The paper “Stress Analysis and Support Design for Longwall Mine-Through Entries” by West Virginia University Associate Professor Keith Heasley, Eastern Associated Coal Company senior mining engineer Phillip Worley and West Virginia University research assistant Yunqing Zhang, undertook a detailed case study of Harris #1 mine’s experience of mining through three entries.

The paper was presented at the 22nd International Conference on Ground Control in Mining held last week in the US.

The Harris #1 Mine is operated by Peabody Coal subsidiary Eastern Associated Coal Company and extracts the Eagle Seam in Wharton, Boone County, West Virginia.

Recently, the mine decided to speed development by driving a set of three entries across the middle of the longwall. Consequently, when retreating the mine needed to decide whether it was most economic to properly support and then mine through these mid–panel entries or to move the longwall around them.

To complicate matters, there was a previously extracted longwall panel in the No 2 Gas Seam which overlaid the cross-panel entries and which was expected to produce multiple-seam stress concentrations on the Eagle Seam.

After doing a time and cost analysis, the mine decided to mine through the three East cross-panel entries.

In order to accurately evaluate the multiple-seam stresses and modify the mine-through support system appropriately, the boundary element program LaModel was used to simulate the stress and displacements in the two mines.

During stress mapping the program found the roof, ribs and floor of the mine-through entries looked solid.

According to the paper, when designing the supports three factors were addressed. Firstly, the design of the pillar widths in order to isolate the individual entries were taken into consideration and secondly, the design of the bolt system in order to minimise roof falls and assure skin control was addressed. Lastly standing support capacity and yield ability in order to transfer abutment loads across the entries and eliminate weighting failure needed to be addressed.

For designing the intrinsic roof support, the team decided to maintain the integrity of the immediate roof in order to minimise roof falls and to successfully transfer abutment loads across the entry.

When designing the standing support three criteria were met including the capability of supporting the applied loads and displacements during the approach and mine through of the longwall face; the ability be easily cut and removed from the face area by the longwall equipment; and economy of the process. To meet all criteria pumpable cement cribs using 3000 psi cement were chosen.

The paper found for designing the spacing of the cribs in the entries, a number of factors were considered. Areas where the multiple-seam abutment stress was expected needed greatest safety factor, and therefore the tightest crib spacing, in order to protect against a weighting-type failure of the inter-burden.

Next, within 100-150 feet of the gate roads, it was expected that the abutments loads would be reduced due to the overburden support provided by the adjacent gate road. Finally, in the crosscuts that were perpendicular to the face advance, it was expected to only need the minimum standing support density due to the limited amount of exposed roof.

During the execution of the drive-through a number of observations were made by the operation team.

“When the longwall face became parallel to the first mine-through entry with a 52 foot coal fender separating the face from the entry, the floor started to heave both in front of the shields and in the first entry, from about shield number 60 to 181 (181 shields total),” the paper said.

“At this location, the longwall floor heave was severe enough to cause serious face delays. Not only was the face closing but a substantial number of the pumpable cribs in the first entry were severely cracked by the 2 to 3 feet of floor heave.

“In the first entry, most of the cribs had severely cracked, but no roof fall problems were encountered, and although the mine through was successful in this entry, the magnitude of the floor heave would have hindered any recovery of supports had the goal been to use this entry as a recovery room.”

The magnitude of the floor heave was attributed to the deteriorated condition of the floor, the minimal cure time for the cribs and the multiple-seam abutment stress from the overlying longwall panel. 

Mining then continued through the remaining crosscuts and rooms without major incident.

“The roof bolts and cables maintained roof integrity and eliminated roof falls in the entries, and the pumpable cribs supported the applied loads and allowed the major abutment stresses to transfer over the entries,” the paper concluded.

“The longwall was able to mine through the cribs and the entries will little or no additional delays, saving considerable time and money over moving the wall around the entries.”