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III IIIIIIIIIIIII III tNDUM INTER ggg TO: ERICA CROSBY" FROM: ALLEN SORENSON ~~ SUBJECT: STABILITY OF HIGH PRESSURE GAS PIPELINE, TELLIER GRAVEL PIT, JOHNSON EXCAVATION, INC., FILE NO. M-98-058 DATE: 09/15/98 The proposed mine plan for the Tellier Gravel Pit includes mining to within 200 feet of a high- pressure gas pipeline. Rule G.4.19 of the Constmcdon Materials Rules and Regulations of the Mined Land Reclamation Boazd specifies that when muting will occur within 200 feet of any significant, valuable, or permanent man-made structure, an apphcant may either: • Provide a notarized agreement between the applicant and the person(s) having an interest in the structure, that the applicant is to provide compensation for any damage to the stmcure. • \X/ttere such an agreement can not be reached, the applicant shall provide an appropriate engineering evaluation that demonstrates that such structure shall not be damaged by activities occurring at the mirtirtg operation. In the case of the Tellier Pit, the applicant has provided an engineering evaluation to demonstrate that their mine plan will be fully protective of the pipeline. Presumably the applicant has attempted to reach an agreement vvith the operator of the pipeline to allow mining within 200 feet, and was unsuccessful If not, the applicant should contact the pipeline operator and attempt to reach an agreement, as this is the preferred mechanism for compliance under the regulations. However, given that the application included an engineering demonstration of stability within the pipeline easement, rather than an agreement with the pipeline operator, the Division's review of the application will proceed under the assumption that such an agreement will not be forthcoming. The elements of the mine plan directed toward protection of the pipeline include a five foot mining setback from the pipeline easement boundaries, as well as a commitment to maintain a maximum 1:1 cut slope at the mined face. A slope stability evaluation was prepazed for the applicant by Northwest Colorado Consultants, and was included in the permit application. The Division is in concurrence with the conclusions in the report that the cut slopes will be resistant to failure with adequate safety factors as long as the report's recommendations aze adhered to during mining of the pit. However, the stability analysis provided applies only to relatively lazge-scale earth movements, such as deep- seated circulaz failure. In addition, the report indicates that there is the potential for surficial sloughing on the ]:] cut slopes that aze proposed. The Division is concerned that smaller scales strains, such as may be caused by relief of pre-existing stresses in the bank when the gravel is excavated, may be suffiuent to adversely affect the pipeline. In order to address the Division's concerns related to protection of the pipeline, the applicant must address the following issues: