Highwall mining of multiple seams can be tricky. Issues of roof stability, web pillar dimensions, sequencing and seam interaction all need to be carefully addressed. But according to Agapito Associates’ Tom Vandergrift, with the right geotechnical and pre-production planning in place, a productive highwall can be a great earner.
The majority of Agapito’s highwall mining work has been in the western US, with a little in the eastern US and India. Out of about 20 highwall mining design projects Agapito has performed over the past seven years, all but one have involved some multiple seam considerations.
In the western US and in India, thick seams are commonly encountered, raising the possibility of multiple pass mining (sending the machine in the same opening two or more times to increase the overall mining height) and multiple lift mining (separate openings vertically in the same seam, with a layer of coal separating the openings).
Typical lithology includes shale, sandstone, claystone, mudstone and interbedded combinations. “Weak carbonaceous mudstone or shale immediately above or below the coal seams are common, and have to be evaluated with regard to roof and floor stability. A common way of dealing with weak strata is to specify leaving a small amount of roof and/or floor coal,” Vandergrift said.
When designing a highwall mine Vandergrift said geology and seam setting need to be taken into account.
“Geometric parameters include the cover depth, the seam thickness and seam structure (dip).
These are important to ensure that mining is practical given the capabilities of the highwall mining machine (cutting height range, pull-out force versus down-dip angle and penetration, sumping force versus up-dip angle and penetration, leveling capabilities of the machine front to back and side to side versus the attitude of the pit floor).
The seam height and cover depth ranges that will be mined also directly relate to the web and barrier pillar dimensions that will be required,” he said.
Physical properties of the coal seams and bounding strata are acquired through core sampling and lab testing. In addition to the seams themselves, Agapito concentrates on the physical property testing 10-20ft above and 5-10ft below the seams of interest.
“Joint and cleat spacing and orientation are documented through available data or through general site observations. This affects the likely stability of the roof and ribs in the opening and stand-up time,” Vandergrift said.
“Lithology logs are examined to develop typical strata sequences, and to later assign typical physical property values for numerical modeling.”
Vandergrift added the risks to any structures or resources that are undermined are also taken into account.
“In multiple seam operations, we consider how mining in one seam may affect planned mining in seams either above or below. The primary effect of multiple seam mining is in stress redistribution from one seam to another.
“We perform numerical modeling to examine stress concentrations in each seam arising from the proximity of planned highwall miner openings in other seams, combined with variations in depth of cover. We then develop criteria for pillar design in each seam, and whether pillars need to be columnized (stacked).
“The stability of the interburden is also an important consideration, and we recommend a minimum interburden thickness based on site-specific geometric and physical property parameters.”
One of the biggest factors Agapito must decide on when designing multi-seam highwalls is the sequence in which the seams will be extracted.
“Sequencing of mining is important, as the design is affected by whether mining is planned from the uppermost seam down, or from the lowermost seam up.
From a geotechnical risk standpoint, mining from the top down minimizes the risk of machine entrapment, as the floor of the opening is solid as opposed to mining from the bottom up, where isolated roof failures in underlying seams may pose a floor stability threat,” Vandergrift said.
“However, machine operators perceive mining from the bottom up as safer, as they do not like to work under a highwall with openings above them.
“From a practical standpoint, most operators prefer to have the final highwall established, and then work from the bottom of the pit up, backfilling the pit to reach each successive seam above. This eliminates the need for extensive coordination between the surface mining operation and the highwall mining operation, which can be problematic when mining from the top down. We believe that either sequence can be successfully applied, as long as the stability of the web and barrier pillars is properly accounted for.”
One example where careful attention had to be paid to seam interaction issues was at Rio Tinto Energy America’s Colowyo mine in Colorado where four seams were mined in succession from the lowermost upward. In order to complete the geotechnical study prior to mining at Colowyo, Agapito needed to address the geotechnical issues:
- Identification of geotechnical/geological constraints that could impact or limit highwall mining, including an assessment of roof stability in the highwall miner openings.
- Development of minimum web pillar dimensions to ensure highwall stability both during and after highwall mining.
- Assessment of the potential for “cascading pillar failure” and barrier pillar design to sufficiently isolate extraction panels from one another should web pillars within an individual panel fail.
- Would seam interaction affect the design in multiple seam mining areas, or were the seams separated by enough interburden that the pillar designs in each seam could be treated independently?
When analyzing the roof competency Agapito found roof stability might be an issue where mudstone forms the immediate roof. Because the presence and persistence of the mudstone is difficult to predict, it was recommended that 6-12 inches of roof coal be left to aid roof stability and to reduce dilution.
Using LAMODEL and UDEC modeling Agapito was able to confirm that its web and barrier pillar design curves would enable the roof, floor and interburden to remain stable.
Despite a slow ramp-up mainly due to weather, Colowyo was able to start producing an average of just under 100,000 tons a month with an ADDCAR highwall mining system.
“No major ground instabilities occurred, and other than reduced penetration resulting from seam undulations and a rock parting, the highwall mining is exceeding production expectations,” Vandergrift said in his paper presented at the 24th International Conference on Ground Control in Mining.
