|32nd International Conference on Ground Control in Mining|
|The Interaction of Unique Geology and Retreat Room-and-Pillar Mining on Overburden Barrier Performance at the Grove No. 1 Mine|
|Anthony T Iannacchione, Michael Keener, Luke Mignogna, Blaise Bucha, University of Pittsburgh, Pittsburgh, United States, Joe Matyus, Pennsylvania Department of Environmental Protection, California, United States, Brent Means, Office of Surface Mining, Harrisburg, United States|
Add to My Library
|[Conference] 32nd International Conference on Ground Control in Mining|
|[Price] Free [Comments] 0|
|[Topical Area] Fundamental Research Studies|
|[Author] Anthony T Iannacchione, Michael Keener, Luke Mignogna, Blaise Bucha, University of Pittsburgh, Pittsburgh, United States, Joe Matyus, Pennsylvania Department of Environmental Protection, California, United States, Brent Means, Office of Surface Mining, Harrisburg, United States|
|Key Conclusions: |
|The Key Factors responsible for the performance of the these barriers was found to be:
• Geology – Dips averaging 15 degrees and the associated structures such as bedding plan slips were present within the Grove No.1 mine property prior to mining
• Mine layout – The mine was designed to drive main entries down-dip towards the Johnstown syncline. Development sections were driven along the strike of the formation on both sides of the main entry. Once a development section was driven to its full length, the panel’s pillars were recovered.
• Strata control – The combination of high dips, bedding plane slips, and pillar recovering mining methods produced challenging strata control conditions that resulted in tremors and vibrations at the surface. These tremors were large enough to generate significant horizontal and vertical fractures in the overlying strata.
• Mine flooding – When the mine began to flood and fill with water, the mining induced failures provided pathways for the impaired waters to reach Ben’s creek.
• Strata barrier design issues – In this case, the strata barrier designed to protect Ben’s creek proved inadequate. In the future, when these conditions are encountered, addition thicknesses of strata barriers or restricting pillar recovery mining are recommended.|
|Key Findings: |
|The Grove No.1 mine was designed to prevent pooled waters from directly discharging to the surface. All of the mine workings were located below drainage so the only threat for unwanted discharge was through the overlying strata.
The Upper Kittanning coalbed at the Grove No.1 mine has overburdens ranging from zero at the portals to over 800-ft. In this area, the coalbed strikes N70°E and dips average 15-degrees to the southeast (vertical change of 900-ft over 1 mile) as it plunges from highs along the limbs of the Laurel Hill anticline into the bottom of the Johnstown syncline (Figure 1). This dip is very rare for room-and-pillar mines in the region where most dips average less than 5-degrees. The excessive bending of the strata has induced complex geologic structures not normally found in southwestern Pennsylvania. One of the authors visited this mine many times when it was in development and noted an abundance of bedding plane slips, both within the coalbed and the overlying strata.
At the time of mining, strata conditions within the Grove No.1 mine were considered to be challenging, especially when development sections were mined down-dip of a pillar recovery panel. The previously mentioned bedding plane slips undoubtedly aided in the down-dip movement of strata that was observed in several of these development sections.
Perhaps the most telling evidence of these challenging conditions occurred in October and November of 1988 when a series of ‘tremor and vibrations’ rocked surface above the Grove No.1 mine (Wu, et al., 1988). The Mine Safety and Health Administration (MSHA) conducted an investigation of the reported tremors and vibrations and found that they had occurred 1.5 months before the writing of the memorandum. These events were thought to emanate from the No. 15 Left and between the No. 11 and 12 Left sections. Both of these areas were in the process of recovering pillars (Figure 1). Both areas were under relatively steep ridges and had removed the ‘safety barrier’, something that is ‘different from regular mining procedures.’
The authors also find it interesting that a magnitude 2.6 earthquake was registered near Jennerstown, Pa on February 3, 1982 (Scharnberger, 2003). Jennerstown is only a few miles from the Grove No.1 mine and this area, according to the Scharnberger report (see Figure 2) has not previously registered an earthquake. Is it possible that this earthquake was caused by pillar recovery mining at the Grove No.1 mine similar in mechanism to the ‘tremors and vibrations’ investigated by MSHA in 1988?
This historical account of how geology and mining interacted to cause considerable deformation of the strata associated with the mining of the Upper Kittanning Coalbed is important for the following reason. After the mine closed and flooded, a major discharge of iron-containing water occurred along Bens Creek (Figure 1). This was not expected since the only mining that occurred under Bens Creek was a main entry system used to access reserves to the northeast. These mains were adequately designed. They should not have damaged the overlying strata. Therefore it is highly unlikely that mine waters could flow upward over 300-ft vertically to the surface.
It is more likely that a significant portion of the strata within and above the Upper Kittanning coalbed was damage to such a degree during pillar recovery events that water flowed vertically and horizontally several hundred feet from the edge of mining to Bens Creek. When the mine began to flood and the fractures and bedding planes were filled with water, discharges occurred along Bens Creek. This mechanism is important to understand if adequate mine layouts are to be designed in the future that will provide a low risk of mine water discharge to surface waters.
[SEET SUPPORTING DOCUMENT FOR FIGURE]|
|Objective of the Paper: |
|The objective of this study is to examine how mine layout, mining method, and geology interacted to allow water from the Grove No.1 mine pool to discharge into local surface waters. The Appalachian Research Initiative for Environmental Sciences (ARIES) has funded a study at the University of Pittsburgh to determine the factors responsible for the successful design of strata barriers. Data analyzed in this study was provided by the PA Department of Environmental Protection.|
|Problem Statement: |
|Section 516(b)(12) [30 USC 1266(b)(12)] of the Surface Mining Control and Reclamation Act of 1977 prohibits the discharge of acid or iron-producing waters from underground coal mines. State regulatory agencies and mining companies have focused on the design of down-dip coal barriers whose main purpose is to prevent mine waters from discharging to the surface. Recently, more attention has been focused on mine layouts that prevent discharges through the strata surrounding the mined coalbed.|