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t <br /> <br />• <br />Memo to Tom Schreiner 3 April 23, 1999 <br />sections in the stability analyses attached to this memo. The geometry of the ditch bank and the mining <br />setback illustrated in the stability analysis provided in the application would translate to a 65-foot <br />setback to the top of the ditch bank as suggested by the Division. Stability analyses conducted by the <br />Division (results attached) indicate that such a setback would be acceptable subject to the provision of <br />certain additional information discussed in item 5 below. <br />4. Mining Setback from Sinclair Gas Pipeline <br />The application states that the Sinclair pipeline will be protected by implementation of the boundazy <br />setback discussed in item 2 above. The additional information required and a list of potential options <br />available to the applicant to establish an acceptable boundazy setback have been discussed previously. <br />Additional structural protection issues applicable to the gas pipeline are discussed in this item (4). <br />The applicant's stability analyses apply only to relatively large-scale earth movements, such as deep- <br />seatedcircular failure. The Division is concerned that smaller scales strains, such as may be caused by <br />relief of pre-existing stresses in [he bank when the gravel is excavated, may be sufficient to adversely <br />affect the pipeline. For this reason the installation of strain gauge emplacements on the pipeline would <br />be a reasonable and prudent measure to detect any small movements in the pipeline that may occur. <br />Early detection of small movements in the pipeline would allow for timely stress relieving measures <br />that would prevent damage. In summary, the Division's position on the installation of strain gauges is <br />that: <br />• The gas pipeline is a critical structure, and [he consequences of a failure of the pipeline could be <br />catastrophic. As such, a high degree of conservatism is called for in determining appropriate <br />measures to protect the pipeline. <br />• The strain gauges are an accepted and easy-to-use pipeline-monitoring device that can be installed <br />at a reasonable cost. The added degree of pipeline protection provided by strain gauges can be <br />realized relatively inexpensively; requiring their installation is a prudent addition to the proposed <br />mine plan. <br />Strain gauge installation and monitoring will require the cooperation of the pipeline operator. If such <br />cooperation cannot be secured, the Division would consider proposals for alternate monitoring <br />methods. Stain gauge specifics and alternate monitoring devices are discussed further below. <br />Strain gauges can be used to monitor the pipeline and provide early warning of stress accumulation <br />allowing for mitigation prior to pipeline rupture. An article on the use of strain gauges is attached to <br />this memorandum. If the applicant agrees to install the strain gauges and is able to gain the cooperation <br />of the pipeline operator, then an installation plan (number and location of strain gauge emplacements), a <br />monitoring and reporting plan, and a threshold criteria for mitigative action must be developed and <br />submitted for Division review and approval. It is the Division's understanding that a single strain <br />gauge emplacement is adequate to monitor approximately 200 linear feet of pipeline (Mike <br />Bukovansky, personal communication). Threshold criteria for mitigation can be developed by <br />considering that maximum allowable stresses in the pipeline should not exceed 50 percent of the <br />