By Lee Buchsbaum
A few years ago the Trapper surface mine, located near Craig in northwest Colorado, was almost entirely dependent on its three identical Page 752 LR (long range) draglines for all of its overburden removal needs. Following a massive landslide in October 2006, Trapper redeveloped its mining plans and is now producing more coal with a mixture of new mobile surface equipment and draglines than ever before in its long history.
The Trapper mine, which has been in operation since the mid-1970s, has four owners: the Salt River Project (32.10%), Tri-State Generation and Transmission (26.57%), PacifiCorp (21.40%) and the Platte River Power Authority (19.93%). All of Trapper’s coal is shipped to the adjacent Craig power station, operated by Tri-State.
Trapper’s first dragline was assembled in 1975 and coal deliveries began to the Craig station in 1977. From 1978 through 1988, Trapper supplied the Craig Station with an average of 2.27 million tons of 9,800 Btu/lb coal. Base contract tonnage requirements fell throught the late 1980s through the early 2000s to just under 1.5 million tons per year (tpy). During many of those years, Trapper sold varying amounts of spot coal to Craig Station, softening the adverse effects of declining base contract production.
Today, Trapper is contractually committed to mining more than 2.3 million tpy through 2020, which is also why the mine has purchased and deployed a more flexible mobile surface mining fleet. The contract is somewhat flexible in that it can fluctuate by 200,000 tpy on any given year.
Trapper mines up to eight seams, each of which has a different sulfur content and calorific value. Sequencing is key, and, as the mine uncovers various seams, it employs a coal quality engineer to conduct field samples. The power plant stockpiles the various coal types so it can regulate the amount of Btus while limiting the ash. Contract specifications call for Trapper to deliver coal with a minimum calorific value of 9,500 Btu/lb. Trapper’s calorific value peaks between 10,300-10,400 Btu/lb. The power plant stacks Trapper’s coal and blends it on site.
“We work closely with Tri-State and communicate what we’re doing as we learn what they need on a daily basis,” said Stephen Hinkemeyer, production and engineering manager, Trapper. “That helps us develop our mining plan both long and short term. Typically we’ll mine hotter coal in the morning, and coal for blending in the afternoon.”
Trapper’s surface coal reserves are part of the Upper Williams Fork member of the Williams Fork Formation. The Upper Williams Fork overlies the Twentymile Sandstone (from which Peabody Energy’s nearby Twentymile longwall mine gets its name). The Upper Williams Fork is comprised of inter-bedded sandstone, siltstone, shale and coal.
Trapper’s surface reserves are bound to the south by the Williams Fork Mountains which rise to an elevation of 7,700 ft. The mine’s topography drops at an average grade of approximately 15% before descending to a flatter slope to the north. Coal seams dip at an average of approximately 16% generally to the north/northwest with localized areas in excess of 25%.
Coal seam nomenclature descends alphabetically. Seam groups of interest include the F, G, H, I, K, L, M, Q and R with frequent seam splits. Mineable seam thicknesses vary from 18 in. to 14 ft. Trapper split its surface mine into the West and East panels. The mine was opened on the West Panel reserves and operated there until the late 1990s. Primary stripping equipment in the west include several Page 752 LR draglines. From 1994-1995, the mine upgraded the coal loading and hauling fleet with the purchase of two Cat 5130 excavators and five Cat 777 haul trucks.
The East panel has an adjusted strip ratio of between 7 yd3/ton and 15 yd3/ton. Coal from the H, I, K, L, M, Q and R seams averages 9,760 Btu/lb. In addition to the Page draglines, Trapper uses Cat D10R and D11R dozers for primary stripping and several Cat 5130 hydraulic backhoes and Cat 777 haul trucks for removal.
Nearby, the still Rio Tinto-owned Colowyo surface mine produces another 2.3 million tpy for the Craig power station as well. All that is left of the former Rio Tinto America mine fleet (the rest is now part of Cloud Peak Energy or Arch Coal), Colowyo had annually produced more than 5 million tpy but no longer mines to that level as Rio Tinto determines the mines’ future. Trapper, Colowyo and the Twentymile longwall mines supply all of the coal needed for the Yampa River Valley’s two hungry power plants that burn a combined 7 million tpy. Together the industry directly employs more than 1,000 people in the area. Trapper alone accounts for nearly 20% of those jobs.
Landslide! All Hands on Deck!
On the morning of October 8, 2006, workers at Trapper started seeing some cracks in the ground and ridge above one of the pits. Earlier that month, Trapper had experienced a “100-year storm event” and a recent hard rain had further soaked the ground. Ken Raschke, Trapper’s truck and loader supervisor, in charge of day to day planning and fleet coordination, remembers clearly that foggy morning. “We were on the backside of a hill and an employee hunting on our land called the mine office to say he had fallen into a crack. At the time he was calling, I kept encountering other cracks, marking them as I went. I kept saying, ‘wow there’s another one we missed, there’s another one,’” said Raschke.
When Raschke and another co-worker drove up to the top of the hill to investigate the crack the hunter had discovered, things really started to let loose. The hunter “was by the fence line and we heard the fence pop as the staples started coming off. As we started to walk over, we saw a topsoil pile start to open up,” said Raschke.
Raschke jumped into his truck and drove down the road to where the new cracks were forming. “There was probably about a 2-ft drop I had to bust through with the truck. I hopped on a dozer to punch through the crack so I could drive back across and hook up with another worker. We made a couple pushes with the dozer and then the whole mountain just went, falling about 15 to 20 ft. It roared like the sound of a river. It was just utter chaos after that with people scrambling to save equipment,” he said.
In seconds, roughly 225 to 250 acres of the mountain, more than 35 million yd3 suddenly slid about 400 ft covering haul roads and exposed coal blocks, and everything else in its path to an average depth of 100 ft. Trapper had a couple dozers, drills and a Cat 5130 in the pit. One of the draglines on the other side of the pit started to back away from the pit. “I don’t think it was ever really in any danger. But it was all hands on deck and we were calling the other active pits telling them to evacuate because we didn’t know what was going on for sure,” said Raschke. Miraculously when the mountain finished sliding, nobody was hurt.
As the ground stabilized, Trapper employees found one generator that had been tossed around, tipped over and was raised 80 ft into the air. “We built a road to recover it after things dried out. We were very, very fortunate not to have any other losses,” said Raschke.
There was a fear there could be other landslides. In cooperation with a geotechnical firm, Trapper redoubled its surveying efforts to figure out why that specific area had failed. Once there was a working theory, Trapper reviewed the entire reserve area. “We determined the water in combination with some geologic characteristics of the reserve and this particular spot led to the slide,” said Hinkemeyer.
Just prior to the slide, Trapper had opened up a pit that was basically about 350 feet wide, and was currently 15 feet above the level where the slide took place. “We had some unstable clay seams located along a really steep floor above more stable ground. When the water got into that, the weight of the material in the unstable area caused the more stable ground below to break free and allow the whole block to move. When the earth broke loose, it slide basically right into the open hole. Some of the area below the slide had recently been backfilled and it was loose and un-compacted. When water lubricated everything, the landslide block slid into the backfilled area and compressed it as well,” said Hinkemeyer.
The Colorado Geologic Survey said this was the third largest landslide in Colorado history. “I’ve seen minor slides where you maybe get 15 acres move on a shallower clay layer. I’ve never seen anything like this 100-ft deep 250 acre block move, and I don’t want to again,” said Hinkemeyer.
The slide actually moved about 35 million yards of mostly top layer materials from the L seam up. The interval between the K and L seams in the slide area is about 15- to 20-ft thick. On average, the base of the slide level was about 7 ft above the L seam. But in the slide process, there is now 6 ft to 8 ft of rubbelized rock. “As it gave way, everything was pulverized in-between because of the weight of material above it,” said Raschke.
Prior to the slide, Trapper’s remaining reserve base was approximately14 million tons. However, the slide covered 10 million tons of those reserves. Under contract to make deliveries to the plant, Trapper had little choice but to actually mine in the slide area. “There was no excess capacity anywhere. The slide had already affected the lowest ratio reserves left in the entire mine. Everywhere else we were mining 10 or 12 yards of material to get a ton of coal. This area had a five to six to one ratio,” said Hinkemeyer.
Slide Hastens Truck-Shovel Move
With stripping ratios growing prior to the landslide, Trapper had already begun evaluating transitioning to a mobile stripping fleet. Trapper was already using truck-shovel pre-stripping operations ahead of the draglines. “We realized that the economics were still there, all we lacked was the equipment fleet. In 2006, we had identified maybe about 50 million yd3 of material that could be moved that would increase our reserve base by quite a bit. We were in the process of looking at the economics of that decision when the slide happened,” said Hinkemeyer.
To fund the purchase of the trucks, loaders and necessary equipment, Trapper developed a workable mine plan and negotiated an agreement with its owners that significantly increased production under long term contracts from 1.5 million tpy to 2.3-2.5 million tpy beginning in 2010. New surface mining equipment includes a Le Tourneau L-2350 front-end loader with a 53-yd3 bucket and four Komatsu 830E haul trucks.
“We also realized that if we were to buy this large stripping fleet, not only do we get back 10 million tons of reserve covered in the slide area, but we could actually more than double overall reserves since we have areas where there’s more coal that’s too deep for our draglines to access,” said Hinkemeyer.
But changing tactics meant increasing production costs. “We estimated it would cost two to three times per yard to move something with the truck and loaders than it does with our draglines,” said Hinkemeyer. Trapper’s new contract calling for a long-term jump in production became a way to finance the new fleet. Trapper retrained over half of its existing production crews and added another 10 to 12 miners.
Revising Plans for Post-Slide Environment
Almost exactly three years after the slide, in October 2009, Trapper gained approval for a new ground control plan. During that time, Trapper spent more than two years doing geo-technical research, drilling holes, gathering more rock data and analyzing the results to determine the cause of the failure. Trapper worked on mine planning, “but until you figure out why the thing failed, it’s difficult to go forward,” said Hinkemeyer.
The new plan calls for Trapper to mine down to the L seam with the mobile equipment. The draglines would follow behind, picking up the M, Q and R seams. The draglines, however, are actually setting up on stable ground below the slide plane.
Following a highwall failure years before, Trapper changed its pit orientation strategies. Once the strike pit had failed, it determined the section it was mining would work for strike mining, but the area above it now appeared to be too steep. Management figured if the mine would have to change six cuts in the future, it was better to make that change now. “We figured let’s not try to find our way back into this thing. So we made the decision to return to the dip cut technique,” said Hinkemeyer.
Often the seam slope is so steep they have to pull the coal down because they are unable to get the trucks up next to it. “So we pull it down onto the pads that the loaders have created and we haul it off that way,” said Hinkemeyer.
Shovel cuts are going to be digging down the hill at a 7,100-ft elevation in 30-ft drops. Trapper is taking the slide material first with mobile equipment, and then moving the dragline down the hill behind after it removes the material. Once on the main block of the landslide, Trapper’s geo-technical analysts determined that, because of the thickness of the seams, the mine should be able to recover more than 50% of the affected coal. In practice, however, Trapper is actually recovering about 75%-80% of that coal because as it slid, it moved almost in unison.
Buchsbaum is a Denver-based freelance writer and photographer specializing in industrial subjects. He can be reached through his Web site at www.lmbphotography.com or by phone at 303-746-8172.