principles in reclamation at La Plata Mine has many long-term benefits. During reclamation, topdressing to varying depths and using multiple seed mixes provides topographic diversity and also promotes plant community diversity. Wildlife habitats are improved by the grading of slopes to re-create natural landforms, which establishes windbreaks and security areas. The resulting landscape is in harmony with the natural surrounding terrain.
Positive results
Around 100 people from the La Plata Mine team and San Juan Coal Company environmental department are involved in the reclamation phase, which is expected to continue until 2007 when major earthwork should be completed and the land returned to wildlife habitat.
A target had been set to regrade 142 hectares by June 2004. At the time of reporting, 255 hectares have been regraded, including 31 hectares that were completed in the calendar year 2003. A total of 34 hectares have so far been reclaimed through regrading, topdressing and seeding.
Based on research involving on-site inspections undertaken in 2001, 2002 and 2003 after periods of significant rainfall, all of the constructed drainage channels are stable, with minor scouring and deposition as predicted. No repair work has been necessary. The test area has stood up well to erosion. It is much more visually appealing and, as vegetation takes hold, is blending well with the natural terrain.
For the innovative approach to reclamation work, La Plata Mine was recognised in 2001 for achieving ‘Excellence in Reclamation’ by the New Mexico Energy, Minerals and Natural Resource Department, Mining and Minerals Division.
Mr Willis L. Gainer, Director of the Albuquerque Field Office of the Department of Interior, Office of Surface Mining, stated in a letter to San Juan Coal Company, ‘Everyone was extremely impressed by your success at geomorphic restoration of the reclaimed topography. In my view, the innovative backfilling, grading, and drainage restoration work that you ... have implemented at your mines has set a new standard for reclamation technology’.
The La Plata Mine project has demonstrated that reclamation grading using fluvial geomorphic principles can be successfully implemented in conjunction with surface mining and truck-and-shovel open-pit operations.
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