Laserfiche WebLink
October 11, 2011 Page 23 <br />4.10 Structures on Ground - Embedded Metal Columns <br />4.10.1 Subsidence Control Plan for Structures on Ground - Embedded Metal Columns <br />Another class of wellhead structures identified was embedded into the ground using <br />metal columns /poles, e.g., wellhead soundproof walls, electrical panels /meters, and electrical <br />outlets for the well. Since no specific subsidence damage criterion was found for such structures <br />during AAI's literature review on the subject, an allowable horizontal strain of 5 millistrain was <br />deemed appropriate for the purpose, given that these structures are embedded in the ground and <br />are coupled to any potential mining- induced movement. To prevent situations where horizontal <br />strains in excess of 5 millistrain may affect such a structure, no retreat mining should be <br />performed within a 5° vertical offset angle. The vertical offset angle was derived from the <br />subsidence prediction results, which indicated that the furthest a maximum horizontal strain of <br />5 millistrain is observed on the surface is at a vertical offset angle of 2.5° from the retreat line at <br />the seam level. <br />4.10.2 Monitoring Plan for Structures on Ground - Embedded Metal Columns <br />plan: <br />To ensure that the control plan is effective, AAI recommends the following monitoring <br />The structure owners should be informed as mining approaches, and data generated <br />during monitoring should be shared with them. <br />• Visual monitoring should be performed weekly as long as the retreat front is active and <br />within a 20° vertical offset angle of the structures on metal columns embedded in the <br />ground. Any appearance of tilt or change in elevation in these structures and any large <br />cracks that may destabilize them should be noted. <br />• Structural anomalies reported by the surface owner(s) will be investigated. <br />4.10.3 Worst Possible Consequences of Damage to Structures on Ground - Embedded Metal <br />Columns <br />Horizontal strains in excess of the allowable strain levels experienced by the soundproof <br />enclosures for wellheads could twist or bend the metal panels. High strains may damage the <br />electrical panel/meter house causing loss of power supply and, hence, disruption in well <br />production. Damage to electrical outlets for the wellhead could sever power supply to the well <br />equipment and disrupt production. <br />4.10.4 Structures on Ground - Embedded Metal Columns Damage Mitigation Plan <br />In case monitoring indicates onset of damage or potential for damage to the structures on <br />metal columns /poles, the following mitigation plan should be adopted: <br />Appropriate personnel at mine and regulatory authorities, as well as the owners of the <br />structure, should be informed in case anomalies are observed that may indicate an <br />affected structure. <br />Agapito Associates, Inc. <br />