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DIVISION OF MINERALS AND GEOLOGY Dep~rtrnent of Natural Resources 131 3 Shcnnan 51.. Koom ?15 Denver. ~ O 80.04 Phone: l:V(13) 866-:;Sb7 FAX: (303) 83?-8106 DATE T0: FROM RE: STATE OF COLORADO May 17, 1993 Lorraine Stepenske James Dillie Slope Stability Analyses, 83rd Joint Venture, M-92-069 pF COQ ~~~; ~. . ~~~` ~ ,q 76 Boy Romer Govemo~ hLChael B. long Division DireUOr Since there are concerns regarding the integrity of an embankment housing a gas transmission pipeline that traverses a proposed sand and gravel pit (the 83rd Joint Venture Pit) it was requested that a slope stability analysis be conducted by the Division to determine whether or not the embankment, given certain preset conditions, would fail to protect the pipeline. Soil properties, embankment dimensions and water information (phreatic surface etc.) supplied by the operator were used in the analyses. SB slope versions 2.0 and 3.0 were used in the evaluations. The analyses were run using the Spencer method for circular shear failure planes. The forced failure mode was used for the analyses. A minimum FOS at the exact location of the pipeline was not determined. In the first computer run a failure plane was forced across the entire width of t:he embankment. As shown on the attached printout, the probability of this event is very low because the factor of safety (FOS> is very high (15.489). During the second run a failure plane was induced at, or near, the estimated location of the pipeline. Again, since the FOS is high (6.206), the probability of a slope failure near the pipeline is very small. Forced failure planes were evaluated at shallower depths (see runs 3, 4, 5 and 6). As expected, because of the type of material in the embankment and the l:l slope, the FOS becomes smaller as the failure plane gets shallower. Indicating, in my opinion, that the probability of a shallow failure along the active mining side of the embankment is very high and, as a result, a failure will occur. The severity of the failure will depend upon whether the operator continues to excavate near the toe of the embankment during, or after, sloughing events. If the sloughed material is not removed the material will eventually reach its angle of repose and remain stable (assuming no severe water or erosion problems). A FOS of 1.5 or above is generally accepted as indicating a low probability of slope failure. The smaller the FOS the higher the probability of slope failure. The FOS of runs 3, 4, 5 and 6 are 1.192, 1.021, 0.865 and 0.828 respectively. In conclusion, the probability of a single slope failure causing structural damage to the pipeline is very small unless mining activities continue at the toe of the embankment after the pit has reached its permitted depth. Cleanup, or mining, of the sloughed material at the toe of the embankment may cause the embankment to fail at, or near, the pipeline. /ern 7446E